Anaerobic digestion is a collection of processes by which microorganisms
break down biodegradable material in the absence of oxygen.
The process is used for industrial or domestic purposes to manage waste and/or
to produce fuels. Much of the fermentation used industrially to produce food and drink products, as
well as home fermentation, uses anaerobic digestion.
Anaerobic digestion occurs naturally
in some soils and in lake and oceanic basin sediments, where it is usually
referred to as "anaerobic activity".
This is the source of marsh gas methane
as discovered by Volta in 1776.
The digestion process begins with bacterial
hydrolysis
of the input materials. Insoluble organic polymers,
such as carbohydrates, are broken down to soluble derivatives that become
available for other bacteria. Acidogenic bacteria
then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids.
These bacteria convert these resulting organic acids into acetic acid,
along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens
convert these products to methane and carbon dioxide.
The methanogenic archaea populations play an indispensable role in anaerobic
wastewater treatments.
It is used as part of the process to
treat biodegradable waste and sewage sludge.
As part of an integrated waste management
system, anaerobic digestion reduces the emission of landfill gas
into the atmosphere. Anaerobic digesters can also be fed with purpose-grown
energy crops, such as maize.
Anaerobic digestion is widely used
as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide
and traces of other ‘contaminant’ gases.
This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate
also produced can be used as fertilizer.
With the re-use of waste as a
resource and new technological approaches which have lowered capital costs,
anaerobic digestion has in recent years received increased attention among
governments in a number of countries, among these the United Kingdom (2011),Germany
and Denmark (2011).
History
Scientific
interest in the manufacturing of gas produced by the natural decomposition
of organic matter was first reported in the 17th century by Robert Boyle and Stephen Hale,
who noted that flammable gas was released by disturbing the sediment of streams and
lakes. In 1808, Sir Humphry Davy determined that
methane was present in the gases produced by cattle manure. The first anaerobic
digester was built by a leper colony in Bombay, India, in 1859. In 1895, the technology was
developed in Exeter, England, where a septic tank
was used to generate gas for the sewer gas destructor lamp, a type of gas lighting. Also in England, in
1904, the first dual-purpose tank for both sedimentation and sludge treatment
was installed in Hampton. In 1907, in
Germany, a patent was issued for the Imhoff tank, an early form of
digester.
Research
on anaerobic digestion began in earnest in the 1930s.
Process
Many
microorganisms affect anaerobic digestion, including acetic acid-forming
bacteria (acetogens) and methane-forming
archaea (methanogens). These organisms
promote a number of chemical processes in converting the biomass to biogas.
Gaseous oxygen is excluded from the reactions by physical containment. Anaerobes utilize electron acceptors from sources other than oxygen gas. These acceptors 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. In the presence of specialised methanogens, the intermediates are converted to the 'final' end products of methane, carbon dioxide, and trace levels of hydrogen sulfide. In an anaerobic system, the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.
Populations
of anaerobic microorganisms typically take a significant period of time to
establish themselves to be fully effective. Therefore, common practice is to
introduce anaerobic microorganisms from materials with existing populations, a
process known as "seeding" the digesters, typically accomplished with
the addition of sewage sludge or cattle slurry.
Process stages
There are four key biological and
chemical stages of anaerobic digestion:
In
most cases, biomass is made up of large organic polymers. For the bacteria in
anaerobic digesters to access the energy potential of the material, these
chains must first be broken down into their smaller constituent parts. These
constituent parts, or monomers, such as sugars, are readily available to other
bacteria. The process of breaking these chains and dissolving the smaller
molecules into solution is called hydrolysis. Therefore, hydrolysis of these
high-molecular-weight polymeric components is the necessary first step in
anaerobic digestion.Through hydrolysis the complex organic
molecules are broken down into simple sugars, amino acids, and fatty acids.
Acetate
and hydrogen produced in the first stages can be used directly by methanogens.
Other molecules, such as volatile fatty acids (VFAs) with a chain length
greater than that of acetate must first be catabolised into compounds that
can be directly used by methanogens.
The
biological process of acidogenesis results in further
breakdown of the remaining components by acidogenic (fermentative) bacteria.
