ABSTRACT
Solid wastes are collected and disposed off at a large number of
unprotected sites in Nigeria. About 85% of solid wastes disposed on land in
Nigeria are organic in nature. Biogas (Anaerobic Digestion –AD) technology can
be used to implement a sustainable waste management programme suitable for both
urban and rural areas as biodegradable wastes are transformed into biogas and
slurry. Biogas technology can serve as a waste disposal technology and help to
solve garbage and solid waste environmental problems. Biogas technology is a
renewable, alternative and sustainable form of energy. It is an attractive
method of solid waste and waste water treatment. Biogas technology can thus
play a vital role in Nigeria in solving some of the major environmental
problems such as solid waste, chemical fertilizer use, deforestation(use of
wood fuel). Biogas technology is extremely appropriate to ecological and
economic demands of the future as it can provide pollution free environment,
efficient energy for cooking, lighting, and improve health conditions of the
people. Highlighted in this paper are the fundamentals of the biogas(Anaerobic)
process, feed stocks for anaerobic digestion, environmental impacts of
anaerobic digestion, benefits of anaerobic digestion and biogas.
I
n t r o d u c t i o n
Around the world, pollution of the air
and water from municipal, industrial and agricultural operations continues to
grow. The concept of the ‘four R's’, which stands for Reduce, Reuse, Recycle,
and Renewable energy, has generally been accepted as a useful principle for
waste handling. The emission of CO2 and other greenhouse gases (GHG) has become
an important issue. Governments and industries are therefore increasingly on
the lookout for technologies that will allow for more efficient and
cost-effective waste treatment while minimizing Green House Gases (GHG).The
CO2-trade will even further increase the need for CO2-neutral technologies(IEA,Bioenergy,
2005). The increasing
population and the development demands in the Third World have caused an
increasing demand on traditional fuels. The fast rate of forest destruction and
low rate of reforestation has simultaneously reduced the availability of
firewood. To arrest the environmental and agricultural deterioration it is
imperative to introduce other sources of renewable energy, such as hydro-power,
wind and solar energy and biogas. Biogas is considered one of the cheapest
renewable energies in rural areas in developing countries. Production of biogas
would not only save firewood but also be beneficial for integrated farming
systems by converting manure to an improved fertilizer for crops or ponds
for fish and water plants. Other benefits of biodigestion include the reduction
of manure smell, elimination of smoke when cooking and the alleviation of
pathogens and thereby improving hygiene on farms (Bui Xuan, 2002).
The environmental aspects and the need for renewable
energy are receiving interest and considerable financial support in both
developed and developing countries, leading to an increase in research and
development in this area. Many systems using biodigestion have been constructed
in different countries. Despite the potential benefits, the expansion of biogas
technology has gone slowly, especially in countries where there has been no
financial support (subsidy) from governments or development agencies. The main
constraint has been the high cost of the biogas plant for people in rural areas
with limited financial resources (Bui Xuan, 2002).
Anaerobic digestion (AD) is the most promising
method of treating the organic fraction of municipal solid waste (MSW) and
other organic wastes. Anaerobic bacteria convert the biomass into a biogas or
landfill gas that can be used to generate energy (Mshandete and Parawira,
2009).
One technology that can successfully treat the
organic fraction of wastes is AD. When used in a fully-engineered system, AD
not only provides pollution prevention, but also allows for energy, compost and
nutrient recovery. Thus, AD can convert a disposal problem into a profit
centre. As the technology continues to mature, AD is becoming a key method for
both waste reduction and recovery of a renewable fuel and other valuable
co-products. Worldwide, there are now approximately 150 AD plants in operation
and a further 35 under construction using MSW or organic industrial waste as
their principal feedstock. The total annual installed capacity is more than
five million tonnes, which has the potential to generate 600 MW of electricity.
Waste managers have found that AD provides environmental benefits allowing
waste disposal facilities to meet increasingly stringent regulations.
Controlling odour and recovering nutrients are major drivers in their decision
making. The use of AD for sewage sludge stabilization is well established and
the use of AD as a (pre-) treatment step for industrial wastewater is
increasing rapidly, to the point where there are now more than 2,500
vendor-supplied systems in operation or under construction throughout the world
(IEA, Bioenergy, 2005).
