/smash/get/diva2:603009/FULLTEXT01.pdf

/smash/get/diva2:603009/FULLTEXT01.pdf

Sludge from pulp and paper mills for biogas production

Strategies to improve energy performance in wastewater treatment and sludge management

Alina Hagelqvist

Faculty of Health, science and technology environmental and energy systems

Dissertation | Karlstad University studies | 2013:9

Sludge from pulp and paper mills for biogas production

Strategies to improve energy performance in wastewater treatment and sludge management

Alina Hagelqvist

DISSERTATION | Karlstad University Studies | 2013:9

Sludge from pulp and paper mills for biogas production - Strategies to improve energy performance in wastewater treatment and sludge management

Alina Hagelqvist

DISSERTATION

Karlstad University Studies | 2013:9

ISSN 1403-8099

ISBN 978-91-7063-484-0

©

The author

Distribution:

Karlstad University

Faculty of Health, Science and Technology

Department of Engineering and Chemical Sciences

SE-651 88 Karlstad, Sweden

+46 54 700 10 00

Print: Universitetstryckeriet, Karlstad 2013

WWW.KAU.SE

Abstract

The production of pulp and paper is associated with the generation of large quantities of wastewater that has to be purified to avoid severe pollution of the environment. Wastewater purification in pulp and paper mills combines sedimentation, biological treatment, chemical precipitation, flotation and anaerobic treatment, and the specific combination of techniques is determined by the local conditions. Wastewater treatment generates large volumes of sludge that after dewatering can be incinerated and thus used for bio-energy production. Sludge is currently viewed as biofuel of poor quality due to its high water content, and some mills treat it solely as a disposal problem.

Two strategies have been identified as feasible options to improve the energy efficiency of sludge management. One is drying using multi-effect evaporation followed by incineration. The other is anaerobic digestion of the wet sludge to produce methane.

This thesis explores the energy balances of sludge management strategies in pulp and paper mills with special focus on anaerobic digestion. The first part consists of a system analysis, used to evaluate some wastewater treatment processes and sludge management, and the second part of empirical studies of anaerobic digestion of pulp and paper mill sludge. It was shown that the use of energy for aeration in aerobic biological treatment should be kept to the minimum required for acceptable quality of the processed water. Additional aeration for reduction of the generated sludge will only result in reduced energy generation in a subsequent methane generation stage. In the second part of the thesis, it is shown that anaerobic digestion is a feasible option for sludge management as it leads to production of high value biogas. Co-digestion with grass silage, cow/pig manure or municipal sewage sludge should then be used to counteract the low nitrogen content of pulp and paper mill sludge.

II

Sammanfattning

Vid produktion av pappersmassa och papper alstras stora mängder avloppsvatten som måste renas för att undvika en allvarlig miljöförorening.

Reningsteknikerna som används kombinerar processer som sedimentering, biologisk behandling, kemisk fällning, flotation och anaerob behandling. Den exakta kombinationen av olika tekniker beror på lokala förhållanden.

Behandlingen av avloppsvatten genererar stora mängder slam som kan användas för bioenergiproduktion. För närvarande behandlas slam som lågkvalitativt biobränsle för samförbränning, och vissa fabriker behandlar det enbart som ett avfallsproblem.

En vanligt förekommande energiåtervinningsteknik är förbränning men den är ineffektiv om vattenhalten i slammet är hög. Två strategier har identifierats som möjliga alternativ för att förbättra energieffektiviteten vid slamhantering. En är energieffektiv termisk avvattning med flereffektsindustning följd av förbränning och den andra är att använda rötning av vått slam för att producera biogas.

Denna avhandling behandlar energieffektiva strategier för slamhantering i massa-och pappersbruk med särskilt fokus på rötning. Den består av en systemanalys, som används för att utvärdera avloppsreningsprocesser och slamhantering, och empiriska studier av rötning av massa-och pappersbruksslam. Det visade sig att användningen av energi för luftning i aerob biologisk behandling bör begränsas till det minimum som krävs för att nå acceptabel kvalitet på det renade vattnet. Ytterligare luftning för minskning av mängden slam skulle bara resultera i ett lägre energiutbyte i efterföljande energiutvinningssteg. Det visade sig också att rötning är ett realistiskt alternativ för slamhantering på massa-och pappersbruk eftersom det leder till produktion av högvärdig biogas. Samrötning bör då användas för att motverka näringsbrist.

III

Preface

The work presented in this thesis was carried out at the Department of Energy,

Environment and Building Technology, Karlstad University, during the years

2006-2012. It mainly deals with the production of biogas by the use of anaerobic digestion of sludges produced in the pulp and paper industry.

At the start of my project, I chose pulp and paper mill sludge as the object of study. The main perspective was energy as such and efficient use of energy, which is in line with the department’s tradition. The reason it became an anaerobic digestion project was the interesting results we obtained in the initial part of the study that later became Paper I. Originally, this doctorate thesis was intended to focus on energy efficient thermal dewatering of sludge from the pulp and paper industry, but the energy balances in Paper I showed that thermal dewatering should generally be avoided. However, anaerobic digestion implied energy recovery without the use of thermal dewatering, and it resulted in a fuel that had the potential to replace fossil fuel. There were (and still are) knowledge gaps regarding anaerobic digestion of this substrate. The original track of energy efficient thermal dewatering was abandoned and the biogas production track with the use of pulp and paper mill sludge was pursued instead.

IV

Acknowledgements

I was fortunate enough to receive a lot of support during this work, and I would like to thank each and every one of those who have made it possible.

First and foremost I want to thank Dr. Karin Granström for sticking with me in spite of it sometimes being very emotional. I also want to give my special thanks to Prof. Thomas Nilsson for having the courage to take this project on in the first place and for always asking the hard questions. It has been a great privilege to work with both of you.

I want to thank Prof. Ulf Germgård for encouraging me to explore the bigger picture.

I would also like to thank my dear colleagues and friends at the Department of

Energy, Environment and Building Technology at Karlstad University for making this endeavour worthwhile. Special thanks are due to Lars Pettersson who helped me with my equipment and to Maria Sandberg who worked with me on my first paper and played a key role in this work.

I also want to thank the personnel at Stora Enso Skoghall AB, Billerud

Gruvöns Bruk, Rottneros AB, Nordic Paper Seffle AB, Säffle Bioenergy HB,

Kristinehamn Wastewater Treatment Plant and Sjöstadsverket in Karlstad for their invaluable help and collaboration. Thank you for the sludge!

Finally, I want to thank my family and friends for always having faith in me.

V

List of publications

This thesis is based on the following papers, which are referred to in the text by their Roman numerals (I-IV). The papers are attached at the end of the thesis.

The reprint is published with the permission of Bioresource Technology.

I. Stoica*, A., M. Sandberg and O. Holby, 2009. Energy use and recovery strategies within wastewater treatment and sludge handling at pulp and paper mills. Bioresource Technology 100, 3497-3505.

II. Hagelqvist, A., K. Granström and D. Mehmood. Improving the biogas potential of pulp and paper mills while decreasing the electricity demand for wastewater treatment. Accepted for presentation at the 1st International IWA Conference on Holistic

Sludge Management, 6-8 May 2013, Västerås, Sweden

III. Hagelqvist, A., in press. Batchwise mesophilic anaerobic co-digestion of secondary sludge from pulp and paper industry and municipal sewage sludge. Waste Management.

IV. Hagelqvist, A. and K. Granström, submitted. Co-digestion of manure with grass silage and pulp and paper mill sludge using the

BMP methodology. Submitted to International Journal of

Environmental Science and Technology.

V. Hagelqvist, A. and M. Murto, submitted. Anaerobic self-degradation of pig and dairy manure using co-digestion with grass silage and pulp and paper mill sludge to shorten the start-up time. Submitted to

Waste Management.

The following related publication is not included in the thesis:

Stoica* A., Handling strategies for mixed sludge from pulp and paper industry,

Proceedings of the 12th European Biosolids and Organic Resources

Conference, Workshop and Exhibition, Manchester England, 2007

* Stoica is the maiden name of the author of this thesis, Alina Hagelqvist.

VI

My contribution to the papers

Paper I: I performed all the work concerning sludge management and roughly half of the work concerning the writing of the manuscript. Dr. Maria Sandberg performed the calculations on the wastewater treatment, and I made the model for sludge management. Together we collected the necessary data from the three sites and synthesised the models of wastewater treatment and sludge management into one model capable of taking into account the necessary parameters to compare different scenarios.

Paper II: I planned the experiments and I wrote the manuscript together with

Karin Granström.

Paper III: I performed all the work on this paper.

Paper IV: I performed most of the experimental work, and I wrote the manuscript together with Karin Granström.

Paper V: I performed all of the experimental work and wrote the first draft of the manuscript. I also adapted and corrected the manuscript after the comments received from Marika Murto.

VII

Table of contents

1 Introduction ....................................................................................................................... 1

1.1 Objectives ................................................................................................................... 3

1.2 Delimitations .............................................................................................................. 4

2. Background ....................................................................................................................... 5

2.1.1 Wastewater treatment in pulp and paper mills generates sludge ...................... 5

2.1.2 Current sludge management ............................................................................... 9

2.2 Issues concerning methane production from pulp and paper mill sludge ............. 12

2.2.1 Reaction pathways ............................................................................................. 13

2.2.2 Inhibition ............................................................................................................ 16

2.2.3 Process consideration ........................................................................................ 17

2.2.4 Environmental risks associated with anaerobic digestion ................................. 19

2.3 The main research approaches of this thesis .......................................................... 20

3 Methods ........................................................................................................................... 20

3.1 System analysis of combined wastewater treatment and sludge management ... 20

3.1.1 Wastewater treatment and production of bio-sludge ....................................... 21

3.1.2 Mechanical dewatering ...................................................................................... 21

3.1.3 Thermal dewatering. .......................................................................................... 22

3.1.4 Anaerobic digestion. .......................................................................................... 22

3.1.5 Incineration ........................................................................................................ 23

3.1.6 Thermal gasification ........................................................................................... 23

3.2 Methane production from pulp and paper mill sludge ........................................... 23

3.2.1 Volumetric biogas monitoring............................................................................ 24

3.2.2 Methane content of gas ..................................................................................... 25

3.2.3 Total solids ......................................................................................................... 25

3.2.4 Volatile solids ..................................................................................................... 25

3.2.5 Nitrogen ............................................................................................................. 25

3.2.6 Chemical oxygen demand .................................................................................. 26

3.2.7 Important parameters that were not analysed ................................................. 26

3.2.8 Using response surface model to investigate the effects on the methane production during anaerobic digestion ....................................................................... 26

4 Results and discussion ...................................................................................................... 27

