Anaerobic and Aerobic digestion

1
 Introduction
Organic matter is the vast array of carbon compounds
in nature.
Originally created by plants, microbes, and other
organisms, these compounds play a variety of roles in
nutrient, water, and biological cycles.
Organic matter can be divided into two major
categories: stabilized organic matter which is highly
decomposed and stable, and the active fraction which
is being actively used and transformed by living plants,
animals, and microbes. Two other categories of organic
compounds are living organisms and fresh organic
residue.

2
 Stabilized organic matter
Many soil organisms decompose plant and animal
tissues, and transform the organic matter into new
compounds. After years or decades of these
transformations, what remains are large, complex
compounds that few microbes can degrade.
These hard-to-decompose, or stabilized, substances
make up a third to a half of soil organic matter. These
hard-to-decompose, or stabilized, substances make up
a third to a half of soil organic matter.
Stabilized organic matter are often divided into three
chemical groups: humic acids, fulvic acids, and humins.

3
 Humic Substances
Soil humic substances (HS) are defined as amorphous,
polymeric, brown coloured substances that are
differentiated on the basis of solubility properties into
humic acids (HAs, precipitated when aqueous alkaline
extracts from soil are adjusted to pH 1), fulvic acids
(FAs, soluble in aqueous media at all pH values) and
humins (insoluble in aqueous media).
Humins are considered to be the major components of
HS, and to compose 50% and more (Stevenson, 1994)
of the transformed or humified components (that bear
no morphological resemblances to the structures from
which they were derived) of organic materials in soil
organic matter (SOM).
4
 Humus
Humus (coined 17901800; Latin: earth, ground) refers to any organic
matter that has reached a point of stability, where it will break down no
further and might, if conditions do not change, remain as it is for
centuries, if not millennia.
Humus is separated from the non-humic substances such as carbohydrates
(a major fraction of soil carbon),fats, waxes, alkanes, peptides, amino
acids, proteins, lipids, and organic acids by the fact that distinct chemical
formulae can be written for these non-humic substances.
Most small molecules of non-humic substances are rapidly degraded by
microorganisms within the soil. In contrast soil humus is slow to
decompose (Degrade) under natural soil conditions. When in combination
with soil minerals soil humus can persist in the soil for several hundred
years.
Humus is the major soil organic matter component, making up 65% to 75%
of the total. Humus assumes an important role as a fertility component of
all soils, far in excess of the percentage contribution it makes to the total
soil mass.

5
 Humic and Fulvic acids
Humic acid: It is a principal component of humic substances, which
are the major organic constituents of soil. It is produced by
biodegradation of dead organic matter; is a complex mixture of
many different acids containing carboxyl and phenolate groups.
FULVIC ACIDS - are a mixture of weak aliphatic and aromatic organic
acids which are soluble in water at all pH conditions (acidic, neutral
and alkaline). Their composition and shape is quite variable. The
size of fulvic acids (FAs) are smaller than humic acids (HAs), with
molecular weights which range from approximately 1,000 to
10,000.
Fulvic acids (FAs) have an oxygen content twice that of humic acids
(HAs). They have many carboxyl (-COOH) and hydroxyl (-COH)
groups, thus fulvic acids (Fas) are much more chemically reactive.
The exchange capacity of fulvic acids (FAs) is more than double that
of humic acids (HAs). This high exchange capacity is due to the total
number of carboxyl (- COOH) groups present.

6
 Stabilized organic matter
In general there are two ways to stabilize organic
matter in nature:
Composting: decomposes waste aerobically into CO2,
water and a humic fraction; some carbon storage also
occurs in the residual compost
Anaerobic Digestion: anaerobic digestion produces biogas
(CH4 + CO2) and biosolids.
Anaerobic digestion is particularly appropriate for wet wastes,
while composting is often appropriate for drier feedstocks.
The two methods form part of biological digestion of waste.
Compost products and digestion residuals can have potential
horticultural or agricultural applications

7
 Compost
Composting is a managed aerobic (i.e. in the presence of oxygen)
microbial process that breaks down organic wastes into compost.
The process is focused on breaking down or decomposing those parts of
the waste stream that are most easy to decompose. This includes sugars,
starches, fats and proteins. At the end of the process all that is left are the
parts of the waste stream that are more resistant to composting.
Composting is said to stabilize waste. This means that the resultant
compost will continue to break down but at a very slow rate.
A key advantage of the composting process is that its high temperature
essentially kills all pathogens and weed seeds that might be found in
wastes.
Bacteria, fungi and actinomycetes are the microorganisms responsible for
the composting process. While they all play different roles they have
essentially the same requirements.
Composting is about creating a suitable environment for the
microorganisms.

8
 Compost
Compost is biologically active. An overabundance of soil organisms
is responsible for transforming the organic matter in compost into
carbon dioxide, water, humic substances capable of releasing
inorganic plant nutrients and energy in the form of heat. These
organisms are especially numerous and active in the initial phases
of composting, but many remain in smaller numbers even in the
finished product.
In a mature compost, enough of the original organic material will
have been consumed to prevent any substantial increase in the
activity (and therefore heat-generating capacity) of the remaining
microbes. This microbial stability is a prerequisite to compost
maturity.
Since stabilized compost is no longer subject to sudden chemical
changes, it may be safely handled, stored and applied. Mature
compost is normally dark brown in color and should have an even
texture and a pleasant, earthy aroma.

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 Compost process
It is an aerobic biological process which uses naturally
occurring microorganisms to convert biodegradable
organic matter into a humus like product.
The process destroys pathogens, converts N from
unstable ammonia to stable organic forms, reduces the
volume of waste and improves the nature of the waste.
It also makes waste easier to handle and transport and
often allows for higher application rates because of the
more stable, slow release, nature of the N in compost.
The effectiveness of the composting process is
influenced by factors such as temperature, oxygen
supply (i.e. aeration) and moisture content.
There are two fundamental types of composting
aerobic and anaerobic: 10
 Anaerobic Composting
This is the decomposition of organic wastes in the
absence of O2, the products being methane (CH4),
CO2, NH3 and trace amounts of other gases and
organic acids.
Anaerobic composting was traditionally used to
compost animal manure and human sewage
sludge, but recently is has become more common
for some municipal solid waste (MSW) and green
waste to be treated in this way.
This method of composting is discouraged for un-
enclosed decomposing matter due to foul smell.