Here, VFAs are created, along with ammonia, carbon dioxide, and hydrogen sulfide, as well as other
byproducts.The process of
acidogenesis is similar to the way milk sours.
The
third stage of anaerobic digestion is acetogenesis. Here, simple
molecules created through the acidogenesis phase are further digested by
acetogens to produce largely acetic acid, as well as carbon dioxide and
hydrogen.
The
terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, methanogens
use the intermediate products of the preceding stages and convert them into
methane, carbon dioxide, and water. These components make up the majority of
the biogas emitted from the system. Methanogenesis is sensitive to both high
and low pHs and occurs between pH 6.5 and pH 8. The remaining,
indigestible material the microbes cannot use and any dead bacterial remains
constitute the digestate.
A
simplified generic chemical equation for the overall processes outlined above
is as follows:
C6H12O6
→ 3CO2 + 3CH4
Configuration
Anaerobic
digesters can be designed and engineered to operate using a number of different
process configurations:
- Batch or continuous
- Temperature: Mesophilic or thermophilic
- Solids content: High solids or low solids
- Complexity: Single stage or multistage
Batch or continuous
Anaerobic
digestion can be performed as a batch process or a continuous process.
In
a batch system biomass is added to the reactor at the start of the process. The
reactor is then sealed for the duration of the process.
In
its simplest form batch processing needs inoculation with already
processed material to start the anaerobic digestion. In a typical scenario,
biogas production will be formed with a normal
distribution
pattern over time. Operators can use this fact to determine when they believe
the process of digestion of the organic matter has completed. There can be
severe odour issues if a batch reactor is opened and emptied before the process
is well completed.
A
more advanced type of batch approach has limited the odour issues by
integrating anaerobic digestion with in-vessel
composting.
In this approach inoculation takes place through the use of recirculated
degasified percolate. After anaerobic digestion has completed, the biomass is
kept in the reactor which is then used for in-vessel
composting
before it is opened
As
the batch digestion is simple and requires less equipment and lower levels of
design work, it is typically a cheaper form of digestion.Using more than one
batch reactor at a plant can ensure constant production of biogas.
In
continuous digestion processes, organic matter is constantly added (continuous
complete mixed) or added in stages to the reactor (continuous plug flow; first
in – first out). Here, the end products are constantly or periodically removed,
resulting in constant production of biogas. A single or multiple digesters in
sequence may be used. Examples of this form of anaerobic digestion include continuous stirred-tank reactors, upflow anaerobic sludge blankets, expanded granular sludge beds and internal circulation reactors.
Temperature
The
two conventional operational temperature levels for anaerobic digesters
determine the species of methanogens in the digesters:
- Mesophilic digestion takes place optimally around 30 to 38 °C, or at ambient temperatures between 20 and 45 °C, where mesophiles are the primary microorganism present.
- Thermophilic digestion takes place optimally around 49 to 57 °C, or at elevated temperatures up to 70 °C, where thermophiles are the primary microorganisms present.
A
limit case has been reached in Bolivia, with anaerobic
digestion in temperature working conditions of less than 10 °C. The
anaerobic process is very slow, taking more than three times the normal
mesophilic time process. In experimental work
at University of Alaska Fairbanks, a 1,000 litre
digester using psychrophiles harvested from
"mud from a frozen lake in Alaska" has produced 200–300 litres
of methane per day, about 20 to 30% of the output from digesters in warmer
climates.
Mesophilic
species outnumber thermophiles, and they are also more tolerant to changes in
environmental conditions than thermophiles. Mesophilic systems are, therefore,
considered to be more stable than thermophilic digestion systems.
Though
thermophilic digestion systems are considered to be less stable and the energy
input is higher, more biogas is removed from the organic matter in an equal
amount of time. The increased temperatures facilitate faster reaction rates
and, hence, faster gas yields. Operation at higher temperatures facilitates
greater pathogen reduction of the digestate. In countries where legislation,
such as the Animal By-Products Regulations in the European
Union, requires digestate to meet certain levels of pathogen reduction there
may be a benefit to using thermophilic temperatures instead of mesophlic.