VIII

4.1 Improving the overall energy balance of combined wastewater treatment and

sludge management ....................................................................................................... 27

4.1.1 Paper I - Energy use and recovery strategies within wastewater treatment and sludge management at pulp and paper mills .............................................................. 27

4.1.2 Paper II- Improving the biogas potential of pulp and paper mills while decreasing the electricity demand for wastewater treatment ................................... 31

4.2 Anaerobic biogas production with pulp and paper mill waste as part of the

feedstock ........................................................................................................................ 32

4.2.1 Paper III - Batchwise mesophilic anaerobic co-digestion of secondary sludge from pulp and paper industry and municipal sewage sludge ..................................... 33

4.2.2 Paper IV - Co-digestion of manure with grass silage and pulp and paper mill sludge using the BMP methodology ........................................................................... 34

4.2.3 Paper V - Anaerobic self-degradation of pig and dairy manure using codigestion with grass silage and pulp and paper mill sludge to shorten the start-up time ............................................................................................................................. 35

5 Conclusions and future perspectives ............................................................................... 37

5.1 Improving the overall energy balance of combined wastewater treatment and

sludge management ....................................................................................................... 37

5.2 Anaerobic biogas production with pulp and paper mill waste as part of the

feedstock ........................................................................................................................ 38

5.3 Future research ......................................................................................................... 38

6 References ........................................................................................................................ 40

IX

1 Introduction

The economics and environmental impacts of the pulp and paper industry all depend on the efficient treatment of wastewater produced in the processes. The pulp and paper industry discards vast amounts of biomass that has been generated during wastewater treatment. This biomass could be used more efficiently for energy generation and play an important role in replacing fossil fuels if it was treated properly.

An estimated 400 million tonnes of paper and paperboard was produced globally in 2012 according to FOEX Indexes Ltd. This production is associated with the yearly generation of an estimated 30-90 billion tonnes of polluted wastewater that needs to be treated yearly (Nemerow and Dasgupta, 1991). The pulp and paper industry is the sixth largest polluter in the world (after the oil, cement, leather, textile, and steel industries) and emits a variety of gaseous, liquid, and solid wastes to the surroundings (Ali and Sreekrishnan, 2001).

Bleaching stages are particularly large sources of pollutant release, whereas the paper making process is much less harmful from an environmental perspective.

The current water pollution issues are primarily related to suspended solids, chemical or biological oxygen demand, and colour. Historically, chlorine used in the bleaching process was the major environmental problem, causing considerable controversy as it contributed with significant amounts of toxic chlorinated organic compounds, among others, dioxins and furans (collectively referred to as adsorbable organic halides). Modern mills using chlorine dioxide or completely chlorine free bleaching processes have dramatically decreased the occurrence of chlorinated compounds in the wastewater (Badar and Farooqi,

2012). Nevertheless, it is still vital that wastewater is treated before being release into the water recipient as other remaining pollutants will cause unacceptable environmental disturbances. Although associated with large investments and high operating costs, wastewater treatment is a necessity in modern pulp and paper manufacturing. It should be noted, however, that there are differences between the practices adopted by the pulp and paper mills in the developing countries and those that are common in the developed world, particularly

Europe and North America. Some old mills in the developing world still use elemental chlorine for bleaching; they release large volumes of wastewater and practice little or no effluent treatment. In the developed countries, the volumes of effluent have been reduced considerably and secondary aerobic biological treatments are employed (Ali and Sreekrishnan, 2001).

The flows of material and energy within an integrated pulp and paper mill are schematically described with figure 1. Natural resources like wood, water and energy are converted to pulp and paper products. Several by-products are also produced, i.e. low grade energy, tall oil, turpentine, waste material and polluted water. The polluted water, i.e. the wastewater, is the focus of this thesis.

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Figure 1. Flows of material and energy within integrated pulp and paper mills.

Purification techniques used for treating the polluted water usually include combinations of sedimentation, biological treatment, chemical precipitation, flotation and anaerobic treatment. The specific combination of techniques depends on the local conditions. The processes of pulp- and papermaking require large amounts of water and freshwater is usually taken from a river or lake. Some organic material ends up in the process water during the papermanufacturing processes; resulting in polluted water that requires purification before being released to the recipient. Some of this organic material settles easily and is removed using sedimentation. The rest of the organic material must first aggregate either biologically or chemically in order to then be easily separated from water. Some of the dissolved organic material is utilised in biological wastewater treatment for growth of various micro-organisms. These micro-organisms are readily separated from water and will then form sludge. A schematic overview covering the processes of main interest within this project is provided in figure 2.

Energy for growth is obtained by oxidation of part of the dissolved organic material with oxygen in cellular respiration. This part is converted to carbon dioxide and water, and the remainder is converted to cellular biomass.

2

Figure 2. Schematic overview of sludge generation and biogas production.

Sludge is thus the solid residue formed during wastewater treatment (mixed with water). Disposal of solid organic waste (especially if it is wet) is expensive, whereas thermal destruction is both expensive and energy-demanding. Efforts have therefore been made to minimise the production of sludge by using advanced biological treatment processes (Mahmood and Elliott, 2006). The dominating method used to decrease sludge formation is the use of a prolonged sludge retention time in the activated-sludge processes, this was also recommended by Mahmood and Elliot (2006). More of the organic material is then metabolised by aerobic micro-organisms and converted to carbon dioxide and water. The method has been shown to successfully decrease the sludge production, but at the expense of an increased use of electrical energy for aeration.

Incineration is a commonly used energy recovery process that deals with two major energy issues: energy recovery and reduced need for transportation of solid residue (Paper I). Increasing energy prices have led to an increased interest for energy issues in wastewater treatment, in particular energy recovery from sludge. The need for efficient sludge management was increased in 2005 when a change of regulations in Sweden banned landfill of untreated sludge

(Miljödepartementet, 2001).

1.1 Objectives

The overall objective of this thesis is to investigate the potential for improvement of energy efficiency in integrated pulp and paper mill wastewater treatment. This includes energy efficiency in wastewater treatment that produces sludge, efficient processing of sludge and energy recovery from sludge.

3

Specific objectives include:

1. To identify and evaluate different strategies for the management of sludge from wastewater treatment in the pulp and paper industry.

2. To investigate whether decreased use of electricity for aeration in aerobic treatment, and the resulting larger production of sludge biomass, can be combined with methane production using anaerobic digestion to improve the net energy balance of wastewater treatment and sludge management.

3. To investigate whether it is an advantage to co-digest pulp and paper mill sludge with substrates that are rich in nitrogen, to overcome poor digestibility that is due to nitrogen deficiency.

Paper I describes the energy situation within pulp and paper mill wastewatertreatment and examines the potentials for bioenergy production from subsequent sludge management. The succeeding papers are based on the findings of Paper I and explore the hypothesis that the energy balance of wastewater treatment and sludge management can be significantly improved.

Paper II addresses the hypothesis that the total energy balance of combined wastewater treatment and sludge management can be enhanced by allowing greater sludge production in aerated biological wastewater treatment. Papers III and IV address the hypothesis that anaerobic digestion is a viable option for treating sludge from secondary wastewater treatment, in particular with respect to anaerobic co-digestion to overcome nutrients deficiency of pulp and paper mill sludge. Paper V finally, tests the hypothesis that pulp and paper mill sludge can shorten the start-up phase of a manure-based anaerobic digester.

1.2 Delimitations

The thesis includes sludge and wastewater from four pulp and paper mills.

These were chosen specifically to be very different and thus to represent a broad spectrum of wastewaters, biological wastewater treatment technologies and sludge management strategies.

Out of the three energy recovery technologies investigated in Paper I, only anaerobic digestion was chosen for further investigation. The choice was made based on the results obtained in Paper I, and a preference to avoid thermal dewatering. Anaerobic digestion is used directly on wet biomass which makes it especially suitable for biomass with poor dewatering properties.

Batch reactors were used for the methane generation experiments in this thesis.

Although continuous reactors dominate in industrial applications, the batch configuration enabled a much higher number of mixtures to be tested in a limited time frame.

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2. Background

2.1 Sludge generation and sludge characteristics

2.1.1 Wastewater treatment in pulp and paper mills generates sludge

In the Swedish forest industry, 505 million m

3

of water is polluted annually in the manufacturing process of pulp and paper mills and thus requires subsequent treatment (Eriksson et al., 2011). The water flows that are of main concern to pulp and paper mills are shown in figure 3. The dominating part of the water used for pulp and paper making is taken from a surface water body adjacent to the mill. The water stream studied in this work is the process-water stream that is delivered to the wastewater-treatment plant for treatment before being released back to a surface-water body.

Figure 3. Schematic presentation of the water flows in pulp and paper making (Eriksson et al., 2011).

Figure 4 presents wastewater treatment in some detail and shows the points of origin for the three most common types of sludge produced in wastewater treatment. Primary treatment is always used; the sludge from this treatment unit is mainly composed of fibres, fines and fillers (Pokhrel and Viraraghavan, 2004;

Mahmood and Elliott, 2006). A primary clarifier achieves some removal of solids, neutralisation and cooling. The primary process unit consists of a sedimentation basin and aims to protect secondary treatment from excessively large loads and shocks and to provide a cost-efficient purification of the effluents. In most cases effluents from pulp and paper mills are then treated with aerobic methods, where the most commonly used methods are the aerated lagoon and the activated-sludge process. These processes are very energy demanding and as much as 80% of the electricity need for wastewater treatment might be used for aeration in aerobic biological treatment (Åsa Sivard, 2010).

Some mills also use chemical flocculation units to further treat the wastewater.

5

The result of secondary treatment will either be bio-sludge from bio-treatment or chemical-flocculation sludge from chemical flocculation treatment, or both.

Phosphorous and nitrogen might need to be added in biological treatment because these substances are required by the micro-organisms that degrade organic substances in the wastewater aerobically, and pulp and paper mill wastewater is deficient in such nutrients. Bio-sludge is often, but not always, separated into two streams, out of which one is recirculated to the bioreactor to increase the concentration of micro-organisms within the reactor and the other is removed from the treatment process, as can be seen in figure 4.

Figure 4. Schematic presentation of production sites and nomenclature of pulp and paper mill sludge. *An activated-sludge process uses recirculation of sludge to increase microorganism concentration.

In table 1, the main organic content of the three different types of sludge are presented. This work is mainly focused on sludge from biological and chemical wastewater treatment, a mix that is often referred to as secondary sludge.

Although, it would be more accurate to call chemical-flocculation sludge

“tertiary sludge”, since it often originates from tertiary treatment. It does in all the cases investigated here.

Table 1. Main components of pulp and paper mill sludges, adjusted from Pokhrel and

Viraraghavan (2004).