Digestion
In southern
countries e.g. Italy, Portugal, Spain the technology is widely used only as a
wastewater
treatment technology. Climatic conditions and
intensive production systems result in the use of
large quantities of water. Thus manure contains a
very low dry matter content and is handled
and treated similarly to wastewater. In countries
such as Germany, Denmark, Switzerland the
composition of manure is different: it is
concentrated, with higher solids content and is
regarded as a fertilizer. This difference makes it
necessary to look at the biogas technology in
two ways:
1. Biogas or anaerobic technologies as an option for
slurry/wastewater treatment
2. Biogas technology as an option for improvement of
the value of the residues as organic fertilizers
There are inevitable overlaps between the two, as
the environmental aspects of importance in
option 2 is also relevant tor option 1. However,
they are generally of less importance when the
treatment type is selected.
Considering biogas technology as a wastewater
treatment system following major
environmental impacts are mentioned1:
· low sludge production: 50 to 75 % less excess sludge is
produced. Although sludge can be
used as soil conditioner and fertilizer, the handling
(post composting, spreading) causes
additional costs. In some cases sludge is also
incinerated or just dumped. The reduction of
excess sludge can be considered as an environmental
positive impact of anaerobic digestion
(AD).
· low land demand: Depending on the anaerobic system
applied land demand can be reduced
up to 5 times compared with aerobic systems.
Especially in those areas where land is rare
and/or expensive AD is given the priority and thus
preserves valuable land resources.
· positive energy balance: Anaerobic treatment processes
have only a small demand for
process energy. The gas generated during the
treatment process, depending on the type of
wastewater, allows a surplus of energy and saves
fossil fuels (see also option 2).
· low emissions: Capturing and utilization of
biogas contributes to the protection of our
climate (see also option 2). (GTZ, 1998).
Biogas
is generated when bacteria degrade biological material in the absence of
oxygen, in a process known as anaerobic digestion. Since biogas is a mixture of
methane (also known as marsh gas or natural gas, CH4) and carbon dioxide it is
a renewable fuel produced from waste treatment. Anaerobic digestion is
basically a simple process carried out in a number of steps that can use almost
any organic material as a substrate - it occurs in digestive systems, marshes,
rubbish dumps, septic tanks and the Arctic Tundra. Humans tend to make the
process as complicated as possible by trying to improve on nature in complex
machines but a simple approach is still possible (University of Adelaide,
2010). Biogas can provide a clean, easily controlled source of renewable energy
from organic waste materials for a small labour input, replacing firewood or
fossil fuels (which are becoming more expensive as supply falls behind demand).
During the conversion process pathogen levels are reduced and plant nutrients
made more readily available, so better crops can be grown while existing
resources are conserved (University of Adelaide, 2010). Biogas technology may
be a possible means of reducing or eliminating the menace and nuisance of solid
wastes in many cities of Nigeria (Akinbami et al., 2001).
Fundamentals of the Anaerobic
Process:
Biogas generation basically takes
place in four successive phases (Hydrolytic, Acidogenic, Acetogenic and
Methanogenic) by decomposition of organic substances to water and Biogas
(methane). Mixed cultures of facultative anaerobic and strictly anaerobic
microorganisms are involved in the decomposition process, with different
microorganisms involved in each step of degradation. Biogas quality is highly
dependent on the composition of the substrate. Biogas is a renewable biofuel
produced by anaerobic digestion or fermentation of biodegradable materials
(Stefan, 2009; Wikipedia, 2010).
The
typical composition of Biogas consists of
Methane(CH4): 50-75%; Carbon dioxide(CO2):25-50%;
Nitrogen(N2): 0-10%; Hydrogen(H2): 0-1%; Hydrogen
sulphideH2S): 0-3%; Oxygen(O2): 0-2% (Stefan, 2009;
Wikipedia, 2010).
Available
feed stocks for Anaerobic Digestion
In principle, most types of biomass can be used as
feedstock for biogas production. Manure from many types of domestic animals,
waste from kitchens, gardens, agriculture and slaughterhouses, and even human
excreta have all been used for anaerobic
digestion. Gas yields will depend on the feedstock used. One tonne of cow
manure produces around 36m3 biogas. Figures from a demonstration biogas plant
in Ludlow, Shropshire, suggest that one tonne of UK household kitchen waste
produces 140m3.