Additional
pre-treatment can be used to reduce the necessary retention time to produce
biogas. For example, certain processes shred the substrates to increase the
surface area or use a thermal pretreatment stage (such as pasteurisation) to
significantly enhance the biogas output. The pasteurisation process can also be
used to reduce the pathogenic concentration in the digesate leaving the
anaerobic digester. Pasteurisation may be achieved by using a Landia BioChop
hygienisation unit or similar method of
combined heat treatment and solids maceration.
Solids content
In
a typical scenario, three different operational parameters are associated with
the solids content of the feedstock to the digesters:
- High solids (dry—stackable substrate)
- High solids (wet—pumpable substrate)
- Low solids (wet—pumpable substrate)
High
solids (dry) digesters are designed to process materials with a solids content
between 25 and 40%. Unlike wet digesters that process pumpable slurries, high
solids (dry – stackable substrate) digesters are designed to process solid
substrates without the addition of water. The primary styles of dry digesters
are continuous vertical plug flow and batch tunnel horizontal digesters.
Continuous vertical plug flow digesters are upright, cylindrical tanks where
feedstock is continuously fed into the top of the digester, and flows downward
by gravity during digestion. In batch tunnel digesters, the feedstock is
deposited in tunnel-like chambers with a gas-tight door. Neither approach has
mixing inside the digester. The amount of pretreatment, such as contaminant
removal, depends both upon the nature of the waste streams being processed and
the desired quality of the digestate. Size reduction (gringing) is beneficial
in continuous vertical systems, as it accelerates digestion, while batch
systems avoid grinding and instead require structure (e.g. yard waste) to
reduce compaction of the stacked pile. Continuous vertical dry digesters have a
smaller footprint due to the shorter effective retention time and vertical
design.
Wet
digesters can be designed to operate in either a high-solids content, with a total
suspended solids
(TSS) concentration greater than ~20%, or a low-solids concentration less than
~15%.
High
solids (wet) digesters process a thick slurry that requires more energy input
to move and process the feedstock. The thickness of the material may also lead
to associated problems with abrasion. High solids digesters will typically have
a lower land requirement due to the lower volumes associated with the moisture. High solids
digesters also require correction of conventional performance calculations
(e.g. gas production, retention time, kinetics, etc.) originally based on very
dilute sewage digestion concepts, since larger fractions of the feedstock mass
are potentially convertible to biogas.
Low
solids (wet) digesters can transport material through the system using standard
pumps that require significantly lower energy input. Low solids digesters
require a larger amount of land than high solids due to the increased volumes
associated with the increased liquid-to-feedstock ratio of the digesters. There
are benefits associated with operation in a liquid environment, as it enables
more thorough circulation of materials and contact between the bacteria and
their food. This enables the bacteria to more readily access the substances on
which they are feeding, and increases the rate of gas production.
Complexity
Digestion
systems can be configured with different levels of complexity:
In
a single-stage digestion system (one-stage), all of the biological
reactions occur within a single, sealed reactor or holding tank. Using a single
stage reduces construction costs, but results in less control of the reactions
occurring within the system. Acidogenic bacteria, through the production of
acids, reduce the pH of the tank. Methanogenic bacteria, as outlined earlier,
operate in a strictly defined pH range. Therefore, the
biological reactions of the different species in a single-stage reactor can be
in direct competition with each other. Another one-stage reaction system is an anaerobic lagoon. These lagoons are
pond-like, earthen basins used for the treatment and long-term storage of
manures. Here the anaerobic
reactions are contained within the natural anaerobic sludge contained in the
pool.
In
a two-stage digestion system (multistage), different digestion vessels
are optimised to bring maximum control over the bacterial communities living within
the digesters. Acidogenic bacteria produce organic acids and more quickly grow
and reproduce than methanogenic bacteria. Methanogenic bacteria require stable
pH and temperature to optimise their performance.
Under
typical circumstances, hydrolysis, acetogenesis, and acidogenesis occur within
the first reaction vessel. The organic material is then heated to the required
operational temperature (either mesophilic or thermophilic) prior to being
pumped into a methanogenic reactor. The initial hydrolysis or acidogenesis
tanks prior to the methanogenic reactor can provide a buffer to the rate at
which feedstock is added. Some European countries require a degree of elevated
heat treatment to kill harmful bacteria in the input waste.In this instance,
there may be a pasteurisation or sterilisation stage prior to digestion or
between the two digestion tanks. Notably, it is not possible to completely
isolate the different reaction phases, and often some biogas is produced in the
hydrolysis or acidogenesis tanks.