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 Aerobic Composting
Composting is the decomposition of organic wastes in the
presence of oxygen (air); products from this process include
CO2, NH3, water and heat.
This can be used to treat any type of organic waste but,
effective composting requires the right blend of ingredients
and conditions. These include moisture contents of around
60-70% and carbon to nitrogen ratios (C/N) of 30/1. Any
significant variation inhibits the degradation process.
Generally wood and paper provide a significant source of
carbon while sewage sludge and food waste provide
nitrogen.
To ensure an adequate supply of oxygen throughout,
ventilation of the waste, either forced or passive is
essential.
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 Aerobic Composting
Key Parameter Optimum values
Oxygen 10-15%
Moisture (supplementary moisture can also be 50-55%
added to composting mass)
Carbon to nitrogen (C:N) ratio 30:1
pH 6-9
Porosity (spaces to allow air in compost) 1-5cm

Oxygen is supplied in two ways:

by turning the compost, either by front-end loader or a specialized compost turner;
by building the pile correctly, so surface air can diffuse into the center. When a pile gets too little
oxygen, it will go anaerobic, and offensive odors may result.

13
 Phases of composting
1. Mesophilic phase (I).
2. Thermophilic phase (II).
3. Cooling phase (III).
4. Maturing phase (IV).
Important Parameters of composting process
1. Water content.
2. Oxygen Demand.
3. Nutrients.
4. Temperature.
5. PH.
6. Time.

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 Suitable materials for composting
Can be composted Cannot be composted
Sewage sludges Coal ash
Industrial wastes (e.g. food, pulp & paper) Metal, glass and plastic
Yard and garden wastes Nappies.
Municipal solid wastes (up to 70% organic The roots of persistent weeds, like
matter by weight) bindweed and couch grass
Kitchen waste like fruit, peelings, teabags Meat or fish
and egg shell.
Paper shredded, mixed with grass cuttings Cooked food, especially meat

15
 Advantages of composting
Reduces mass and volume: E.g. 50% reduction in mass and
80% reduction in volume
Waste stabilization
Pathogens are reduced
Improves soil structure makes it more friable i.e. gives
it crumbly texture, beneficial for root growth.
Improves water-retention in soils, helping to keep plants
healthier for longer in dry conditions
Provides a source of slow-release, organic fertilizer for your
plants
Boosts the community of microorganisms and other
creatures beneficial for enhancing nutrient uptake and
fighting plant diseases
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 Biological stabilization of liquid waste
A lot of waste produced in nature, industry and in
homes is in liquid form and cannot be
composted.
Under carefully controlled conditions, the waste
can be stabilized under aerobic conditions known
as aerobic digestion or in the absence of air
known as anaerobic digestion.
The latter method has economic value as it
generates biogas rich in methane.
17
 Anaerobic digestion (AD)
In an anaerobic system there is an absence of gaseous
oxygen; gaseous oxygen is prevented from entering the
system through physical containment in sealed tanks.
Anaerobes access oxygen from sources other than the
surrounding air, which can be the organic material
itself or may be supplied by inorganic oxides from
within the input material.
When the oxygen source in an anaerobic system is
derived from the organic material itself, the
'intermediate' end products are primarily alcohols,
aldehydes, and organic acids, plus carbon dioxide.

18
 Anaerobic digestion
The oxygen source for these microorganisms
can be the organic material itself or
alternatively may be supplied by inorganic
oxides from within the input material.
When the oxygen source in an anaerobic
system is derived from the organic material
itself, then the 'intermediate' end products are
primarily alcohols, aldehydes, and organic
acids plus carbon dioxide.

19
 Types of Anaerobic Digesters:
Mesophilic digestion
It is an established and relatively simple
technology.
During this process decomposition of the
organic matter takes 15 to 40 days and as a
result requires a large tank.
The tank is heated to a relatively low
temperature of between 30 to 40C.

20
 Types of Anaerobic Digesters:
Thermophilic digestion
It is an emerging technology with a lower
retention time (12 to 14 days) and with a faster
and higher rate of biogas production.
It also has the benefit that a smaller tank can be
used.
The tank needs to be heated to a higher
temperature (55C) therefore using more energy.
The thermophilic digestion process is currently
more expensive and is technically more complex.
21
 Anaerobic digestion occurs in four
steps
Hydrolysis
Fermentation or Acidogenesis
Acetogenesis
Methanogenesis

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 Stages of Anaerobic digestion

Cabohydrates Monosaccharides

Carbonic acids
& alcohols
Fatty Acids Hydrogen Methane
Fats
Acetic Acid Carbon Dioxide
Carbon Dioxide
Hydrogen
Carbon Dioxide
Ammonia

Proteins Amino Acids

Hydrolysis Acidogenesis Acetogenesis Methanogenesis

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 Anaerobic digestion:Hydrolysis
Complex organic matter is decomposed into
simple soluble organic molecules using water to
split the chemical bonds between the substances
Hydrolysis of the input materials occurs in order
to break down insoluble organic polymers such as
carbohydrates and make them available for other
bacteria
Through hydrolysis the complex organic
molecules are broken down into simple sugars,
amino acids, and fatty acids
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 Anaerobic digestion: Fermentation or
Acidogenesis:
The chemical decomposition of carbohydrates
by enzymes, bacteria, yeasts, or molds in the
absence of oxygen.
Acidogenic bacteria then convert the sugars
and amino acids into carbon dioxide,
hydrogen, ammonia, and organic acids
The process of acidogenesis is similar to the
way that milk sours.

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Anaerobic digestion: Acetogenesis
Acetogenic bacteria then convert these
resulting organic amino acids into acetic acid,
along with additional ammonia, hydrogen,
and carbon dioxide

26
Anaerobic digestion: Methanogenesis
methanogenic bacteria are finally are able to
convert acetate, hydrogen to methane and
carbon dioxide.

