Module 5: Anaerobic Digester Start

Module 5:
Anaerobic Digester Start-up,
Operation and Control
5.1: Four steps of AD
5.2: AD start-up
5.3: AD operation & process monitoring
5.4: Reasons for AD failure
5.5: Safety concerns
5.6: Understanding & managing H2S
This curriculum is adapted from: eXtension Course 3: AD, University of Wisconsin
vtc.edu
Module 5:
Anaerobic Digester Start-up,
Operation and Control
5.1: Four steps of AD
5.2: AD start-up
5.3: AD operation & process monitoring
5.4: Reasons for AD failure
5.5: Safety concerns
5.6: Understanding & managing H2S
This curriculum is adapted from: eXtension Course 3: AD, University of Wisconsin
vtc.edu
The four steps of AD
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Overview of AD
AD is a complex series of biochemical reactions that are the result of the
interaction of several types of bacteria. These bacteria function in the
absence of oxygen gas and produce biogas, mainly methane and carbon dioxide.
The AD process
occurs in 4 steps.
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Step 1: Hydrolysis
Biomass is made of of very large organic polymers (aka biomolecules):
•  Proteins
•  Fats (lipids)
•  Carbohydrates
Hydrolysis is a biochemical process that breaks polymers down into smaller
organic molecules:
•  Proteins
amino acids
processed in step 2
•  Lipids
fatty acids
acidogenesis
•  Carbohydrates
simple sugars
some acetate & hydrogen
that goes directly to step 4, methanogenesis
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Step 2: Acidogenesis (aka fermentation)
Simple organic molecules are broken down & pH drops.
•  ammonia (NH3)
•  hydrogen gas (H2)
Used immediately in step 4, methanogenesis.
•  carbon dioxide (CO2)
•  hydrosulfuric acid (H2S)
These acids cause pH to fall.
•  volatile fatty acids (VFAs)
•  carbonic acids
Processed in step 3, acetogenesis.
•  alcohols
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Step 3: Acetogenesis
Creation of acetic acid from remaining simple organic compounds
•  volatile fatty acids (VFAs)
•  carbonic acids
•  alcohols
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acetate
(also H2 and CO2)
to step 4
Step 4: Methanogenesis
Methanogenic bacteria turn acetate, CO2 and H2 into methane (CH4).
minor pathway
CO2 + 4 H2 à CH4 + 2 H2O
major pathway
CH3COOH à CH4 + CO2
acetate
Note that the biogas produced in step 4 contains water and carbon dioxide
as well as methane.
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Composition of biogas
Methane content varies somewhat with the type of feedstock used.
Chemical
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%
methane(CH4)
45-65
carbondioxide(CO2)
30-40
hydrosulfuricacid(H2S)
0.3–3
ammonia(NH3)
0–1
water(H2O)
0-10
nitrogen(N2)
0-5
oxygen(O2)
0-2
hydrogen(H2)
0-1
Assessment!
Please answer the questions in section 5.1 of the Module 5 Assessment.
vtc.edu
Module 5:
Anaerobic Digester Start-up,
Operation and Control
5.1: Four steps of AD
5.2: AD start-up
5.3: AD operation & process monitoring
5.4: Reasons for AD failure
5.5: Safety concerns
5.6: Understanding & managing H2S
This curriculum is adapted from: eXtension Course 3: AD, University of Wisconsin
vtc.edu
AD start-up
vtc.edu
Start-up process
Monitoring the start-up processes is critical, and easier than fixing a problem
created by lack of care and attention.
“Seed” feedstock from an operating AD plant is often used to jumpstart AD
start-up. Seeding the tank with 20 – 25% volume transfers bacteria that can grow
and expand.
•  Alternatively, begin with conditioned manure: manure stored for
at least 2 weeks in anaerobic conditions.
•  Fresh manure is then fed in slowly increasing volumes over the first 6 – 8
weeks of operation.
•  Biogas should be produced around the fourth week of operation.
•  2 – 3 months should be allowed for bacterial populations to expand before
a normal feeding schedule is started.
Flooding the gas space with carbon dioxide (or other non-oxygen gas) may
speed the rate of start-up by displacing oxygen in the head space and
rapidly creating anaerobic conditions.
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Monitoring start-up
Monitoring several parameters helps the operator gauge the success and
rate of start-up:
parameter
pH
VFAlevels
biogascomposiMon
temperature
asmethaneproduc3onbegins
warningsign?
approaches7
pHdrops
decrease
VFAlevelsincrease
CO2falls&CH4rises
noriseinCH4
remainsstable
becomesunstable
AD can occur at several different temperatures:
•  Psychrophillic aka room temperatures
•  Mesophillic temperatures should be about 100 °F.
•  Thermophillic temperatures should be about 135 °F.
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First feedings
Ideally, the first feedings after start-up should occur continuously and at
low rates (or low concentrations of organic matter).
•  Alternatively feed small volumes multiple times per day.
•  Feeding once daily is the least desirable option.
Calculation of loading (or feeding) rate was discussed in Module 3:
Underfeeding produces less methane but has few negative consequences.
Overfeeding (volume or concentration of volatile solids):
•  Inhibits production of biogas; and
•  Pushes undigested VS into the digestate or effluent.
vtc.edu
Cold weather start-up
While all start-ups require supplemental heating, winter start-ups require
very significant amounts of energy to raise the temperatures of large
volumes of cold feedstock. This represents a significant added expense and
should be avoided if possible.