Available
Sewage
Sludge
Digestion of sewage sludge provides
significant benefits when recycling the sludge back to land. The digestion
process sanitizes and also reduces the odour potential from the sludge.
Typically between 30 and 70% of sewage sludge is treated by AD depending on
national legislation and priorities. In countries like Sweden and Switzerland
limitations for the field application of sludge have been introduced. However,
AD is still considered an important step since it produces renewable energy and
improves the ability of the sludge to settle which makes it easier to dry.
In less developed countries, direct AD
is the only treatment of waste water. If the digester is adequately designed and
the retention time of the water is long enough, the quality of the treated
water can be excellent.
Agricultural
Wastes
Digestion of animal manure is probably
the most widespread AD application worldwide. It produces a valuable fertilizer
as well as biogas. Today more and more organic industrial waste materials are
added to the manure which brings increased gas production and creates an
additional income from the gate fee. In countries like Denmark, Austria and
Germany the easily degradable wastes are becoming scarce and farmers are
looking for alternative substrates (energy Cattle manure is an excellent
substrate for biogas production. The yield is not the highest however; it
guarantees high methane content of up to 63% and makes the process more stable.
Industrial Wastes
Organic solid wastes from industry are
increasingly treated in biogas plants. Even if some of the substances might be
difficult to digest as a sole substrate, in mixture with manure or sewage
sludge they don’t pose any problem. The combined digestion of different wastes
is called co-digestion. Most of the waste products from the food industry have
excellent gas potential and therefore are in demand by plant operators. Until
recently the industry paid the operators reasonably high gate fees (up to 35
Euro per ton) to accept the waste products. Now, the operators are starting to
pay for the waste materials with the highest gas potential like fat and
vegetable oil. With current high feed-in tariffs they can easily recover the
cost of these wastes. AD of industrial waste waters is becoming a standard
technique. Whilst AD is only an initial stage in the treatment of high quality
water discharge, it can significantly reduce the cost and size of plant
compared to wholly aerobic treatments.
Municipal
Solid Wastes(MSW)
Organic wastes from households and
municipal authorities provide potential feedstock for anaerobic digestion. The
treatment of clean source separated fractions for recycling of both the energy
content and the organic matter is the only method in which the cycle can be
completely closed. In most of the participating countries, the source
separation of MSW is actively encouraged. This includes separation of the
putrescible organic fraction, also known as ‘green waste’ or ‘biowaste’.
Experience has shown that source separation provides the best quality feedstock
for AD. The digested material is a valuable fertilizer and soil improver,
especially after aerobic post-treatment. Where source separation has been
widely introduced, the results are encouraging. Alternatively, the unsegregated
wastes or the ‘grey waste’ after separation of the ‘biowaste’ can be treated to
gain the biogas from the waste as well as stabilizing it to prevent further
problems in landfill. The latter technologies are called mechanical biological
treatment (MBT).The EU has set the goal of reducing the amount of organic waste
to landfill by 65% by 2014. Some countries have completely banned the disposal
of untreated organic waste. Odour
free storage and efficient collection of source separated waste are the key
components for a successful introduction of an MSW system (Pulford, 1988;
Zamani, 2009).
gas Production
Farm-scale digestion plants treating
primarily animal wastes have seen widespread use throughout the world, with
plants in developing and technically advanced countries. In rural communities
small-scale units are typical; Nepal has some 50,000 digesters and China is
estimated to have 8 million small-scale digesters. These plants are generally
seed for providing gas for cooking and lighting for a single household. In more
developed countries, farm-scale AD plants are generally larger and the gas is
used to generate heat and electricity. These farm-scale digestion plants are
simple stirred tank designs that use long retention times to provide the
treatment required. In Germany more than 2,000 farm-scale biogas digesters are
in operation; Austria has approximately 120, and Switzerland 69.Two designs
are prevailing throughout Europe: the
so-called rubber top digester, and the concrete top digester usually built in
the ground. Both have a cylindrical form with a height to diameter ratio of 1:3
to 1:4.They are intermittently mixed tank reactors with hydraulic retention
times (HRT) of the waste in the digester of 15 to 50 days. The longer HRT
applies where an energy crop is used as a co-substrate or even the only source
of energy. There are digesters with a single and a double membrane cover. The
advantage of the rubber top digester is the price. A membrane is cheaper than a
concrete cover. At the same time, the membrane serves as gas storage whereas
concrete top digesters need additional gas storage. On the other hand, the
latter are easy to insulate and can take high snow The most applied digester
technology is the family sized Chinese dome digester with over 8 million
plants. This modern version of ADRA in Suck Chon (DPRC) is built in a glass
house to maintain elevated temperature in the cold winters of North Korea.