27
 Anaerobic digestion: Two groups of
methane forming bacteria

Methanobacterium
Convert CO2 and H2 to CH4
Reductive methane formation
About 30% of methane formed
Methanosarcina
Convert acetate to CH4 and bicarbonate
Acetate decarboxylation
About 70% of methane formed

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Anaerobic digestion: Methane Formers
Methane forming bacteria control the anaerobic
digestion process.
Sensitive to environmental changes
Reproduce slowly
Easy to kill and hard to grow
Methane formers
Strict anaerobes
No stabilization until methane is formed.
AD Process revolves around methane formers.
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Path of Anaerobic Digestion

30
Bacteria involved in Anaerobic digestion

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 Anaerobic Digestion: Digestate
Digestate is the solid remnants of the original
input material to the digesters that the
microbes cannot use.
It also consists of the mineralised remains of
the dead bacteria from within the digesters.
Digestate can come in three forms: fibrous,
liquor, or a sludge-based combination of the
two fractions.

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 Anaerobic Digestion: Digestate
The acidogenic digestate is a stable organic
material consisting largely of lignin and
cellulose, but also of a variety of mineral
components in a matrix of dead bacterial cells;
some plastic may be present.

33
 Anaerobic Digestion: Digestate
The solid digestate can also be utilized as
feedstock for ethanol production.
The methanogenic digestate is a liquid that is
rich in nutrients and can be used as a fertiliser
dependent on the quality of the material
being digested.

34
 Anaerobic Digestion
The final output from anaerobic digestion
systems is water.
This water originates both from the moisture
content of the original waste that was treated
but also includes water produced during the
microbial reactions in the digestion systems.
This water may be released from the
dewatering of the digestate or may be
implicitly separate from the digestate.

35
 Anaerobic Digestion
The wastewater exiting the anaerobic digestion
facility will typically have elevated levels of
biochemical oxygen demand (BOD) and chemical
oxygen demand (COD).
These are measures of the reactivity of the
effluent and show an ability to pollute.
If this effluent were put directly into
watercourses, it would cause eutrophication.
As such, further treatment of the wastewater is
often required.
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 Some Definitions in Anaerobic Digestion
Biochemical oxygen demand or B.O.D. is a chemical
procedure for determining the amount of dissolved
oxygen needed by aerobic biological organisms in a
body of water to break down organic material present
in a given water sample at certain temperature over a
specific time period.
It is most commonly expressed in milligrams of oxygen
consumed per litre of sample during 5 days of
incubation at 20 C and is often used as a robust
surrogate of the degree of organic pollution of water.
Moderately polluted rivers may have a BOD value in
the range of 2 to 8 mg/L. Untreated sewage varies, but
averages around 600 mg/L .
37
 Some Definitions in Anaerobic Digestion

Chemical oxygen demand (COD) test is

commonly used to indirectly measure the
amount of organic compounds in water. Most
applications of COD determine the amount of
organic pollutants found in surface water (e.g.
lakes and rivers), making COD a useful
measure of water quality. It is expressed in
milligrams per liter (mg/L), which indicates the
mass of oxygen consumed per liter of solution.
38
Conditions for Anaerobic Digestion
Environmental conditions: Neutral pH (actually
between 6.8- 7.2)
Below 6: unionized volatile acids become toxic to
methane formers.
-Above 8: unionized ammonia becomes toxic.
Alkalinity 2,000 5,000 mg/L
Volatile acids 50 300 mg/L
Volatile acid/alkalinity ratio Range: 0.2 0.3
(Increase above 0.3 0.4 indicate upset)

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Conditions for Anaerobic Digestion
Temperature: Mesophilic 32 - 38C
Thermophilic range 50 - 60 C
**Tank temperature must not change more than 1 C per
day.
Causes of Toxicity
Organic compounds
Heavy metals
Ammonia
Sulfide
Oxygen
Salts

40
Important factors in AD of solid waste
Several factors can affect the performance of
the anaerobic digestion, either by process
enhancement or inhibition, influencing
parameters such as specific growth rate,
degradation rates, biogas production or
substrate utilisation.
These factors are: pH, temperature, substrate,
retention time, organic loading, mixing
condition and inhibitory substances.

41
 Important factors in AD: PH
The pH value of the digester content is an important indicator of
the performance and the stability of an anaerobic digester. In a
well-balanced anaerobic digestion process, almost all products of a
metabolic stage are continuously converted into the next breaking
down product without any significant accumulation of intermediary
products such as different fatty acids which would cause a pH drop.
Many aspects of the complex microbial metabolism are greatly
influenced by pH variations in the digester. Although acceptable
enzymatic activity of acid-forming bacteria can occur at pH 5.0,
methanogenesis proceeds only at a high rate when the pH is
maintained in the neutral range.
Most anaerobic bacteria including methane forming bacteria
function in a pH range of 6.5 to 7.5, but optimally at a pH of 6.8 to
7.6, and the rate of methane production may decrease if the pH is
lower than 6.3 or higher than 7.8
Anaerobic digestion of kitchen wastes with controlled pH value at
7.0 resulted in a relatively high rate of hydrolysis and acidogenesis
with about 86 % of TOC and 82 % of COD were solubilized.
42
 Important factors in AD: PH
Alkalinity and pH in anaerobic digestion can be
adjusted using several chemicals such as sodium (bi)
carbonate, potassium (bi) carbonate, calcium
carbonate (lime), calcium hydroxide (quick lime) and
sodium nitrate. Addition of any selected chemical for
pH adjustment should be done slowly to prevent any
adverse impact on the bacteria.
Because methanogenic bacteria require bicarbonate
alkalinity, chemicals that directly release bicarbonate
alkalinity are preferred ( sodium bicarbonate and
potassium bicarbonate are more preferred due to their
desirable solubility, handling, and minimal adverse
impacts).
Lime may be used to increase digester pH to 6.4, and
then either bicarbonate or carbonate salts (sodium or
potassium) should be used to increase the pH to the
optimum range 43
Important factors in AD: Temperature
Temperature is one of the major important parameters
in anaerobic digestion. It determines the rate of
anaerobic degradation processes particularly the rates
of hydrolysis and methanogenesis.
Moreover, it not only influences the metabolic
activities of the microbial population but also has a
significant effect on some other factors such as gas
transfer rates and settling characteristics of biosolids.
Anaerobic digestion commonly applies two optimal
temperature ranges: mesophilic with optimum
temperature around 35 C and thermophilic with
optimum temperature around 55 C.