• 
Plans for supplemental heat should be made if cold weather start-up
cannot be avoided. cold weather.
Cold weather can also cause problems with installation of valves, sensors and
pumps. Ideally these items are installed, tested, and optimized prior to both
start-up and the onset of cold weather.
vtc.edu
Assessment!
Please answer the questions in section 5.2 of the Module 5 Assessment.
vtc.edu
Module 5:
Anaerobic Digester Start-up,
Operation and Control
5.1: Four steps of AD
5.2: AD start-up
5.3: AD operation & process monitoring
5.4: Reasons for AD failure
5.5: Safety concerns
5.6: Understanding & managing H2S
This curriculum is adapted from: eXtension Course 3: AD, University of Wisconsin
vtc.edu
AD operation & control
vtc.edu
Seven key to operational success
AD is a complex process depending on many coordinated bacterial and
biochemical processes that transform complex organic material into methane.
Zickefoose & Hayes (1976) developed seven operational procedures to help
operators reach and maintain steady state AD.
1.  Set up a feeding schedule;
2.  Control loading rates;
3.  Control operating temperature;
4.  Control mixing rates;
5.  Control the quality of the slurry;
6.  Control the HRT (length of digestion); and
7.  Use lab tests and data to monitor the AD process and guide controls.
* See their paper for detailed checklists.
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1. A feeding schedule
The keys are:
•  Minimizing excess water in feedstock; and
•  Feeding continuously, or at regular intervals.
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2. Control the loading rate
The loading rated depends on the mass of volatile solids fed each day and
the total volume of the AD tanks. Remember this?
Calculating manure volume
cylinder = (π)(r2)(h) = (π)(252)(20) = 39,250 ft3
cone = (1/3)(r2)(h) = (1/3)(252)(5) = 3,217 ft3
total = 42,521 ft3
Calculating loading rate
pounds TS/day = (gallons/day)(8.34 lb/gallon)(%TS)
= (5000)(8.34)(0.065) = 2,710 lb TS/day
pounds VS/day = (lb TS/day)(%VS) = (2,710 lb TS/day)(0.69) = 1,869 lb VS/day
loading rate = (lb VS/day) / volume of manure = 1,869 lb/day / 45,521 ft3
= 0.04 lb / day / ft3
Average loading rates are 0.02 – 0.37 lb VS / ft3 volume.
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2. Organic loading rate & biogas yield
Volatile solids: generally, AD systems are loaded at a rate of 8% VS per day.
VTCAD is fed 16,000 gallons per day at full operational capacity.
16,000 gallons 8.34 lb 1 kg = 60,654.5 kg
1 gallon 2.2 lb
•  8% of 60,654.6 kg = 4,852.4 kg of VS.
•  Feedstock is typically 8% TS and 85% VS à 6.8% VS
•  6.8% of 60,654.5 kg is 4,124.5 kg, a bit short of 8% VS.
Biogas yield: typically 0.75 – 1.12 m3 biogas/kg VS destroyed
VTCAD: We’ve been seeing about 70% destruction of VS, so:
(0.70)(4,124.5 kg VS) = 2,887.2 kg VS destroyed
Biogas range: (0.75 m3/kg VS)(2,887.2 kg VS) = 2,165.1 m3
(1.12 m3/kg VS)(2,887.2 kg VS) = 3,233.2 m3
vtc.edu
WPCF(1987)
3 - 6. Temp, mixing, slurry & HRT
3. Temperatures must be kept constant!
4. Mixing should be sufficient to mix bacteria with feedstock, and to prevent
foaming and the formation of scum.
5. Feedstock controls slurry characteristics.
6. HRT and withdrawl of digestate should ensure complete digestion and
destruction of odors.
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7. Lab tests and operational data
Together, these data help operators to measure AD progress and to predict
impending instability. In order of importance with optimal ranges.
parameter
hydrolysistank
ADtank
VFA:alkalinityraMo
higherthanAD
0.2–0.6
%methane
>60%
%carbondioxide
<40%
%H2S
pH
%destrucMonofVS
<200ppm
4.5–6.0
6.8–7.4
>>40%
Trends and rates of change are far more important than absolute values.
vtc.edu
7. On-site testing
The operational tests suggested by Zickefoose & Hayes can, and should, be
done on-site (at the digester facility). On-site testing can be done quickly and
this allows problems to be nipped in the bud.
•  Gas quality (methane, carbon dioxide, and H2S levels) can be measured by
sensors in the gas line or at the generator.
•  VFA: alkalinity (aka the Ripley ratio) is measured by acid titration of slurry
sampled from the hydrolyzer or digester tank.
•  Destruction of VS is determined by drying and combusting slurry samples.
Rodrigo Labatut and Curt Gooch of Cornell University created simplified
testing procedures that can be performed with minimal training and using
simple equipment. We’ve adapted their protocols for use at VTCAD and
present some results here.
vtc.edu
hYp://www.manuremanagement.cornell.edu/
Pages/Topics/Anaerobic_DigesMon.html
7. Destruction of VS
Volatile solids (VS) are organic materials converted to biogas by AD.