Environmental
Impacts of Anaerobic Digestion (AD):
Considering
the technology as a fertilizer treatment option environmental impact can be
divided into impacts at farm and public level :
Farmers Interests
·
Quality improvement of organic fertilizer/Reduction
of mineral fertilizer
·
Risk of increased NH4 loss
·
Reduction of phytotoxic substances
·
Reduction of the use of pesticides
·
Fertility/reduction of desertification
P ublic Interests
·
Reduction of pollutants
·
Reduction of odour
·
Positive impact on resource protection
·
Positive impact on climate protection
·
Compared to other fuels positive
emission behavior of biogas (Klinger, 2000).
Quality improvement of organic fertilizer
The most important advantage of organic fertilizer
is their participation in the natural nutrient
cycle, while inorganic fertilizers are additional to
it. A characteristic of organic fertilizers is the
wide nutrient spectrum, which is very similar to the
demand of the plants.
Although in many European regions surpluses of
manure cause groundwater and air pollution,
farmers are still using additional mineral fertilizer.
This is, mainly due to the comparative
ease of calculating the nutrient (especially Nitrogen)
content and the more convenient handling.
After anaerobic digestion about 25 to 40% of the
organic dry matter is converted to methane
(CH4)
and carbon dioxide (CO2). This reduction results in a
decreased carbon/nitrogen
proportion and improves the quality of manure.
In addition the fluidity is increased, which allows
for easier handling (less clogging) and an
increased infiltration after spreading2.
Odour causing compounds are degraded and weed seeds
and pests are reduced.
With increasing retention time the ammonia content
increases. Thus anaerobically treated manure acts more like mineral fertilizer
and can be utilized more on demand by plants (Knuz, 1996; Munasinghe, 1996a, 199 6b ; GTZ,
1998; Parawira , 2009 ).
This effect was also measured in an 8 years trial
with digested manure. The ammonia content
increased from 58 to 62 %. An average increase of
ammonia from 5 to 10 % was found.
This effect and a slightly increased nutrient
release measured in the soil resulted in an improved
fertilizer effect from digested manure compared to
untreated manure and generally allows
fertilization more appropriate to the plant needs.
Many farmers have observed improved yields after
using digested manure. Measurements
show a yield increase of 2 to 3 % compared to untreated
manure. The effect is more
significant if the manure is directly injected into
or covered with soil and when applied before
seeding.
Another observation from farmers after use of
digested manure is that the grass variety is
higher than with untreated manure.
Through co-fermentation (addition of agro-industrial
residues such as slaughterhouse wastes,
fat etc.) the nutrient content can easily be
increased. Through addition of co-substrates and
improved fertilization techniques an average of 20%
more ammonia (ca. 0.7 kg/m³ biomass)
and 30% more phosphate (ca. 0.3 kg/m³ biomass) is
achieved, which substitute the use of
mineral fertilizer (Taftrup, 1993).
To produce one kg of nitrogen, 2 kg mineral oil is
needed (Haber Bosch System). An average
of 30 MJ per m³ of biomass (manure and
co-substrates) can be saved through co fermentation.
The anaerobic treatment of animal manure with
co-substrates increases the quality of the
digested manure, which results in a better handling
and increased yield. In Addition the range
of application possibilities and the acceptance by
farmers is increased(Taftrup, 1993).
Increased risk of ammonia losses
The increased ammonia content of digested manure
combined with a slightly increased pH
causes a higher risk of ammonia losses in treated
manure compared to untreated manure.
Therefore digested manure must be handled more
carefully and farmers have to follow manure
handling instructions given by the national advisory
services. Anaerobically treated manure
should be injected into the soil or covered to minimize
ammonia emissions.