44
 Influence of temperature on the rate
of anaerobic digestion process.

Optimum temperature for mesophilic around 30

40 C and for thermophilic 50 60 C
45
 Mesophilic AD
Mesophilic bacteria are supposed to be more robust
and can tolerate greater changes in the environmental
parameters, including temperature.
Smaller digesters, poorly insulated digesters, or
digesters in cold climates are susceptible for extreme
temperature fluctuations thus these would be
beneficial if the digester is being run in the mesophilic
range to minimize system crashing.
Although it requires longer retention time, the stability
of the mesophilic process makes it more popular in
current anaerobic digestion facilities

46
 Thermophilic AD
Thermophilic process offers faster kinetics, higher
methane production rates and pathogen removal. This
method, however, is more sensitive to toxic substances
and changes of operation parameters.
A study comparing the performance of thermophilic
and mesophilic treating mechanically sorted municipal
solid waste (by Cecchi in 1991) found that thermophilic
process yielded 100 % more methane production and
better volatile solids elimination compared to
mesophilic process.
However, thermophilic process is sometimes
considered as less attractive from the energy point of
view since it requires more energy for heating
47
 Heat Exchangers in AD
The most common method for maintaining the
temperature in anaerobic digester is an external heat
exchanger. This method has the benefit of enabling to
mix recirculating digestate with raw slurry before
heating, and in seeding the raw slurry with anaerobic
microorganisms.
Among three types of external heat exchangers
frequently used (water bath, tubular and spiral
exchangers), both tubular and spiral exchangers are
mostly preferred for their countercurrent flow design
and heat transfer coefficients. The hot water used in
the heat exchangers is commonly produced in a boiler
fueled by biogas that comes from the digester.
At the startup and/or under conditions of insufficient
biogas production, an alternative fuel source such as
natural gas must be provided 48
 Psychrophilic AD
Reasonable methane yields still can be expected
from anaerobic digestion at low temperatures (14
23 C) if the organic loading of the digester is
reduced by mean of extending the hydraulic
retention.
A relative stable operation of an anaerobic
digester treating mixture of animal manure can
be achieved at low temperature (18 25 C) with
an optimum OLR of 4 6 kg VS m-3 d-1 and a
methane content of 47 55 % in the biogas.

49
Anaerobic Digestion: Substrate Characteristics

The characteristics of solid wastes determine

the successful anaerobic digestion process (
high biogas production potential and
degradability).
In municipal solid waste, substrate
characteristics may vary due to the method of
collection, weather season, cultural habits of
the community .

50
 Anaerobic Digestion: Substrate
Characteristics
The degradability and biogas production potential
from solid waste in an anaerobic digester are
dependent on the amount of the main
components: lipids, proteins, carbohydrates such
as cellulose and hemicelluloses as well as.
Among them lipids are the most significant
substances in the anaerobic digestion, since the
methane yield from lipids is higher than from
most other organic materials.

51
 Anaerobic Digestion: Substrate
Characteristics
Lignocellulosic (cellulose and hemicelluloses which are
tightly bound to the lignin) waste can be found in
abundant amount in the form of garden waste, paper
residue or agricultural waste. Due to the presence of
lignin, lignocellulosic waste is considered to be quite
resistant to anaerobic digestion and hydrolysis is the
rate limiting step in the overall process.
In order to improve the rate of enzyme hydrolysis and
increase yields of fermentable sugars from cellulose or
hemicellulose in lignocellulosic waste, several
pretreatment methods such as thermal (steam or hot
water), chemical (acid, lime or ammonia addition) or
combination of both methods can be used.
52
 Anaerobic Digestion: Substrate
Characteristics
The composition of waste also determines the relative amounts of
organic carbon and nitrogen present in the waste substrate (C/N
ratio).
A solid waste substrate with high C/N ratio is not suitable for
bacterial growth due to deficiency of nitrogen. As a result the gas
production rate and solids degradability will be low. On the other
hand, if the C/N ratio is very low, the degradation process leads to
ammonia accumulation which is toxic to the bacteria.
A C/N ratio (based on biodegradable organic carbon and nitrogen)
within the range of 2530 is considered to be optimum for an
anaerobic digester. To maintain the C/N level of the digester
material at optimum levels, substrates with high C/N ratio can be
codigested with nutrient-rich organic wastes (low C/N ratio) like
animal manure or foodwaste.

53
 Anaerobic Digestion: Substrate
Characteristics
The particle size has a significant role in anaerobic
digestion of solid waste, especially during hydrolysis
since a smaller particle size provides a greater area for
enzymatic attack .
The increase of the average particle size in anaerobic
digestion of food waste was reported to decrease the
maximum substrate utilization rate coefficient.
It has been reported that by reducing the size to 2 mm,
the potential methane production of sisal fiber waste
will improve to more than 20 % and the total fiber
degradation increased from 31% to 70% compared to
the untreated fibers.

54
 Anaerobic Digestion: Hydraulic
Retention time
The hydraulic retention time (HRT) is a measure to describe
the average time that a certain substrate resides in a
digester. In a digester with continuous mixing, the contents
of the reactor have a relative uniform retention time. In this
system, the minimum HRT is dictated by the growth rate of
the slowest growing, essential microorganisms of the
anaerobic bacterial community. If the HRT is shorter, the
system will fail due to washout of the slowest growing
microorganisms that are necessary for the anaerobic
process.
Shortening the HRT consequently reduces the size of the
digester, resulting in capital cost savings. Furthermore, a
shorter HRT yields a higher biogas production rate, but less
efficient degradation of organic matter (as volatile solids or
COD), associated with less process stability must be
anticipated. 55
 Anaerobic Digestion: Hydraulic
Retention time
The HRT of anaerobic digesters treating solid wastes
varied from 3 to 55 days, depending on the type of
waste, operational temperature, process stage(s) and
configuration of the digesters.
The HRT for dry anaerobic digestion ranges between 14
and 30 days and for wet anaerobic processes it can be
as low as 3 days. Longer retention time of 50 100
days have been used for a digester treating solid waste
from poultry slaughterhouse.
However, at a shorter retention time (13 to 25 days),
the process appeared to be inhibited, as indicated by
the buildup of long chain fatty acids and a lower
methane yield. 56
 Anaerobic Digestion: organic loading
rate (OLR)
The organic loading rate (OLR) is defined as the amount
of organic matter (expressed as volatile solids or COD
of the feeding substrate) that must be treated by a
certain volume of anaerobic digester in a certain period
of time.
The value of the OLR is mostly coupled with the HRT
value. If the concentration of organic matter in the
feedstock substrates is relatively constant, the shorter
the HRT the higher value of OLR will be achieved. On
the other hand, the value of the OLR will vary at the
same HRT if there is a variation of organic matter
concentration in the feeding substrate.

