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. While composting is often appropriate for dry feedstocks, anaerobic digestion is particularly appropriate for wet wastes.
Biodegradation is a natural, ongoing biological process that is a common occurrence in both human-made and natural environments. The term composting, or aerobic biological treatment, is used to define biological degradation under controlled aerobic conditions. The process is used to stabilize wastewater solids prior to their use as a soild amendment or mulch in landscaping, horticulture and agriculture (EPA, 2000). Figure 1 illustrates the process of composting. The waste is decomposed into CO2, water and the soil amendment or mulch. In addition, some carbon storage also occurs in the residual compost. The teaser image above shows the finished product of the process. The process destroys pathogens, minimizes odors, and reduces vector attraction potential.
Three composting techniques that are available to compost biosolids are windrow, aerated static pile, and in-vessel composting. Supporting techniques such as sorting, screening and curing also play an important role in the process. Each technique uses the same scientific principals but varies in procedures and equipment needs. Other variations between the technologies are issues such as air supply, temperature control, mixing, and the time required for composting. Moreover, their capital and operating costs also differ widely.
In-vessel composting (EPA, 2000)
The technique of in-vessel composting occurs within a contained vessel. This technique allows the operator to maintain close control over the process. Other advantages of this technique compared to other composting techniques are: the effects of weather are diminished, the quality of the resulting product is more consistent, less manpower is required to operate the system and public acceptance of the facility may be better. In addition, due to the smaller space requirement, in-vessel technology is more suitable in suburban and urban technologies compared to the other composting technologies. In addition, the in-vessel system allows for detailed containment and treatment of air to remove odors before release.
Disadvantages of this technique are that it is generally more costly than the other methods, particularly with respect to capital expenditures. The higher level of mechanization with this technique also results in more maintenance requirements which increases operational costs.
In this technique, organic waste is placed into rows of long piles called windrows and are aerated by turning the pile periodically by either manual or mechanical methods. The ideal pile height is approximately 4 to 8 feet and the ideal width is between 14 and 16 feet. This height allows for a pile large enough to generate sufficient heat yet small enough to allow oxygen to flow to the windrow’s core.
This method has the advantage that it is applicable to large volumes of diverse wastes, including yard trimmings, grease, liquids and animal byproducts. However, this requires frequent turning of the pile and careful monitoring. The technique is suitable for large quantities such as that generated by entire communities and collected by local governments and high volume food-processing businesses.
Windrow composting can work in both warm, arid climates and in cold climates. In warm, arid climates it is sometimes necessary to cover or shelter the pile to prevent water evaporation. Rainy seasons sometimes require adjustment of the shape of the pile to ensure that the water runs off the top of the pile rather than being absorbed into the pile. In cold climates, the pile might freeze at the outside, but will remain warm in the core.
It is important to collect and treat the leachate that is released during the composting process. Otherwise, it might contaminate local ground-water and surface-water supplies. Windrow composting often requires large tracts of land, sturdy equipment, a continual supply of labor. In addition, the technique requires some experimentation with various materials mixtures and turning frequencies. The technique has the advantage that it results in large quantities of compost.
Aerated static pile (EPA, 2000)
Aerated static pile composting mixes organic waste together in one large pile instead of rows. To ensure adequate flow of oxygen throughout the pile, layers of loosely piled bulking agents such as wood chips are added so that air can pass from the bottom to the top of the pile. Oxygen can also be delivered mechanically into the pile with the use of air blowers and a network of pipes which are placed into the piles. This technology is applicable for a relatively homogenous mix of organic waste and works well for larger quantity generators of yard trimmings and compostable municipal solid waste. The technology is therefore suitable for local governments, farms or landscapers. However, the technique is not suitable for composting animal byproducts or grease from food processing industries.
In a warm, arid climate it is sometimes required to cover the aerated static pile to prevent evaporation of water. Sometimes, aerated static piles are placed indoor with proper ventilation to exclude climate or seasonal influences. Since the technique doesn’t use physical turning of the pile it is essential to carefully monitor the pile to ensure that the outside of the pile heats up as much as the core. One method to achieve this is to apply a thick layer of finished compost over the pile which additionally helps to prevent bad odors. The technique requires equipment such as blowers, pipes, sensors and fans. The use of this equipment might cause significant costs and require technical assistance. The advantage of the technique is that it requires less land than the windrow method. Additionally, the method has a high production rate – it only takes about 3 to 6 months to produce compost.