Effective AD will convert a significant percentage of VS to methane;
conversion is called ‘destruction’ of VS.
Testing: Samples of feedstock and AD slurries are:
•  First, dehydrated to determine total solids (TS); and
•  Second, combusted to determine total volatile solids (VS).
Calculations: for feedstock vs. effluent
•  TS as a percentage of initial (wet) sample mass
•  VS as a percentage of initial (wet) sample mass
•  Percent of TS that is VS (volatile TS)
•  Percent volatile TS remaining and destroyed
vtc.edu
www.renewwisconsin.org/biogas/AD/
performanceanalysisdigesMonsystems_finalreport.pdf
VTCAD VS data
5/18/15
prep$pit
hydrolyzer
digester
effluent$1
effluent$2
solids
vtc.edu
dish)(g)
133.14
132.53
131.01
131.04
134.38
133.27
dish)+)wet)(g) dish)+)dry)(g) dish)+)ash)(g)
364.25
158.55
137.38
343.00
149.57
135.60
340.45
141.06
133.81
338.95
138.60
133.58
348.06
141.53
136.94
217.20
165.39
136.70
VTCAD VS data + results
5/18/15
prep$pit
hydrolyzer
digester
effluent$1
effluent$2
wet((g)
231.11
210.47
209.44
207.91
213.68
dry((g)
25.41
17.04
10.05
7.56
7.15
ash((g)
4.24
3.07
2.80
2.54
2.56
%(TS
10.99
8.10
4.80
3.64
3.35
%(VS volatile(TS((%) %(VS(destroyed
83.31
9.16
0.0
81.98
6.64
27.5
72.14
3.46
62.2
66.40
2.41
73.6
64.20
2.15
76.5
=(dry/wet)(100)
Subtractmassofdish
frommassofsample+dish
=(ash/wet)(100)
=(%TS)(%VS/100)
=(%TS-%VS)[100]
%TS
vtc.edu
VS destruction by VTCAD
In mid-May of 2015 feedstock was mainly manure, brewery waste, grease trap
waste and glycerol.
Results shows that the digestion process was working efficiently with significant
destruction of VS occurring in the hydrolyzer, and more in the digester.
%"VS"destroyed"
%"VS"destruc,on"
100"
80"
60"
40"
20"
0"
prep"pit" hydrolyzer" digester" effluent"1" effluent"2"
vtc.edu
Assessment!
Please answer the questions in section 5.3 of the Module 5 Assessment.
vtc.edu
Module 5:
Anaerobic Digester Start-up,
Operation and Control
5.1: Four steps of AD
5.2: AD start-up
5.3: AD operation & process monitoring
5.4: Reasons for AD failure
5.5: Safety concerns
5.6: Understanding & managing H2S
This curriculum is adapted from: eXtension Course 3: AD, University of Wisconsin
vtc.edu
Reasons for AD failure
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Most failure occurs in the design phase
Prior to 1998, rates of failure for on-farm AD systems were astonishing:
•  70% failure for complete mix
•  63% failure for plug-flow in the feedstock
Today, long-term failure rates have been lowered by improved system design,
better construction practice and the participation of more qualified
and experienced companies.
However, short-term failure and underperformance are still common.
Four reasons explain most AD failures:
1.  Bad design / poor installation
2.  Bad choice of components
3.  Poor farm management / AD operation
4.  AD toxins in the feedstock
vtc.edu
1. Bad design / poor construction
For example, hydraulics are often overlooked.
•  While manure is flowable and pumpable it is still a semi-solid.
•  Consistency can change with diet, management or season.
AD technology providers should be thoroughly vetted.
Grants for AD installation should include engineering reviews to:
•  Validate design
•  Ensure that AD design is appropriate for the farm and its management
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2. Bad choice of equipment & materials
In the interests of keeping costs down, builders are tempted to choose less
expensive or robust equipment. Farmers tend to be fiscally conservative and
very cost-conscious.
There is often a cost to this approach: “You get what you pay for.”
•  Equipment failure or breakdown
•  Increased need for maintenance and thus increased downtime
•  Pumps and engines need to be properly sized and robust.
Examples:
•  In New England, it appears that many AD facilities were over-sized using
the premise that farms would expand and the extra capacity would be
useful. This resulted in genset downtime and decay.
•  In hindsight we can see that simplifying plumbing, using more robust
valves, and having interchangeable pumps, or spare parts on hand, would
have simplified early operations!
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3. Poor farm management
Farmers must be committed to running and AD in addition to running the
farm.
Successful operation requires consistent monitoring, operation and
maintenance. or some farmers.
•  Once the AD system is up and running this is an average daily commitment
of 15 – 60 minutes per day for the simplest AD systems using manure only.
farmers.
•  However, more time will be required during start-up, when significant
changes are made, or when feedstock transportation is required.
Failures have occurred when operators cut corners. For example:
•  Overfeeding for a day and then hoping the AD will run itself for 2 – 3 days.
•  Poor performance can becomes lead farmers to believe that the system
itself is at fault and not worth their time: a self-fulfilling prophecy.
•  Turning AD operations over to a professional operator may be a better
approach for some farmers.
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4. AD toxins in feedstock
Common farm chemicals and products are often toxic to the bacteria needed
for AD.
Example:
•  High-protein diets can produce toxic levels of ammonia in the AD.