57
 Anaerobic Digestion: organic loading
rate (OLR)
The potential danger of a rapid increase in the
OLR would be that the hydrolysis and acidogenic
bacteria would produce intermediary products
rapidly.
Since the multiplication time of methanogenic
bacteria is slower, they would not be able to
consume the fatty acids at the same rate.
The accumulation of fatty acids will lead to a pH
drop and hampering the activity methanogenic
bacteria, causing a system failure.
58
Anaerobic Digestion: Mixing condition
Mixing plays an important role in anaerobic digestion of
solid waste.
Mixing provides an adequate contact between the
incoming fresh substrate and the viable bacterial
population and also prevents the thermal stratification and
the formation of a surface crust/scum buildup in an
anaerobic reactor.
Furthermore, mixing ensures that solids remain in
suspension avoiding the formation of dead zones by
sedimentation of sand or heavy solid particles.
Mixing also enables the particle size reduction as digestion
progresses and the release of produced biogas from the
digester contents.
Mechanical mixing systems generally use lows peed flat-
blade turbines and are most suited for digesters with fixed
covers. 59
 Anaerobic Digestion: Biogas
recirculation
Biogas recirculation is a successful method of
mixing the digester content and avoids the
buildup of scum.
Biogas mixing systems can be confined and
unconfined.
In unconfined systems, the gas is collected at the top
of the digestion tank, compressed and then released
through a pattern of diffusers or a series of radially
placed lances suspended from the digester cover.
In confined systems the gas is collected at the top,
compressed and discharged through confined tubes
and gas bubbles rise, creating an airlift effect.

60
 Anaerobic Digestion: Inhibitory
Substances
Inhibition in anaerobic digestion process by the
presence of toxic substances can occur to varying
degrees, causing upset of biogas production and
organic removal or even digester failure.
These kinds of substances can be found as
components of the feeding substrate (organic
solid waste) or as byproducts of the metabolic
activities of bacteria consortium in the digester.

61
 Anaerobic Digestion: Inhibitory
Substances
The main reason for these variations is the significant
influence by microbiological mechanisms such as
acclimation, antagonism, and synergism.
Acclimation is the ability of microorganism to rearrange their
metabolic resources to overcome the metabolic block produced
by the inhibitory or toxic substances when the concentrations of
these substances are slowly increased within the environment.
Antagonism is defined as a reduction of the toxic effect of one
substance by the presence of another.
Synergism is an increase in the toxic effect of one substance by
the presence of another.
Several substances with inhibitory/toxic potential to
anaerobic digestion, such as ammonia, sulfide, light metal
ions, heavy metals and organic substances, will be
discussed shortly.
62
 Anaerobic Digestion: Inhibitory
Substances
Ammonia is a hydrolysis product formed during anaerobic
digestion of solid waste by degradation of nitrogenous
matter in the form of proteins, phospholipids and
nitrogenous lipids.
The inhibition mechanisms of ammonia are presumably
due to the change of intracellular pH, the increase of
maintenance energy requirement to overcome the toxic
conditions, and inhibition of specific enzyme reactions.
In a solution, ammonium exists in the form of ammonium
ion and free ammonia. Free ammonia is reported to have a
more pronounced inhibition effect since it is freely
membrane permeable and may diffuse passively into the
cell, causing proton imbalance and/or potassium deficiency.

63
 Anaerobic Digestion: Inhibitory
Substances
Ammonia Toxicity
50 200 mg/L: beneficial
200 1,000 mg/L: no adverse effects
1,500 3,000 mg/L: inhibitory at pH 7.4 7.4- 7.6
3,000 mg/L: toxic

64
 Anaerobic Digestion: Inhibitory
Substances
The formation of hydrogen sulfide in anaerobic digestion is the
result of the reduction of oxidized sulfur compounds and of the
dissimilation of sulfur containing amino acids such as cysteine by
sulfate reducing bacteria. The reduction is performed by two major
groups of SRB including incomplete oxidizers, which oxidize
compounds such as lactate to acetate and CO2 and complete
oxidizers (acetoclastic SRB), which completely convert acetate to
CO2 and HCO3. Both groups utilize hydrogen for sulfate reduction.
Inhibition caused by sulfate reduction can be differentiated into two
stages. Primary inhibition is indicated by lower methane production
due to competition of SRB and methanogenic bacteria to obtain
common organic and inorganic substrates. Secondary inhibition
results from the toxicity of sulfide to various anaerobic bacteria
groups.

65
 Anaerobic Digestion: Inhibitory
Substances
The light metal ions including sodium, potassium,
calcium, and magnesium are commonly present in the
digestate of anaerobic reactors. They may be produced
by the degradation of organic matter in the feeding
substrate or by chemicals addition for pH adjustment.
Moderate concentrations of these ions are needed to
stimulate microbial growth, however excessive
amounts will slow down growth, and even higher
concentrations can cause severe inhibition or toxicity.
Salt toxicity is primarily associated with bacterial cells
dehydration due to osmotic pressure.