Residence time
The
residence time in a digester varies with the amount and type of feed material,
the configuration of the digestion system, and whether it be one-stage or
two-stage.
In
the case of single-stage thermophilic digestion, residence times may be in the
region of 14 days, which, compared to mesophilic digestion, is relatively
fast. The plug-flow nature of some of these systems will mean the full
degradation of the material may not have been realised in this timescale. In
this event, digestate exiting the system will be darker in colour and will
typically have more odour.
In
two-stage mesophilic digestion, residence time may vary between 15 and
40 days.
In
the case of mesophilic UASB digestion, hydraulic residence times can be 1 hour
to 1 day, and solid retention times can be up to 90 days. In this manner, the
UASB system is able to separate solids and hydraulic retention times with the
use of a sludge blanket.
Continuous
digesters have mechanical or hydraulic devices, depending on the level of
solids in the material, to mix the contents, enabling the bacteria and the food
to be in contact. They also allow excess material to be continuously extracted
to maintain a reasonably constant volume within the digestion tanks.
Feedstocks
The
most important initial issue when considering the application of anaerobic
digestion systems is the feedstock to the process. Almost any organic material
can be processed with anaerobic digestion;however, if biogas
production is the aim, the level of putrescibility is the key factor in its
successful application. The more putrescible
(digestible) the material, the higher the gas yields possible from the system.
Feedstocks
can include biodegradable waste materials, such as waste paper, grass
clippings, leftover food, sewage, and animal waste. Woody wastes are the exception, because they
are largely unaffected by digestion, as most anaerobes are unable to degrade lignin. Xylophalgeous
anaerobes (lignin consumers) or using high temperature pretreatment, such as
pyrolysis, can be used to break down the lignin. Anaerobic digesters can also
be fed with specially grown energy crops, such as silage, for dedicated
biogas production. In Germany and continental Europe, these facilities are
referred to as "biogas" plants. A codigestion or cofermentation plant
is typically an agricultural anaerobic digester that accepts two or more input
materials for simultaneous digestion.
Anaerobes
can break down material with varying degrees of success from readily, in the
case of short-chain hydrocarbons such as sugars, to over longer periods of
time, in the case of cellulose and hemicellulose. Anaerobic
microorganisms are unable to break down long-chain woody molecules, such as
lignin.
Anaerobic
digesters were originally designed for operation using sewage sludge and
manures. Sewage and manure are not, however, the material with the most
potential for anaerobic digestion, as the biodegradable material has already
had much of the energy content taken out by the animals that produced it.
Therefore, many digesters operate with codigestion of two or more types of
feedstock. For example, in a farm-based digester that uses dairy manure as the
primary feedstock, the gas production may be significantly increased by adding
a second feedstock, e.g., grass and corn (typical on-farm feedstock), or
various organic byproducts, such as slaughterhouse waste, fats, oils and grease
from restaurants, organic household waste, etc. (typical off-site feedstock).
Digesters
processing dedicated energy crops can achieve high levels of degradation and
biogas production. Slurry-only systems
are generally cheaper, but generate far less energy than those using crops,
such as maize and grass silage; by using a modest amount of crop material
(30%), an anaerobic digestion plant can increase energy output tenfold for only
three times the capital cost, relative to a slurry-only system.
Moisture content
A
second consideration related to the feedstock is moisture content. Dryer,
stackable substrates, such as food and yard waste, are suitable for digestion
in tunnel-like chambers. Tunnel-style systems typically have near-zero
wastewater discharge, as well, so this style of system has advantages where the
discharge of digester liquids are a liability. The wetter the material, the
more suitable it will be to handling with standard pumps instead of
energy-intensive concrete pumps and physical means of movement. Also, the
wetter the material, the more volume and area it takes up relative to the
levels of gas produced. The moisture content of the target feedstock will also
affect what type of system is applied to its treatment. To use a high-solids
anaerobic digester for dilute feedstocks, bulking agents, such as compost,
should be applied to increase the solids content of the input material. Another key
consideration is the carbon:nitrogen ratio of the input material. This ratio is
the balance of food a microbe requires to grow; the optimal C:N ratio is
20–30:1. Excess N can lead to
ammonia inhibition of digestion.