Primary sludge

Wood fibre

(cellulose, hemicellulose, lignin)

Bio-sludge

Micro-organisms

Unsettled fibre from primary sedimentation

Chemical-flocculation sludge

Unsettled micro-organisms from the aerated lagoon

Undigested organics

Primary sludge is relatively easy to dewater mechanically, whereas both biosludge and chemical-flocculation sludge are difficult to dewater mechanically to an operationally advantageous content of total solids. This is especially true if the organic substances released back to the return water need to be limited.

Bio-sludge is difficult to dewater mechanically because much of the water is

6

bound as intracellular water or by a surrounding matrix of highly hydrated extracellular polymers. Chemical-flocculation sludge is composed of flocs that are sensitive to shear forces in mechanical dewatering, which limits the use of high pressure and, thus, results in limitation of mechanical dewatering.

Enhanced dewatering of secondary sludge comes at the price of increased organics release, which puts extra strain on wastewater treatment as the polluted water is returned to the bio-treatment.

2.1.1.1 Bio-sludge production

Two biological processes for wastewater treatment are described in this section, the aerated lagoon and the activated-sludge process. These two biological processes are the most common within pulp and paper industry. In these processes, dissolved organic material is converted to carbon dioxide, water and cellular biomass by microbial growth sustained by aerobic respiration.

The aerated lagoon has a large volume, a long retention time for the water and no continuous removal of bio-sludge. The lagoon has to be large to accommodate the conversion of dissolved organics by slow-growing microorganisms. The growth of microbes requires oxygen, which is introduced into the water almost exclusively by using mechanical aeration equipment. The aeration equipment also provides mixing, which is required to keep solids in suspension and to enhance the microbial nutrient uptake. The aerated lagoons are built as earth basins because of the large area and large volume they require.

The aerated lagoon process does not involve recirculation of biomass, which is the primary difference between an aerated lagoon and the activated-sludge process. The settled sludge is removed once every 1-10 years. The use of aerated lagoons has recently become less common for several reasons. In comparison with the activated-sludge process, the aerated lagoons require large land areas and basin volumes; they have high energy requirements and low energy efficiency as regards aeration and mixing. In addition, there can sometimes be problems with effluent foam and smell. The removal and disposal of settled sludge can also generate problems.

The treatment efficiency varies widely depending on the type of effluent, design of the treatment plant and operating conditions. Typical values are 30-60% decrease of chemical oxygen demand. The removal of solids is very case specific and, in some instances, there is a higher concentration of suspended solids in the outlet effluent than in the inlet stream of the biological treatment unit. The cause of this phenomenon is that microbial growth sometimes produces biomass with poor sedimentation characteristics.

The activated-sludge process consists of two main units, an aeration basin and a sedimentation basin. In the aeration basin, water is treated with a culture of micro-organisms that is called activated sludge and is present in high concentration. The subsequent sedimentation basin removes sludge from the effluent. The residence time of water is typically a few hours, which means that

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the volume requirement is much lower than for an aerated lagoon. The sludge is separated from treated effluent in the sedimentation basin and the main part of it is then recycled back to the aeration basin. This recycling is necessary to keep a high sludge concentration in the aeration basin and will decide the sludge retention time, and, as a consequence, the composition of microorganisms in the microbial culture. The retention time of sludge in activatedsludge processes is defined as the average period of time during which the sludge resides within the system (Tchobanoglous et al., 2003). A representation of how the sludge retention time and the hydraulic retention time (retention time of water) are related is shown in

,

(1) where SRT is the sludge retention time [d

-1

], R is the volumetric share of activated sludge recirculation [m

3

/m

3

], V is the tank volume [m

3

], Q is the volumetric flow of wastewater in to the reactor [m

3

/d] and HRT is the hydraulic retention time [d

-1

].

Oxygen and mixing is supplied to the aeration basin by mechanical aeration equipment. The activated-sludge process is used in approximately 60-75% of all the biological-treatment plants in pulp and paper industry. This is also the most common process used in recently built treatment plants. The potential of high or very high treatment efficiencies, the possibilities to control the process

(particularly the oxygen concentration), and the relatively small demands on space are all advantages of the activated-sludge process. Disadvantages include high vulnerability to disturbances with the subsequent risk of operational instability unless protective measures are taken such as installing an equalisation basin. There are also other issues like high production of biological waste sludge and high operating costs. Treatment efficiencies vary and depend on the type of effluent, the design of the treatment plant and the operating conditions.

Typical values are within the range of 60-85% of chemical oxygen demand removal. The overall efficiency of total solids suspended-removal of primary and secondary treatment combined is about 85-90%.

2.1.1.2 Chemical-flocculation sludge production

In some cases, it is necessary to include a tertiary step, which is commonly some type of chemical precipitation. This is common in Swedish pulp and paper mills. Metal ions, mostly iron(III) or alumina(III), are used to decrease the repulsive forces between the negatively charged organic ions that are still present in the wastewater. This allows them to form flocs. Polymers are then used to form and reinforce larger units (of flocs and/or micro-organisms), which can be removed either in a sedimentation basin or a flotation unit depending on their density. Flotation should be avoided if possible, as the requirement of electricity is almost as high as that of aeration in biological wastewater treatment. The cost of chemicals is substantial and the purification

8

is selective; neutral substances cannot be captured as efficiently as dissociated ions (European_Commission, 2001).

2.1.2 Current sludge management

Sludge is the wet solid waste resulting from the cleaning of process wastewater.

From an energy point of view, it holds a considerable potential for bioenergy production. Energy recovery by incineration is however difficult because of the high water content. Water removal is a crucial part of sludge management, and the most energy efficient way to remove water is by using mechanical dewatering. Much of the water in sludge, however, is unavailable for removal with mechanical dewatering. The two sludge management strategies that are currently dominating within the industry are (1) mechanical dewatering followed by composting in order to make material for soil amendment or covering material for landfill and (2) mechanical dewatering and incineration with deposition of the ashes on landfill. The mills are not compelled to perform sludge treatment on site, and some mills choose to outsource the composting or drying to an external operator, with a subsequent increase in transportation requirements. The cost is directly proportional to the mass of solid waste for both strategies.

Most of the problems with sludge management are caused by the dewatering characteristics of sludge. In general, the sludge has poor mechanical dewaterability. The dewatering properties will vary depending on the type of sludge, with an acceptable dewaterability for primary sludge and very poor dewaterability for biological and chemical-flocculation sludge. The mechanical dewatering of pulp and paper mill sludge is usually performed using a series of process units, such as gravity table or rotary thickener followed by a belt press or a screw press, as each process unit operates in different ranges of the total solids content. Pure secondary sludge and digestate from anaerobic digestion generally require a centrifuge for the mechanical dewatering process. Filtrate from the mechanical dewatering units often has a high content of organic substances and requires renewed wastewater treatment.

Composting is the biological decomposition of biodegradable organic matter under aerated conditions, and it is carried out in either windrows or reactors.

Wastes with a high moisture and low fibre content need considerable amounts of moisture sorbing material and structural support to compost well (Tritt and

Schuchardt, 1992). The process generates mostly carbon dioxide, water and some heat, the latter of which is almost never collected. The process may also generate methane and other environmentally harmful gases when released to the atmosphere untreated. Emissions to air, water, and land may present a problem, especially in windrow composting, and it may also reduce the nitrogen

(fertilising) content in the compost (Tritt and Schuchardt, 1992). Presently the composted pulp and paper mill sludge is mostly used to cover landfills that have been discontinued. The European Commission Council Directive on the

9

use of sewage sludge in agriculture (04/07/1986 ) should cover sludge from pulp and paper mill. Pulp and paper mill sludge contains a smaller amount of the regulated metals than are allowed by the maximal limits of concentration; it also, however, contains less nitrogen and phosphorus than sewage sludge. The nutrients content needs to be enhanced if pulp and paper mill sludge is to be used as a fertiliser.

Incineration is commonly used within the pulp and paper industry. The process achieves a complete stabilisation of the sludge and delivers thermal energy and a solid residue in the form of ash. Gyllenhammar et al. (2003) presented an extensive study on emission and operational issues connected with the incineration of sludge. A high content of water disturbs the incineration of biosludge and chemical-flocculation sludge. There is often a need for supporting fuel to avoid a temperature drop in the bed caused by a low net calorific value.

2.1.2.1 Addressing suboptimal sludge management

There is often great potential to improve the energy balance and the economy of wastewater treatment and subsequent sludge management by using other sludge-management strategies than those that are used today. This is due to limitations on the total solids content that can be achieved by using mechanical dewatering, especially for secondary sludge. Strategies that can deliver a higher total energy recovery could include the use of thermal dewatering to remove water before energy recovery by incineration or thermal gasification. They could also include energy recovering technologies that do not depend on water removal, for example anaerobic digestion. Separate treatment of primary and secondary sludge should be aimed at, see figure 5, as the mechanical dewatering of primary sludge is much easier than mechanical dewatering of bio-sludge and chemical-flocculation sludge.

Figure 5. Schematic presentation of sludge production using separate mechanical dewatering for primary and secondary sludge. *An activated-sludge process uses recirculation of sludge to increase micro-organism concentration.

10

There is an upper limit to how far bio-sludge and chemical-flocculation sludge can be dewatered mechanically, and this limit is less than 20% of total solids.

There are two major causes to the poor mechanical dewaterability of biosludge: intracellular water and extracellular polymeric substances with high capacity to bind water (Neyens and Baeyens, 2003). Wet bio-sludge is a viscous fluid composed of micro-organisms suspended in a matrix of extracellular polymeric substances and water. Extracellular polymeric substances are composed of proteins, polysaccharides, DNA from dead cells, lipids, humic substances and heteropolymers (Neyens and Baeyens, 2003; Adav et al., 2008).

The main contributions to extracellular polymeric substances are thought to be cell lysis, the active secretion from micro-organisms and adsorption from the surrounding water body of for instance, artificial polymers, which have been intentionally added and naturally occurring resin acids (Laspidou and Rittmann,

2002; Makris and Banerjee, 2002). Flocs formed in an activated-sludge process are held together by weak forces and small changes in ionic strength or mechanical stress can easily disintegrate them (Keiding and Nielsen, 1997).

Managers of pulp and paper mills are aware that, potentially, some primary sludge can be reused in paper products of lower grade like paper sacks. It can also be mechanically dewatered to make a biofuel of reasonably good quality.

Although primary sludge could be a valuable resource on its own, it is currently common to mix primary and secondary sludge prior to the mechanical dewatering to enhance the dewatering properties of secondary sludge. The resulting mixed sludge will reach total solids contents that are more suitable for incineration than secondary sludge alone; it is not, however, better than pure primary sludge.