66
 Anaerobic Digestion: Inhibitory
Substances
Although the cations of salts in solution must
always be associated with the anions, the toxic
action of salts was found to be predominantly
determined by the cation.
The role of the anions was relatively minor and
largely associated with their effect on properties
such as the pH of the media.
If compared on a molar concentration basis,
monovalent cations, such as sodium and
potassium, were less toxic than the divalent
cations, such as calcium and magnesium

67
 Anaerobic Digestion: Inhibitory
Substances
Similar with light metal ions, the presence of heavy metals in trace
concentration will stimulate the growth of anaerobic digesters flora.
However, unlike other toxic substances, heavy metals are not
biodegradable and can accumulate to potentially toxic concentrations.
Extensive studies (in 1969 by Swanwick and 2008 by Chen) on the
performance of anaerobic reactors found that heavy metal toxicity is one
of the major causes of anaerobic digester upset or failure.
The toxic effect of heavy metals is attributed to their ability to inactivate a
wide range of enzyme function and structures by binding of the metals
with thiol (sulfhydryl) and other groups on protein molecules or by
replacing naturally occurring metals in prosthetic groups of enzymes.
The toxicity of heavy metals in anaerobic digestion depends upon the
various chemical forms which the metals may assume under anaerobic
conditions at the temperature and pH value in the digester. For instance,
heavy metals in the precipitated form have little toxic effect on the
biological system 68
 Anaerobic Digestion: Inhibitory
Substances
Many organic compounds were reported to have a
inhibitory potential to anaerobic digestion processes. The
accumulation of hydrophobic organic pollutants in bacterial
membranes causes the membrane to swell and leak,
disrupting ion gradients and eventually causing the
breaking of cellular membranes.
The toxicity concentration of organic compounds ranges
vary widely and is affected by many parameters, including
toxicant concentration, biomass concentration, toxicant
exposure time, cell age, feeding pattern, acclimation and
temperature.
Several important organic substances which are inhibitory
to anaerobic digestion are: chlorophenols, halogenated
aliphatic, nitrogensubstituted aromatic, longchain fatty
acids and lignins/lignin related compounds.
69
 Anaerobic Digestion: Minimizing the effect of
inhibitory substances
Several strategies to minimize the effect of inhibitory
substances can be summarized as follows;
Removal of potential inhibitory/toxic substances from the
feeding substrate.
Dilution of the feeding substrate in order to reduce the
concentration of inhibitory substances below the
threshold.
Addition of chemicals to precipitate or insolubilize the
inhibitory substances.
Change of the chemical form of inhibitory substances
through pH control.
Addition of material that is antagonistic to the inhibitory
substances in order to counteract the inhibitory effect.

70
 Types of AD reactors for solid waste
Typically anaerobic reactors or processes of solid
waste can be distinguished into several types,
mostly according to the feeding mode
(continuous mode: single stage, two stages and
batch mode) and the moisture content of the
substrate (wet or dry digestion).
Furthermore with those basic types, the
anaerobic reactors can be arranged according to
the digestion process temperature (mesophilic or
thermophilic) and the shape of the reactors
(vertical or horizontal).

71
Large Scale Anaerobic Digestion

72
Small Scale Anaerobic Digestion

73
A typical Anaerobic Digester site

74
 Anaerobic Digestion: Biogas
Biogas is the name given to the mixture of gases
formed during the anaerobic digestion of organic
wastes.
Biogas consists of methane (c70%) and carbon dioxide
(c30%).
It can be used in stationary engines to generate
electricity, but it is not suitable as a vehicle fuel
After removing the carbon dioxide (and other trace
gases using a variety of methods in a process known as
upgrading) the remaining methane is known as
Renewable Natural Gas or Biomethane.

75
 Typical data on composition of biogas
Compound Anaerobic Digestion
biogas
Methane, CH4 55-75 %
Carbon dioxide, CO2 25-45 %
Carbon monoxide, CO 0-0.3 %
Nitrogen, CO2 1-5 %
Oxygen, CO2 Traces
Hydrogen, CO2 0-3 %
Hydrogen sulphide, H2S 0.1-0.5 %
Chlorine, Cl2 -
Fluorine, F2 - 76
Refining Bio-Gas into Biomethane
The bio-gas produced in the methane digester
is primarily methane and carbon dioxide, with
traces of hydrogen sulfide, and other gasses.
Bio-gas by itself can be used as-is for heating
and for cooking. However, use of raw bio-gas
in heating equipment and in internal
combustion engines will cause early failures
because of the corrosive nature of the
hydrogen sulfide and water vapor.

77
Refining Bio-Gas into Biomethane
Carbon dioxide in the bio-gas lowers the
heating value of the gas. It should be noted
that the bio-gas from the digestion of animal
wastes does not have some of the
contaminants of bio-gas from landfills or
municipal waste water treatment plants and is
therefore easier to clean up

78
 Risks Associated with Bio-Gas
While methane is a very promising energy
resource, the non-methane components of
bio-gas (hydrogen sulfide, carbon dioxide, and
water vapor) tend to inhibit methane
production and, with the exception of the
water vapor, are harmful to humans and/ or
the environment. For these reasons, the bio-
gas produced should be properly cleaned
using appropriate scrubbing and separation
techniques.
79
 Risks Associated with Bio-Gas
In addition, the methane itself represents a
serious danger, as it is odorless, colorless, and
difficult to detect. Methane is also highly
explosive if allowed to come into contact with
atmospheric air at proportions of 6 to 15 percent
methane. For these reasons, it is recommended
that buildings be well ventilated; motors, wiring,
and lights should be explosion-proof; flame
arrestors should be used on gas lines; and alarms
and gas detection devices should be used.

80
 Anaerobic Digestion: Digester Failure
Unbalanced microbiological growth
Acid-formers out produce the methane-formers
Over-production of acids
Process Failure Indicators
VA concentration increases
Alkalinity drops
VA/ALK ratio increases
pH falls
Gas production rate drops,
CO2 Percent increases

81
 Anaerobic Digestion: Digester Failure
Causes of Process Failure
Hydraulic overload [Hydraulic residence time, HRT (in
hours) or tau, is a measure of the average length of time that a
soluble compound remains in a constructed bioreactor = Volume of
aeration tank (m3)/inflow rate (m3/h)]
Dilute feed sludge
Excessive sludge production
Grit and scum accumulation
Alkalinity washout
82
 Anaerobic Digestion: Digester Failure
Organic overload
Increase in sludge production
Increase in sludge concentration
Change in sludge characteristics
Infrequent feeding
Too rapid startup

83
 Anaerobic Digestion: Digester Failure
Toxic overload
Heavy metals
Detergents
Chlorinated organics
Oxygen
Cations
Sulfides

84
 Anaerobic Digestion: Digester
Solutions
Adjust alkalinity
Adjust feed schedule
thicken feed sludge
Industrial pretreatment
Clean digester

85
 Advantages of Anaerobic Digestion
Wastewater pollutants are transformed into
methane, carbon dioxide and smaller amount
of bio-solids.
The biomass growth is much lower compared
to those in the aerobic processes.
They are also much more compact than the
aerobic bio-solids

86
 Advantages of Anaerobic Digestion
Anaerobic digestion reduces the emission of
landfill gas into the atmosphere.
Anaerobic digestion is a renewable energy
source because the process produces a
methane and carbon dioxide rich biogas
suitable for energy production helping replace
fossil fuels.
The nutrient-rich solids left after digestion can
be used as fertilizer.