Contamination
The
level of contamination of the feedstock material is a key consideration. If the
feedstock to the digesters has significant levels of physical contaminants,
such as plastic, glass, or metals, then processing to remove the contaminants
will be required for the material to be used. If it is not
removed, then the digesters can be blocked and will not function efficiently.
It is with this understanding that mechanical biological treatment plants are
designed. The higher the level of pretreatment a feedstock requires, the more
processing machinery will be required, and, hence, the project will have higher
capital costs.
After
sorting or screening to remove any physical contaminants from the feedstock,
the material is often shredded, minced, and mechanically or hydraulically
pulped to increase the surface area available to microbes in the digesters and,
hence, increase the speed of digestion. The maceration of solids can be
achieved by using a chopper
pump
to transfer the feedstock material into the airtight digester, where anaerobic
treatment takes place.
Substrate composition
Substrate
composition is a major factor in determining the methane yield and methane
production rates from the digestion of biomass. Techniques to determine the
compositional characteristics of the feedstock are available, while parameters
such as solids, elemental, and organic analyses are important for digester
design and operation.
Applications
Using
anaerobic digestion technologies can help to reduce the emission of greenhouse
gases in a number of key ways:
- Replacement of fossil fuels
- Reducing or eliminating the energy footprint of waste treatment plants
- Reducing methane emission from landfills
- Displacing industrially produced chemical fertilizers
- Reducing vehicle movements
- Reducing electrical grid transportation losses
- Reducing usage of LP Gas for cooking
Waste treatment
Anaerobic
digestion is particularly suited to organic material, and is commonly used for
effluent and sewage treatment. Anaerobic digestion,
a simple process, can greatly reduce the amount of organic matter which might
otherwise be destined to be dumped at sea, dumped in landfills, or burnt in incinerators.
Pressure
from environmentally related legislation on solid waste disposal methods in developed countries has increased the
application of anaerobic digestion as a process for reducing waste volumes and
generating useful byproducts. It may either be used to process the
source-separated fraction of municipal waste or alternatively combined with
mechanical sorting systems, to process residual mixed municipal waste. These
facilities are called mechanical biological treatment plants.
If
the putrescible waste processed in anaerobic digesters were disposed of in a
landfill, it would break down naturally and often anaerobically. In this case,
the gas will eventually escape into the atmosphere. As methane is about 20
times more potent as a greenhouse gas than carbon dioxide,
this has significant negative environmental effects.
In
countries that collect household waste, the use of local anaerobic digestion facilities
can help to reduce the amount of waste that requires transportation to
centralized landfill sites or incineration facilities. This reduced burden on
transportation reduces carbon emissions from the collection vehicles. If
localized anaerobic digestion facilities are embedded within an electrical
distribution network, they can help reduce the electrical losses associated
with transporting electricity over a national grid.
Power generation
In
developing countries, simple home and farm-based anaerobic digestion systems
offer the potential for low-cost energy for cooking and lighting. From 1975, China and India have both
had large, government-backed schemes for adaptation of small biogas plants for
use in the household for cooking and lighting.At present, projects
for anaerobic digestion in the developing world can gain financial support if
they are able to show they provide reduced carbon emissions.
Methane
and power produced in anaerobic digestion facilities can be used to replace
energy derived from fossil fuels, and hence reduce emissions of greenhouse
gases, because the carbon in biodegradable material is part of a carbon cycle. The carbon released
into the atmosphere from the combustion of biogas has been removed by plants
for them to grow in the recent past, usually within the last decade, but more
typically within the last growing season. If the plants are regrown, taking the
carbon out of the atmosphere once more, the system will be carbon neutral. In contrast, carbon
in fossil fuels has been sequestered in the earth for many millions of years,
the combustion of which increases the overall levels of carbon dioxide in the
atmosphere.