2.1.2.2 Available technologies that can improve the total energy balance of sludge management

Thermal dewatering is the removal of water by evaporation, as opposed to mechanical water removal by centrifugation, sieving or pressing, which is used in mechanical dewatering. Evaporation demands about 600 times more energy for every unit of water that is removed. The quality of energy required in the two processes differs, since kinetic energy, which is usually delivered using electricity, is the only acceptable form of energy for mechanical dewatering.

Thermal dewatering requires heat, which can be supplied at for example temperatures above 100 ºC, around 70 ºC or at ambient temperature, depending on the technique used. Basically, it is possible to leave the sludge to dry outdoors where the sun or just the ambient air, supplies the required energy for evaporation. On the plus side should be noted that this energy is considered free of cost and is actually being used industrially, although it is not feasible for all mills. Far from it actually, since slow drying requires much space.

Furthermore, the evaporation will not be limited to water, also malodorous substances are evaporated and spread, which increases the risk for odour problems. Thermal dewatering is often inefficient from an energy point of view and there are several ways to enhance the efficiency. One method is the multieffect evaporation, which increases the efficiency of the dewatering process by

11

reusing evaporation heat within the drying system. Mills that currently use multi-effect evaporation for bio-sludge dewatering do so within their black liquor recovery system.

Anaerobic digestion is a process that can be used for energy recovery from wet sludge; it is, however, very unusual in the pulp and paper industry (Mahmood and Elliott, 2006). Methane is produced in this biological process. The process requires electricity for mixing and pumping purposes and some low temperature heat to sustain the proper temperature within the reactors as the process generates too little heat to be self-supporting in climates such as the

Swedish climate. The methane potential of different sludge types should be known in order to make informed decisions on energy efficient wastewater treatment and sludge management. Anaerobic digestion is described in more detail in subsequent parts of this thesis.

Thermal gasification converts biomass through partial oxidation into a gaseous mixture of syngas (hydrogen, carbon monoxide, methane and carbon dioxide).

Wang et al. (2008) concluded that thermal gasification is a competitive way to use lignocellulosic biomass of low value to produce syngas for combined heat and power generation, for synthesis of liquid fuels, and for the production of hydrogen. The authors concluded that more research is needed to improve the syngas quality for commercial uses in gas turbines, fuel cells, and for the production of liquid fuels and hydrogen.

2.1.2.3 Emerging technologies

Supercritical water treatment of pulp and paper mill sludge was tested by Zhang et al. (2010). This is a promising technique to produce high quality fuels like methane, hydrogen and heavy oils from wet sludges. Sludge of 2% total solids was tested. However, for higher dryness, more than 25% total solids is advisable in order to obtain a positive energy balance (Rönnlund et al., 2011).

The technique is especially interesting because it does not require extensive dewatering beyond what can be achieved with mechanical dewatering. Some of the products may also be useful as replacements for fossil based oils in incinerators.

2.2 Issues concerning methane production from pulp and paper mill sludge

This section aims to outline the issues concerning methane production from pulp and paper mill sludge. It presents all the components that have been deemed relevant to explain the results of Papers II-V. Methane production from sludge is the result of the concerted action of a variety of micro-organisms that use different stages in the process to sustain growth.

12

2.2.1 Reaction pathways

Methane production is the final reaction stage and can take place via two main pathways, i.e., acetoclastic methane production or the hydrogen-consuming pathway where carbon dioxide is used as a respiratory electron acceptor, and hydrogen or formic acid is used as an electron donor (Demirel and Scherer,

2008). Methane production is seldom the rate determining step in anaerobic digestion. Some of the preceding reaction stages, such as hydrolysis and cell lysis, are more often recognised as rate determining (Angelidaki et al., 2009).

The reaction stages in the production of methane from sludge are summarised in figure 6. The first step of anaerobic degradation of cellular material is cell lysis, which means that the cell walls of micro-organisms are broken. Complex polymers (mainly carbohydrates, proteins and lipids) contained within the cell walls will thereby be released in the fluid and made accessible for hydrolysing bacteria. The hydrolysis process that follows is extracellular and uses excreted enzymes to degrade complex polymers into smaller compounds like amino acids, sugars and long chain fatty acids. These products can then be transported across the cell membranes of anaerobic micro-organisms. Volatile fatty acids are formed as intermediate products through fermentation and they are then excreted from the micro-organisms. Carbon dioxide and hydrogen are formed throughout the entire intracellular chain of degradation steps. Hydrogen and formate are used as electron carriers throughout much of the anaerobic digestion process. The species are believed to be in enzyme-assisted equilibrium and either species is accepted by hydrogen utilising methanogens. The four carboxylic acids, C2–C5, are the volatile fatty acids. Acetic acid has a special status because it is the only volatile fatty acid that is used by methanogens to produce methane and therefore it is presented separately from the rest in figure

6. Propionic acid, butyric acid and valeric acid are presented in figure 6 as volatile fatty acids.

13

Figure 6. Schematic presentation of anaerobic digestion process. Sugars, amino acids and long chain fatty acids metabolise to either acetate, volatile fatty acids or formic acid, which then can be converted in accordance with the reactions in the dotted box or end up as methane and carbon dioxide.

2.2.1.1 Methanogenesis

The acetoclastic reaction, formula 1, represents the major pathway of methane production in a well-functioning anaerobic digester. The pathway is predominately used by Methanosaeta, especially at very low concentrations of acetate, since they have a very high affinity to acetate. The pathway is also used by Methanosarcina under favourable conditions, especially at acetate concentrations above 10

-3

M (Stams, 1994; Batstone et al., 2002; Madigan and

Martinko, 2006). Acetate degradation has been shown to occur by way of acetate oxidation formula(2) when Methanosaeta is absent and when inhibiting concentrations of substances, such as ammonia, are present (Zinder and Koch,

1984; Batstone et al., 2002; Karakashev et al., 2006). The thermophilic area of operation, with a moderate total ammonia level and a high acetate level, promotes acetoclastic methanogenesis performed primarily by Methanosarcinales

(Karakashev et al., 2006). Acetate oxidation, formula 2, is thermodynamically feasible only at a very low concentration of hydrogen, and it is thus dependent on the presence of hydrogen-consuming organisms. The second methaneproducing reaction is the reduction of bicarbonate by hydrogen, formula 3. This reaction consumes hydrogen that can be obtained from acetate oxidation or in the acidogenic steps (see 2.2.1.2). The situation where the growth of one species

14

is dependent on the production or removal of hydrogen by another species is labelled syntrophy. The interspecies hydrogen transfer allows growth under conditions that would not sustain growth of either species alone, see formulas 2 and 3.

CH

3

COOH → CH

4

+ CO

2

CH

3

COO

-

+ 4H

2

O → 4H

2

+ 2HCO

3-

+ H

+

4H

2

+ HCO

3-

+ H

+

→ CH

4

+ 3H

2

O

∆G°’=-31kJ/mol

∆G°’=-135kJ/mol

(1)

∆G°’=+104kJ/mol (2)

(3)

Hydrogen-consuming methanogens will be supplied with additional hydrogen from the acidogenesis. These micro-organisms, for example Methanomicrobiales, grow very slowly but they are quite robust and dominate the methanogenic population where the environment is unfavourable to acetoclastic organisms

(Kobayashi et al., 2009).

2.2.1.2 Acidogenesis

Acidogenesis is a collective name for intracellular degradation of released monomers from hydrolysis into volatile fatty acids. Sugars and amino acids will be fermented at the low redox potentials that prevail in the anaerobic digesters.

Long chain fatty acids will be degraded via the path of fatty acid oxidation, which cleaves the fatty acid into acetate units.

Fermentation is an intracellular activity and occurs when the supply of respiratory electron acceptors, such as oxygen, sulphate, nitrate and iron(III), is insufficient. It is the redox-balanced catabolism of an organic compound where

ATP synthesis occurs only by substrate level phosphorylation. This requires the production of a phosphate compound capable of directly transferring a phosphate group to ADP. The phosphate transfer from acetyl phosphate is one example of this. The latter can be formed from acetyl-coenzyme A that is produced in the catabolism of carbohydrates, fatty acids and some amino acids.

Redox balance is maintained by the production and excretion of fermentation products, i.e., volatile fatty acids, alcohols, carbon dioxide, hydrogen, formic acid or ammonia (Madigan and Martinko, 2006). Hydrogen is produced by reduction of protons. This path, however, requires a low concentration of hydrogen to be feasible. Some micro-organisms that use fermentation include members of the Clostridium, Lactobacillus, Streptococcus and Propionibacterium genera.

2.2.1.3 Hydrolysis

Complex polymers are degraded extracellularly into their monomers in the hydrolysis step. The most probable conceptual model for hydrolysis is that organisms attach to particles. Once there, the micro-organisms will release

15

enzymes into the surrounding fluid and benefit from the soluble hydrolysis products that are released by the enzymatic reaction (Batstone et al., 2002).

2.2.1.4 Cell lysis and disintegration

Cell lysis and disintegration can be enhanced by using pre-treatment technology. The most used technologies for pre-treating sludge include systems that are based on sonication, thermal processing, chemical processing and mechanical disintegration. Sludge pre-treatment technology, in combination with an increased methane recovery, a reduced excess sludge generation, and the introduction of carbon credits under the Kyoto Accord, appears to have increased the potential of transferring the anaerobic digestion technology to the pulp and paper industry (Elliott and Mahmood, 2007). The effects of various pre-treatments on lignocellulosic substances have been reviewed by Nizami et al. (2009), who found that the surface area that is accessible for hydrolysis was enhanced by almost all pre-treatment technologies, and some even dissolved hemicellulose and lignin as well, thus enabling a faster hydrolysis process.

Others have also investigated pre-treatment as means to enhance methane production from pulp and paper mill sludge (Lin et al., 2009; Yunqin et al.,

2010; Saha et al., 2011).

2.2.2 Inhibition

Anaerobic digestion is inhibited by several common substances if the concentration is high enough, for example, ammonia, hydrogen and volatile fatty acids. The details on the inhibition mechanisms of each compound are found in Appels et al. (2008).

A high total ammonia level inhibits acetate-utilising methanogens and promotes syntrophic acetate oxidation with hydrogen-consuming methanogens. Hansen et al. (1998) tested the inhibition by ammonia at different temperatures using swine and cattle manure as well as an addition of NH

4

Cl. Ammonia inhibition has been investigated by many researchers since this factor is of great importance (Angelidaki et al., 1993; Vavilin et al., 1995; Poggi-Varaldo et al.,

1997; Hansen et al., 1998; Angelidaki et al., 1999; Hafner and Bisogni Jr, 2009).