87
 Advantages of Anaerobic Digestion
Almost any organic material can be processed
with anaerobic digestion.
This includes biodegradable waste materials
such as waste paper, grass clippings, leftover
food, sewage and animal waste.
The exception to this is woody wastes that are
largely unaffected by digestion as most
anaerobes are unable to degrade lignin found
in wood.

88
 Advantages of Anaerobic Digestion
Stabilization in the absence of oxygen
Pathogen reduction
Reduction in mass
Production of methane

89
Disadvantages of Anaerobic Digestion
Anaerobic Digestion (AD) produces certain
emissions and effluents, to air, ground and water,
which need treatment to avoid damage to human
health and the environment.
Water produced during process can be
contaminated with nitrates and other chemicals
needs to be processed before release to the
environment
An expensive technology requiring grant to
encourage development under current
circumstances

90
 Disadvantages
Longer start-up time to develop necessary biomass inventory
May require alkalinity and/or specific ion addition
May require further treatment with an aerobic treatment process
to meet discharge requirements
Biological nitrogen and phosphorus removal is not possible
Much more sensitive to the adverse effect of lower temperatures
on reaction rates
May need heating (often by utilisation of process gas) to achieve
adequate reaction rates
May be more less stable after toxic shock(eg after upsets due to
toxic substances in the feed)
Increased potential for production of odours and corrosive gases.
Hazards arise from explosion.

91
 Aerobic Digestion
The process uses organic matter, nutrients, and
dissolved oxygen, and produces stable solids,
carbon dioxide, and more organisms.
The microorganisms which can only survive in
aerobic conditions are known as aerobic
organisms.
This is the natural biological degradation and
purification process in which bacteria that thrive
in oxygen-rich environments break down and
digest the waste.
92
Aerobic Digestion

93
 Aerobic Digestion: Flow pattern

Supernatant: The liquid lying above a solid residue after

crystallization, precipitation, centrifugation, or other
process 94
 Aerobic Digestion
In an aerobic system, such as composting, the
microorganisms access free, gaseous oxygen
directly from the surrounding atmosphere.
The end products of an aerobic process are
primarily carbon dioxide and water which are the
stable, oxidised forms of carbon and hydrogen.
Of all the biological treatment methods, aerobic
digestion is the most widespread process that is
used throughout the world.
95
 Aerobic Digestion
Under aerobic conditions, bacteria rapidly consume
organic matter and convert it into carbon dioxide. Once
there is a lack of organic matter, bacteria die and are
used as food by other bacteria. This stage of the
process is known as endogenous respiration. Solids
reduction occurs in this phase. Because the aerobic
digestion occurs much faster than anaerobic digestion,
the capital costs of aerobic digestion are lower.
However, the operating costs are characteristically
much greater for aerobic digestion because of energy
costs for aeration needed to add oxygen to the process.

96
Path of Aerobic Digestion

97
 Aerobic Digestion: Activated Sludge
In the activated sludge process, the dispersed-
growth reactor is an aeration tank or basin
containing a suspension of the wastewater and
microorganisms, the mixed liquor.
The contents of the aeration tank are mixed
vigorously by aeration devices which also supply
oxygen to the biological suspension .
Aeration devices commonly used include
submerged diffusers that release compressed air
and mechanical surface aerators that introduce
air by agitating the liquid surface.

98
 Aerobic Digestion: Activated Sludge
Following the aeration step, the microorganisms are
separated from the liquid by sedimentation and the
clarified liquid is secondary effluent.
A portion of the biological sludge is recycled to the
aeration basin to maintain a high mixed-liquor
suspended solids (MLSS) level.
The remainder is removed from the process and sent
to sludge processing to maintain a relatively constant
concentration of microorganisms in the system.

99
 Advantages of Aerobic Digestion
Aerobic bacteria are very efficient in breaking
down waste products.
Aerobic treatment usually yields better
effluent quality that that obtained in
anaerobic processes.
The aerobic pathway also releases a
substantial amount of energy.
A portion is used by the microorganisms for
synthesis and growth of new microorganisms.
100
Disadvantages of the Aerobic digestion
The major disadvantages of the aerobic digestion process
are that :
(1) a high power cost is associated with supplying the
required oxygen,
(2) a digested sludge is produced with poor mechanical
dewatering characteristics,
(3) the process is affected significantly by temperature,
location, and type of tank material and
(4) a useful by - product such as methane is not recovered
(5) More Sludge produced for disposal

101
 Waste stabilisation ponds (WSPs)
Waste stabilisation ponds (WSPs) are shallow man made basins into
which wastewater flows through. After a retention time of many
days, a well-treated effluent is discharged.
They are a good alternative in countries where land and sunshine is
plentiful.
BOD, COD and SS removal efficiencies of 53%, 53% and 74%
respectively and performances increased with an increase in
temperature.
WSPs can withstand shock loads without significant loss of
efficiency in terms of BOD removal and bacterial pathogen removal,
although nutrient removal by maturation ponds may be reduced.
WSPs have been classified according to the availability of oxygen for
the stabilization process as anaerobic, facultative and aerobic.
There are three types of ponds; anaerobic ponds, facultative ponds
and maturation/oxidation ponds.

102
 Anaerobic ponds
These serve as a pre-treatment step for high BOD
organic loading with high protein, fat and suspended
solids content, they more or less function like open
septic tanks.
They are normally 2-5 m deep sometimes up to 9 m
with long detention times of 20 50 days.
Where temperatures are higher than 20 0C, they have
shorter retention times of 1 5 days and can achieve
60% to 75% BOD removal. A consortium of
microorganisms, mostly bacteria, is involved in the
transformation of complex materials into simple
molecules.

