Many developed and developing countries practice composting  and anaerobic digestion of mixed waste or biodegradable waste fractions (kitchen or restaurant wastes, garden waste, sewage sludge). Both processes are best applied to source-separated waste fractions: anaerobic digestion is particularly appropriate for wet wastes, while composting is often appropriate for drier feedstocks. Aerobic digestion is favoured when the waste-water has a low concentration of biodegradable content, low temperature, high effluent quality needs, and nutrient removal requirements. Waste-water with high biodegradable content and high temperature, such as industrial waste-water, is more suitable for anaerobic processes.
The process of anaerobic digestion is decomposition of biodegradable  material by micro-organisms in the absence of oxygen. Anaerobic digestion is often used for industrial or domestic purposes to manage waste streams. Three principal products are produced through the proccess of anaerobic digestion. First, the process produces a biogas, consisting mainly of CH4 and CO2, which can be used for energy production in a Combined Heat and Power  plant. Second, the process results in a nutrient-rich digestate which is similar to compost. Finally, the process results in a liquid liquor that can be used as a fertilizer. Video 1 is a brief introduction clip of how anaerobic digestion works.
As illustrated in Figure 1, a biogas facility with an anaerobic digester has four main components:
1) a manure (or waste-water) collection system. Existing liquid/slurry manure management systems can readily be adapted to deliver manure to the anaerobic digester.
2) the anaerobic digester itself. The production of the biogas consisting of methane and CO2 occurs here.
3) A biogas handling system.
4) A device that puts the biogas to use such as a combined heat and power plant.
Types and variations of anaerobic digesters
There are two basic types of digesters: batch and continuous. Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated. In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. They may be better suited for large-scale operations. Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable biogas.
Many different variations of anaerobic digesters exist. Many different variations of anaerobic digesters exist. The three most common variations are: the covered lagoon, the completely mixed reactor, and the plug flow anaerobic digester. This section will briefly discuss these common variations, but many more options exist such as the induced blanket reactor .
Anaerobic covered lagoons design is simple and easy to build. Anaerobic lagoons are covered ponds (figure 3). Manure enters at one end and the effluent is removed at the other. The lagoons operate at psychrophilic [verwijzing invoegen] or ground temperatures. Consequently the reaction rate [verwijzing invoegen] is affected by seasonal variations in temperature.
Due to the low reaction temperature, the rate of conversion of manure to gas is also low. Moreover, little or no mixing of the manure occurs. Therefore, lagoon utilization is poor (Burke, 2001). Gas production rates have been low and seasonal. A considerable amount of energy potential is lost with the removal of particulate solids. The main advantage of the anaerobic lagoons is the low cost.
The most common form of an anaerobic digester is the completely mixed reactor. This technology is frequently used by sewage treatment plants and industrial treatment plants. The completely mixed reactor is a tank that is heated and mixed (figure 4). Most reactors operate in the mesophilic range (Burke, 2001). The cost of mixing is high. Throughout the European Union completely mixed thermophilic digesters are used to treat animal manure (Ahring, Ibrahim et al, 2001). The main advantage of this variation of digester is that it is a proven technology that achieves reasonable conversion of solids to gas. The disadvantage is the high cost of installation and the energy cost associated with mixing (Burke, 2001).
The plug flow anaerobic digester is the simplest form of anaerobic digestion and the least expensive (Burke, 2001). However, applications of the technology are limited. For instance, within the dairy manure sector applications are limited to dairy manure containing a minor amount of sand or silt (Burke, 2001). If stratification of the manure occurs within the tank significant operating cost, due to the required cleaning of the tank, significant operating costs will be incurred.
The process itself is carried out by a group, or consortia, of micro-organisms (Burke, 2001). The anaerobic digestion process converts organic waste streams into methane and carbon dioxide. The inorganic contents of the waste stream can be converted into beneficial products when the process is executed correctly. The anaerobic digestion process consists of four steps: Hydrolysis, Acidogenesis, Acetogenesis, and Methanogenesis. The first step of hydrolysis, or the liquification of insoluble materials is the rate-limiting step in anaerobic digestion of waste slurries (Burke, 2001). The second step is the conversion of decomposed matter to organic acids. And finally, the acids are converted to methane gas. The most important factor in converting waste to gas is the bacterial consortia. The bacterial consortia are essentially the "bio-enzymes" that accomplish the desired treatment. A poorly developed or stressed bacterial consortium will not provide the desired conversion of waste to gas and other beneficial products.