Reduction of phytotoxic
substances and odour
Phytotoxic substances in the manure can cause
necroses and scleroses when applied to growing
plants. This is the main the reason why overhead fertilizing
of a growing field is not done with
organic fertilizer, but mineral fertilizer.
Untreated manure with a dry matter content of 7 to
10% spread on plants, covers leaves and reduces
assimilation.
Through anaerobic digestion phytotoxic acids are degraded
and dry matter content is
decreased. Therefore digested manure can be applied
to a growing field (e.g. maize) which
usually has a high demand for nutrients. Farmers are
able to reduce their amount of mineral
fertilizer (Kunz, 1996).
Odour causing substances are degraded in the same
way. This is the major reason to install a
biogas plant for many farmers. . In particular,
farmers situated near housing areas face
problems with the local population due to odour
complains. Anaerobically treated manure
allows farmers to spread manure also close to
villages and thus increase the application
possibilities.
Anaerobically treated manure increases the range
application possibilities in terms of time,
crops, housing.
Reduction of weed seeds and pests
Organic fertilizer generally contains weed seeds and
microorganisms causing pests. The
anaerobic digestion process lowers the ability of
seeds to germinate. This loss of ability to
germinate can occur after 10 to 16 days. Differences
have been registered for different weeds.
In general the longer seeds are kept in an anaerobic
environment, the lower the ability to
germinate and greater the reduction in the
infectious potential of manure(Taftrup, 1993; Kunz, 1996).
This effect is a major reason for German organic
farmers to integrate a biogas plant in their
farming system, since they are not allowed to use
herbicides and pesticides in general.
To quantify the effect and whether biogas technology
is able to reduce the amount of pesticides
additional scientifically measurements are needed.
Improvement and stabilization of
soil fertility
Organic fertilizer (liquid and solid animal manure)
and mineral fertilizer differentiate not only
in nutrient content, composition and variance but
also in qualitative aspects. While organic
fertilizer contributes directly to the humus
household of the soil, mineral fertilizer does not.
Lack of humus ultimately results in desertification.
A productive soil system needs a balance
between incoming humus and degradation of humus. Manure,
compost or any organic fraction
will increase or stabilize the humus level in soil.
Humus is one of the most important compounds in the
soil since it is responsible for the
temperature of the soil;, the water capacity,
structure and pore volume (which is very important
against desertification) and absorption of
nutrients.
During the anaerobic process most of the low
molecular substances (less than 1000 g/Mol) are
degraded, while lignin substances still contribute
to the humus pool of the soil . Thus fermented manure still contributes with
its humus building substances to soil fertility.
Findings from a study in southern Germany on the use
of digested manure confirmed the above:
· 100
% of the farmers found digested manure easier to handle, as they are able to
use the
digested manure more on demand.
· 100%
experienced the odour reduction, which was a very important side effect for
farmers
near housing areas.
· 81
% stated a higher crop yield through the better and demand driven handling.
· 75%
agreed that digested manure is similar to mineral fertilizer and 50 % were able
to
reduce the amount of mineral fertilizer.
· 40%
of the farmers said that digested manure reduces weed seeds and pests (GTZ,
1998; Klinger, 2000).
Reduction of pollutants
Reduction of heavy metals is not feasible. Organic
compounds can be reduced through the
anaerobic treatment process. Tests have shown that
organic carbon compounds, mainly
resulting from the use of pesticides, can be degraded.
Organic compounds that can be degraded through anaerobic treatment includes
Trichlormethane
Tetrachlormethane
1, 2-Dichlorethane
3-Chlorbenzoesäure
3, 5-Dichlorbenzoesäure
Phenol
Cresol
Xylol,
Toluol
2-,3-,4-Chlorphenol
2,4-Dichlorphenol
3,4-Dichlorphenol
3,5-Dichlorphenol
Pentachlorophenol
low condensate PCB
low condensate PAC
There is no information available on whether benzol,
dioxin and furans (PCDD, PCDF) are
degraded through anaerobic processes. But
similarities to the structure of other carbohydrogens
compounds suggest there is a possibility that they
could be.
