P Victor Cucarella Cabañas HOSPHORUS RECYCLING FROM

P  Victor Cucarella Cabañas HOSPHORUS RECYCLING FROM
PHOSPHORUS RECYCLING FROM
WASTEWATER TO AGRICULTURE USING
REACTIVE FILTER MEDIA
Victor Cucarella Cabañas
May 2007
TRITA-LWR LIC Thesis 2039
ISSN 1650-8629
ISRN KTH/LWR/LIC 2039-SE
ISBN 978-91-7178-715-6
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
ii
Phosphorus recycling from wastewater to agriculture using reactive filter media
A CKNOWLEDGMENTS
During the first year of research, funding was received from the European Commission through
a Marie Curie Training Site at the Foundation for Materials Science Development, Krakow,
headed by Prof. Ryszard Ciach. The experimental work was performed at the Agricultural University of Krakow, Poland. I would like to express my gratitude to Dr. Tomasz Zaleski, Dr.
Ryszard Mazurek and other co-workers at the Department of Soil Science and Soil Protection for
their help and support during my fellowship.
In the second year, funding was received from the Swedish company Bioptech AB. I appreciate
both their financial support and their motivating cooperation. I am particularly grateful to Claes
Thilander, MD of Bioptech, for his support and encouragement.
I would like to thank my advisor, Assoc. Professor Gunno Renman at the Department of Land
and Water Resources Engineering, since he is the person who made all this possible from the
beginning. I really appreciate his help and support. I would also like to thank Professor Zygmunt
Brogowski at the SGGW Warsaw and Assoc. Professor Lars Hylander at Uppsala University for
their advice and comments on manuscripts.
I would like to kindly thank my wife Aneta for her patience and understanding, especially, since
she has devoted time and efforts to letting me accomplish this work.
Finally, yet importantly, I want to thank all co-workers and Ph.D. students at the Department of
Land and Water Resources Engineering, for their company, understanding, interesting discussions and help.
Victor Cucarella
Stockholm, May 2007
iii
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
iv
Phosphorus recycling from wastewater to agriculture using reactive filter media
L IST OF P APERS
This thesis is based on the following papers, which are referred to in the text by their Roman
numerals and can be found in Appendix 1-3.
I.
Cucarella, V., Zaleski, Z., Mazurek, R., 2006. Phosphorus sorption capacity of different types of opoka. Journal of Polish Agricultural Universities (accepted for publication).
II.
Cucarella, V., Zaleski, Z., Mazurek, R., 2007. Fertilizer potential of calcium-rich substrates used for phosphorus removal from wastewater. Polish Journal of Environmental Studies (accepted for publication).
III.
Cucarella, V., Zaleski, Z., Mazurek, R., Renman, G., 2007. Effect of reactive substrates used for phosphorus removal from wastewater on the fertility of acid soils.
Submitted to Bioresource Technology.
Articles published or in press are reproduced with kind permission from the respective journals.
v
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
vi
Phosphorus recycling from wastewater to agriculture using reactive filter media
T ABLE OF C ONTENTS
ACKNOWLEDGMENTS ............................................................................................................................... III
LIST OF PAPERS ............................................................................................................................................. V
ABSTRACT......................................................................................................................................................... 1
1
INTRODUCTION..................................................................................................................................... 1
1.1
2
PHOSPHORUS IN THE ENVIRONMENT ..........................................................................................3
2.1
2.2
2.2.1
2.2.2
2.2.3
2.3
2.4
2.4.1
2.4.2
3
Objectives and scope............................................................................................................................. 2
Phosphate resources and extraction implications ................................................................................... 3
Phosphorus in agriculture...................................................................................................................... 3
Soil phosphorus ...................................................................................................................................................... 3
Plant uptake.......................................................................................................................................................... 4
Crop production and P fertilizers ............................................................................................................................ 4
Phosphorus pollution............................................................................................................................ 5
Phosphorus removal and recovery......................................................................................................... 5
Phosphorus recovery from wastewater streams........................................................................................................... 6
Phosphorus recovery from sludge .............................................................................................................................. 6
ONSITE WASTEWATER TREATMENT SYSTEMS.............................................................................7
3.1
Reactive filter media .............................................................................................................................. 8
3.1.1
Phosphorus sorption capacity................................................................................................................................... 8
3.1.2
Recycling of reactive media....................................................................................................................................... 9
4
MATERIALS AND METHODS ..............................................................................................................11
4.1
Materials ............................................................................................................................................. 11
4.1.1
Opoka................................................................................................................................................................. 11
4.1.2
Polonite................................................................................................................................................................ 11
4.1.3
Natural wollastonite............................................................................................................................................. 12
4.1.4
Filtra P ............................................................................................................................................................... 12
4.1.5
Soils .................................................................................................................................................................... 12
4.2
Methods.............................................................................................................................................. 12
4.2.1
Chemical analysis................................................................................................................................................. 12
4.2.2
Phosphorus sorption capacity................................................................................................................................. 14
4.2.3
Pot cultivation experiments ................................................................................................................................... 15
5
RESULTS AND DISCUSSION ............................................................................................................... 16
5.1
5.2
5.2.1
5.2.2
5.3
5.3.1
5.3.2
5.3.3
5.4
6
Reactive media composition................................................................................................................ 16
Phosphorus sorption capacity.............................................................................................................. 16
Batch experiments ................................................................................................................................................ 16
Column experiments............................................................................................................................................. 20
Fertilizer potential of the materials ...................................................................................................... 20
Effect on yield and composition of barley and ryegrass............................................................................................. 20
Effect on soil pH and soil P availabiity................................................................................................................. 22
Effect on other soil properties................................................................................................................................. 23
Suitability of the substrates as soil amendments................................................................................... 24
CONCLUSIONS ...................................................................................................................................... 24
REFERENCES ................................................................................................................................................ 25
vii
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
viii
Phosphorus recycling from wastewater to agriculture by using reactive filter media
A BSTRACT
This thesis focused on testing the suitability of reactive filter media used for phosphorus (P)
removal from wastewater as fertilizers, thus recycling P to agriculture. The work compared the P
sorption capacity of several materials in order to assess their suitability as a source of P for plants.
The selected materials (Filtra P, Polonite and wollastonite) were saturated with P and used as soil
amendments in a pot experiment. The amendments tended to improve the yield of barley and
ryegrass compared with no P addition. The amendments also increased soil pH, P availability and
cation exchange capacity in the studied soils. The substrates studied here can be of particular
interest for acid soils. Of the materials studied, Polonite appears to be the most suitable substrate
for the recycling of P from wastewater to agriculture.
Keywords: Filtra P, phosphorus, Polonite, recycling, sorption, wollastonite
1
instead of conventional sewage systems
(Jantrania and Gross, 2006). Reactive materials with a high affinity for P are used as
filter media for improving the quality of
septic tank effluents. Such media can efficiently retain P by sorption processes on the
surface of the material. The advantage of
using reactive media is that, once saturated
with P, they can be used as soil amendments
in agriculture, thus recycling the nutrients
(Fig. 1).
The amendments may be directly applied to
soils if the content of toxic compounds and
pathogenic bacteria does not restrict their
use according to the EU directive on the use
of sludge in agriculture (86/278/EEC) or
other criteria. The fertilizer potential of such
amendments depends on the amount and
form of P in the substrates and the soil P
status. In addition to direct P supply for
plant uptake, the reactive substrates might
have other potential benefits for soils and
crops, such as increasing the pH of acid soils
or improving soil structure and conditioning
and thus enhancing soil fertility. A large
number of reactive substrates with the ability
to remove P from wastewater are described
in literature (e.g. Johansson Westholm,
2006). However, little is known about their
suitability as soil amendments and their
effectiveness as fertilizers. Therefore, the
effect of such substrates on soils and plants
requires further investigation.
I NTRODUCTION
In a sustainable society, the existing resources must be managed appropriately, in
particular, those that are finite and nonrenewable. This is the case for phosphorus
(P), which is a key element in all living forms
as a component of cell membranes, nucleic
acids and ATP (adenosine triphosphate); and
which is therefore, essential for crop plants.
The increasing world population is demanding and increase in crop productivity and
phosphate rock deposits are being progressively depleted for the fertilizer industry to
meet this demand (Steen, 1998). The excessive application of phosphate fertilizers
together with P from increasing human
waste discharges has altered the natural P
cycle, causing the excessive accumulation of
P in waters and sediments. Conventional
wastewater treatment has significantly reduced point sources of P pollution in the
last two decades. On the other hand, large
volumes of sludge have been deposited in
landfills or incinerated, thus postponing the
pollution problem and losing its potential as
a nutrient (Günther, 1999). Attempts for the
recovery of P from wastewater, although
technically possible, are often economically
unfeasible, especially in small communities
and rural areas. In such places, onsite treatment facilities such as constructed wetlands
and wells or infiltration beds are often used
1
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
Phosphate ores
(P2O5)
Agriculture
Fertilizer
Other
Detergents
Food
Soil P
P run-off
Manure
Septic tank
Wastewater
Eutrophication
Onsite
Reactive
filter media
P recycling
Clean effluent
Figure 1. Sustainable P cycle
1.1 Objectives and scope
The objective of this thesis was to evaluate
the fertilizer potential of reactive substrates
used for the removal of P from wastewater.
A great deal of the work focused on opoka
and its commercial derivative product Polonite, since previous work has shown its
efficiency and promising suitability as a
fertilizer.
The first investigation studied the variation
in the composition of opoka deposits and its
influence on P sorption ability and sorption
mechanisms and, in particular, the relationship between the content and form of Ca
and the P-sorption capacity of three different deposits of opoka, wollastonite tailings
and the commercially available products
Polonite and Filtra P.
The second stage and main part of the work
focused on estimating the fertilizer potential
of three selected materials, Filtra P, Polonite
and wollastonite tailings, saturated with P in
infiltration columns. The materials were
selected for their known ability to remove P
from wastewater. They are characterized by
a high content of Ca and alkaline pH values.