Many different factors influence the quality of the biogas and therefore the efficiency and profitability of the anaerobic digester facility. Burke identifies these factors: a) Temperature, b) The type of waste being digested; c) The concentration of the waste being digested; and d) several other factors which can be found in Burke 2001, p. 20  (Burke, 2001).
a) The bacteria of the anaerobic digester operate under three general temperature ranges. The bacterial consortia that function under 68 degrees Fahrenheit, called psychrophilic bacteria, have the least amount of bacterial action. Between 68 degrees and 105 degrees Fahrenheit, or mesophilic conditions, has more bacterial action. The optimum mesophilic temperature is between 95 and 98 degrees Fahrenheit. The third temperature range is the thermophilic range between 140 and 145 degrees Fahrenheit. Bacterial growth and waste decomposition is fastest under thermophilic conditions. However, thermophilic digestion produces an odorous effluent when compared to mesophilic digestion. Seasonal and diurnal temperature fluctuations affect performance of the anaerobic digestion and consequently the quantity of gas produced. Temperature influences the level of bacterial action, the quantity of moisture in the biogas, the quantity of gas, the volatile organic substances, and the concentration of ammonia and hydrogen sulfide gas.
b) Different types of waste decompose or convert to gas through anaerobic digestion at different rates. For instance, anaerobic bacteria do not degrade lignin and some other hydrocarbons. Also, the digestion of waste containing high nitrogen and sulfur concentrations might produce toxic concentrations of ammonia and hydrogen sulfide. Dairy wastes have been reported to degrade slower than swine of poultry manure.
c) Waste characteristics can be altered by simple dilution. Adding water will limit the inhibitory effects to the anaerobic digestion process of nitrogen and sulfur. Therefore, some dilution can have positive effects. Greater reduction efficiencies usually occur when the concentration of solids is slightly reduced below the “as excreted” value of the manure.
The recovery of biogas through anaerobic digestion systems is a proven technology. Both in the United States and the European Union the anaerobic digestion of animal waste streams has been used extensively. Figure 6 shows the implementation of this technology in the United States. Currently, the U.S. has 151 projects which utilize this technology with a total estimated energy production of 392 000 MWh/year equivalent (AgSTAR, 2010). Figure 7 shows the current number of projects as a percentage of the potential in the U.S. market. In addition, the figure shows the energy production and the methane reduction as a percentage of the potential. Clearly, the U.S. market still offers a large variety of opportunities for further implementation of this technology.
This technology contributes to socio-economic development and environmental protection in a variety of ways. In general, six goals of an anaerobic digestion system can be identified (Burke, 2001): a) to reduce the mass of solids; b) reduce the odors associated with the waste products; c)produce clean effluent for recycle and irrigation; d) to concentrate the nutrients in a solid product for storage or export; e) to generate energy; f) to reduce pathogens associated with the waste. While the benefits will be discussed separately in further sections below, Table 1 shows the general results of a U.S. based study  of a farm with an anaerobic digestion facility and a farm without such a facility.
|Parameter||With Anaerobic Digestion (Comparison between |
AA. Dairy farm and Patterson Farms)
|Greenhouse gas emissions||Methane - Substantial reduction (3.03 tons per cow-year |
on a carbon dioxide equivalent basis)
Nitrous oxide - No evidence of emissions with or without
|Ammonia emissions||No significant reduction|
|Potential water quality impacts|
Oxygen demand - substantial reduction
Pathogens - substantial reduction
|Economic impact||Significant increase in net farm income (US$ 82 per cow-year)|
The technology contributes to social development in a variety of ways.
The sector in which the technology is mainly applied is the agriculture sector. Rural enterprises are often an important part of the economy of developing nations, producing a considerable part of the GDP and employing a vast number of people. Projects using anaerobic digestion technology, such as the projects outlined in the Clean Development Mechanism section, improve the viability of these rural enterprises. The technology is therefore capable of strengthening the backbone of the economy and subsequently improves social development.
The current waste stabilization technique most often used at farms and industrial locations is the open anaerobic lagoon. Next to emitting methane directly into the atmosphere, this technique has several disadvantages that would be solved by the introduction of a anaerobic digester facility. The workplace at an open lagoon system is unhealthy and unpleasant to work at. Local air quality at such facilities is poor and the a strong odor is produced by the open lagoon. The implementation of an anaerobic digester facility makes the workplace safer and healthier. Local air quality is significantly improved, as outlined in the environmental protection section, and the strong odor is considerably reduced (see e.g.: ERG, 2003).
Several economic development benefits are associated with this technology. These benefits mostly arise from the energy production of the technology.
National energy self sufficiency is increased due to the local energy production. This can be a major benefit in countries which are highly dependent on fossil fuel imports for their energy. Reducing the dependence on foreign energy sources leads to an improved economic balance sheet of the country and a higher level of energy security. For example, the introduction of an anaerobic digestion facility at a Colorado pork farm generated 342,414 kWh of electricity with an estimated gross value of US$ 22,907 for onsite use (ERG, 2003).