Common on-farm AD toxins:
•  Rumensin ® and similar products (detergents)
•  Copper sulfate and formaldehyde used in dairy foot-baths
•  Pesticides
•  Herbicides
vtc.edu
Assessment!
Please answer the questions in section 5.4 of the Module 5 Assessment.
vtc.edu
Module 5:
Anaerobic Digester Start-up,
Operation and Control
5.1: Four steps of AD
5.2: AD start-up
5.3: AD operation & process monitoring
5.4: Reasons for AD failure
5.5: Safety concerns
5.6: Understanding & managing H2S
This curriculum is adapted from: eXtension Course 3: AD, University of Wisconsin
vtc.edu
Implementing safety procedures
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AD operational precautions
Over a million AD systems are in use worldwide. Despite the obvious hazards
posed their safety record is good.
Safety depends on planning and precautions.
•  Fire prevention
•  Mechanical hazards
•  Statically sound construction
•  Electrical safety
•  Lighting protection
•  Thermal safety
•  Noise emission protection
•  Asphyxiation & poisoning prevention
•  Hygienic & veterinary safety
•  Air pollution hazards
•  Protection of ground & surface waters
•  Nutrient overload of soil & water
•  Flooding safety
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Human health risks
The main risks to human health involve:
•  Asphyxiation
•  Explosion
•  Burns
•  Electrical shock
•  Falls
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Hydrogen sulfide, toxin in biogas
The AD process produces low to significant levels (0.05 – 4,000 ppm) of
hydrosulfuric acid (H2S or hydrogen sulfide) in biogas. H2S smells like rotten
eggs and is:
•  Inflammable
•  Colorless
•  Highly poisonous: lethal at 1.2 – 2.8 mg/L (0.117%)
•  Soluble in water, acting as a weak acid
H2S inhibits the blood’s ability to transport oxygen, causing victims to
‘suffocate internally’. Initial symptoms:
•  Irritation of the eyes & mucous membranes
•  Nausea
•  Vomiting
•  Difficulty breathing
•  Cyanosis
•  Delirium & cramps
•  Respiratory paralysis & cardiac arrest
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Hydrogen sulfide exposure limits
Because H2S is hazardous to human health, NIOSH and other agencies
recommend limiting human exposure to H2S:
PEL
(permissible limit)
20 ppm
REL
(recommended limit)
10 ppm
IDLH
(intermediate danger)
100 ppm
Personal or room monitors can be used to
measure exposure to H2S.
Conversion between common units:
1 ppm = 1.40 mg/m3 = 1.40 mg/1000 L = 1.40 ug/L
vtc.edu
www.cdc.gov/niosh/npg/npgd0337.html
Entering confined spaces
Improper training on entering and working in confined spaces is the most
common cause of accidents related to AD systems. There have been no ADassociated deaths in the US, but plenty of confined space-related deaths on
farms.
No entry to, or work in, confined spaces may occur without training.
Once trained via OSHA, remember that:
•  Never work alone in confined spaces;
•  Monitor air quality for oxygen and explosive risks;
•  Wear self-contained breathing apparatus (SCBA);
•  Provide a continuous flow of fresh ventilated air by explosion-proof blower;
•  Maintain constant contact and communication with the worker to
monitor their state of mind;
•  Wear a harness or safety belt with a lifeline attached to a support outside the
tank;
•  Keep ignition sources far from the confined space; and
•  Never go in after an unconscious or injured worker unless all above conditions
are met.
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Explosions / burns
Methane is odorless, colorless, difficult to detect and highly explosive when in
contact with oxygen (or atmospheric) gas.
•  Lighter than air.
•  …therefore collects in the top of confined space so detect leaks high.
AD plants have been destroyed by fire and explosion!
Precautions:
•  All AD buildings should be well ventilated;
•  All electrical wiring (lights, motors, pumps) should be explosion proof;
•  Gas lines should be equipped with flame-arrestors; and
•  Gas-detection systems should be installed throughout to detect leaks.
vtc.edu
hYp://www.organics-recycling.org.uk/uploads/arMcle2165/
IntegraMng%20lessons%20learned%20from%20accidents%20into
%20operators_%20behaviour%20and%20equipment%20design.pdf
Other risks at AD plants
Burns:
Contact with heating and cooling systems, the flare or genset
Electric shock:
From improper installation of equipment
Slips & falls
Collision with moving parts
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Proper training, signage & fencing
Signage providing clear warning of hazards and required precautions is an
essential element of a well-run AD system.
Particularly hazardous areas, like effluent ponds, and flares should be fenced in.
AD operators and all personnel must be trained, equipped with personal
protective equipment, and monitored to ensure continued good practice.
vtc.edu
AD hazards self-assessment tool
Nellie Brown of Cornell’s School of Industrial and Labor Relations has developed
an assessment tool for farmers, AD operators and personnel:
“ConducMngaSafetyWalk-throughonaFarm:HazardsoftheManureHandling
System,AnaerobicDigester,andBiogasHandlingSystem(ASelf-Assessment
GuidelineforFarmers)”
hYp://digitalcommons.ilr.cornell.edu/manuals/13/
vtc.edu
Assessment!