The aim was to study the effect of the Penriched substrates when used as soil
amendments on yield and composition of
barley and ryegrass, and to evaluate the
effect of the amendments on soil pH, availability of P, K and Mg, hydrolytic acidity and
cation exchange capacity in soils after harvest.
2
Phosphorus recycling from wastewater to agriculture using reactive filter media
2
of Cd, which is more abundant in sedimentary deposits, involves further processing
costs to phosphate fertilizer prices.
Phosphates are mostly used to produce
mineral fertilizers, accounting for 80% of the
ore utilisation worldwide, but are also used
in detergents (12%), animal feeds (5%) and
special applications (3%).
P HOSPHORUS IN THE
ENVIRONMENT
In nature, P is released to the environment
by weathering of rocks and is transported by
surface runoff until it reaches water bodies
and soils, thus becoming available to all
living organisms. However, the increasing
demand for P, mainly from agriculture, has
altered the natural cycle, resulting in the
progressive depletion of phosphate ores and
an increase in P concentrations in waters and
sediments.
2.2 Phosphorus in agriculture
Phosphorus is an essential macronutrient for
crop production and together with N and K
is one of the main limiting factors for plant
growth. However, P in soils is poorly available for plants and the application of P
fertilizer is necessary in many agricultural
systems in order to ensure plant productivity.
2.1
Phosphate resources and extraction
implications
Phosphorus is the eleventh most abundant
element in the lithosphere. Phosphate rock
deposits are found throughout the world,
the largest reserves being in Morocco, USA
and China. There are two types of deposits,
igneous and sedimentary, widely differing in
mineralogical, textural and chemical characteristics. The most prevalent phosphate
minerals in these rocks are species of apatite.
Igneous rock is generally low grade (low
phosphate concentration) and therefore,
about 80% of world phosphate is derived
from sedimentary deposits (Steen, 1998).
The phosphate in these rocks is built around
Ca and PO4 structures with varying degrees
of Ca substitution by other elements such as
Na, Mg and heavy metals (Pb, Cd, Cr, As).
This substitution restricts the ability to extract phosphate content so that P2O5 values
may range from 28% in highly substituted
concentrates to 42% in a good quality calcium phosphate rock (Duley, 2001). The
processing methods range from simple milling and screening to extensive washing or
calcinations, depending on the composition
and structure of the sedimentary rocks.
The annual global production of phosphate
is about 50 million tonnes of P2O5 and 75%
of the rock is surface mined. Phosphate ores
are being progressively depleted and production costs are increasing. The current economically exploitable reserves may have a
lifetime of about 100 years (Steen, 1998). In
addition, Cd impurities represent a serious
threat to the environment and the removal
2.2.1
Soil phosphorus
In neutral and calcareous soils, the relative
concentration of phosphate in the soil solution depends mainly on the concentration of
Ca2+ ions and soil pH, which governs the
formation and dissolution of calcium phosphates. The lower the Ca:P ratio of calcium
phosphates, the higher the solubility in water; thus, hydroxyapatite is regarded as quite
insoluble (Mengel and Kirkby, 2001).
Ca(H2PO4)2 + Ca2+ ↔ 2CaHPO4 + 2H+
(calcium monohydrogen phosphate)
3CaHPO4 + Ca2+ ↔ Ca4H(PO4)3 + 2H+
(calcium octophosphate)
Ca4H(PO4)3 + Ca2+ + H2O ↔
Ca5 (PO4)3OH + 2H+ (hydroxyapatite)
From these equilibria it can be seen that
increasing H+ groups in the soil solution has
a positive effect on the solubility of calcium
phosphates but increasing Ca2+ has the opposite effect. These calcium phosphate
products may be present in different crystalline forms. However, in the upper layer of
calcareous and alkaline agricultural soils,
amorphous calcium phosphates generally
dominate. In neutral and acid soils, phosphate adsorption is the dominant process
affecting phosphate availability to plants.
Phosphate ions are adsorbed on Fe and Al
3
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
soil solution is usually quite low; in fact,
more than 80% of soil P becomes immobile
and unavailable for plant uptake because of
adsorption, precipitation and conversion to
organic form (Schachtman et al., 1998).
Plant roots take up P from the soil solution
as ortho-phosphate anions, HPO42- or
H2PO4- depending on the pH. The optimum
pH range for the uptake of P by plants lies
between 5.5-7 (Fig. 2). In addition to the low
availability of soil P, the low diffusion rate of
P in soil (10-12 to 10-15 m2 s-1) creates a depleted zone around the root (Schachtman et
al., 1998). During active growth, plants
maintain between 0.3 and 0.5 % of P in dry
matter. In cases of P deficiency, symptoms
appear as a purplish colouration in the older
tissues of plants due to the formation of
anthocyanins (Valsami-Jones, 2004).
Table 1. Common soil P tests (from Valsami-Jones, 2004)
Name
Composition
Bray 1
Bray 2
DL
Olsen
Mehlich I
Mehlich II
0.03M NH4F + 0.025M HCl
0.03M NH4F + 0.1M HCl
0.02M Ca-lactate + 0.02M HCl
0.5M NaHCO3 - pH 8.5
0.05M HCl + 0.0125M H2SO4
0.015M NH4F+0.2M CH3COOH
+0.25M NH4Cl+0.012M HCl
0.54M CH3COOH
+ 0.7M NaC2H3O3
Morgan
hydrous oxides by ligand exchange in which
OH- groups are replaced by phosphate ions
(Mengel and Kirkby, 2001). Phosphate adsorption is stronger the lower the OH- concentration, i.e. the lower the soil pH. Therefore, the adsorbed phosphate fraction is
dominant in acid soils.
To differentiate between ‘pools’ of phosphorus in soil, a variety of soil P tests have
been developed. Each test dissolves a specific P-pool using acids or alkalis as Pextractants. There is no single accepted
method to determine plant-available soil P in
any soil. Most methods seek to extract P that
is weakly-bound to soil or P in those chemical compounds thought to predominate in
different types of soil, i.e. acidic extractants
for acid soils and alkaline/neutral extractants
for alkaline soils. One of the first Pextractants used to estimate plant-available
soil P was citric acid (1%). The most common methods used nowadays are summarized in Table 1.
The ammonium lactate (AL)-extractable P in
acetic acid (Egner et al., 1960) is the standard commonly used method in Europe.
Water- and CaCl2-extractable P are also
used. However, these chemical extractants
do not always indicate the P status satisfactorily (Hylander et al., 1996).
2.2.2
2.2.3
Crop production and P fertilizers
The application of fertilizer guarantees that
soil contains sufficient readily available P to
allow a crop to achieve the optimum daily
uptake rate for each growing stage.Both the
P status of the soil and the amount and form
of P in the fertilizer influence the contribution from the soil P solution to total plant
uptake (Morel and Fardeau, 1990).
The principal P fertilizers in use today are
triple superphosphate (TSP) 47% P2O5,
diammonium phosphate (DAP) 18% N,
46% P2O5, and monoammonium phosphate
(MAP) 12% N, 52% P2O5. Other sources of
P inputs to agriculture include organic manures such as farmyard manure and slurry,
biosolids (sewage sludge), and recovered
phosphates from wastewater streams. Manures contain around 2.0-2.5 % P on a dry
matter basis. Applied instead of inorganic
fertilizers, manures may reduce P losses
(Smith et al., 2007). However, manures
contain more P relative to N and the application of manures has resulted in P enrichment of soils on farms with animal production. In areas with high animal densities, this
becomes a major potential source of diffuse
losses of P to surface waters (Sharpley et al.,
1994). ‘Mining’ soil P by growing deeprooting crops without any additional P fertilization has been proposed as a possible
Plant uptake
From the point of view of plant nutrition,
soil P can be considered in terms of ‘pools’
with varying accessibility to plants. Phosphorus in the soil solution is fully available
to plants but the concentration of P in the
4
Phosphorus recycling from wastewater to agriculture using reactive filter media
other products. Typical P concentrations in
municipal wastewater range from 6-12 mg
P·dm-3. According to the EU directive on
urban wastewater treatment (91/271/EEC),
the total P effluent concentrations must be
reduced to 1-2 mg·dm-3 with a minimum
reduction of 80%.
2.4 Phosphorus removal and recovery
The principle of phosphorus removal is
based in the transfer of soluble phosphorus
to the solid phase, with a subsequent separation process. There are several alternatives
for removing P from wastewater. Chemical
precipitation of phosphates is carried out by
the addition of coagulants such as alum,
lime, FeCl3 and FeSO4. The final products
are Al, Ca or Fe phosphates precipitated in
the chemical sludge (Brett et al., 1997). Precipitation with Ca salts can also be used but
has a high dependence on pH variations.
The final product in this case is hydroxyapatite. The choice of chemical depends mainly
on the pH of the effluent, cost of chemicals
and the nature of the secondary biological
processes. These are the main reactions
involved in the precipitation of phosphate
and the solubility constants of the phosphate
compounds (Sincero and Sincero, 2003):
Figure 2. Phosphorus availability in relation
to soil pH
strategy for P-enriched soils to decrease the
risk of P leaching (Koopmans et al., 2004).
In addition to P, N and K, other important
plant macronutrients include Ca, Mg and S.
Other elements such as B, Cl, Cu, Fe, Mn,
Mo, and Zn are needed in small or trace
amounts. Factors such as soil structure or
water supply can limit yields irrespective of
the amount of nutrients applied.
2.3 Phosphorus pollution
Elevated phosphorus concentrations in
surface waters can sometimes be of natural
origin (bedrock), but are often the result of
soil erosion, agricultural runoff and discharges of municipal and industrial wastewaters. Agricultural runoff is the major diffuse
source of P in surface waters. Transport of P
from soils to surface waters takes place in
both chemical (dissolved) and physical (particulate) forms. In freshwaters P is usually
the limiting growing factor and high concentrations of P accelerates eutrophication.