It has been shown that anaerobic treatment is able
to eliminate a number of organic
pollutants. Further research is needed to quantify
the effect.(Klinger, 2000).
Contribution to the water
resource protection
Within the agricultural sector anaerobic digestion
may contribute through a double effect to
save water resources
1. Through the increased ammonia content in the
digested manure an accelerated plant up take
occurs. Roots prefer ammonia than nitrate, if they
are available at the same time. When
ammonia is more and faster absorbed by the plants it
cannot be transformed into nitrate and
leached downwards in direction of the groundwater
level. Thus groundwater pollution through
nitrate is prevented.
Trials on cereals have demonstrated the higher uptake
of ammonia from digested manure which
resulted in an equivalent of mineral fertilizer
between 79 to 101 % (mineral fertilizer is 100%).
Raw manure achieved an equivalent of 35 to 42% only.
This increased up take of accessible
nitrogen during the growing period implies for the
following period (autumn and winter) a
reduced nitrogen content in the soil. Unwanted mineralization
of nitrogen at this time is reduced
and the risk of transfer to the groundwater is also
reduced (Taftrup, 1993; Klinger, 2000).
2. Decentralized biogas plants with a co-generation
unit reduce the technical water demand for
electricity supply. For example: 1988 in Germany
about 360 TWh electricity was produced
resulting in a water demand of 300 Bill m3 fresh water. For about 18% of the
final electricity
use, half of Germans water demand was spent!
Decentralized energy production saves water
resources.
Contribution to the climate
change protection
Methane is the second most important greenhouse gas
in the world, with a global warming
potential (GWP) of 25 times higher than CO2 (in a time horizon of 100 years).
Methane
emissions occur in any anaerobic processes with
organic materials. It has been estimated that
methane emissions from agriculture contribute about
33% to the global greenhouse effect.
About 7 % alone result from animal excrement which
is similar to 20-30 Million
tonnes of methane per year( Cassada and Safley,
1990; Kunz, 1996).
Through anaerobic treatment of animal excrement a
renewable source of energy is generated,
which has an important dual climatic effect: The use
of renewable energy reduces the CO2-
emissions through a reduction of the demand for
fossil fuel (1 m3
biogas substitutes
0,5 kg oil
for energy purposes reducing 2,6 kg CO2-emissions,
see also table 4). At the same time the
process can diminish uncontrolled methane generation
by capturing methane (Japan Environment Agency, EPA, 1990).
Half of the methane emissions could be reduced
through implementation of biogas technology.
In particular in developing countries smaller
agricultural biogas units reduce the use of forest
resources for household energy purposes and thus
slow down deforestation (about 1 ha of
forest per rural biogas plant), soil degradation and
resulting natural catastrophes like flooding
or desertification.
Nitrous oxide emissions that are much more harmful
to the climate change through their very
high GWP of 320 are also diminished through the
anaerobic process.
Nitrous oxide generation is a natural microbial
process. It is produced during nitrification and
denitrification processes in soils, stables and
animal waste management systems. In general
nitrous oxides emissions appear in soils without
anthropogenic influence. Fertilizing as well as
special conditions during storage can immensely
increase the emissions.
Very few detailed information is available about the
reduction potential of nitrous oxides
through anaerobic digestion of animal waste. There
is still a need for further research.
Nevertheless recent research results lead to the
opinion that anaerobic digestion of animal
waste significantly reduces nitrous oxide emissions by
1. Avoidance of emissions during
storage of animal waste.
2. Avoidance of anaerobic conditions
in soils.
3. Less N2O-losses through increased
nitrogen availability for plants and faster nitrogen absorption through crop plants.
4. Reduced application of inorganic
nitrogen fertilizer and N2O avoidance during production of
nitrogen fertilizer.
5. Avoidance of changes of land use
(saving of forest resources).
Considering all these effects an N2O-reduction potential through
anaerobic treatment of about
10 % could be assumed.
Anaerobic treatment of agricultural residues reduces
the global climate change effect (Taftrup, 1993; Klinger, 2000).
Biogas composition and emissions
after burning in comparison with other fuels
There are four major gas components in biogas:
methane, carbon dioxide, nitrogen and oxygen.