Biogas
from sewage works is sometimes used to run a gas engine to produce
electrical power, some or all of which can be used to run the sewage works. Some waste heat from
the engine is then used to heat the digester. The waste heat is, in general,
enough to heat the digester to the required temperatures. The power potential
from sewage works is limited – in the UK, there are about 80 MW total of
such generation, with the potential to increase to 150 MW, which is
insignificant compared to the average power demand in the UK of about
35,000 MW. The scope for biogas generation from nonsewage waste biological
matter – energy crops, food waste, abattoir waste, etc. - is much higher,
estimated to be capable of about 3,000 MW. Farm biogas plants
using animal waste and energy crops are expected to contribute to reducing CO2
emissions and strengthen the grid, while providing UK farmers with additional
revenues.
Some
countries offer incentives in the form of, for example, feed-in tariffs for feeding
electricity onto the power grid to subsidize green energy production.
In
Oakland, California at the East Bay Municipal Utility District’s main
wastewater treatment plant (EBMUD), food waste is currently
codigested with primary and secondary municipal wastewater solids and other
high-strength wastes. Compared to municipal wastewater solids digestion alone,
food waste codigestion has many benefits. Anaerobic digestion of food waste
pulp from the EBMUD food waste process provides a higher normalized energy
benefit, compared to municipal wastewater solids: 730 to 1,300 kWh per dry ton
of food waste applied compared to 560 to 940 kWh per dry ton of municipal
wastewater solids applied.
Grid injection
Biogas
grid-injection is the injection of biogas into the natural gas grid. As an alternative,
the electricity and the heat can be used for on-site
generation, resulting in a
reduction of losses in the transportation of energy. Typical energy losses in
natural gas transmission systems range from 1–2%, whereas the current energy
losses on a large electrical system range from 5–8%.
In
October 2010, Didcot Sewage Works became the first in the UK to produce biomethane gas supplied to the
national grid, for use in up to 200 homes in Oxfordshire.
Fertiliser and soil conditioner
The
solid, fibrous component of the digested material can be used as a soil
conditioner to increase the organic content of soils. Digester liquor can be
used as a fertiliser to supply vital nutrients to soils instead of chemical
fertilisers that require large amounts of energy to produce and transport. The
use of manufactured fertilisers is, therefore, more carbon-intensive than the
use of anaerobic digester liquor fertiliser. In countries such as Spain, where many soils are organically
depleted, the markets for the digested solids can be equally as important as
the biogas.
Cooking gas
By
using a bio-digester, which produces the
bacteria required for decomposing, cooking gas is generated. The organic
garbage like fallen leaves, kitchen waste, food waste etc. are fed into a
crusher unit, where the mixture is conflated with a small amount of water. The
mixture is then fed into the bio-digester, where the bacteria decomposes it to
produce cooking gas. This gas is piped to kitchen stove. A 2 cubic meter
bio-digester can produce 2 cubic meter of cooking gas. This is equivalent to
1 kg of LPG. The notable advantage of using a bio-digester is the sludge
which is a rich organic manure.
Products
The
three principal products of anaerobic digestion are biogas, digestate, and
water.
Biogas
Typical composition of biogas
|
|
Matter
|
%
|
Methane, CH4
|
50–75
|
Carbon dioxide, CO2
|
25–50
|
Nitrogen, N2
|
0–10
|
Hydrogen, H2
|
0–1
|
Hydrogen sulfide, H2S
|
0–3
|
Oxygen, O2
|
0–2
|
Biogas
is the ultimate waste product of the bacteria feeding off the input
biodegradable feedstock (the methanogenesis stage of anaerobic
digestion is performed by archaea - a micro-organism
on a distinctly different branch of the phylogenetic tree of life to
bacteria), and is mostly methane and carbon dioxide, with a small amount
hydrogen and trace hydrogen sulfide. (As-produced, biogas also contains water
vapor, with the fractional water vapor volume a function of biogas
temperature). Most of the biogas
is produced during the middle of the digestion, after the bacterial population
has grown, and tapers off as the putrescible material is exhausted. The gas is normally
stored on top of the digester in an inflatable gas bubble or extracted and
stored next to the facility in a gas holder.