The optimum level for volatile fatty acids degradation is in the pH range of 6.5 to 7.2 (Appels et al., 2008). High levels of volatile fatty acids in solution result in a low pH. Low pH will by itself be inhibitory for some organisms. In addition, the protonated volatile fatty acids, which are predominant at low pH, cause free acid inhibition (Batstone et al., 2002). This is especially hard on propionate and butyrate/valerate-oxidising organisms and hydrogen and acetate-utilising methanogenic organisms, as these organisms use metabolic reactions with a low energy yield (Batstone et al., 2002). Because volatile fatty acids degradation is inhibited at low pH-levels the levels of volatile fatty acids are further increased.

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2.2.3 Process consideration

The most important process parameters for commercial anaerobic digestion plants are methane production, digester temperature, pH-value, alkalinity, and concentration of volatile fatty acids, since they all affect or are connected to the rates of the different steps of the digestion process.

2.2.3.1 Operating conditions

The methane production of the reactor can be monitored, for example, by using any type of gas flow measuring device to measure the total amount of biogas and combine it with a device that uses a near dispersion infrared method to analyse the content of methane in the biogas. It might, however, prove tricky to measure the gas flow accurately. Odour risks, poor dewatering properties and pathogens in digestate are potential operational problems with anaerobic digestion that could be managed with process control.

Two temperature ranges are generally considered for industrial application; the mesophilic range that is roughly between 30 °C and 40 °C and the thermophilic range that is roughly between 40 °C and 60 °C. Thermophilic digestion is generally faster than mesophilic digestion because the biochemical reaction rates increase with increasing temperature. Other advantages of thermophilic anaerobic digestion are: increased reduction of volatile solids (such as fats, proteins, cellulose), improved dewatering, and increased destruction of pathogenic organisms. Thermophilic digestion, however, requires more energy, has a higher potential for odour problems and poor process stability (Appels et al., 2008). Operation at either temperature regime requires support heating, as the reaction heat is not sufficient to compensate for the heat loss from the ambient heat exchange. In tropical climates, however, mesophilic anaerobic digestion may not require additional heating.

The pH-value is easy and useful to monitor as both a pH that is too high and a pH that is too low indicate process disturbances. Increasing concentrations of volatile fatty acids can lower the pH to less than 6 and cause process disturbances. The phenomenon can be confirmed by using a quantitative volatile fatty acids analysis with differentiated acids. This disturbance can be managed by a decrease in influent organic load or by adjusting the pH with hydroxide. Currently it appears that there are no online monitoring devices on the market that can monitor the individual volatile fatty acids. Near infrared spectroscopy, however, might prove a feasible solution if combined with suitable multivariate calibration/prediction models (Holm-Nielsen and

Esbensen, 2011). To date, titration methods are commonly used to assess the total amount of volatile fatty acids in the reactor. Some examples of usable titrimetric techniques are presented in Feithenhauer et al. (2002), Lahav and

Morgan (2004) and Madsen et al. (2011).

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High total ammonium content in combination with a high pH is suspected to cause ammonia inhibition, especially in thermophilic reactors. The substrate is certainly the cause of this disturbance and a remedy would be to add a substrate with a higher carbon to nitrogen ratio.

Anaerobic digestion is most often considered to be rate limited by the hydrolysis and cell lysis stages. Various sludge disintegration methods have been studied for pre-treatment purposes in order to increase the rate of methane production; including thermal, chemical, mechanical and biological action.

These are discussed in more detail in Marjoleine and Weemaes (1998), Elliot and Mahmood (2007), Nizami et al. (2009) and Appels et al.(2008).

2.2.3.2 Substrates

Pulp and paper mill sludge has been considered for anaerobic digestion in the past. The biomethane potential of Swedish pulp and paper mill sludges has mostly been focused on bio-sludge (Karlsson et al., 2011), whereas the focus in other countries has been mainly on primary sludge (Rintala and Puhakka, 1994;

Jokela et al., 1997; Lin et al., 2009; Yunqin et al., 2010). It appears that chemical-flocculation sludge has not been investigated separate from the other sludge types as of yet.

Previous work has revealed that nitrogen deficiency is a major concern with anaerobic digestion of pulp and paper mill sludge. Co-digestion with a nutrient rich waste material is considered a low-cost option to improve the nutrient status of anaerobic digestion of pulp and paper mill sludge. Lin et al. (2011) used co-digestion of pulp mill sludge and monosodium glutamate waste liquor to successfully eliminate the nitrogen deficiency problem. Berg et al. (2011) presented a preliminary investigation of anaerobic co-digestion of bio-sludge with a large number of co-substrates, including cows manure. Municipal sewage sludge is an especially suitable co-substrate as it is rich in nitrogen and other required macro and micro nutrients. It is also readily available close to most pulp and paper mills and there is a great deal of accumulated experience of anaerobic digestion operation with this substrate. Furthermore, municipal sewage has been successfully tested in earlier studies of anaerobic digestion of wastewater-treatment sludges from pulp and paper mills (Jokela et al., 1997;

Ghosh and Taylor, 1999).

There are several other reasons why municipal sewage sludge is suitable for codigestion. The solid residue after anaerobic digestion of municipal sewage sludge is often used as a soil amendment, partly in the agricultural sector, and partly in green areas, such as public parks and golf courses. In the county of

Värmland, Sweden, its main areas of use are as covering material for closed landfills where 55% was used and as soil amendment for non-food producing lands where 45% was used (Kanth and Bergström, 2010). The agricultural use of municipal sewage sludge in Sweden is controversial because it contains potentially harmful contaminants, such as heavy metals, pathogens, organic

18

contaminants such as pharmaceuticals, flame retardants, PAHs and PCBs

(Harrison et al., 2006; Smith, 2009; Kanth and Bergström, 2010).

Manure has been identified as a cheap nutrient source for anaerobic digestion as it contains high concentrations of both macro and micro nutrients that might be low in other agricultural products. It is often present at farms where anaerobic digestion is considered, and there are several advantages with using manure as a base load for an anaerobic digestion plant. The process of anaerobic digestion adds value to manure as a fertiliser and helps to decrease some of the problems associated with management of manure. Much work has been done on manure as a substrate for anaerobic digestion, however not so much on co-digestion with pulp and paper mill sludge. Möller et al. (2004) have performed an extensive analysis of pigs and cows’ manure and calculated a theoretical ultimate methane yield to compare with experimental data from batch trials. Lehtomäki et al. (2007) carried out long term continuously fed tank reactor trials with cows’ manure and binary mixtures of cows’ manure and sugar beet tops, grass silage and oat straw. Several others have also investigated anaerobic digestion of pigs and cows’ manure (Angelidaki et al., 1993; Murto et al., 2004; Møller et al., 2004; Amon et al., 2007; Macias-Corral et al., 2008;

Vedrenne et al., 2008; Xie et al., 2011). There is also some unpublished work on grass and beetroot silage storing (Lehtomäki, 2006).

2.2.4 Environmental risks associated with anaerobic digestion

Biogas is a renewable fuel with better environmental footprint than both bioethanol and biodiesel (Börjesson and Mattiasson, 2008). The unintentional release of methane into the atmosphere, however, poses a great environmental risk since methane has high potential as a greenhouse gas, about 20 times that of carbon dioxide. Methane is a rather tricky gas to control, since it is a small molecule and small leakages are common in anaerobic digesters, as well as in composts. Furthermore, it is imperative that the digestate is properly degassed after digestion, since methane continues to be released even after the digestate leaves the digestion chamber, and some methane is also dissolved into the digestate. This phenomenon is called post-methanisation and some anaerobic digestion plants implement anaerobic post-digestion at a lowered temperature to extract as much methane as is economically viable, while allowing the digestate to release most of its dissolved methane within a controlled environment with a gas collection system. The potential for post-methanisation depends on how far the digestion process is allowed to proceed in the main digester. Different operational parameters in the anaerobic digester will therefore give different risks for post-methanisation of the digestate. The methane yield was shown to be highly influenced by the organic loading rate and by feedstock quality whereas the hydraulic retention time had only limited effects (Menardo et al., 2011).

19

The heavy metal content of solid residue have been identified as the main issue of concern when it comes to pulp and paper mill sludges, and needs to be addressed when considering co-digestion with municipal sewage sludge or manure. The solid residue obtained after anaerobic digestion of bio-sludge and chemical sludge is expected to have a lower content of phosphorus and nitrogen as compared to municipal sewage sludge and its digestate. The heavy metal content is generally lower than the recommended levels for bio-fertilisers, except for the cadmium level, which is slightly too high in the untreated sludge.

This represents a major concern if the solid residue is to be used on soil where food is grown; it is therefore not suitable for such purposes. Presently there are no certification rules that can be applied on the use of residue from the pulp and paper industry as an organic fertilizer. This issue is however under discussion (Berg et al., 2011).

2.3 The main research approaches of this thesis

This thesis consists of two interrelated parts. The first is a systems analysis, covering wastewater treatment and sludge management. Energy and materials balances are used to determine whether the current wastewater treatment has the potential to be more energy efficient, and which of the seven modelled sludge management strategies would best serve the studied site. The second part concerns methane production using anaerobic digestion of pulp and paper mill sludge and how methane potential can be increased. The second part is partly an attempt to validate the anaerobic digestion unit of the sludge management model developed in the first part of the thesis (Paper I).

3 Methods

The methods described in this section describes the system analysis used in

Paper I and the analytical methods used to explore the suitability of pulp and paper mill sludge as feedstock for anaerobic digestion (Papers II-V) are described.

3.1 System analysis of combined wastewater treatment and sludge management

The evaluation of different options in the production and management of sludge requires a framework for estimating the overall energy balance for different scenarios for sludge management. Several unit processes that can be combined in different sludge management strategies were thus identified. These are shown in figure 7 together with suitable interconnections. However, in each pulp and paper mill, only a few of these unit processes are utilized. Models for materials and energy balances were developed for each process unit. These models are described briefly in sections 3.1.1-3.1.6.

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Figure 7. Outline of the pulp and paper mill sludge-management model.

3.1.1 Wastewater treatment and production of bio-sludge

The model of wastewater treatment differs somewhat from the subsequent processes. Most of the model is based on calculations of the actual processes and only a small part is theoretical: theoretical oxygen demand and bio-sludge production. Materials and energy balances were calculated based on actual operational data collected on-site. Modelling of the process was based on the work presented by Tchobanoglous et al. (2003). The model relates treatment efficiency (i.e. reduction of dissolved organics), production of bio-sludge, residence time of solids, and the demand for aeration. Aeration accounts for the major part of energy input in this module. Therefore, calculations of theoretical oxygen demand and bio-sludge production as functions of solids retention time within the biological treatment, calculated according to Tchobanoglous et al.

(2003) are included. Further details regarding oxygen demand and bio-sludge production are presented in appendix A.