Besides those other gases such as ammonia, chloride,
etc. could be a minor part of the biogas
and disturb either the burning process or harm the
environment. Recent research on biogas
composition confirmed that biogas from animal
excrement and co-fermentation does not have
an increased content of disturbing gases. There was
a non-significant correlation visible
between co-fermentation and other gases in the
biogas.
For H2S
values under 100 mg/m³ have been found, when the normal desulphuration with air
is
installed.
For ammonia, chloride, fluoride, mercaptane
detectable limit of 0,1 mg/m³ was not exceeded.
The same stands for benzol, toluol, ethylbenzol,
xylol and cumol and PAC with its limit of 0,01
micrograms/m³.
These data are supported by the emission data after
burning from different fuels. Biogas shows
positive emission behaviour in comparison to other fossil
and renewable fuels. Measurements
on a cogeneration unit showed that parameters like
NOx and SO2
can fulfill emission
standards
like in Germany (GTZ, 1998). Problems have occurred with carbon monoxide.
In comparison to other fuels biogas has cleaner
emissions after burning.
Resource protection, an
appropriate technology
Fossil fuels are limited and contribute to the
greenhouse effect. Biogas is renewable and can
help to reduce the climate change as well as support
the protection and conservation of limited
resources.
Gases as fuels have one big advantage in comparison
to other fuels. There is no need for
refining and processing of the fuel; and the exhaust
usually does not need an expensive and
sophisticated cleaning facility before use. The
output after burning is nothing more than the
exhaust gases.
With co-generation the highest efficiency in the
energy supply is achieved. This way of
electricity and heat generation saves water and
energy resources in comparison to conventional
power stations.
Biogas can be stored and transformed to energy when
it is needed.
Biogas is in terms of its application
mode a competitive energy source and protects limited
fuel resources(Taftrup, 1993; Klinger,
2000).
Reduction of waste
disposal
Through the introduction and support by
the EC of separated waste collection the amount of
organic residuals has been increased
tremendously, while possibilities to re-use have not been
considered. Therefore disposal problems
are evident in many communities. Composting is
limited, mainly due to limited market
potential. Incineration does not always contribute to the
common understanding of sustainable
development.
In many of the agricultural biogas
plants in Europe additional organic residues from separated
household waste, or agro-industrial
wastes are co-treated with manure. This contributes
strongly to a closed nutrient cycle
system, where nutrients are not lost but re-used in the
agriculture (see figure 6). At the same
time energy is generated. However the re-use of waste in
the agriculture is limited due to many
factors, such as public acceptance, input of pollutants,
overload of nutrients, organization and
infrastructure.
The participation of the agricultural
sector is and should be a major and important step in a
sound waste management.
There are a number of
benefits resulting from the use of Anaerobic Digestion(AD) technology.
The Anaerobic
Digestion Process
In the absence of oxygen, anaerobic
bacteria will ferment biodegradable matter into methane and carbon dioxide, a
mixture called biogas. Approximately 90% of the energy from the degraded
biomass is retained in the form of methane. Hence, very little excess sludge is
produced. Biogas is formed solely through the activity of bacteria.
The AD process occurs naturally in the
bottom sediments of lakes and ponds, in swamps, peat bogs, intestines of
ruminants, and even in hot springs. Methane formation is also the process which
stabilizes landfill sites. The widespread natural occurrence of methane
bacteria demonstrates that anaerobic degradation can take place over a wide
temperature range from 10°C to over 100°C and at a variety of moisture contents
from around 50% to more than 99%. The potential to operate digesters at
temperatures above 50°C makes the AD process particularly interesting for
promoting hygiene. In addition to temperature, the anaerobic chemical
environment multiplies the sanitation effect (IEA, Bioenergy, 2005; Mshandete and Parawira, 2009).