On the other hand, point sources of P account for more than half of the phosphates
discharged in Europe (Farmer, 2001). Phosphorus in municipal wastewater originates
mainly from human sources (accounting for
about 2 g P person-1 day-1), but also from
detergents, food waste, food additives and
Fe3+ + PO43- → FePO4 (s) ↓
Ks=10-21.9
Al3+ + PO43- → AlPO4 (s) ↓
Ks=10-21
5Ca2+ + 3 PO43- + OH→ Ca5(PO4)3OH- (s) ↓
Ks=10-55.9
In the biological P removal process, phosphate ions are taken up by bacteria. The
mechanism is based on the importance of
phosphorus as an essential nutrient for microorganisms because of its role in the storage and transfer of energy (Brett et al.,
1997). Conventional activated sludge only
uses enough phosphorus to satisfy their
basic metabolism requirements, resulting in
typical removal rates of 20-40 %. However,
in a treatment plant designed to remove
phosphorus, a particular environment is
created for the proliferation of bacteria that
accumulate phosphorus in excess of normal
5
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
metabolic requirements. An example is the
enhanced biological phosphorus removal
(EBPR) process in which, alternating conditions from initially carbon-rich strictly anaerobic incubation using polyphosphates as
a source of energy are followed by a carbonpoor aerobic environment that enhances the
uptake of a larger amount of orthophosphate by bacteria (Bashan and Bashan,
2004). In both cases, the end product is a
chemical or biological sludge to which P is
tightly bound.
From both chemical precipitation and biological removal processes, P can be recovered either from the supernatant or from the
sludge by different technologies.
2.4.1
may show promising results, but different
factors affect the processes in larger pilot
plants. This was demonstrated by Angel
(1999) for a new process in which a product
containing 18% P in the laboratory was
found to contain far below that amount in
field conditions. In both laboratory and field
experiments the removal of P was satisfactory (>98% and >95% respectively).
2.4.2
Phosphorus recovery from sludge
With the full implementation of the Urban
Wastewater Directive, sludge volumes will
subsequently increase. This sewage sludge
could be recycled directly to agriculture.
However, increasing limitations on sludge
disposal imposed by the EU (Directive
86/278/EEC) are imposing constraints on
this alternative. This is because wastewater
treatment plants inevitably receive not only
household but also industrial waste, some of
which may contain toxic and/or persistent
non-biodegradable compounds, pathogens,
hormones and other undesirable substances.
Land application, landfilling and incineration
are the dominant methods for sludge disposal nowadays and the costs of the last two
options are important (Stark, 2005b). In
addition to elevated costs, these major
routes of waste disposal are not acceptable
in a sustainable society, where the recovery
of nutrients must be achieved. In addition,
dumping biodegradable waste must be reduced according to the Landfill of Waste
Directive (99/31/EC), so landfilling of
sludge will be limited. Some EU countries
such as Sweden, Germany and the Netherlands, have already announced national
objectives on P recovery from sewage. The
Swedish EPA has proposed a target of at
least 60% P recycling from wastewater by
2015 (SEPA, 2000).
There are different options to recover P
from sewage sludge, either as a chemical
precipitate or concentrated in biomass. One
of the most well-known methods of sludge
fractionation is KREPROTM, which treats
the digested sludge with acid hydrolysis at
high temperatures producing iron phosphate. However, the fertilizer potential of
iron phosphate is unclear (Stark, 2005a).
Phosphorus recovery from wastewater streams
Phosphorus can be recovered from wastewater streams as calcium phosphate, which
can be directly utilized in the phosphate
industry. A good example is the DHV CrystalactorTM system in the Geetmerambacht
enhanced biological treatment (the Netherlands), which recovers calcium phosphate as
a pellet formed around a silica sand seed
particle, with a P content of up to 11%
(Duley, 2001). Another possible pathway is
magnesium ammonium phosphate (struvite),
which forms spontaneously in wastewater
with high concentrations of soluble phosphorus and ammonium, low concentrations
of supended solids and pH above 7.5 (Bashan and Bashan, 2004). Struvite has a potential application as a slow-release fertilizer
for direct application in agriculture. The use
of struvite as source of P for plants in a pot
experiment was found to produce similar
effects on the yield of ryegrass to monocalcium phosphate (MCP) (Johnston and Richards, 2003). A recent innovative seedinduced crystallisation process has been
shown to efficiently remove P from wastewater (80-100% P removal) yielding a product containing 10% P (w/w) that could be
recycled by the phosphate industry or even
used directly as a fertilizer (Berg et al., 2005).
The economic viability of these processes
depends significantly on the P content in the
recovered product. Laboratory scale tests
6
Phosphorus recycling from wastewater to agriculture using reactive filter media
Another method is the Aqua-Reci process
with supercritical water oxidation (SCWO),
which decomposes organic matter contaminants, followed by chemical processes to
recover components including iron or calcium phosphates in the residual ash. A novel
technology using phosphate-solubilizing
microorganisms (PSB or PSF) together with
non-soluble phosphate compounds such as
iron phosphate may become a feasible alternative to recover the nutritive value (Bashan
and Bashan, 2004).
If the use of sludge in agriculture is not
possible, the sludge is dewatered and incinerated. Phosphorus can be then recovered
from the ash by hydrochloric acid digestion
as phosphoric acid. Ash usually contains
high concentrations of Zn and Cu, which
limit its use as a fertilizer.
Sewage sludge typically contains between 15% P and reliable technologies may allow
50-80% recovery of sewage phosphates. The
implementation of P recovery, although
technically feasible, involves elevated investment costs, which gives uncertainty to
the economic feasibility. For instance, the
estimated investment cost for the
KREPROTM system in 1999 was 7.3 million
EUR and for the supercritical water oxidation process (SCWO) 8.5 million EUR
(Stark, 2005a).
Furthermore, it is probable that P recovery
from conventional wastewater treatment
systems in rural areas cannot be economically justified.
3
O NSITE WASTEWATER
TREATMENT SYSTEMS
In some countries, water discharges from
private households represent an important
source of P pollution, and it is estimated that
these discharges are of the same magnitude
as the total discharges from all municipal
sewage treatment plants. Decentralized
wastewater treatment systems are a costeffective and long-term option for meeting
public health and water quality goals,
particularly in rural areas (Jantrania and
Gross, 2006). The difference with traditional
septic tanks is the level of treatment and
consequently, the dependence on soil and
site conditions. In order to meet the targets
on nutrient removal from wastewater
imposed by the EU, the quality of septic
tank effluents must be improved. Advanced
onsite treatment is a feasible option to meet
the target and may consist of different
systems grouped as follows:
-
Media filters
Natural systems (wetlands, greenhouse)
Aerobic treatment units (ATUs)
Waterless toilets (dry toilets)
Disinfection systems (UV light, chlorination/dechlorination)
Among the different alternatives, focus is
being put on media filters for its efficiency
and simplicity. Media filters are prepackaged units usually located after a septic
tank that can improve substantially the qual-
Table 2. Typical components and their concentrations in raw wastewater, septic tank
and media filter effluents (from Jantrania and Gross, 2006)
Effluent
BOD
(mg/l)
TSS
(mg/l)
NO3-N
(mg/l)
NH4-N
(mg/l)
D.O.
(mg/l)
Fecal coliform
(cfu/100ml)
PT
(mg/l)
Sewage
Septic
tank
Media
filter
155-286
130-250
155-330
30-130
<1
0-2
4-13
25-60
<2
106-108
105-107
6-12
4-20
5-25
5-30
15-30
0-4
3-5
102-104
(*)
* It strongly depends on the media filter used
7
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
ity of effluents (Hedström, 2006; Jantrania
and Gross, 2006).
The performance of an onsite wastewater
system using media filters depends on different factors such as incoming wastewater
properties, pre-treatment step, size and
arrangement of the system, hydraulic loading, contact time, temperature, etc. Table 2
shows typical effluent concentrations from
media filters. The P removal efficiency depends mostly on the media filter used, although it can be affected by other factors
too. Sand and gravel filters have been used
for many years, but clean sand may remove
some P for only a short period of time. A
material with a strong affinity for P is necessary to remove it efficiently. Natural systems
such as constructed wetlands may also incorporate such media to improve the
performance of the system. A large number
of reactive materials have been lately
proposed as suitable filter media for P removal.
3.1 Reactive filter media
Reactive media may consist of a porous
material with a high affinity for P. Such
media are often called P-sorbents or reactive
substrates. A large variety of reactive substrates with the ability to adsorb P are described in the literature (Mann and Bavor,
1993; Zhu et al., 1997; Baker et al., 1998;
Sakadevan and Bavor, 1998; Drizo et al.,
1999; Brooks et al., 2000; Drizo et al., 2002;
Brogowski and Renman, 2004; Johansson
Westholm, 2006; Ádám et al., 2007). The
studied substrates can be classified in three
groups including natural materials, industrial
by-products and manufactured commercial
products. Table 3 gathers some of the reactive filter media reported in literature.
The substrate must have an appropriate size
and consistency for the filter system to work
properly. The P removal efficiency of a
reactive substrate depends on its structure,
particle size, porosity, pH, and amount of
reactive groups or sorption sites. Substrates
are usually rich in Ca, Fe or Al compounds,
which favour the interaction with P. The
mechanisms of P retention involve sorption
processes at the surface of the material.
3.1.1
Phosphorus sorption capacity
The term sorption was described by
McBride (1994) as a continuous process that
ranges from adsorption to precipitation
reactions. Adsorption can be defined as the
net accumulation of matter at the interface
between a solid phase and an aqueous solution phase and it can take place via different
mechanisms. In the case of P adsorption,
ligand exchange is the predominant mechanism. The binding forces involved in the
ligand exchange are covalent bonding, ionic
bonding, or a combination of the two.
Phosphate ions form inner-sphere complexes on the solid surface (Fig. 3). These
forces are much stronger than those involved in anion exchange, and the phosphate anion is therefore said to be specifically adsorbed. In contrast to nonspecifically adsorbed ions, specifically adsorbed ions are not considered readily exchangeable.