103
 Anaerobic ponds
Anaerobic biodegradation yields methane
through methanogenesis, carbon dioxide
(CO2) and other gases such as hydrogen
sulphide (H2S).
Anaerobic ponds require temperatures above
10 oC, and so work very well in warm climate

104
 Facultative Ponds
Facultative ponds can be primary or secondary. They
are 1 3 m deep, designed for low BOD loading of 100
400 kg /ha/day and a retention time of 5 50 days.
The biological wastewater treatment processes are
carried out mainly by heterotrophic and autotrophic
bacteria, algae and zooplankton, in a mixture of
anaerobic, aerobic and facultative conditions.
The biological activity in the facultative ponds can be
classified into photic, heterotrophic and anaerobic
zones

105
Facultative Ponds

106
 Facultative Ponds
The Photic Zone
This zone is aerobic, mainly dominated by algae, which
are involved in nitrogen fixation, nutrient uptake
through photosynthesis and therefore produce oxygen
for the heterotrophic bacteria.
Photosynthesis leads to an increase in pH, if alkalinity is
low, and this favours phosphorus precipitation as
calcium phosphate, and ammonium ion may be lost as
ammonia.
Good wind mixing ensures a more uniform distribution
of BOD, DO, bacteria and algae and hence a better
degree of waste stabilisation.

107
 Facultative Ponds
Heterotrophic activity
Heterotrophic bacteria are the principal microbial agents
for degradation of organic matter. The aerobic activity
produces CO2 and micronutrients for algae growth and
utilises O2 produced by algae by wind mixing, as electron
acceptor. The anaerobic activity results in the production of
gases such as methane (through methanogenesis),
hydrogen sulphide, carbon dioxide and nitrogen (through
denitrification).
The anaerobic zone
The degradation in this zone is similar to that described
earlier in this leacture (using controlled mesophilic and
thermophilic conditions in industrial tanks).

108
 Maturation/Tertially Ponds
A series of maturation ponds receives effluent
from facultative ponds, the size and number is
governed by the bacteriological quality of the
final effluent.
These are 1 2 m deep and has a significant
role in pathogen and nutrient removal but less
significant in BOD removal.
Oxygen is typically supplied by surface
reaeration.

109
 Pathogen and Parasite Removal in WSPs

Bacterial die-off (pathogen removal) is

dependent on long retention times, high pH
and DO generated by algal photosynthesis,
necessary for photo-oxidative damage, which
in turn is promoted by high light intensity.
Zooplankton predation is also important.
The protozoan cysts and helminthes eggs are
removed by sedimentation mainly in
anaerobic and facultative ponds.

110
 Nutrient Removal in WSPs
The nutrient removal processes taking place in
the ponds are purely biological processes, in
which diversified groups of organisms are
responsible i.e. bacteria, fungi and algae.
The origin of these microorganisms could be from
the air, soil, and animals living near the system
and the wastewater itself.
In waste stabilisation ponds, nitrogen removal is
both assimilative (by algal uptake) and
dissimilative (through nitrification and leading to
denitrification and ammonification).

111
 Factors affecting the Stabilisation Process,
growth and activities in the WSPs
Natural purification of wastewater by stabilisation
ponds begins immediately after wastewater enters a
pond. Settleable solids, suspended solids and colloidal
particles either settles to the bottom of the pond by
gravity or may be precipitated by action of soluble salts
due to the rise in pH. Soluble materials are oxidised by
bacteria. Settled organic matter is converted to inert
residue and soluble substances diffuse into the bulk of
the water above, where further decomposition is
carried out by bacteria. Degradation of dead bacterial
biomass with release of ammonia promotes algae
growth.

112
 Factors affecting the Stabilisation Process,
growth and activities in the WSPs
The stabilisation processes in WSPs is dependent on various
factors. These include types of ponds (anaerobic,
facultative or maturation).
The type of ponds is generally determined by pond depth.
In deeper ponds, light cannot penetrate the water because
of the excessive water depth; algae photosynthesis is
therefore inhibited, rendering the microbiological activity
to be more heterotrophic than photosynthetic.
Reduction of pathogenic organisms may be minimized as a
result of inadequate exposure to direct sunlight. On the
other hand maturation ponds are shallow (0.5 -1 m) and
are good in destruction of faecal bacteria.

113
 Factors affecting the Stabilisation Process,
growth and activities in the WSPs
Detention time is another important factor. Algae
require sufficient time to grow and multiply through
binary fission. Each pond cell (compartment) should
provide a minimum retention time to avoid premature
cell washout.
Detention time will also influence the achievement of
the desired level of coliform and organic matter
removal.
Sufficiently long detention times (dependent on type
of pond) should be provided to allow parasites (faecal
and helminths eggs) to die off and sink to the bottom
and to allow sufficient nutrient removal and organic
matter transformations.
114
 Factors affecting the Stabilisation Process,
growth and activities in the WSPs
Temperature influences the organism yield coefficient, die-
off rates, availability of nutrients and dominant species, all
of which are influencing the organic matter decomposition.
Temperature does not only influence the degradation of
soluble matter in the supernatant, but also the settled
sludge that is most often not considered during the design.
At temperatures below 17 oC fermentation may be neglible,
and above 23 oC it is so intense that sludge may float to the
surface.
Light intensity has a great influence on photosynthesis,
which in turn can influence the stabilisation process. As the
concentration of algae increases, light penetration
decreases so that the phytoplankton becomes self-shading,
and thereby restricting photosynthesis in the lower layers.
115
 Wastewater Stabilisation Ponds in Ruai
Birds at the sewage inlet

116
Wastewater Stabilisation Ponds in Ruai: Automated robot prepares to
remove harmful solids from influent

117
Wastewater Stabilisation Ponds in Ruai: Gotcha! Trapped
polythenes etc are being removed from the influent

118
Wastewater Stabilisation Ponds in Ruai:
Anaerobic pond (about 5 m deep!)

119
Wastewater Stabilisation Ponds in Ruai:
Facultative pond (1-2m deep)

120
Wastewater Stabilisation Ponds in Ruai: Excuse
me.., who is living in the sewage pond?

121
Wastewater Stabilisation Ponds in Ruai:
One of the maturation ponds

122
Wastewater Stabilisation Ponds in Ruai: Treated sewage
about to be discharged! (Notice the green colour)

123
Wastewater Stabilisation Ponds in Ruai: Water from
treatment being discharged to Nairobi River!

124