3.1.2 Mechanical dewatering

On-site data on dewatering equipment was used for the current dewatering process. For simulating mechanical dewatering of pure bio-sludge, chemical sludge and anaerobically digested sludge, it was assumed that these materials reach a total solids content of 20 % in accordance with empirical data from

Tchobanoglous et al. (2003) and Krogerus et al. (1999). The electricity demand of the simulated mechanical dewatering process is calculated assuming that the available sedimentation equipment is complemented with a centrifuge, as secondary sludge and anaerobically digested sludge respond poorly to treatment

21

in belt press and screw press. The most common process units that are used in pulp and paper industry for mechanical dewatering are presented in figure 8 along with their respective ranges of operation.

Figure 8. Water removed by mechanical dewatering units, based on data from Krogerus et al. (1999).

After mechanical dewatering, it is assumed that all solid material is present in the wet sludge. This means that only negligible amounts of solid material will be removed with the filtrate.

3.1.3 Thermal dewatering.

Mill 1 currently uses multi-effect evaporators to dry the bio-sludge before incineration and the actual energy requirement and total solids content that was reached in the drying process were used in the thermal dewatering part of the model for Mill 1. For Mills 2 and 3, the model simulates drying by use of low pressure indirect contact rotating drum dryer run in batch in accordance with the results of Eklund and Eriksson (2002). Negligible heat loss was assumed for the sake of simplicity.

3.1.4 Anaerobic digestion.

The anaerobic digestion process is described in detail in section 2.2. Volatile solids reduction and gas production for anaerobic digestion of secondary sludge were calculated in the model using the results from Puhakka et al. (1992).

Initially, the model presented by Dalemo (1997) was also used. The former model was, however, preferred due to its stronger theoretical foundation. The eight most significant energy fluxes including heat and electricity as well as the production of methane were used in the energy balance model for the

22

anaerobic digestion part of the model in accordance with the energy fluxes discussed in Tchobanoglous et al. (2003) and Dalemo (1997).

3.1.5 Incineration

The incineration model used a net calorific value of 22.8 MJ per kg dry, ash free sludge for Mill 1, in accordance with data on bio-sludge produced in the wastewater treatment of Mill 2. A net calorific value of 21 MJ per kg dry, ash free sludge was used for Mills 2 and 3; this is mixed sludge data from Mill 2.

Complete combustion was assumed, giving pure ash as solid residue.

3.1.6 Thermal gasification

The thermal gasification model uses the sum of the specific values of the energy use and energy generation that were presented by Groß et al. (2008); it used 552 kWh of ignition oil and generated 2496 kWh of excess heat and 588 kWh of excess electricity per tonne of total solids of sludge. This process handles sludge of approximately 2.5% of TS and includes mechanical dewatering, thermal dewatering, thermal gasification process, gas cleaning and electricity generation.

The solid residue was assumed to be pure ash, the same as in the incineration part of the model.

3.2 Methane production from pulp and paper mill sludge

The second part of this work concerns suitable conditions for anaerobic digestion and methane production from pulp and paper mill sludge. This requires methods to study anaerobic digestion under a large number of conditions. A short description of the methods is followed by more in-depth description in section 3.2.1-3.2.8.

In Paper II bio-sludge, which was used for substrate in subsequent anaerobic digestion, was produced in laboratory scale. Sequential batch reactor setup

(Mata-Alvarez et al., 2000) was used to control the length of solids retention time. It was chosen because it provides bio-sludge with a controlled solids retention time and it is closely related to the continuous mode of operation used in activated-sludge process.

Batchwise anaerobic digestion (0.5 L reactors) was chosen as the main approach for methane production to facilitate the parallel study of many variations. The variable of primary interest is the amount of methane produced during the course of a batch run. In order to analyse this, the volume of biogas was measured, and its concentration of methane was determined. The experiments described in Papers II, III, IV and V were all performed using 500 ml Erlenmeyer-flasks, with rubber stoppers perforated with glass tubes. Gas samples were extracted for methane content analysis using a gas tight syringe.

The samples were collected from sampling ports with rubber septum that were provided to all reactors in Papers II, III, IV and V. Temperature control was

23

achieved by using a stationary water bath for Papers II and III, a shaking water bath for Paper V and a stationary water bath or an oven for Paper IV. Two experimental setups were used in the experiments described in Paper IV due to problems with diffuse leakages. Biogas production was measured continuously by a gas flow meter with water displacement in the experiments described in

Papers II, III and IV (see figure 10) and, in the experiments described in Papers

IV and V, by collecting the biogas in gas tight bags and manually emptying the gas bags with a gas tight glass syringe (see figure 9).

Figure 9. Parts a and b show the reactor vessels and the shaking water bath used for the experiments described in Paper V. Part c shows the reactor vessels used for Paper IV. All reactors are 500 ml Erlemeyer-flasks connected to gas-tight aluminium-lined plastic bags.

3.2.1 Volumetric biogas monitoring

A continuous gas flow meter (see figure 10) used water displacement to measure the very slow gas flow of the reactors, i.e., biogas-filled plastic cups submerged in water that flipped and released the gas upon being filled with 5 ml of gas. A magnet, placed on top of each cup, was lifted to switch a reed switch placed outside of the water container, thereby generating a digital signal that was read and monitored with a mini measurement laboratory and a personal computer. The volume required to flip each cup of the gas flow meter was calibrated with a gastight glass syringe of 50 ml. Lacquer was used to seal all tubing connections to minimise biogas leakage.

24

Most of the carbon dioxide present in the biogas expelled from the headspace was expected to dissolve in the water because it is highly soluble in water

(Carroll et al., 1991).

Figure 10. A gas flow measuring device that uses water displacement to measure low gas flows continuously and automatically.

3.2.2 Methane content of gas

The methane content of the biogas was measured by using a gas chromatograph equipped with a flame-ionisation detector. This method is widely used within anaerobic digestion. A Hamilton gastight 1 ml syringe was used to take gas samples from the headspace and the syringe was rinsed with acetone between samples to ensure that it was not clogged with foreign material. The inserted gas volume was 0.5 ml, except during calibration.

Calibration was made with synthetic biogas for calibration purposes: Geotech precision check and calibration gas 60.22 mole% of methane and the rest carbon dioxide. Different methane concentrations were simulated by inserting smaller gas volume. The biogas gave two peaks, one for methane and one for acetone, which were completely separated.

3.2.3 Total solids

The total solids value (TS) represents the residue remaining after the sample has been dried at 103°C to 105 °C until constant weight. The TS value is characteristic of sludge and is often used to express the efficiency of the dewatering process. The detailed standard of this test are presented as standards

2540 D and G (APHA, 2005).

3.2.4 Volatile solids

The volatile solids value (VS) is the mass from the pre-dried residue combusted upon treatment at 550 °C in air. The detailed standard of this test is presented as standard 2540 G (APHA, 2005).

3.2.5 Nitrogen

The nitrogen species that are relevant to wastewater treatment and applications in anaerobic digestion are organic nitrogen, ammonia/ammonium, nitrate and nitrite. Total Kjeldahl nitrogen value, which is normally used in the area of

25

anaerobic digestion, includes ammonia and organically bound nitrogen at the same oxidation state, whereas nitrite and nitrate will be excluded. Organically bound nitrogen includes common biological materials, such as proteins, peptides, urea and nucleic acids. The detailed standard of this test is presented as standard 4500-Norg (APHA, 2005).

3.2.6 Chemical oxygen demand

Chemical oxygen demand (COD) is defined as the amount of a specified oxidant, usually dichromate, which reacts with a fluid sample containing organic substances under controlled conditions. The reduced dichromate is translated to an oxygen equivalent that is then reported as the COD value of the sample.

The detailed standard of this test is presented as standard 5220 (APHA, 2005).

3.2.7 Important parameters that were not analysed

Volatile fatty acids play a key role in the anaerobic digestion process. It would have been advantageous to analyse these, but such analysis equipment has not been available during this work.

Hydrogen is present at very low concentrations (see section 2.2.1). The flameionisation detector would not record any hydrogen peak, as hydrogen is used as combustion gas by the detector. However, it is highly improbable that hydrogen was present in large quantity, as methane production would have been delayed initially due to the slow growth of hydrogenotrophs, see section 2.2.1.1.

3.2.8 Using response surface model to investigate the effects on the methane production during anaerobic digestion

Experiments made with anaerobic digestion of complex substrates and mixtures of these include several factors that could be varied. Design of experiments (DOE) in combination with multivariate data analysis is an efficient approach in these cases (Capela et al., 2008). DOE is a structured approach used to evaluate the effects of factor variation; with efficient use of information from a limited number of experiments. D-optimal design is a feasible option of DOE when one uses data points with an irregular pattern or when qualitative factors are mixed with formulation factors and quantitative factors. Empirical models of specific methane production were made using a multilinear regression method.

26

4 Results and discussion

4.1 Improving the overall energy balance of combined wastewater treatment and sludge management

4.1.1 Paper I - Energy use and recovery strategies within wastewater treatment and sludge management at pulp and paper mills

To identify the processes with greatest potential for energy saving and recovery, different strategies for the production and processing of sludge were investigated in Paper I. The energy balance is of primary interest; however, the analysis would be incomplete without addressing the solid remaining after sludge management as this represents a cost that is proportional to its mass.

Three pulp and paper mills were compared with respect to actual and theoretical bio-sludge production and oxygen demand in biological wastewater treatment. The mills, one NSSC/kraft, one kraft/CTMP and one mechanical pulp mill are presented in table 2. The biological treatment processes of two of the investigated mills differed from the activated-sludge process described in section 2.1.1. Mill 1, which is a NSSC/kraft mill, used a multi-bio concept for wastewater treatment, which is a five-stage configuration of aerobic bioreactors, some with and some without sludge recirculation, to achieve low sludge production with compact reactor volume and moderate sludge retention time (Sandberg and Holby, 2008). Mill 2, which is a kraft/CTMP mill, used a biological treatment unit that was originally designed to be used as an aerated lagoon and which had since been customised to better suit on-site requirements. It operated as an activated-sludge unit with long retention time of water, owing to the very large reactor volume. Mill 3, which is a mechanical pulp mill, used a regular activated-sludge process.

27

Table 2. Descriptive data for case mills.

Pulp process

Pulp production

(t/year)

Paper and board production (t/year)

Mill 1

NSSC a

and kraft pulp

570,000

Mill 2

CTMP b

and kraft pulp

524,000

Mill 3

TMP c

and CTMP b

147,000

517,000

Wastewater treatment

Sludge management strategy

Spent process water flow (m3/day)

Total COD red.