Ion exchange involves non-specific electro-
Table 3. Different types of reactive filter media for phosphorus removal
Natural materials
Industrial by-products
Commercial products
-
Bauxite
Fe-rich sands
Limestone
Opoka
Shale
Shell sands
Wollastonite
Zeolites
- Blast furnace slag (BFS)
- Electric arc furnace slag
(EAF)
- Fly ash
- Red mud (Bauxite residue)
- Ochre
- Steel furnace slag
8
- Filtralite® P (from LECA)
- Light expanded clay
aggregates (LECA)
- Nordkalk Filtra P
- Polonite® (from opoka)
- UTELITETM (from LWA)
Phosphorus recycling from wastewater to agriculture using reactive filter media
3.1.2
static forces that render the phosphate ion
readily exchangeable, i.e. other anions can
displace the phosphate ion. This is important, since this exchange is an important
means of providing readily available nutrient
anions to higher plants (Brady and Weil,
1996). Precipitation, or the formation of
moderately soluble phosphate minerals, is
closely related to the pH of the substrate.
Precipitation of a solid phase cannot occur
until the solubility product of that phase has
been exceeded, i.e. some degree of super
saturation is required. Depending on the
degree of saturation, non-crystalline to
highly crystalline solids are formed
(McBride, 1994). Precipitation mechanisms
are in general much slower than adsorption
reactions.
The P sorption capacity of a substrate can
be determined in batch and column experiments and is expressed as the amount of P
sorbed per unit (usually mass) of substrate.
The P sorption capacity of a substrate can
range from a few hundred milligrams up to
several grams of P per kg of substrate. However, this depends appreciably on the particle size of the material as well as the procedure used to estimate the capacity. Some
industrial by-products and manufactured
products have shown a high to very high P
sorption capacity, for example some types of
fly ash (Xu et al., 2006; Li et al., 2006), different slag materials such as BFS (Sakadevan
and Bavor, 1998; Johansson and Gustafsson,
2000) and EAF (Drizo et al., 2002, 2006),
recovered ochre (Heal et al., 2005), products
derived from light weight aggregates such as
UTELITETM (Zhu et al., 1997), LECA (Johansson, 1997; Drizo et al., 1999) and Filtralite® P (Ádám et al., 2007), the opoka rock
derivative Polonite® (Brogowski and Renman, 2004; Renman et al., 2004) and the
commercial product Filtra P (Gustafsson et
al., 2007).
Not only the sorption capacity but also the
percentage of P removal is relevant when
choosing an appropriate substrate. In some
cases, P removal efficiencies from wastewater can be as high as 95%. This is true for
Polonite (Renman et al., 2004) and Filtra P
(Gustafsson et al., 2007).
Recycling of reactive media
Just like the sludge from conventional
wastewater treatment works, the media used
in onsite filter systems may be recycled directly to agriculture if the content of toxic
compounds and pathogenic bacteria does
not restrict their use according to the EU
Directive 86/278/EEC. Household derived
wastewater from normal human activities
usually has no hazardous components and
therefore, the saturated media from onsite
treatment systems may not be a threat to the
receiving environment.
Land and agricultural application of waste
products has always been regarded as a
possible solution for the disposal of different industry-derived sub-products. In many
cases, the application of such amendments
has improved soil structure, conditioning
and/or even fertility. In the particular case
of acid soils, different amendments have
Inner-sphere
P
Oxygen
OH
O
Metal
F
Water
molecule
Cu+
H2+
Cl -
H
-
Na+
Solid
Diffuse
layer
Outer-sphere
Figure 3. Phosphate complexation at the
surface of a substrate (from Schnoor, 1996)
9
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
shown good results in increasing soil pH.
Some of these include alkaline biosolids
(Sloan and Basta, 1995), wood ash (Demeyer
et al., 2001), fly ash, a by-product of the coal
combustion process (Matsi and Keramidas,
1999; Mittra et al., 2004), and the steel works
by-product blast furnace slag (Kühn et al.,
2006). It is known that cattle manures can
also increase the pH of acid soils and, additionally, recycle P, N and other nutrients to
soils (Whalen et al., 2000). However, as
mentioned before, the excessive application
of manures leads to the accumulation of P in
soils, with the subsequent risk of P leaching
to surface waters. Some of reactive materials
have been proposed as appropriate soil
amendments to retain P thus reducing P
leaching (Summers et al., 1993; Cheung et
al., 1994). Bauxite residue (red mud), an
alkaline by-product from the alumina industry, has been shown to reduce P leaching
from P-enriched sandy soils (Summers et al.,
1996) and to improve P uptake by plants
(Snars et al., 2004). The efficiency of bauxite
residue as nutrient source for plants has
recently been studied (Eastham et al., 2006).
These are some of the examples of beneficial outcomes from waste disposal. In some
cases, the increase in soil pH results in inPlant root hair
Soil P solution
Fertilizer P
creasing P availability, thus improving soil
fertility and crop yield.
In the case of P-saturated media, their fertilizer potential in agriculture has to be tested.
Alternatively, such products may be recovered by the fertilizer industry depending on
their composition. The solubility of P in the
substrate varies depending on its composition and form but it should be in a form
capable of desorbing and being released to
the soil P solution, thus becoming available
to plants. Both the soil P status and the
amount and form of P in the substrate influence the contribution from the soil solution
to total plant uptake (Morel and Fardeau,
1990).
A number of pot experiments have recently
been conducted in order to study the plant
availability of P from different substrates
used for onsite wastewater treatment. In
most cases, the P-saturated substrates improved the yield compared with no P addition. Among the substrates studied, blast
furnace slag and Polonite have been shown
to efficiently improve the yield of barley
(Hylander et al., 2006). Studies on Fe-rich
sands and LECA have shown that P sorbed
to these substrates is as available to ryegrass
P
P P
P
P P
P
P
P
P
P
P
P
P
P
P
P
P
P P
P
P
P P
P PP
P PP P
Soil particle (Fe, Al hydrous oxides, Ca)
Substrate P
P P
P P P P
PP P P
P
P
P
P
P
P
PP
P
Reactive media saturated with P
Figure 4. Schematic representation of phosphorus fertilizing conditions
10
Phosphorus recycling from wastewater to agriculture using reactive filter media
but also in Ukraine, Lithuania and Russia.
Opoka mainly consists of SiO2 and CaCO3
but also contains significant amounts of
Al2O3 and Fe2O3 (Brogowski and Renman,
2004). There is great variation in opoka
deposits in terms of the silica and carbonate
content, which ranges from 37.5 to 52.1 %
silica and 34.5-50.4% calcium carbonate.
Thus, opoka can be classified as light-weight
(more SiO2) and heavy-weight opoka (more
CaCO3). Polish literature gives a wider range
of silica content from 17.06 to 51.88 %
(Bolewski and Turnau-Morawska, 1963).
This type of rock can also be classified as
geza when the silica dominates.
The ability of opoka to remove P from
wastewater is well-known. The process
occurs mainly through Ca-P interactions
(Johansson and Gustafsson, 2000). Opoka
has a moderate to low P sorption capacity;
while some studies have also shown a poor
P-availability for plant uptake from Psaturated opoka (Hylander and Simán,
2001).
Natural opoka was acquired from three
different quarries located in the region of
Miechów, about 60 km north of Krakow,
Poland. The Strzezów quarry contains large
deposits of opoka, which are mainly used for
construction purposes (Fig. 5), while at the
Cisie (Antolka) quarry, a confined layer of
less than one metre depth is present (Fig. 6).
The Widnica quarry is abandoned but opoka
can be acquired at surface level. The materials were crushed and sieved to appropriate
fractions. A particle size of 2-5.6 mm was
used in the column infiltration experiment.
as a water-soluble P compound (Kvarnström
et al., 2004). Phosphorus-saturated ochre, a
by-product from iron mining, has been
shown to function as a slow-release fertilizer
being as effective as conventional P fertilizer
for grass and barley crops (Heal et al., 2003;
Dobbie et al., 2005).
It has been shown that P bound to Ca compounds is more plant-available than P bound
to Al and Fe for some substrates (Hylander
and Simán, 2001). Therefore, calcium derivates might be more attractive from the
point of view of nutrient recycling effectiveness. In addition, such substrates usually
have high pH values, which efficiently reduce the bacteria content in wastewater
(Renman et al., 2004) and may increase soil
pH when used as soil amendments.
Only a fraction of fertilizer P is taken up
immediately by crops, while the remainder
becomes adsorbed and, possibly after further reactions, absorbed to soil particles (Fig.
4). The speed at which this sorption and
other reactions occur depends on the type
and size of the soil particles and the presence of other elements such as Al, Fe, Ca,
soil acidity and organic matter. Phosphorus
in many of the substrates is not as soluble as
in most mineral fertilizers and therefore,
more investigation about their value as a
source of P for crop production is necessary.
4
M ATERIALS AND METHODS
4.1 Materials
The materials used in this study were opoka,
Polonite, wollastonite tailings and Filtra P.
They were chosen for their suitability as
filter media for the removal of P from
wastewater. Polonite and Filtra P are commercially available products used in Scandinavia for the removal of P in onsite wastewater treatment systems.
4.1.1
4.1.2
Polonite
Polonite (Polonite®) is the product of opoka
processing, which consists of thermal treatment at high temperatures for an appropriate period of time. By heating the material,
most of the calcium carbonate is transformed into calcium oxide, which has a
higher solubility product than calcium carbonate and is therefore more reactive in
aqueous solutions. The material is then
sieved to the appropriate fraction to be used
in filter systems.