Please answer the questions in section 5.5 of the Module 5 Assessment.
vtc.edu
Module 5:
Anaerobic Digester Start-up,
Operation and Control
5.1: Four steps of AD
5.2: AD start-up
5.3: AD operation & process monitoring
5.4: Reasons for AD failure
5.5: Safety concerns
5.6: Understanding & managing H2S
This curriculum is adapted from: eXtension Course 3: AD, University of Wisconsin
vtc.edu
Understanding & managing H2S
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Lowering H2S levels in biogas
Biogas naturally contains H2S in addition to methane and CO2.
2,000 – 5,000 ppm of H2S in dairy farm AD systems are typical & depend on:
1.  Levels of sulfur in feedstock
•  Manure has abundant sulfur, generally from high-protein feed.
2.  Levels of sulfur in the water supply
3.  The ‘substrate to sulfate’ ratio: in other words, the ratio of high-energy
compounds to sulfate in feedstock.
•  Methanogenic bacteria grow and function best when they have access to
high-energy feedstock. So increasing the ‘substrate to sulfate’ ratio
gives methanogens the ability to effectively compete with the sulfur
reducing bacteria (SRBs) that make H2S.
4.  The presence of oxygen in the biogas generation or storage tank.
•  Some oxygen allows sulfur-oxidizing bacteria to metabolize H2S and
convert it to elemental sulfur.
vtc.edu
hYp://www.bioenergyconsult.com/hydrogen-sulphide-removal-from-biogas/
Why is H2S a problem?
H2S (aka hydrosulfuric acid or hydrogen sulfide) is an acid that is a gas at room
temperature. The gas poses two distinct and serious dangers:
1.  H2S is toxic to human and animal health, & to microbial function.
•  H2S begins to affect human health at 20 – 30 ppm.
•  H2S enhances methane production at 0.5%, but prevents it at 6%.
2.  H2S is corrosive and damages metallic surfaces it comes in contact with.
Equipment
H2Sstandards(ppm)
boiler
1,000
internalcombusMonengine
turbine
100
70,000
phosphoricacidfuelcell
20
moltencarbonatefuelcell
10
solidoxidefuelcell
SMrlingengine
purifiedbiomethane
vtc.edu
Magomnang&Villanueva(2014);Chynoweth&Issacson,1987)
1
1,000
4
Acetate & H2 are used to make H2S
Reactions of sulfur-oxidizing bacteria that produce sulfides
4H2 + SO4-2 à HS-1 + 4H2O
CH3COO-1 + SO4-2 à HS-1 + 2HCO3-1
acetate
4CH3CH2COO-1 + 3SO4-2 à 4[CH3COO-1] + 4HCO3-1 + 3HS-1
propionate
2CH3CH2CH2COO-1 + SO4-2 à 4[CH3COO-1] + HS-1 + H+1
butyrate
General summary from the sulfur’s point of view
anaerobes
SO4-2 + organics
S-2 + H2O + CO2
HS-1
S-2 + H+1
H2S
HS-1 + H+1
vtc.edu
Farhan&Farhan(2006);Spreece(1996)
H2S: a methane competitor
Digester feedstock contains a wide variety of bacteria. Two functional groups of
bacteria compete for acetate feedstock (substrate) and the reducing power of
hydrogen (H2).
•  Methanogens (the good guys)
•  Sulfur reducing bacteria (SRBs) (the competition)
CO2
CH4
H2
methanogens
acetate
CH4
acetate
alcohol
H2
SO4-2
SRBs
H2S
Competition from SRBs is a problem for anaerobic digesters because:
•  SRBs are more robust bacteria & cope better with environmental challenge;
•  SRBs reproduce more quickly than methanogens (rapid doubling);
•  H2S produced by SRBs inhibits methanogensesis.
•  H2S has a less inhibitory effect on acetogenesis. vtc.edu
Gerardi(2003)
Methods of controlling H2S
Prevention (pro-active):
•  Low protein / low sulfur diet (feedstock)
•  Addition of FeCl3 to slurry to lower formation of H2S
Treatment (reactive):
•  Addition of tract amounts of oxygen to biogas space
•  Post-digester scrubbing of biogas
•  Biological
•  Chemical
vtc.edu
hYp://www.bioenergyconsult.com/hydrogen-sulphide-removal-from-biogas/
Diets that minimize H2S production
To minimize production of H2S by anaerobic digestion consider:
1.  Control the amounts of protein in feedstock.
•  Protein is the main source of sulfur in AD diets.
2.  Increase the substrate to sulfate ratio; the percentage of high-energy
feedstock to sulfur containing feedstock.
•  ‘Substrate’ means high-energy feedstock that can be quickly & easily
hydrolyzed and metabolized into simple organics with high levels of
volatile solids.
•  High levels of VFAs give methanogens an advantage and allow them to
compete effectively with SRBs.
vtc.edu
Gerardi(2003);Yoda(1987)
Using FeCl3 to prevent formation of H2S
When ferric chloride (FeCl3) is added to feedstock, it reacts with sulfide (S-2)
generated as hydrolytic bacteria breakdown proteins by fermentation.
Extracellular hydrolysis releases sulfides where the iron can react with them.
2FeCl3à2Fe+3+3Cl-1à+3S-2àFe2S3+6Cl-1
The ferric sulfide (Fe2S3) is a precipitate, an ionic compound whose chemical
bonds are so strong that bacteria cannot access the sulfide. So SRBs cannot
metabolize the sulfur and produce H2S.