(t/day)

Bio

Primary sed. MultiBio

Mech. dew., thermal dew. and incinerating with black liquor

45,000

30

30

Chemical

SRT in Bio stage with sludge reduction

-

6

Total sludge produced (t TS/day) 11

Primary sludge

Bio sludge

Chemical sludge

Mixed sludge yield

(t/t COD red.)

Secondary sludge yield (t/t COD red.)

Bio

-

11

-

0.36

0.36

0.36

Chemical

Electrical power

WWT (kW)

Aeration in bio

Flocculation and flotation

Total for WWT

(kWh/COD red.)

-

1778

1130

-

Bio

Chemical

Total for WWT

(kWh/t sludge)

1390

884

-

3885

Aeration in bio

Flocculation and

2469 flotation - a

NSSC = Neutral sulphite semi chemical. b

CTMP = Chemo thermo mechanical pulp. c

TMP = Thermo mechanical pulp. d

AS = Activated-sludge treatment.

SRT= solids retention time.

692,000

Primary sed. Aerated

Lagoon with sludge return, chemical floc.

Mech. dew. and incineration with bark

58,000

32

30

2

21

49

31

8

10

1.52

0.54

0.25

5.21

1873

1300

-

1357

1000

-

947

4162

-

-

Primary sed. AS d

, chem.floc.

Mech. Dew. Sludge management off-site

8200

26

21

5

4

32

6

9

16

1.20

0.96

0.45

3.07

762

442

100

674

486

455

647

486

455

28

The energy balances of the present sludge management strategy as well as six other sludge management strategies were estimated for each mill, using on-site data and the mathematical model that was outlined in section 3.1. Further presentation of the separate strategies is made in table 3.

Table 3. Presentation of the seven sludge management strategies studied in Paper I.

Strategy Process units Energy use Energy recovered Type of solid residue

Strategy 1 MD

Strategy 2 MD->I

Strategy 3 MD->TD->I

Electricity

Electricity

-

Solid wet fuel

Electricity, heat Solid dry fuel

Wet sludge

Ash

Ash

Strategy 4 MD->AD->MD

Strategy 5 MD->AD->MD->I

Electricity, heat Methane Wet digestate

Electricity, heat Methane, wet fuel Ash

Strategy 6 MD->AD->MD->TD->I Electricity, heat Methane, dry fuel Ash

Strategy 7 G Fossil oil Electricity, heat Ash

MD = mechanical dewatering, TD = thermal dewatering, I = incineration, AD = anaerobic digestion and G = gasification.

For the sake of comparison, energy consumption and energy recovery at the mills were normalised with respect to the reduction of chemical oxygen demand

(COD) at each mill. In all mills, it was found that the process of treating wastewater was currently the most energy demanding. In this process, Mill 3 had the lowest energy demand and produced most bio-sludge of the three mills, since it had the shortest sludge retention time and used the activated-sludge process. Mill 2 used longer sludge retention time and thus had higher energy demand and lower sludge production. Mill 1 had a higher energy demand and lower sludge production in the biological treatment, as compared to a comparable activated-sludge process. However, this was expected since the mill used a multi-bio configuration in the biological wastewater treatment.

For the overall energy balances, energy recovery from sludge produced in wastewater treatment must be considered. Mill 3 achieved the least favourable total energy balance, since it used no energy recovery. Mill 2 had a very positive energy balance owing to a large fraction of primary sludge in the mixed sludge.

Mill 1 reused its primary sludge (i.e. fibers) in the process, which means that only secondary sludge was available for incineration. Thus, the energy available to be recovered per ton of chemical oxygen demand reduced in wastewater treatment was much less than in Mill 2, where both primary and secondary sludge was incinerated. However, from a resource perspective, it is more efficient to recycle primary sludge to the process where possible compared to energy recovery.

If recycling of primary sludge is not possible, as for example in the manufacture of food- grade packaging, it is well suited for incineration since mechanical dewatering is fairly easy. Primary sludge should therefore be treated separately.

Therefore, management of secondary sludge, without the mixture of primary sludge will be discussed. The energy balances of strategies 3 and 4 were

29

simulated with secondary sludge – no primary sludge – to explore the consequences of using different sludge management strategies on secondary sludge. The results are shown in figure 11. Energy recovery through either incineration or anaerobic digestion is proportional to the secondary sludge production in the wastewater treatment, which means that Mill 3, with the highest secondary sludge production, also had the highest potential for energy recovery per ton of removed chemical oxygen demand.

The experiments performed since Paper I, using secondary sludges from Mills 1 and 2 (Papers II-IV), have shown lower methane yields than predicted by the model. It has also been shown by others that the methane potential of biosludges of different quality vary (Karlsson et al., 2011). The potential for energy recovery by anaerobic digestion of sludge may therefore be overestimated by the model. The sludges of Mills 1 and 2 were found to produce 25-50 % as much methane as predicted by the model (see section 3.1.4) because their sludge was more difficult to digest than the sludge used in Puhakka et al. (1992).

Figure 11. Energy balances for wastewater treatment (sludge production) and sludge management in strategies 3 and 4, expressed as the net energy use and generation per ton of reduced chemical oxygen demand. (WWT = wastewater treatment, MD = mechanical dewatering, TD = thermal dewatering, I = incineration, AD = anaerobic digestion).

From the results of Paper I it can be concluded that the input of energy into wastewater treatment counteracts energy recovery in sludge management, simply by decreasing sludge production. It is thus recommended to aim at sufficient effluent treatment and not use additional electricity for bio-sludge reduction. The increase of secondary sludge production obtained with shortened sludge retention time is favourable from an energy perspective. A prerequisite is that sludge is used for bioenergy production, for example, as fuel for production of heat (and possible electricity) or as substrate for biogas

30

production by anaerobic digestion. When using secondary sludge as fuel, thermal dewatering of the sludge and any technology for energy recovery that is dependent on thermal dewatering should be avoided as it is very energy consuming unless multi-effect evaporators are available. The reason for this is that energy used for thermal dewatering is not easily recovered. Anaerobic digestion is attractive from this perspective, since the technology offers a possibility for energy recovery without the necessity to use dry substrates.

However, anaerobic digestion of pulp and paper mill sludge is not a widely used technology, and knowledge gaps of how this substrate performs during anaerobic digestion need to be further examined. The following sections deal with pulp and paper mill sludge as substrate for anaerobic digestion.

4.1.2 Paper II- Improving the biogas potential of pulp and paper mills while decreasing the electricity demand for wastewater treatment

Paper II deals with anaerobic digestion of sludge produced at shortened sludge retention time in the biological wastewater-treatment step. This corresponds to a scenario where aeration is sufficient for the required reduction of dissolved organics in the wastewater from the pulp and paper mills, but where no additional aeration is used for degradation of the sludge. This study was performed to further investigate the conclusion drawn in Paper I, i.e. that wastewater treatment should only be sufficient to reach the effluent treatment goal, and not for bio-sludge reduction.

In the experimental study bio-sludge was produced using sequential batch reactor mode of operation (Mata-Alvarez et al., 2000). This allows a precise control of the solids retention time. Bio-sludges of 2, 10 and 20-days solids retention times were produced in the lab and their methane formation potential were examined using the biomethane potential method (Angelidaki et al., 2009) under thermophilic conditions (50 °C). The thermophilic temperature was found to be better suited than mesophilic temperature (38 °C) for anaerobic digestion of pulp and paper bio-sludge, as the rate of methane production was increased. The biomethane potentials of the laboratory-produced bio-sludges were then compared to those of three bio-sludges with different sludge retention times that were collected from the wastewater-treatment plant (the multi-bio) of Mill 1. This was done in order to confirm the applicability of the sludge production method and to test the hypothesis that shorter solids retention time within the biological wastewater treatment results in higher methane formation in subsequent anaerobic digestion of the bio-sludge.

It was confirmed that prolonged solids retention time did not further improve the effluent quality, as measured by chemical oxygen demand content and total solids suspended content. The prolonged solids retention time from 2 days to

10 and 20 days only decreased the sludge production and, as a consequence, the potential for methane production, as seen in figure 12. A solids retention time between one and two days gave the highest potential for methane production,

31

although the settleability of the sludge might not be satisfactory at these short solids retention times. The theoretical oxygen demand was calculated according to Tchobanoglous et al. (2003) and it was confirmed that prolonged solids retention time will increase the theoretical oxygen demand and thus the need for aeration, see figure 12. Aeration demand is expected to increase beyond the increase of theoretical oxygen demand, because of the increase of sludge concentration in the reactor with prolonged solids retention time. It has been demonstrated by Sandberg (2010) that aeration efficiency decreases with elevated sludge concentration. It can thus be concluded that more oxygen is required to decrease the sludge output by using a longer retention, i.e. an increased sludge recirculation see equation 1. The mass transfer of this extra oxygen will be impaired by high sludge concentration.

Figure 12. The effect of solids retention time (SRT) on theoretical oxygen demand and methane production from bio-sludge formed in biological wastewater treatment are presented. The striped columns present oxygen demand and the plain columns present methane production. Both parameters have been normalised with respect to COD reduction.

4.2 Anaerobic biogas production with pulp and paper mill waste as part of the feedstock

Pulp and paper mill sludge is nutrient deficient, in particular with respect to nitrogen, and requires additional nutrient sources for good digestibility and for methane production. To investigate whether it is an advantage to co-digest pulp and paper mill sludge with substrates rich in nitrogen, to overcome nitrogen deficiency, pulp and paper mill sludge was co-digested using municipal sewage sludge, cow’s manure, pig’s manure and grass silage. Paper III shows how municipal sewage sludge affects methane potential of pulp and paper mill sludge as co-substrates. Paper IV investigates how manure and grass silage

32

affects methane production when added to pulp and paper mill sludge. Finally,

Paper V is an investigation of how the start-up time of an anaerobic digester can be shortened using pulp and paper mill sludge to increase the carbon to nitrogen ratio of a manure-based reactor.

4.2.1 Paper III - Batchwise mesophilic anaerobic co-digestion of secondary sludge from pulp and paper industry and municipal sewage sludge

The work described in Paper I showed that Mill 2 has the potential for an annual production of biogas with an energy content of 38 GWh, based on its wastewater treatment of mixed (primary and secondary) sludge. However, by using all its sludge for biogas production, the mill would lose 64 GWh of heat, since it is currently incinerating the wet sludge. The investigation of anaerobic co-digestion of municipal sewage sludge and secondary sludge from a pulp and paper mill is described in Paper III. The study is made in order to enhance the methane production. Paper III explores a scenario in which the primary sludge, which is separated from the effluent in a primary clarifier, would be incinerated separately and bio-sludge and chemical sludge (secondary sludge) would be used for anaerobic digestion. In this case, more water can be removed from the primary sludge by way of mechanical dewatering, as the secondary sludge, which is more difficult to dewater, is excluded. Primary sludge can thus be incinerated more efficiently. Furthermore, this would make available muchneeded capacity in the existing mechanical dewatering process, by removing the secondary sludge from the current dewatering process. The aim of Paper III is to investigate whether secondary sewage sludge from Mill 2 is suitable for codigestion with municipal sewage sludge, with regard to specific methane production and solid residue use.