Opoka
The bedrock opoka is a calcium rich sedimentary deposit from the late Cretaceous
period called Mastrych, formed from the
remains of minute marine organisms (diatoms). Deposits are mainly found in Poland,
11
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
∆T
CaCO3 → CaO + CO2 ↑
The P sorption capacity of Polonite is considerably higher than that of opoka. Its Psorption efficiency depends strongly on
particle size and retention time. The powder
fraction of Polonite showed a P-sorption
capacity of 60-80 mg P·g-1 in batch tests
with an estimated maximum capacity of
117.65 mg P·g-1 according to the Langmuir
isotherm (Cucarella Cabañas, 2000). Other
studies have reported a P sorption capacity
of up to 119 g P·kg-1 (Brogowski and Renman, 2004). Polonite used in an appropriate
size fraction (2-5.6 mm) for infiltration of
sewage showed over 98% P removal and
nearly 99.5% bacteria removal (Renman et
al., 2004). Some studies have shown promising results of Polonite saturated with P as a
fertilizer (Hylander et al., 2006).
Polonite (Fig. 7A) is manufactured by the
Swedish company NCC from raw opoka
bedrock extracted in Poland. Polonite used
in this study had a particle size of 2-5.6 mm,
which is the most appropriate fraction for
large-scale production (Renman, pers. com.).
4.1.3
Soil 1 was acquired in Łazy, situated 40 km
south of Krakow, Poland (20°30’ E;
49°58’N; altitude 320 m asl). It was taken
from the A horizon (0-25 cm) of a cultivated
field, classified as a Haplic Luvisol (FAOISRIC-SICS, 1998), and consists of 12%
sand, 56% silt and 32% clay, with a
pHH2O/KCl of 6.88/6.42, C/N 7.8, and ALextractable P and K of 7.5 and 61 μg g-1 dry
soil respectively.
Soil 2 was acquired in Czarny Potok, a region of southern Poland within the Carpathian mountains (20º54’E, 49º24’N, altitude
720 m asl). It was taken from the A horizon
(0-20 cm) of a mountainous meadow classified as a Dystic Cambisol (FAO-ISRICSICS, 1998) and consists of 40% sand, 37%
silt and 23% clay, with a pHH2O/KCl of
4.22/3.66, and AL-extractable P and K of
4.3 and 27 μg g-1 dry soil respectively.
4.2
4.2.1
Natural wollastonite
Filtra P
Filtra P (Fig. 7C) is a commercial product
developed by the Finnish company Nordkalk. It consists of lime, iron compounds
and gypsum, forming spherical aggregates
with a diameter between 2-13 mm. It is
characterized by high pH values and Ca
content, which favours the interaction with
phosphates. Filtra P has a high P-removal
efficiency, but no studies about its fertilizer
potential were found in the literature.
4.1.5
Chemical analysis
The materials and soil samples were crushed
and milled in a mortar. Triplicate 2 g samples were used for analysis after extraction
with nitric and perchloric acids by heating
for 3-4 days followed by filtration. The element content was determined by atomic
absorption and emission spectrophotometry
using an AAS Ssolar M6 and ICP-AES JY
238 Ultrace.
Natural opoka (Opk) and opoka heated to
900 ºC for 1 hour (900Opk) were analysed
for P, Al, Fe, Ca, Mg, Na, K, Mn, Cu, Zn,
Co and Cd. The silica content was calculated
from the remaining weight of the filter after
incineration at a temperature of 900 ºC. The
CaCO3 content was analysed using the
Scheibler method. The pH was measured in
a 1M KCl:water (1:2.5) solution.
Polonite®, Filtra P and wollastonite tailings,
saturated with P and soil samples were analysed for total P, Al, Fe, Ca, Mg, Mn, Cu,
Zn, Pb and Cd. The pH was then measured
in a 1:2.5 (w/v) material:water and KCl 1M
solution suspension.
Natural wollastonite is a calcium metasilicate
compound with reported P-sorption ability
(Brooks et al., 2000). This material was chosen for its mineralogical similarity to Polonite. Wollastonite tailings (Fig. 7B) produced
in 1-3 mm particle size containing 27.3% of
pure wollastonite were used in this study.
4.1.4
Methods
Soils
Two different types of soils were used together with the material amendments in the
pot cultivation experiments.
12
Phosphorus recycling from wastewater to agriculture using reactive filter media
Figure 5. Deposits of opoka in the quarry of Strzezów
Figure 6. Deposits of opoka in the quarry of Cisie
13
Victor Cucarella Cabañas
A
TRITA LWR LIC Thesis 2039
B
C
Figure 7. Reactive filter media: Polonite (A), wollastonite tailings (B), Filtra P (C)
4.2.2
initial concentration (Ci) at equilibrium (eq.
2,4). This gave a straight line and the Langmuir and Freundlich constants were obtained from the y-axis intercept and the
slope (McKay, 1996).
Phosphorus sorption capacity
The P sorption capacity of the materials
(sorbents) was estimated in batch and column experiments. In the batch experiments
0.1 g of powdered material was placed in 50
ml flasks with an artificial P solution
(KH2PO4) containing increasing concentrations of P (0, 10, 25, 50, 100, 200, 300, 500
mg P⋅dm-3) and then mixed in a rotator at 60
rpm for one hour at room temperature (21
ºC). The materials were milled to powdered
fractions in order to keep a comparable
particle size. The effluent pH was then
measured and the samples were centrifuged
at 6000 rpm and filtered through a 0.45 μm
membrane filter for P determination. The P
was analysed using the standard ammonium
molybdate method (Murphy and Riley, 1962)
in a Beckman DU 600 spectrophotometer.
The difference between the initial (Ci) and
final (Cf) P concentrations at equilibrium
was assumed to be sorbed to the material.
Phosphorus sorption isotherms were plotted
using the amount of P sorbed to the material
(Cs, mg⋅g-1), at different influent P concentrations (Ci, mg⋅dm-3). The isotherm may be
curved reaching a ‘plateau’ at which higher P
concentrations do not increase the sorption
and this is assumed to be the maximum Psorption capacity (Csmax). The experimental
data were fitted to Langmuir and Freundlich
isotherms according to equations 1 and 3
respectively. Sorption parameters were
calculated by plotting graphically the inverse
and logarithm, for Langmuir and Freundlich
respectively, of the sorbed portion (Cs)
against the inverse and logarithm of the
Cs = KL · Ci / 1+aL·Ci
1/Cs = 1/Ci · 1 / KL + aL / KL
(1)
(2)
where KL(dm3·g-1) reflects the adsorptivity
and aL (dm3·mg-1) is related to the energy of
adsorption. KL/aL (mg·g-1) is obtained from
the y-axis intercept and reflects the maximum adsorption capacity.
Cs = aF · Ci bF
log Cs = bF · log Ci + log aF
(3)
(4)
where aF (dm3·g-1) expresses the adsorbent
capacity (the larger the value the higher the
Figure 8. Column experiment set-up
14
Phosphorus recycling from wastewater to agriculture using reactive filter media
capacity) and bF, which ranges from 0 to 1, is
the heterogeneity factor.
The column experiments were conducted
placing 200 g of each material in columns
made from 1-litre plastic graduated cylinders
with perforated bases (Fig. 8). The materials
were used in their original fractions (see
section 4.1). The columns were gravity fed
(unsaturated flow) with a 25 mg P·dm-3
solution as KH2PO4 until saturation was
reached, i.e. the influent and effluent P concentrations did not differ significantly. The
effluent pH was measured and the samples
were centrifuged at 6000 rpm and then filtered through a 0.45 μm membrane filter for
P determination according to the standard
ammonium molybdate method (Murphy and
Riley, 1962) in a Beckman DU 600 spectrophotometer.
The amount of P sorbed to the materials
after saturation was determined by atomic
emission spectrophotometry using an ICPAES JY 238 Ultrace after extraction with
nitric and perchloric acids by heating for 3-4
days followed by filtration.
4.2.3
mately 0.9 g) per pot and 0.6 g per pot,
respectively. The pots were randomly distributed in a greenhouse situated in Krakow
(19º51’54,43”N; 50º00’41,30”E), Poland,
with an insolation of 343 hours (ryegrass)
and 440 hours (barley) during the experiment. The plants were watered every one or
two days to maintain an average soil moisture of 30-35 vol.%. The pots were rearranged every one or two weeks. The average
air temperature during the experiment in the
greenhouse was 15-20ºC. Harvesting took
place on 4 July 2006 (50 days) for ryegrass
and on 18 July (64 days) for barley. The
plants were cut manually at approximately 1
cm above the soil surface, dried at 55ºC and
weighed. Next, leaves and stem from ryegrass and spikes from barley were cut and
milled and the total P, Al, Fe, Ca, Mg, Cu,
Mn, Cd, Pb and Zn concentrations were
determined by atomic spectrometry using an
ICP-AES JY 238 Ultrace after extraction
with a mixture of nitric and perchloric acids
at 200ºC under recirculation conditions.
After harvesting, soil samples from each pot
were analysed for AL-extractable P, K and
Mg in acetic acid (Egner et al., 1960). The
pH was measured in a 1:2.5 (w/v) soil:water
and soil:KCl 1M solution suspension. The
hydrolytic acidity (Hh) in soil after harvesting was determined according to the Kappen
method using 1 mol·dm-3 CH3COONa solution. The base cations (Ca2+, Mg2+, K+ and
Na+) were determined by atomic spectrometry using an AAS Ssolar M6 System after
extraction with a 0.5 mol·dm-3 NH4Cl solution. Total exchangeable bases (TEB), cation
exchange capacity (CEC) and percentage
base saturation (BS) were then calculated as
follows:
Pot cultivation experiments
Polonite®, Filtra P and wollastonite saturated with P in the column experiment were
equilibrated in a 100 mg P·dm-3 solution for
2 days in order to ensure a homogeneous Pcontent in the substrates. After that, the
materials were dried at 105 ºC and milled for
analysis according to section 4.2.1.
Plastic pots (5-litre) were filled with 4.0 kg
wet soil (30-35 vol.%) and received 1.5 g
K2SO4, 0.5 g MgSO4 and 1.0 g NH4NO3 as
basic fertilization, incorporated and mixed in
the middle-upper soil layer (5-10 cm). Phosphorus was added as KH2PO4 or as Psorbed to the materials in quantities based
on previous research (Hylander et al., 2006).