In some countries, the locally generated energy might provide a more reliable supply of energy than the energy supplied by the national grid. Moreover, when the technology is applied at many locations, the combined energy production might reduce the need to build new power plants at a national level and therefore preserve national resources.
Large amounts of animal waste can create serious environmental concerns. When animal manure enters rivers, streams or groundwater supplies it can have environmentally detrimental effects. In addition, decomposing manure causes air quality concerns associated with ammonia emissions, and the contribution of methane emissions to global climate change. The implementation of an anaerobic digestion offers a number of air and water quality benefits. Differences exist among the possible variations of anaerobic digester systems as illustrated in Table 2.
Emissions control of ammonia and hydrogen sulfide. These emissions are controlled by covering up the manure storage tanks. For example, a Colorado farm realized a 5,902 pound hydrogen sulfide emission reduction per year through the installation of a anaerobic digestion facility.
Water quality benefits. When an anaerobic digester system is properly managed, phosphorous and metals such as copper and zinc will settle out in the process cells (EPA, 2002). Therefore, the technology reduces phosphorous and metals loadings to surface waters when manure is land-applied. In addition, digester systems isolate and destroy disease causing organisms that might otherwise enter surface waters and pose a risk to animal and human health. This is especially true for heated digesters. Moreover, anaerobic digesters help protect ground water. Synthetic liners provide a high level of groundwater protection for manure management systems (EPA, 2002). The concrete or steel in plug flow and complete mix digesters also effectively prevent untreated manure from reaching the ground water. For example a Eastern Research Group study found that with anaerobic digestion the densities of fecal coliforms and M.avian Paratuberculosis was substantially reduced (ERG, 2004).
Reduction of landfill requirements. Biological treatment of waste, such as composting and anaerobic digestion reduces volume of waste and therefore the lowers landfill requirements. Recycling of the residual solids as fertilizer further reduces waste volume.
|Options||Odor Control||Greenhouse gas reduction||Water quality protection||Cost Range (per 1000 pounds/live weight)|
|Covered lagoon digesters with open storage ponds||E||H||G||US$ 150 - 400|
|Heated digesters (i.e. complete mix and plug flow) with open storage tanks||E||H||G||US$ 200 - 400|
|Aerated lagoons with open storage ponds||G - E||H||F - G||US$ 200 - 450|
|Seperate treatment lagoons and storage ponds||F - G||L||G||US$ 200 - 400|
|Combined treatment lagoons and storage ponds||P - G||L||F - G||US$ 200 - 400|
|Storage ponds and tanks||P - F||M - H||P - F||US$ 50 - 500|
Key: P=Poor, F=Fair,G=Good,E=Excellent, L=Low,M=Medium,H=High
Aerated lagoon requirements add another US $ 35 - 50 per 1000pounds/live weight per year
Cost Ranges do not include annual operation and maintenance costs
The main climate related benefit of this technology is the prevention of methane emissions into the atmosphere. Conventional manure management practices emit large amounts of methane. As can be seen in Table 3, methane contributes significantly to climate change. In order to prevent methane emissions it is essential that the anaerobic digester facility either puts the produced biogas to effective use (for instance by feeding it into a CHP unit) or that the biogas is flared. Flaring the biogas results in higher CO2 emissions. However, the Global Warming Potential of methane is 21 times stronger than that of CO2 so it still limits the effect on global climate change. In addition, the energy produced by the biogas facility offsets energy derived from fossil fuels. Therefore, anaerobic digesters with a biogas recovery system can help reduce overall quantities of CO2. For example, the Colorado based pork farm with an anaerobic digester was able to reduce fossil fuel derived CO2 emissions by 409 tons per year and methane emissions on a CO2eq basis by at least 3022 tons per year (ERG, 2003).
|Sector||Carbon dioxide (CO2)||Methane (CH4)||N20||Global Total|
|Energy||23 408||1646||237||25 291|
|Global Total||31 686||6021||3114||41 382|
|Percentage of global total||77 %||15 %||8 %|
|Contribution to greenhouse effect (W/M2)||69.9 %||22.9 %||7.1 %|
Installation and operation of an anaerobic digestion facility differ between the type and variation of installation. Moreover, the characteristics of the waste stream also influence the economics of the system. A review of the anaerobic digestion system costs of several U.S. dairies compiled by Lusk has established that the typical anaerobic system constructed in the U.S. had an average cost of US$ 470 per cow. More generally, anaerobic systems for digestion, solids processing, and generation are expected to cost US$ 500 to US$ 800 per cow in the U.S. (Burke, 2001).