•  This prevention also gives the methanogens a competitive advantage.
•  The effect of FeCl3 addition is fairly rapid because it works via chemical,
rather than biological, reaction with feedstock.
vtc.edu
hYp://www.bioenergyconsult.com/hydrogen-sulphide-removal-from-biogas/
FeCl3 caveats
Trace elements, like iron, nickel and cobalt, are required for the microbial
biochemistry that is anaerobic digestion.
Addition of FeCl3 can co-precipitate these ions and limit anaerobic digestion.
So, balance is critical.
H2S partitions between biogas and digester slurry. A slurry concentration of 26
mg/L corresponds to 10,000 ppm H2S in biogas.
•  1 mg/L in slurry ~ 380 ppm in biogas
Realistically, FeCl3 can be used to reduce biogas H2S levels by 50%: mitigation.
Remember, added FeCl3 must:
1)  Precipitate and remove sufficient sulfur to reduce H2S significantly; and
2)  Provide soluble iron ions for bacterial metabolism
vtc.edu
Farhan&Farhan(2006);Speece,1996)
Evidence that FeCl3 enhances AD
Bench-scale case study:
Mini-digester tests using wastewater treatment sludge digested under
mesophillic conditions with a 30-day SRT were used to study the effects of ferric
chloride on H2S levels and efficiency of anaerobic digestion.
A FeCl3 dose of 1.25% w/w produced these results:
•  Control of H2S;
•  Reduction of volatile organic sulfur compounds that cause odor; and
•  Increased volatile solids destruction.
vtc.edu
Park&Novak(2013)
FeCl3 doses used in wastewater AD
Wastewater treatment case study:
A large wastewater plant uses a dose of:
•  0.3 gallons per minute of 40% FeCl3
•  That’s 12 pounds per ton of feedstock TS
•  At $0.10/pound, the annual cost of treatment is $73,000.
•  H2S levels are controlled.
The downside?
•  The expense
•  Some foaming & struvite formation
How does this compare to the dose we use at VTCAD?
16,000 gallons = 133,440 pounds = 66.72 tons
At 8% TS, that’s 5.3 tons of feedstock solids per day.
At the dose above, that’s 64 pounds of FeCl3 or 7.68 gallons.
During 2015, we used 40 – 50 gallons of FeCl3 per day!
vtc.edu
Walker&Barnes:MetroWastewaterReclamaMonDistrict
FeCl3 case study: AA Dairy NY (NYSEDA)
AA Dairy is a 1,000-cow dairy farm in NY with a below-grade, plug-flow AD
AD that operates at mesophilic temperature with a 37 – 40 day HRT.
•  Feedstock: 11,000 gallons of manure slurry per day (sawdust bedding)
•  Biogas yield: 13,200 – 48,500 ft3/day
•  Biogas quality: 34 – 40% CO2, balance is CH4 (60 – 66%), 4,000 ppm H2S
•  Solids are used to create compost for commercial sale
vtc.edu
Farhan&Farhan(2012)
FeCl3 case study: slurry biochemistry
Biochemical testing of manure feedstock and FeCl3-treated & digested effluent:
AADiary
Effluent
EPADairyAvg
TS(mg/L)
74,137
(7.4%)
18,674
(1.9%)
VS(mg/L)
21,128
(2.1%)
10,882
(1.1%)
11,600
(11.6%)
COD(ng/L)
70,800
28,985
100,000
PO4(mg/L)
2,408
1,198
1,550
NH3-N(mg/L)
1,494
1,272
1,250
NO3-N(mg/L)
1,297
274
133
5.1
Sulfate(mg/L)
2,874
4,390
Sulfide(mg/L)
50.0
30.3
Iron(mg/L)
vtc.edu
Feedstock
Farhan&Farhan(2012);EPA(2002)
FeCl3 case study: conclusions
•  The dose of FeCl3 predicted by stoichiometry is far lower than the dose
found to be effective in lab and full-scale study. Why? Hypotheses:
•  Iron is sequestered by organic material in the slurry;
•  Plug flow digesters don’t provide sufficient mixing; and / or
•  Low digester ORP promotes loss of effective iron by absorption.
•  Iron dosing of 150 mg/L in feedstock:
•  Lab-scale: reduced sulfides by 40%
•  In AD: reduced biogas H2S levels by 40%
•  Much higher doses (250 mg/L) were required to achieve 70% reduction in H2S
•  Mixing might increase effectiveness
•  Ferrous chloride was just as effective as ferric chloride on an iron weight basis.
•  Production of methane was neither increased nor decreased by FeCl3.
Bottom line: The effective dose of FeCl3 is best determined empirically
vtc.edu
Farhan&Farhan(2012)
Finding the effective dose of FeCl3?
The dose of FeCl3 required to keep H2S levels at acceptable levels depends on
many factors, including:
•  Sulfur in the diet;
•  Type and design of the anaerobic digester; and
•  Operation of the anaerobic digester.
So the effective dose must be determined empirically: by trial and error.
1.  Monitor H2S levels daily, looking for change.
2.  As soon as H2S levels trend upward, increase the dose of FeCl3.
3.  Once levels of H2S drop, lower FeCl3 to levels a bit higher than before
the upward trend.
vtc.edu
Other iron salts?