The materials were previously crushed and
sieved to a fraction of 0.5-1 mm in order to
keep a comparable size and enhance P release to the soil solution. No P was added to
the control treatment.
Spring barley (Hordeum vulgare cv. Poldek)
was sown in soil 1 and perennial ryegrass
(Lolium multiflorum cv. Mowester) in soil 2 on
16 May 2006 at a rate of 20 seeds (approxi-
TEB=[Ca2+]+[Mg2+]+[K+]+[Na+]
CEC = Hh + TEB
BS (%) = (TEB / CEC) · 100
(5)
(6)
(7)
All the results were analysed statistically for a
sample size of n=3 (yield), n=6 (element
concentration in plant and AL-extractable P,
K, Mg in soil) and n=9 (Hh and base cations
in soil) and a significance level of p<0.05
15
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
was used to compute the confidence level.
Values lying outside the established confidence interval (based on the p<0.05 confidence level) for a given population mean
were considered significantly different.
5
Fe and relatively higher amounts of Pb
compared with the other materials. Wollastonite contained less Ca, its concentration
being comparable with that of light opoka. It
had visibly higher amounts of Mn than the
rest of the materials. Discussion about the
composition of soils is beyond the scope of
this section and is taken into consideration
when discussing the pot experiments using
the filter materials as soil amendments.
R ESULTS AND DISCUSSION
5.1 Reactive media composition
The chemical composition of the studied
materials is shown in Table 4. According to
the content of silica and calcium carbonate,
opoka can be classified as light-weight (more
SiO2) and heavy-weight (more CaCO3)
opoka. The variability in composition of
three different opoka deposits is presented
in Paper I. Opoka from Strzeżów (Opk1)
can be classified as light opoka and the other
two samples, Opk2 from Cisie (Antolka) and
Opk3 from Widnica, with higher carbonate
content, can be classified as heavy opoka.
There was a noticeably higher content of Al
and Fe oxides in light opoka (Opk1). The
rest of the components appeared in similar
proportions for all three sources of opoka.
The pH was slightly higher in heavy opoka
as a consequence of the higher Ca content.
Most of the CaCO3 and other possible Ca
forms in opoka convert to CaO after heating
to 900 ºC. This has been already reported
and it is the basis for a strong Ca-P interaction due to the higher reactivity of CaO
(Johansson and Gustafsson, 2000; Brogowski and Renman, 2004). The reason for the
increasing amounts of Si and Ca in 900Opk
samples is simply the result of a 7-16%
weight lost due to decomposition of carbonate to CO2 (the higher the CaCO3 the higher the weight loss) increasing the relative
amount of other compounds per unit mass
of the material. The pH values increase
drastically after heating, which also favours
the Ca-P reaction.
The composition of Polonite was comparable with that of 900Opk1, indicating that it
had been produced from light opoka. However, it had noticeably higher amounts of Mg
and Fe. The content of Ca in Filtra P lay
between that of light and heavy burned
opoka. It contained substantial amounts of
5.2
5.2.1
Phosphorus sorption capacity
Batch experiments
The batch experiments showed a high Psorption capacity for powder fractions of all
three types of opoka heated to 900 ºC
(900Opk1, 900Opk2 and 900Opk3), Polonite and Filtra P. The natural form of opoka
was not tested in this study since it has already been reported to have a low P sorption capacity (Johansson and Gustafsson,
2000). The observed P sorption capacity was
higher for the samples of burned heavy
opoka, 900Opk2 and 900Opk3, compared
with the other materials. This was expected
from its higher Ca content, although, light
opoka has a higher porosity that could favour P movement through cavities and make
more sorption places available in the long
term. Heavy opoka has a more compact
structure due to its higher content of CaCO3
and this may have some implications when
using the material in filter systems (see section 5.2.2). The amount of P sorbed to the
materials from the highest influent P concentration (Csmax) represents the maximum
sorption capacity (Table 5). The Langmuir
and Freundlich isotherms fitted well to the
experimental data (Fig. 9, 10). The Langmuir
isotherm appeared to model the sorption by
burned heavy opoka (900Opk2 and
900Opk3) and Filtra P more accurately,
while the Freundlich isotherm fitted better
for burned light opoka (900Opk1) and
Polonite. In previous studies, the Freundlich
equation seemed to work better than Langmuir for slags, zeolites (Sakadevan and
Bavor, 1998), limestone and calcareous soils
(Zhou and Li, 2001).
16
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
Table 4. Element concentration and pH in the studied materials and soils
units
SiO2
Ca
Mg
Fe
Al
Na
K
P
Mn
Zn
Cu
Co
Pb
Cd
g·kg-1
g·kg-1
g·kg-1
g·kg-1
g·kg-1
g·kg-1
g·kg-1
g·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
Soil 1 Soil 2
Opk1
Opk2
Opk3
900Opk1 900Opk2 900Opk3 Polonite Filtra P Wollastonite
(Lazy) (Czarny) (Strzeżów) (Cisie) (Widnica) (Strzeżów) (Cisie) (Widnica)
n.a.
n.a.
494.87 326.17 330.69
664.63
480.92
449.37
n.a.
n.a.
n.a.
0.86
0.10
171.34 326.20 311.58
220.79
424.70
419.75
230.44 343.22
162.41
1.64
2.06
2.56
2.75
2.83
2.02
2.87
3.11
5.01
5.61
1.76
15.99 12.65
10.07
6.38
6.87
6.72
5.71
5.76
11.15
44.42
7.55
n.a.
n.a.
8.96
5.85
7.32
6.41
5.55
7.11
27.0*
11.1*
54.6*
n.a.
n.a.
0.19
0.22
0.22
0.13
0.20
0.21
1.46*
1.46*
12.30*
n.a.
n.a.
1.32
0.84
0.99
0.49
0.43
0.46
9.45*
4.89*
26.6*
0.66
0.57
0.42
0.29
0.34
0.39
0.29
0.32
n.a.
n.a.
n.a.
246.43 114.48
52.86 47.55
12.26
5.47
n.a.
n.a.
15.49 20.27
0.16
0.19
41.43
19.87
5.31
1.29
n.a.
< 0.1
51.92
18.14
4.76
0.95
n.a.
< 0.1
42.33
19.58
6.16
1.11
n.a.
< 0.1
36.87
15.19
2.32
1.43
n.a.
< 0.1
65.82
16.00
2.78
1.44
n.a.
< 0.1
43.41
15.74
3.41
0.91
n.a.
< 0.1
49.90
53.18
4.41
n.a.
0.64
< 0.1
195.65
92.84
5.98
n.a.
7.01
< 0.1
394.87
21.34
3.12
n.a.
0.97
< 0.1
pH H2O
6.88
4.22
n.a
n.a
pH KCl
6.42
3.66
7.48
7.64
n.a.= not analyzed; *from Gustafsson et al. (2007)
n.a
7.55
n.a
12.39
n.a
12.46
n.a
12.41
12.0
n.a
12.5
n.a
9.4
n.a
17
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
140
120
Polonite
100
-1
Cs (mg P g )
Filtra P
80
900Opk1
900Opk2
60
900Opk3
40
20
0
0
100
200
300
400
500
-3
Ci (mg P dm )
Figure 9. P-sorption isotherms for different substrates (lines indicating Langmuir isotherms)
140
120
Polonite
-1
Cs (mg P g )
100
Filtra P
80
900Opk1
900Opk2
60
900Opk3
40
20
0
0
100
200
300
400
500
-3
Ci (mg P dm )
Figure 10. P-sorption isotherms for different substrates (lines indicating Freundlich
isotherms)
18
Phosphorus recycling from wastewater to agriculture using reactive filter media
Table 5. Observed maximum P-sorbed (Csmax) and calculated sorption parameters from
Langmuir and Freundlich equations
Observed
Cs
Polonite
Filtra P
900Opk1
900Opk2
900Opk3
max
Langmuir
-1
(mg·g )
60
70
72
100
120
2
3
-1
Freundlich
-1
R
KL(dm ·g )
KL/aL(mg·g )
R
aF (dm3·g-1)
bF
0.9980
0.9990
0.9777
0.9973
0.9991
0.5294
0.5190
0.3909
0.5126
0.4405
62.3319
179.1495
79.3651
136.9863
181.8182
0.9947
0.9745
0.9836
0.9833
0.9994
1.1072
1.1157
0.9768
0.8714
0.8794
0.6860
0.7416
0.6950
0.8192
0.8227
The sorption parameters calculated are
shown in Table 5. The maximum P sorption
capacity according to the Langmuir model
was lower for burned light opoka (900Opk1)
and Polonite and higher for burned heavy
opoka (900Opk2 and 900Opk3) and Filtra
P. The Langmuir maxima correlated well
with the observed maxima (Csmax) for
burned light opoka and Polonite, since these
materials showed a better fit to the Langmuir model. The Freundlich aF values give a
measure of the relative P adsorption capacity. In this study, the values were not directly
related to the adsorption maxima from the
Langmuir equation. This has been previously
observed by Sakadevan and Bavor (1998),
when using other reactive substrates and it
was attributed to the precipitation of P in
addition to P adsorption. Thus, adsorption
mechanisms dominate in burned light opoka
and Polonite, while precipitation may contribute considerably to the P sorption process in burned heavy opoka and Filtra P. The
relationship between the Ca content and Psorption capacity is also discussed in paper I.
The estimated P-sorption capacity showed a
2
linear correlation with the CaO concentration in the materials. It is not known exactly
what Ca-P compounds were formed. Previous work has shown that slag materials and
opoka may form hydroxyapatite when a
certain supersaturation is reached (Johansson and Gustafsson, 2000). Depending on
the Ca:P ratio, pure CaCO3 can form four
main different species (House, 1999). The P
sorbed may be more or less available depending on the strength of the interaction
and type of compound formed. It is known
that hydroxyapatite is a strong component,
in which form P is not available to plants.