The
methane in biogas can be burned to produce both heat and electricity, usually
with a reciprocating
engine
or microturbine often in a cogeneration arrangement where
the electricity and waste heat generated are used to warm the digesters or to
heat buildings. Excess electricity can be sold to suppliers or put into the
local grid. Electricity produced by anaerobic digesters is considered to be
renewable energy and may attract subsidies. Biogas does not
contribute to increasing atmospheric carbon dioxide concentrations because the
gas is not released directly into the atmosphere and the carbon dioxide comes
from an organic source with a short carbon cycle.
Biogas
may require treatment or 'scrubbing' to refine it for use as a fuel. Hydrogen sulfide, a toxic product
formed from sulfates in the feedstock, is released as a trace component of the
biogas. National environmental enforcement agencies, put strict limits on the
levels of gases containing hydrogen sulfide, and, if the levels of hydrogen
sulfide in the gas are high, gas scrubbing and cleaning equipment (such as amine
gas treating)
will be needed to process the biogas to within regionally accepted levels. Alternatively, the
addition of ferrous
chloride
FeCl2 to the digestion tanks inhibits hydrogen sulfide production.
Volatile siloxanes can also contaminate
the biogas; such compounds are frequently found in household waste and
wastewater. In digestion facilities accepting these materials as a component of
the feedstock, low-molecular-weight siloxanes volatilise into biogas. When this
gas is combusted in a gas engine, turbine, or boiler, siloxanes are converted
into silicon dioxide (SiO2), which deposits internally in the
machine, increasing wear and tear. Practical and
cost-effective technologies to remove siloxanes and other biogas contaminants
are available at the present time. In certain
applications, in situ treatment can be used to increase the methane
purity by reducing the offgas carbon dioxide content, purging the majority of
it in a secondary reactor.
In
countries such as Switzerland, Germany, and Sweden, the methane in the biogas
may be compressed for it to be used as a vehicle transportation fuel or input
directly into the gas mains. In countries where
the driver for the use of anaerobic digestion are renewable electricity
subsidies, this route of treatment is less likely, as energy is required in
this processing stage and reduces the overall levels available to sell.
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. In two-stage
systems, different forms of digestate come from different digestion tanks. In
single-stage digestion systems, the two fractions will be combined and, if
desired, separated by further processing.
The
second byproduct (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.
The material resembles domestic compost and can be used as such or to make
low-grade building products, such as fibreboard. The solid digestate
can also be used as feedstock for ethanol production.
The
third byproduct is a liquid (methanogenic digestate) rich in nutrients, which
can be used as a fertiliser, depending on the quality of the material being
digested. Levels of
potentially toxic elements (PTEs) should be chemically assessed. This will
depend upon the quality of the original feedstock. In the case of most clean
and source-separated biodegradable waste streams, the levels of PTEs will be
low. In the case of wastes originating from industry, the levels of PTEs may be
higher and will need to be taken into consideration when determining a suitable
end use for the material.
Digestate
typically contains elements, such as lignin, that cannot be broken down by the
anaerobic microorganisms. Also, the digestate may contain ammonia that is phytotoxic,
and may hamper the growth of plants if it is used as a soil-improving material.
For these two reasons, a maturation or composting stage may be employed after
digestion. Lignin and other materials are available for degradation by aerobic
microorganisms, such as fungi, helping reduce the overall volume of the
material for transport. During this maturation, the ammonia will be oxidized
into nitrates, improving the fertility of the material and making it more
suitable as a soil improver. Large composting stages are typically used by dry
anaerobic digestion technologies.
Wastewater
The
final output from anaerobic digestion systems is water, which originates both
from the moisture content of the original waste that was treated and 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.
The
wastewater exiting the anaerobic digestion facility will typically have
elevated levels of biochemical oxygen demand (BOD) and chemical
oxygen demand
(COD). These measures of the reactivity of the effluent indicate an ability to
pollute. Some of this material is termed 'hard COD', meaning it cannot be
accessed by the anaerobic bacteria for conversion into biogas. If this effluent
were put directly into watercourses, it would negatively affect them by causing
eutrophication. As such, further
treatment of the wastewater is often required. This treatment will typically be
an oxidation stage wherein air is passed through the water in a sequencing
batch reactors or reverse
osmosis
unit.
SUBSCRIBERS - ( LINKS) :FOLLOW
/ REF / 2 /
findleverage.blogspot.com
Krkz77@yahoo.com
+234-81-83195664
No comments:
Post a Comment