Waste treatment benefits:
_ Natural waste treatment process
_ Requires less land than aerobic composting
_ Reduces
disposed waste volume and weight
Energy benefits:
_ Net energy producing process
_ Generates high quality renewable fuel
_ Biogas proven in numerous end-uses to be land filled applications
l Benefits Economic
Benefits
Environmental
benefits:
_ Significantly reduces greenhouse gas
_ Eliminates odours
_ Produces a sanitized compost and Nutrient-rich
liquid fertilizer
_
Maximizes
recycling benefits
Economic benefits:
Considering the whole life-cycle, it is more emissions cost-effective
than other treatment options
Advantages of biogas over wood as a
cooking fuel:-
- Less labour than tree felling
- Trees can be retained
- Biogas is a quick, easily controlled fuel
- No smoke or smell (unless there is a leak - then you
need to know
anyway!) so reduced eye/respiratory irritation - Clean pots
- Sludge is a better fertilizer than manure or synthetic fertilizers
(and
is cheaper then manufactured products) - Reduced pathogen transmission compared to untreated waste(University of Adelaide,2010)
Biogas can provide a clean, easily controlled source of renewable energy
from organic waste materials for a small labour input, replacing firewood or
fossil fuels (which are becoming more expensive as supply falls behind demand).
During the conversion process pathogen levels are reduced and plant nutrients
made more readily available, so better crops can be grown while existing
resources are conserved.
The
benefits of biogas:
_ Biogas systems
make clean energy for household use. After an initial investment in the system,
there is no need to
spend money on fuel and no more smoke from wood or charcoal
_ Cooking on biogas
is quicker and easier than cooking with firewood
_ Biogas systems
kill the bacteria in livestock manure. A farm with a biogas system is a cleaner
and safer place.
_ Biogas systems
produce excellent safe fertilizers for use on the farm
_ Biogas systems
can help in the fight against global warming by allowing us to burn methane
from organic waste,
instead of letting it escape into the atmosphere where it adds to the
greenhouse effect.
It also helps by letting us leave more trees standing!
Biogas and climate change
Carbon dioxide released by burning fossil fuels is
the primary cause of global warming. Burning methane, for example for cooking
or to run a generator, produces carbon dioxide and water. However, all the
carbon contained in biogas (in the form of carbon dioxide and methane) has
previously been absorbed from the atmosphere by the plants that produced the
feedstock. Hence burning biogas will only release as much carbon dioxide into
the air as the plants have taken out of it, and as long as the biomass is
allowed to regenerate, biogas is a carbon-neutral source of energy.
However, methane is a potent greenhouse gas, many
times more powerful than carbon dioxide in causing climate change. It is therefore
important that all gas is burned, with no leakage during production and distribution(Centre
for Alternative Technology, 2007) .
Uses of the slurry
Only a small proportion of the total mass of the
feedstock is converted into biogas. The remainder is converted into
nutrient-rich slurry which can be used as a biofertilizer. The anaerobic
digestion process eradicates most pathogens. Studies show that counts of fecal
coliform bacteria can be reduced by 99.9%. Biogas effluent does not attract flies.
These factors can contribute positively to the health of the people (Ellegard, et al., 1983; Sinha and Kazaglis, 2006; Centre
for Alternative Technology, 2007).
Conclusions
Biogas technology contributes in a wide
range of aspects and effects to the environment.
The major effect for farmers is the
improved quality of anaerobically treated manure. This
results in a better handling (less
clogging), increased ammonia content, increased possibility to
apply on demand of the plants, less odour
and potential decreased use of mineral fertilizer and
pesticides. Considering all positive
impacts an increased crop yield is observed. The overall
and most important effect is the higher
acceptance of organic fertilizers through AD by farmers
and the public.
A potential reduction of the use of
mineral fertilizer and pesticides is feasible but needs to be
quantified scientifically.
The risk of an increased ammonia
emission should be minimized by proper application
(injection, covering).
Anaerobic treatment and gas use
contributes to the reduction of greenhouse gases.
Biogas is a competitive energy source
and protects limited fuel resources.
The participation of the agricultural sector is and
should be a major and important step in a
sound waste management due to its resource saving
potential.
It becomes evident that with the implementation and
promotion of biogas technology in the
agricultural sector interactive implications can be
achieved especially for the protection of the
environment. This is confirmed by the growing number
of agricultural biogas plants in the
European community. In respect of the global harm,
“Climate change” and it's range of positive
impacts for farmers and the public the technology
should gain more importance and should be
included in promotion programmes.
Biogas technology blends with our culture and
society. However the success of promoting this technology depends on careful
planning, management, implementation, training and monitoring. Biogas
technology can help in maintaining the environment and improvement of health
conditions.
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