Recent studies have shown that the Ca-P
phase formed in Polonite and Filtra P is
amorphous in nature and that only a proportion of the phosphorus may have crystallized
to slightly less soluble phases (Gustafsson et
al., 2007).
It is important to remember that this research was performed with artificial P solutions and the results may differ when using
wastewater due to the presence of other
substances that may interfere the accessibil-
Table 6. Physical properties, pH value and P-content in soils and in the saturated reactive
substrates
Particle size (mm)
Bulk density (g·cm-3)
pH H2O / pH KCl
P ± SE (μg·g-1)
AL-extractable P
± SE (μg·g-1)
AL-extractable K
± SE (μg·g-1)
Soil 1 (Lazy)
Soil 2 (Czarny)
Polonite
Filtra P
Wollastonite
6.88/6.42
4.22/3.66
2-5.6
0.7
9.88 / 9.90
2-13
1.0
11.49 / 11.36
1-3
1.4
9.02 / 8.76
663.73±7.85
569.13±11.28
1862.15±53.58
713.64±115.99
253.96±12.73
7.51 ± 0.15
4.32 ± 0.06
49.88 ± 0.31
5.62 ± 0.38
13.93 ± 0.61
61.33 ± 0.24
26.91 ± 0.35
62.13 ± 0.02
23.48 ± 0.58
4.49 ± 0.01
19
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
ity of P to the sorption sites.
These results are not applicable when using
other fractions of the materials and they
should be viewed with caution. Coarser
fractions have higher hydraulic conductivity
and are therefore more suitable for infiltration systems, but they have smaller surface
area and their expected sorption capacity will
be lower than that estimated with powdered
material.
5.2.2
larger compound, which could not pass
through the 0.45 μm filter for P-PO4 determination. Supporting this explanation is the
fact that Filtra P disintegrated in water with
time, giving a yellow colour to the effluent.
The P sorption capacity of wollastonite was
much lower and saturation was reached
quicker. Previous research has shown that
wollastonite can remove over 80% of P
from wastewater with hydraulic residence
times of more than 40 h (Brooks et al.,
2000), which is much longer than the residence time used in this experiment.
Column experiments
The column studies were used to estimate
the P saturation potential of the materials in
a filter system. The columns containing
burned heavy opoka (900Opk2 and
900Opk3) clogged shortly after the beginning of the experiment, showing poor suitability for infiltration when used in a fraction
between 2-5.6 mm. Differences between
light and heavy opoka regarding porosity
and content of Ca may be critical for the
success of a system. Heavy opoka has a
lower porosity and more compact structure,
which often leads to clogging of the system.
The burned light opoka (900Opk1), despite
performing better, reached saturation
quickly at the incoming flow rate. Polonite,
Filtra P and wollastonite tailings performed
well during the whole experiment. Polonite
and especially Filtra P effluents were markedly turbid, while wollastonite effluents were
very clear. The materials were assumed to be
saturated when the effluent concentration
was similar to the influent at the feeding
flow rate. The content of P and pH in the
materials after saturation is shown in Table 6
and these results are discussed in paper II.
The saturation potential of Polonite (nearly
2 mgP·g-1) was considerably higher than that
of Filtra P and wollastonite. Previous work
using Polonite for wastewater treatment
showed a saturation potential of about 1.3
mg P·g-1, suggesting that the material was
not fully saturated (Hylander et al., 2006).
Recently, a filter system with alternating flow
has shown that Polonite has a much higher
P saturation potential (Gustafsson et al.,
2007). The lower P content in Filtra P suggests that P precipitated by either Ca or Fe
may have leaked out of the column as a
5.3 Fertilizer potential of the materials
The three materials Polonite, Filtra P and
wollastonite were used as soil amendments
in a pot experiment. The fertilizer potential
on barley and ryegrass yield is discussed in
Paper II and III respectively. The effect of
the amendments on the properties of acid
soils is broadly described in Paper III.
5.3.1
Effect on yield and composition of
barley and ryegrass
The average yield of barley and element
composition in barley spikes grown under
five different treatments is shown in Table 7.
There was no significant difference between
treatments in terms of dry matter (DM)
production but there was a tendency for
higher yield when P was applied in a watersoluble form (KH2PO4) and P from saturated substrates compared with the control
treatment (No P). In other studies, Polonite
saturated with P from wastewater streams
has been shown to improve the yield of
barley when added to a P-depleted soil
(Hylander et al., 2006).
The average yield and element composition
of ryegrass grown under five different
treatments is shown in Table 8. There was
no significant difference between treatments
but there was a tendency for higher yield of
ryegrass with substrate treatment compared
with both control (No P) and potassium
phosphate treatments (KH2PO4). All plants
showed healthy growth during the experiment and no changes in physiology due to
substrate treatment were observed (Fig. 11).
In previous work, P sorbed to different
20
Phosphorus recycling from wastewater to agriculture using reactive filter media
substrates including Fe-rich sands and
LECA (light expanded clay aggregates) was
as available to ryegrass as a water-soluble P
compound (Kvarnström et al., 2004). For
both
barley
and
ryegrass,
the
yield/amendment ratio was in agreement
with the initial P content in the substrates
giving a comparative idea of substrate effectiveness and decreasing in the order Polonite
> Filtra P > wollastonite. The increase in
yield with substrate treatment compared
with the control may be due to direct P
supply for plant uptake, but there is no
evidence of P release from the substrates in
this study. The substrates may also enhance
soil P availability, as discussed in the next
section. The element concentration in barley
spikes revealed no significant difference
between treatments (Table 7).
It can be concluded that the substrates did
not affect the composition of barley spikes.
As for ryegrass (Table 8), the concentrations
of P and Ca were significantly higher in
ryegrass grown with substrate treatment
Table 7. Amendment rate, yield and composition (spikes) of barley and pH, AL-extractable P, K
and Mg, hydrolytic acidity (Hh), base cations concentration, TEB, CEC and BS in soil 1
Amendment
units
NO P
KH2PO4
POLONITE
FILTRA P
WOLLASTONITE
g·pot-1
-
0.12
22
24
32
BARLEY
Fresh
weight
Yield
g·pot-1
gDM·pot-1
33.3 ± 0.51
19.23 ± 0.33
34.43 ± 1.43
21.00 ± 0.98
33.36 ± 2.17
20.03 ± 1.70
33.10 ± 2.89
20.60 ± 2.72
33.30 ± 0.98
20.73 ± 0.54
Spike mass gDM·pot-1
7.60 ± 0.32
8.26 ± 1.08
7.53 ± 1.09
7.76 ± 1.17
8.06 ± 0.69
-
0.91
0.85
0.64
4.59 ± 0.17
4.09 ± 0.12
4.16 ± 0.17
112.12 ± 10.49 147.43 ± 10.08 129.96 ± 8.698
792.79 ± 48.69 782.24 ± 50.83 736.85 ± 33.43
99.77 ± 3.80 94.55 ± 9.122
90.86 ± 3.79
59.75 ± 1.62
57.58 ± 1.31
51.24 ± 3.53
21.35 ± 2.18
31.86 ± 4.02
21.66 ± 0.82
49.02 ± 1.40
47.50 ± 1.85
43.48 ± 1.53
8.64 ± 0.95
8.84 ± 1.55
8.94 ± 1.68
5.22 ± 0.50
7.83 ± 0.58
8.61 ± 1.31
0.48 ± 0.08
0.40 ± 0.06
0.39 ± 0.03
4.83 ± 0.11
117.22 ± 7.573
867.67 ± 32.19
94.89 ± 1.72
50.69 ± 3.05
17.63 ± 1.67
45.10 ± 1.65
10.0 ± 1.69
5.82 ± 0.51
0.49 ± 0.04
4.48 ± 0.15
146.73 ± 14.96
706.78 ± 31.52
76.69 ± 3.50
45.93 ± 1.33
23.21 ± 3.05
42.32 ± 1.10
9.86 ± 2.07
10.25 ± 1.49
0.32 ± 0.07
7.24
7.17
13.65 ± 0.32
21.15 ± 0.75
709.03 ± 13.24
7.18
6.79
12.89 ± 0.40
22.60 ± 0.48
651.00 ± 40.16
0.60 ± 0.03
29.45 ± 1.16
1.26 ± 0.04
0.34 ± 0.01
0.65 ± 0.07
31.70
32.29
98.14
0.72 ± 0.03
20.40 ± 0.33
1.25 ± 0.01
0.38 ± 0.01
1.27 ± 0.13
23.29
24.01
97.02
Yield ratio
P
Ca
Mg
Fe
Al
Mn
Zn
Cu
Pb
Cd
± SE
g·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
SOIL 1
pH H2O
pH KCl
AL-P
Al-K ± SE
AL-Mg
Hh
Ca2+
Mg2+ ± SE
K+
Na+
TEB
CEC
BS%
mg·kg-1
mg·kg-1
mg·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
6.77
6.83
7.34
6.50
6.38
7.04
11.71 ± 0.26
12.72 ± 0.49
13.88 ± 0.10
24.11 ± 1.49
24.35 ± 0.94
24.13 ± 0.93
657.19 ± 4.48 634.72 ± 16.14 656.94 ± 24.58
1.07 ± 0.05
15.86 ± 0.52
1.32 ± 0.05
0.53 ± 0.06
0.57 ± 0.11
18.28
19.34
94.49
1.20 ± 0.06
16.38 ± 0.38
1.27 ± 0.02
0.40 ± 0.02
0.50 ± 0.08
18.55
19.75
93.92
21
0.67 ± 0.03
20.67 ± 0.48
1.14 ± 0.01
0.38 ± 0.01
0.34 ± 0.06
22.53
23.20
97.13
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
Table 8. Amendment rate, yield and composition of ryegrass and pH, AL-extractable P, K and
Mg, hydrolytic acidity (Hh), base cations concentration, TEB, CEC and BS in soil 2
units
Amendment
g·pot
-1
NO P
KH2PO4
POLONITE
FILTRA P
WOLLASTONITE
-
0.12
22
24
26
RYEGRASS
Fresh
weight
Yield
g·pot-1
gDM·pot-1
81.83 ± 0.60
16.93 ± 0.17
83.63 ± 2.15
16.86 ± 0.41
88.13 ± 3.66
17.36 ± 0.49
93.63 ± 4.44
17.56 ± 0.49
88.66 ± 2.01
17.00 ± 0.15
-
0.79
0.73
0.65
g·kg-1
2.07 ± 0.14
2.17 ± 0.08
3.20 ± 0.07