A number of different salts of iron and other metals can be used to precipitate
sulfur.
For example, iron phosphate can also be used to control H2S levels.
Lab studies showed that this iron salt could reduce levels of H2S in biogas from
2500 to 100 ppm.
Other effects:
•  pH of digester slurry increased from 6.7 to 8.2.
•  Soluble sulfides increased from 18 to 61 mg/L.
vtc.edu
Farhan&Farhan(2006);McFarland&Jewell(1989)
Infusing oxygen to treat H2S
It is crucial to remember combining oxygen with biogas to oxygen levels of
6-12% creates an explosive gas and is extremely dangerous.
However, at lower levels of 2-6%, oxygen in biogas creates a micro-aerophilic
atmosphere that allows the growth of sulfur oxidizing bacteria (SOB) that convert
H2S to elemental sulfur (S).
•  SOBs (Thiobaccilli) colonize and grow on the surface of the digester slurry.
•  These bacteria are found in feedstock and don’t need to be added.
•  Oxidized, elemental sulfur deposits can be seen as yellow deposits on the
surface of the slurry when oxidation is occurring.
vtc.edu
hYp://www.bioenergyconsult.com/hydrogen-sulphide-removal-from-biogas/
Infusing oxygen to treat H2S
If digester design mixes slurry well, oxidized sulfur deposits created by oxygen
treatment can be mixed back into the slurry where sulfur-reducing bacteria can
access the sulfur and metabolize it back into H2S.
•  This may be less problematic in plug flow digesters where mixing is minimal.
2–6%O2
biogasspace
oxidized
S
H2S
SRB
SH
2S
slurry
vtc.edu
Treating biogas with oxygen
in a separate gas storage
tank, or in piping that
delivers biogas to the genset,
avoids sulfur recycling.
However, it requires another
tank and care and feeding
of this bio-treatment
system.
Post-digester biogas scrubbing
A variety of methods, both chemical and biological, can be used to scrub or
purify biogas.
Drying of biogas and purification of methane to ‘biomethane’ is an expensive
process. Biomethane has less than 4 ppm H2S and may be:
•  Injected into natural gas pipelines; or
•  Compressed for storage and / or transportation.
However, lowering (but not removing) H2S is simpler and less expensive.
vtc.edu
How big is your problem?
It’s useful to know how much H2S a digester produces per year in order to:
•  size a scrubber;
•  understand the cost of media / replacements; and
•  understand the costs of disposal of used media / chemicals.
Parameters:
•  Volume of biogas per day
•  Concentration of H2S (ppm)
Calculate kg of H2S produced on an annual basis
•  Remember that 1 ppm = 1.4 mg/m3 and use metric conversions.
Typical chemical H2S scrubbing techniques can handle 200 kg of S/day, but there
is a cost to any scrubbing technology.
vtc.edu
Chemical (absorption) scrubbers
Chemical scrubbers provide a chemical compound that react with H2S, altering
it and removing it from biogas.
•  Metal oxides
•  Absorbents
•  Caustics
•  Granular activated charcoal
•  Water scrubbing
•  Resins
vtc.edu
Soroushianetal.(2006);Farhan&Farhan(2012)
Iron sponge
Iron sponge canisters use metal oxides to precipitate sulfides out of biogas.
•  Metal oxides
XOx + H2S à XSx ê
metal oxide
metal sulfide
precipitate
ironsponge:FeOx
zincoxide:ZnOx
The metal oxide is consumed and must be periodically replace.
An iron sponge uses a bed (canister) of media + active ingredient.
•  Media: steel wool or wood chips (often pine) [wood preferred
•  Active ingredient: hydrated iron oxides (Fe2O3; aka rust)
Iron sulfides precipitate onto the media and can be rinsed off and filtered from
the aqueous rinse solution.
Media must be replaced every 4 – 6 months.
vtc.edu
Soroushianetal.(2006);Farhan&Farhan(2012)
Iron sponge reactions & regeneration
Iron sponge (Fe2O3) reacts with H2S to form iron sulfide precipitate (1)
(1) 2Fe2O3 + 6H2S à 2Fe2S3 + 6H2O
ΔH = -22 kJ/g-mol H2S
Iron sponge can be regenerated by reaction with oxygen gas (2)
(2) 2Fe2S3 + 3O2 à 2Fe2O3 + 6S
ΔH = -198 kJ/g-mol H2S
Partial regeneration by addition of 8% O2 to the gas stream or by spreading the
sponge out in air while continuously wet for 10 days:
•  releases elemental sulfur (S); and
•  enormous amounts of heat (exothermic ΔH); so
•  must be done with care to avoid conflagration.
Operation of iron sponge:
•  Operation without oxygen achieves 85% efficiency (0.56 kg H2S/kg sponge) &
requires regeneration following saturation.
•  Operation with added air allows continuous (simultaneous) regeneration.
vtc.edu
Magomnang,A-A.S.M.andVillanueva,E.P.(2014);Taylor(1956)
Iron sponge: case studies
A 2014 study conducted in Thailand found that simple PVC columns packed with
steel wool were capable of reducing H2S concentrations from 170 to zero ppm.