Table 4 illustrates the capital and operating costs of European digestion systems. Burke notes that the capital, operation and maintenance costs are considerably greater in Europe than those reported in the U.S. However, Burke also notes that the income derived from the sale of the solids is considerably greater in Europe.
|Large 1 MW 5000 Cow Facility||Small 25 kW 125 Cow farm|
|Capital Cost||US $ 9.113.000,-||US $ 500.000,-|
|Annual Operating Cost||US $ 643.000,-||US $ 8.800,-|
|Power Sale Rate $/kW||US $ 0.06||US $ 0.06|
|Heat Sale $/kW||US $ 0.01||US $ 0.01|
|Solids Sales||US $ 700.000,-||US $ 20.000|
The results of a 2004 U.S. based study shows that anaerobic digestion with biogas utilization can produce revenue adequate to recover the required capital investment and increase farm net income through the on-site use and sale of electricity generated (ERG, 2004). The study found that capital investment can be recovered in approximately eleven years and then add about US$ 32 000 annually to net farm income over the remaining useful life of the system (ERG, 2004). The study concludes that there is a “significant economic incentive to realize the environmental quality benefits that the anaerobic digestion of dairy cattle manure can provide” (ERG, 2004).
Within the CDM portfolio, twenty projects use the process of anaerobic digestion. Sixteen of these twenty projects concern the anaerobic biological treatment of swine wastewater and are all located in the Philippines (for an example see: anaerobic digestion swine wastewater treatment with on-site power project ). Normally, within the Philippines, swine wastewater is fed into an open lagoon. The resulting methane emissions from the digestion process are therefore vented into the atmosphere. The CDM projects use a cover system to prevent the escape of methane into the atmosphere. Instead, the methane is used for electricity generation to be used on the swine farm.
Therefore, in the case of anaerobic treatment of swine wastewater, an anaerobic digestor is capable of realizing GHG emission reductions compared to the business-as-usual approach, For calculation of these GHG emission reductions, it is recommended to apply the approved methodology for Methane recovery in animal manure management systems  AMS-III.D version 16. This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. In addition, when the recovered methane is used for electricity generation the CDM methodology g rid connected renewable electricity generation  AMS-I.D version 16 applies.
However, other applications of anaerobic digestion besides animal manure management systems are also possible. The CDM methodologies suitable for these applications differ from the methodologies with animal manure. For instance, industrial processes also generate wastewater suitable for anaerobic digestion. Such is the case with the Bioenergia project , which utilizes the wastewater of a distillery to produce biogas which is subsequently used to power the distillery. In this case CDM methodology Avoided Wastewater and On-site Energy Use Emissions in the Industrial Sector  AM0022 version 4 applies. Another possible methodology is Avoided methane emissions from organic waste-water treatment  AM0013 version 4.
General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html .
AgSTAR, 2010. Anaerobic digesters continue to grow in the U.S. livestock market. AgSTAR: Energy and pollution prevention. Retrieved 25th October from: http://www.epa.gov/agstar/anaerobic/fact.html 
AgSTAR fift national conference, 2010. National conference on anaerobid digestion by the U.S. Environmental Protection Agency. Retrieved 26th October from: http://www.epa.gov/agstar/news-events/events/conference10.html 
Burke, 2001. Dairy waste anaerobic digestion handbook: options for recovering beneficial products from dairy manure. Environmental Energy Company report. Retrieved 25th October from: www.mrec.org/pubs/dairy%20waste%20handbook.pdf 
EPA, 2002. The AgSTAR program: Managing manure with biogas recovery systems – improved performance at competitive costs. United States Environmental Protection Agency. Retrieved 25th October from: http://www.epa.gov/agstar/anaerobic/fact.html 
ERG, 2004. A comparison of dairy cattle manure management with and without anaerobic digestion and biogas utilization. Eastern Research Group submission to AgSTAR program of the United States Environmental Protection Agency. Retrieved 25th October from: http://www.epa.gov/agstar/anaerobic/evaluation.html 
ERG, 2003. An assessment of the Colorado Pork, LCC. Anaerobic digestion and biogas utilization system. Eastern Research Group submission to AgSTAR program of the United States Environmental Protection Agency. Retrieved 25th October from: http://www.epa.gov/agstar/anaerobic/evaluation.html 
Long, Harriet, 2010. Image retrieved 26th of October from: http://water.me.vccs.edu/courses/ENV149/lesson4b.htm 
PSU, 2010. Penn State University Biogas and anaerobic digestion website. Image retrieved 26th of October from: http://www.biogas.psu.edu/coveredlagoon.html 
Virginia Tech, 2010. Virginia Cooperative Extension Biomethane Technology. Image retrieved 26th of October from: http://pubs.ext.vt.edu/442/442-881/442-881.html