3.46 ± 0.26
mg·kg-1 624.55 ± 56.87 608.34 ± 48.36 1891.11 ± 236.13 4060.60 ± 536.68
mg·kg-1 1570.14 ± 77.26 1386.11 ± 42.39 1925.32 ± 97.35 1738.27 ± 67.11
mg·kg-1 398.37 ± 39.32
197.33 ± 7.89
200.33 ± 2.44 192.10 ± 18.45
2.44 ± 0.08
2699.42 ± 166.35
1823.69 ± 87.69
170.43 ± 6.65
Yield ratio
P
Ca
Mg
Fe
Al
Mn
Zn
Cu
Pb
Cd
± SE
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
mg·kg-1
97.07 ± 0.79
420.13 ± 18.08
83.79 ± 1.14
24.66 ± 5.40
6.39 ± 0.70
2.97 ± 1.43
86.44 ± 1.82
374.70 ± 29.10
73.45 ± 1.64
21.17 ± 4.79
24.31 ± 10.82
0.74 ± 0.03
78.19 ± 2.37
227.08 ± 20.54
59.17 ± 0.82
19.64 ± 4.15
23.43 ± 4.06
0.79 ± 0.04
64.52 ± 2.67
148.12 ± 9.08
50.40 ± 2.42
15.70 ± 3.33
32.87 ± 16.19
1.00 ± 0.07
85.14 ± 1.91
242.41 ± 20.64
66.46 ± 1.25
17.31 ± 3.13
48.34 ± 2.02
3.05 ± 1.79
4.60
4.51
5.12
5.52
3.72
3.68
4.28
5.15
5.72 ± 0.10
6.06 ± 0.24
6.48 ± 0.10
6.32 ± 0.12
15.32 ± 0.26
16.38 ± 0.30
15.74 ± 0.11
14.62 ± 0.81
1257.91 ± 70.60 1313.30 ± 20.74 1128.07 ± 19.26 1143.16 ± 46.60
5.02
4.06
6.27 ± 0.38
15.51 ± 0.58
1117.53 ± 66.45
SOIL 2
pH H2O
pH KCl
AL-P
Al-K ± SE
AL-Mg
Hh
Ca2+
Mg2+ ± SE
K+
Na+
TEB
CEC
BS%
mg·kg-1
mg·kg-1
mg·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
cmol·kg-1
8.28 ± 0.05
1.51 ± 0.03
0.58 ± 0.01
0.29 ± 0.00
1.18 ± 0.08
3.57
11.85
30.10
9.90 ± 0.12
1.37 ± 0.03
0.56 ± 0.01
0.29 ± 0.00
0.94 ± 0.10
3.16
13.06
24.20
6.90 ± 0.26
5.71 ± 0.30
0.58 ± 0.02
0.27 ± 0.01
0.45 ± 0.10
7.00
13.90
50.37
3.53 ± 0.20
10.97 ± 0.56
0.62 ± 0.02
0.27 ± 0.01
0.37 ± 0.11
12.23
15.76
77.58
6.35 ± 0.22
4.87 ± 0.24
0.58 ± 0.03
0.28 ± 0.01
0.93 ± 0.42
6.67
13.02
51.21
since all substrates had relatively high pH
values (Table 7). In the case of soil 2 (acid),
the pH increased during the experiment for
all treatments and the increase was higher
with substrate treatment (table 8). In both
cases, there was an increase in ALextractable P during the experiment for all
treatments and the increase was higher with
substrate treatment. The AL-extractable P
from both soils correlated well with the
initial P content in the substrates and it was
highest in soil treated with Polonite. This
suggests that P in the substrates was able to
desorb and dissolve in the soil solution. On
compared with both control and potassium
phosphate treatments. There was a slight
increase in the concentration of Mg in ryegrass grown with substrate treatment and a
slight decrease in Fe and Al compared with
the control treatment. The relationship
between substrate amendment and ryegrass
composition depends strongly on soil properties. This is further discussed in Paper III.
5.3.2 Effect on soil pH and soil P availabiity
The pH of soil 1 (neutral) was unaffected
during the experiment for the control and
potassium phosphate treatments but it
slightly increased with substrate treatment
22
Phosphorus recycling from wastewater to agriculture using reactive filter media
the other hand, soil P availability may increase as a result of a soil pH increase, which
favours the dissolution of P bound to Fe
and Al oxides in soil particles. Several studies have shown that alkaline substrates can
increase the pH of acid soils and, subsequently, soil P availability (Sloan and Basta,
1995; Matsi and Keramidas, 1999; Snars et
al., 2004). In the present study, the increase
in soil P availability was the result of a combination of increased soil pH and direct P
release from the reactive substrates.
There was a decrease in AL-extractable K in
both soil 1 and 2 for all treatments, which
may indicate insufficient K fertilization in
the experiment. However, there was no
significant difference between treatments
regarding AL-extractable K and Mg.
5.3.3
A
Effect on other soil properties
The application of the reactive substrates to
both soil 1 and 2 decreased the hydrolytic
acidity (Hh) and increased the cation exchange capacity (CEC) compared with both
control and potassium phosphate treatments. The increase in soil CEC is mainly
the result of increasing amounts of Ca2+ ions
in the sorption complex derived from the
high Ca content in the substrates applied.
This can be particularly beneficial for acid
soils, although it can reduce the concentration of other ions in the sorption complex.
This was observed in soil 1, where the concentration of K+ and Na+ ions in the sorption complex decreased with substrate
treatment. The reduction in Hh as a result of
substrate treatment involves a decrease in
Al3+ ions in the sorption complex, which
reduces Al availability for plants. This can be
observed as a slight decrease in Al concentrations in barley grown in soil 1 amended
with substrate treatment (Table 7) despite
the noticeable content of Al in the substrates. In the case of acid soils (soil 2), the
substrates can be particularly beneficial since
they can reduce the risk of Al toxicity, which
B
C
D
Figure 11. Pot experiment showing ryegrass
yield for five different treatments, A:No P,
B:KH2PO4,C:Polonite,D:FiltraP,
E:Wollastonite
E
23
Victor Cucarella Cabañas
TRITA LWR LIC Thesis 2039
is one of the primary limitations for agriculture on acid soils (Kochian et al., 2004).
Filtra P amendments greatly increased the
percentage base saturation and significantly
decreased the hydrolytic acidity by introducing large amounts of Ca2+ ions into the sorption complex. This is probably due to the
high solubility of Ca compounds in Filtra P.
The implications of such amendments are
further considered in Paper III.
provide considerable amounts of other
macro- and micronutrients and since it is
derived from a natural bedrock (opoka), it
does not contain hazardous levels of heavy
metals.
6
C ONC LUSIONS
This work shows that reactive filter media
saturated with P can be used as fertilizer
substrates in agriculture. The P content in
the substrates depends on their P sorption
capacity, which depends mainly on the
amount and form of Ca compounds in the
material. The results show that the sorption
capacity increases linearly with the CaO
concentration in the material. The mechanisms of P sorption differ as a consequence
of varying Ca:P ratio. The suitability of the
materials as filter media depends on the
structure, chemical composition and particle
size.
Polonite, Filtra P and wollastonite tailings
saturated with P showed a tendency for
higher dry matter yield of barley and ryegrass
in pot experiments. According to the
yield/amendment ratio, Polonite was the
most effective fertilizer substrate. All three
amendments increased soil pH, availability
of P and cation exchange capacity. The
increase was more obvious in the acid soil
(soil 2). Filtra P amendments produced the
highest increase in soil pH and cation exchange capacity, causing significant changes
in soil properties probably due to its alkaline
pH and high content of soluble Ca. On the
other hand, Polonite produced the highest
increase in P availability in both soils due to
its higher content of P.
The results suggest that the substrates can
act as a source of P for plants and improve
nutrient availability in soils, while also increasing soil pH and acting as soil conditioners. Among the substrates studied here,
Polonite can be regarded as the most suitable material for the purpose of P recycling
from wastewater to agriculture.
5.4
Suitability of the substrates as soil
amendments
The materials did not contain large amounts
of hazardous compounds except for the
content of Pb in Filtra P (Table 4). When
using the materials with real wastewater, the
content of metals may increase since they
can efficiently adsorb metals too (Kietlińska
et al., 2004). However, if the materials are
used for domestic wastewater treatment
only, their levels of metals may be kept low.
Thus, the saturated materials may fulfil the
requirements of the EU directive on the use
of
sewage
sludge
in
agriculture
(86/278/EEC).
The reactive media saturated with P may be
a reliable source of P for plants and can
influence nutrient availability in soils. The
substrates can also increase soil pH and act
as soil conditioners. The high content of Si
in Polonite and wollastonite might improve
soil structure as well. Wollastonite is a natural product and contains substantial amounts
of Mn (Table 4) compared with other substrates. This can be of benefit for plants, but
due to its lower P sorption capacity, wollastonite may not be a reliable source of P in
the long term. Filtra P was able to increase
the amount of soluble P and especially Ca in
the soil solution. However, the elevated
content of Pb and also Hg (Renman, personal communication) may limit its use in
agriculture. Polonite appears to be the most
appropriate substrate for several reasons. It
has a high P saturation potential, which can
ensure P supply for plant uptake during a
reasonable period of time. In addition, it can
24
Phosphorus recycling from wastewater to agriculture using reactive filter media
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