•  Capacity of iron oxide for sulfide absorption ranged from 0.20 – 0.72 kg/kg
•  3 sequential iron oxide canisters were used at a flow rate of 3 – 4 L / min
•  1.9 kg of iron oxide remained effective after 17 kg of biogas were treated
A 1990 study conducted in NY state found that 12 lb/bushel-grade iron sponge
removed 1.84 kg H2S / kg Fe2O3 when operated with 2.29% oxygen.
vtc.edu
Magomnang,A-A.S.M.andVillanueva,E.P.(2014);VeYer(1990)
Absorbents
Absorbent media present a large and porous surface area that H2S absorbs
onto.
Media can be regenerated by:
•  Heating;
•  Lowering pressure; and
•  Flooding with another gas to displace absorbed H2S.
Zeolites are silicates with uniform pore size & dimensions that absorb polar
compounds:
•  H2O
•  SO2
•  NH3
•  carbonyl sulfide
•  mercaptans
vtc.edu
Soroushianetal.(2006);Farhan&Farhan(2012)
Granular activated carbon
Granular activated carbon particles have huge surface areas of 4000 – 5000
square inches per ounce. Polar gases like H2S are absorbed onto these
surfaces.
•  These surfaces can be coated with alkaline or oxide material to increase
reactivity & thus effective removal of gases.
•  NaOH, Na2CO3, KOH, KI, metal oxides are used
•  These coatings can effectively double rates or removal from 10% from
untreated GAC to 20-25%
•  Canisters (sometimes sequential) are packed with charcoal.
•  Biogas is passed through canisters.
•  When H2S levels in treated biogas rise, canisters are repacked or replaced.
•  Addition of O2 increases effectiveness.
vtc.edu
Soroushianetal.(2006);Farhan&Farhan(2012)
Water scrubbing
The components of biogas have varying solubility in water.
Gascomponent
methane
carbondioxide
hydrogen
hydrosulfuricacid
nitrogen
ammonia
oxygen
sulfurdioxide
Solubilityinwater@20°C
(ggas/kgwater)
0.023
1.70
0.0016
3.90
0.018
520
0.044
110
Bubbling biogas through water will remove CO2 and H2S as they will
dissolve in the water.
•  Scrubbing water will be acidic & may need to be treated before discharge.
vtc.edu
hYp://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html
Biological (SOB) scrubbers
Post-AD biological scrubbers use sulfur-oxidizing bacteria (SOBs) to destroy
H2S by oxidation of sulfur prior to combustion.
•  A system tested in CA in 2006 kept H2S levels below that state’s stringent air
quality maximum of 40 ppm under normal conditions.
Advantages:
•  Low energy use;
•  No chemicals to store and handle;
•  Automated; and
•  Long life expectancy of overall system.
Cautions: Bacteria that degrade or oxidize H2S can produce acids like H2SO4.
Accumulation of acid lowers pH and can kill bacteria in the biological scrubber.
two-stage chemical / biological scrubbers sold commercially.
Example:
The IEUA RP-1 was developed as a simple single-stage scrubber in contrast to
the two-stage chemical / biological scrubbers sold commercially.
vtc.edu
Soroushianetal.(2006)
IEUA RP-1
System: fiberglass tank filled with plastic media; water pump; air blower
Air was added at 5% v/v or less.
The system worked very well for > 1 month.
Drop in performance may have been due to drop in temperature from October
to December (30 to 22C in scrubber) or lack of nutrients; none ever added.
vtc.edu
Soroushianetal.(2006)
Economic comparison of H2S technologies
Cost comparison of H2S removal technologies:
Technology
Capital
($)
Annual
O&M($)
TotalNPV
5years($)
TotalNPV
10years($)
FeClx
1.5–2K
8–10K
(45.8K)
(91.0K)
Ironsponge
40–50K
1.5–2K
(52.3K)
(61.1K)
Carbonfilter
12–14K
3.5–4.5K
(32.2K)
(52.3K)
CausMcscrubber
12–15K
2.5–3.8K
(28.5K)
(44.4K)
NPV = net present value calculated from average capital and O&M costs.
vtc.edu
Farhan&Farhan(2012)
Assessment!
Please answer the questions in section 5.6 of the Module 5 Assessment.
vtc.edu
References:
Gerardi (2003) The Microbiology of Anaerobic Digesters, Wiley & Sons, New Jersey
Chynoweth & Issacson (1987)
http://www.bioenergyconsult.com/hydrogen-sulphide-removal-from-biogas/
Madigan (2000)
Magomnang, A-A.S.M. and Villanueva, E.P. (2014) Removal of hydrogen sulfide from
biogas using dry desulfurization systems, International Conference on Agricultural,
Environmental & Biological Sciences, April 2014, Phuket Thailand
McFarland & Jewell (1986) Water Research, 23(12): 1571-7.
Park, C.M. and Novak, J.T. (2013) The effect of direct addition of iron (III) on anaerobic
digestion efficiency and odor causing compounds. Water Science Technology, 68(11):
2391-6.
Revell (1997)
Speece (1996) Anaerobic Biotechnology for Industrial Wastewater, Archae Press, p. 394Taylor (1956)
Vetter & Friederick (1990) Full scale anaerobic digester and waste management system
for a 300 cow dairy. Proceedings of the Sixth International Symposium on Agriculture
and Food Processing Wastes, ASAE, Chicago, IL: 236-249.
Yoda (1987) Water Research, 21(12): 1547vtc.edu
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