The experiment was performed as a batch operation under mesophilic conditions (38 °C), with the use of inoculum and no buffer or nutrient supplement. Biogas production and methane content were measured during the experiment, and pH-value, total solids and volatile solids were measured before and after digestion.

It was found that methane production was largely the same for municipal sewage sludge as for mixtures of municipal sludge and up to 50% of pulp and paper mill secondary sludge, see figure 13. The same behaviour was observed for degradation of volatile solids. A mixture of bio-sludge and chemicalflocculation sludge from the pulp and paper industry is thus feasible to use for anaerobic co-digestion with municipal sewage sludge. As previously mentioned, the solid residue also needs to be considered as it represents a major cost.

Elemental analysis presented in Paper III has revealed that cadmium poses the greatest risk of all the metals present. The levels found were in the same range as those found in residues from municipal sewage sludge (Naturvårdsverket,

2010). In Sweden, there is much concern about cadmium levels in agriculture

33

and the residue is therefore deemed unsuitable as soil supplement in agricultural areas,

see also section 2.2.4

. It could, however, be used for improving soil quality where food production is not of interest.

Figure 13. Specific methane yield and volatile solids degradation after 20 days of mesophilic batch operation. WAS = municipal sewage sludge, FSS = bio-sludge and chemical-flocculation sludge from pulp and paper mill.

4.2.2 Paper IV - Co-digestion of manure with grass silage and pulp and paper mill sludge using the BMP methodology

The work described in Paper IV aimed to investigate whether co-digestion enhances methane production by affecting other factors than previously known ones, i.e., nutrient deficiency, low buffering capacity, inadequate dilution, and an insufficient activity and amount of inoculum. Manure, grass silage and pulp and paper mill sludge were investigated. The biomethane potential methodology (Angelidaki et al., 2009) was used, both in order to eliminate these known factors, and to examine whether the methane production is affected by unknown factors.

34

The biomethane potential methodology described by Angelidaki et al. (2009) was used. A test duration of 20 days was used in combination with a high inoculum to substrate ratio in order to minimise the risk of a long lag phase at start-up, while working within an industrially feasible time frame. The experiment was performed under mesophilic conditions (38 °C). It was found that the behaviour of combined substrates could be well described by linear combinations of the behaviour of the constituents, which suggests that there are no other significant factors from co-digestion other than those accounted for by the biomethane potential methodology.

An interesting additional finding is that the season of grass silage and manure collection proved to be an important, previously unreported, factor affecting methane production. Short-term methane production from dairy manure and grass silage was found higher in the spring when the grass silage had been stored for a longer period of time. In addition, the dairy manure methane potential co-varied with that of grass silage. It was found always beneficial to methane production to add grass silage to manure. In certain cases, even more slowly degradable substrates, such as sludge, can be added with maintained specific methane production.

4.2.3 Paper V - Anaerobic self-degradation of pig and dairy manure using co-digestion with grass silage and pulp and paper mill sludge to shorten the start-up time

Paper V deals with the start-up procedure of anaerobic digesters. The aim of this paper was to find conditions that shorten start-up times in manure based anaerobic digester using co-digestion with mixed primary and secondary sludge from pulp and paper mills and grass silage. Self-degradation was tested to simulate the setting of many sites where the use of external inoculum is not feasible because of the long distances to anaerobic digestion plants where the appropriate start-up material could be collected. Co-digestion of pig and dairy manure with grass silage and pulp and paper mill sludge was tested using various compositions.

The experiment was setup in laboratory scale (500 ml) batch reactors. Pigs’ manure, dairy manure, grass silage and pulp and paper mill sludge at a total solids concentration of 6% were used to produce mixtures of different compositions. Biogas production and methane content were measured several times during the incubation time (44 days). Total solids, volatile solids and pHvalue were measured before and after digestion, whereas the total ammonia content and total alkalinity value was also measured, but in the digestate only.

The results showed that the mixture with lowest proportion of manure (one third) combined with the largest proportion (two thirds) of pulp and paper mill sludge gave a considerably shorter start-up time (about 15 days) as well as a

35

higher methane yield as compared to other mixtures tested (120 ml CH4/g VS added within the 44 days of batch operation performed), see figure 14.

Figure 14. Accumulated methane yield for the different studied mixtures of manure, pulp and paper mill sludge (PPMS) and grass silage (silage). Manure is the basis of all mixtures.

Dilution of ammonium cannot alone account for the higher methane yield observed with pulp and paper mill sludge. Much less methane was obtained after dilution with grass silage to the same ammonium concentration, see figure

15. This is probably due to build-up of inhibitory volatile fatty acids when grass silage is present. It is therefore preferable to use pulp and paper mill sludge, rather than grass silage, for dilution of manure in order to lower the ammonium concentration.

36

180

160

140

PPMS

Grass silage

PPMS and Grass silage

Manure

120

100

80

60

40

20

0

0 1

Ammonium concentration [g/l]

2

Figure 15. Relation between methane production after 44 days and ammonium content of digestate. PPMS stands for pulp and paper mill sludge. The standard deviation of four replicates is presented for the mixture with the highest methane production.

In conclusion, co-digestion of manure with mixed pulp and paper mill sludge is a simple strategy to shorten the start-up time of a mesophilic (38 ºC) anaerobic digester without the use of external inoculum. Manure dilution with substrates that have high carbon to nitrogen ratios and slow degradability is recommended in order to shorten the lag phase, as (1) the growth phase of the methanogen culture is expected to slow down at total ammonia levels greater than 0.7 g/L, and (2) the culture present in the manure appeared sensitive to easily degradable material like grass silage. It is thus recommended to start-up a full-scale process using low proportion of manure and high proportion of pulp and paper mill sludge. Rapid start-up is then promoted by a moderate concentration of ammonium (0.4 g/l) combined with slowly degradable material like pulp and paper mill sludge, see figure 15.

5 Conclusions and future perspectives

5.1 Improving the overall energy balance of combined wastewater treatment and sludge management

The results obtained in Paper I reveal two approaches to energy efficient secondary sludge management. The first approach relies on efficient thermal dewatering using multi-effect evaporation, followed by incineration of the dried sludge. The second approach uses anaerobic digestion to produce a fuel, i.e. methane, directly from wet sludge.

37

There are several advantages of separate management of primary and secondary sludge. Primary sludge is a potential raw material for paper or board and it can therefore be recycled to the pulp mill in some cases, which is the best use of this sludge. If this is not possible, a second option is mechanical dewatering followed by incineration. Primary sludge is, however, not suited for drying in multi-effect evaporators and it might cause decreased methane production in anaerobic digestion due to its high lignin content (Chen et al., 2008). Separate sludge management would also make it possible to increase the production of secondary sludge, as the ratio of primary to secondary sludge must not necessarily be kept at as high ratio as possible that is today commonly the case with combined sludge management. Increased secondary sludge production makes it possible to significantly improve the energy efficiency in wastewater treatment and sludge management. The results obtained in Papers I and II demonstrate for example that the activated-sludge process should be operated at a short sludge retention time, to decrease energy demand for aeration and to maximise the production of substrate for anaerobic digestion.

5.2 Anaerobic biogas production with pulp and paper mill waste as part of the feedstock

The process of anaerobic digestion offers small margins to energy balances and in Sweden the use of biogas is also limited by poor process economy and lack of an energy efficient national gas distribution system (Lantz et al., 2007).

Development this far has been directed towards local solutions to gas distribution in locations where a grid for distribution of natural gas is absent.

Local production of biogas and local use of the solid residue are essential to this development. Local production and use helps to avoid excessive transportation of gas and wet solids, which would otherwise negatively affect economy and energy balance of the process. Pulp and paper mills sludges offer a large point source of biomass that can enhance the local production capacity considerably.

Efficient anaerobic digestion of pulp and paper mill sludge requires addition of nutrients (in particular nitrogen). The result in Papers III-IV demonstrate that co-digestion with nitrogen-rich substrates, such as manure or municipal sludge, has the capacity to deal with this issue. Moreover, pulp and paper mill sludge can also enhance the digestibility of other substrates by dilution of inhibitory compounds.

5.3 Future research

More knowledge is needed within the area of energy recovery processes for very wet biomass. In the present work, the following areas of further interest have been identified: anaerobic wastewater treatment, thermophilic anaerobic digestion, continuous anaerobic digestion and the subsequent management of solid residue.

38

In Paper I, the contributions of wastewater treatment and sludge management to the pulp and paper mill net energy balance were evaluated using exclusively aerobic wastewater treatment technologies. It is possible to treat at least some of the wastewater anaerobically and thus decrease the need for aeration while increasing the potential for methane production. This has not been investigated in this thesis as anaerobic wastewater treatment is very unusual in Swedish pulp and paper industry; only one mill is using it currently. It would, however, make a big difference for the energy performance of wastewater treatment and sludge management and would thus be interesting to evaluate.

In this thesis, anaerobic digestion was limited to batch operation, which is not the preferred mode of operation in industrial anaerobic digestion. It would be useful to investigate co-digestion of pulp and paper mill sludge with municipal sewage sludge in continuous operation where the inherent nutrient supply of the inoculum is removed. There is also need for deeper understanding of the interplay between regulations, public opinion and economic considerations in management of the solid residue.

39

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Sludge from pulp and paper mills for biogas production

Production of pulp and paper is associated with the generation of large quantities of wastewater that has to be purified to avoid a severe pollution of the environment. Wastewater purification generates sludge which can be used for bioenergy production. The main focus of this thesis is on energy efficient strategies for sludge management, with special focus on anaerobic digestion which produces biogas to replace fossil fuels. Pulp and paper mill sludge is currently treated as a poor quality biofuel and some mills treat it solely as a disposal problem.

The results show that electricity used for aerators in the wastewater treatment plant should be kept to the minimum required for sufficient reduction of dissolved organics. If more electricity is used, i.e. if more air is used for wastewater treatment, this will reduce the energy value of the sludge as the bio-sludge will be degraded. Biogas production using anaerobic co-digestion of pulp- and paper mill sludge together with either municipal wastewater treatment sludge or with manure and grass silage has been studied from several perspectives. It was found that anaerobic digestion is a feasible option for pulp and paper mill sludge management, provided that co-digestion with other substrates is used for nutrient supplementation.

isBn 978-91-7063-484-0 issn 1403-8099

Dissertation | Karlstad University studies | 2013:9

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