The micro-organisms which perform the composting process are influenced by their biological, chemical, and physical needs (EPA guide).
In composting, micro-organisms such as bacteria, fungi and actinomycetes play an active role. In addition, larger organisms such as earthworms and insects also play a role, albeit a less significant role compared to the micro-organisms. The biological needs of the micro-organisms relate to the type of organic material they consume. The composting process may proceed slowly at first, due to smaller microbial populations, but as populations grow the process speeds up. Generally, the number and kind of micro-organisms are not a limiting environmental factor. Feedstock, such as agricultural materials or municipal solid waste, generally contains an adequate diversity of micro-organisms. However, microbial levels could be a limiting factor if the feedstock is generated in a sterile environment.
Microbial activity is key in the composting process. If the conditions are right composting will occur rapidly. It is therefore essential to establish the conditions favorable to the micro-organisms.
The material to be composted determines the chemical environment. In addition, modifications can be made to the composting process to create an ideal chemical environment leading to rapid decomposition. The chemical environment is mainly determined by the following factors: a) carbon content ; b) a balanced amount of nutrients; c) the moisture content; d) the level of oxygen; e) the pH levels; f) the biodegradability in the form of the lignin  content.
a) Microorganisms rely on the carbon in organic material as their energy source. The biodegradability of carbon sources differ. For instance, lignin has a low degradability. In other words, there are only a few types of micro-organisms capable of decomposing lignin. In contrast, sugars are easily biodegraded. Most municipal solid waste and agricultural residues contain adequate amounts of biodegradable forms of carbon. Therefore, carbon is generally not a limiting factor in the process.
b) Besides a carbon source for their energy, the microorganisms require nutrients such as nitrogen, phosphorous and potassium. A possible limiting factor in the composting process is nitrogen. The other nutrients are usually not a limiting factor. A critical ration in the decomposition rate is the ratio of carbon to nitrogen. The ratio must be established based on available carbon. Generally, an initial ratio of 30:1 carbon to nitrogen is considered ideal. The ratio can be influenced by adding nitrogen rich materials such as yard trimmings, animal manure, or biosolids. It also works to add partially decomposed or composted materials.
c) A moisture content of 50 to 60 percent of total weight is ideal. Higher moisture levels can create anaerobic conditions which results into rotting and obnoxious odors.
d) While decomposition will occur under both aerobic and anaerobic conditions, the process of composting requires oxygen. Oxygen can naturally enter into the compost if there is enough void space within the compost pile. In addition, air may be mechanically forced into the compost pile. It is also possible to turn the pile to expose the inner parts of the pile to the atmosphere. The oxygen concentration considered adequate is about 10 to 15 percent.
e) A pH between 6 and 8 is optimal.
a) Particle size reduces naturally throughout the decomposition process. Smaller particles allow more microbial activity due to their increased surface per unit of weight. However, when particles become too small they allow few open spaced for air to circulate. There is therefore a optimum particle size which has enough surface area for microbial activity but which also allows void space for air circulation.
b) The composting optimum temperature range is between 32° and 60° C. Pathogens are destroyed when temperatures are higher than 55o C for at least three days. However, the windrow technique requires 55o C for a minimum of fifteen days to ensure pathogen destruction. During those fifteen days it is essential to turn the windrows at least 5 times. The longer time frame and increased turning are necessary to achieve pathogen destruction throughout the entire pile.
Composting is a proven technology and is being utilized throughout the world. The dedicated capacity for composting, together with incineration and Mechanical Biological Treatment (MBT) capacity, for several EU countries and the Flemish region is illustrated in Figure 5. For instance, the European Environment Agency notes that about 40 % of households in the Flemish region use home composting, Germany composts approximately 3-7 million tons of organic waste each year. In Germany this equals about 40 – 85 kilograms per capita (EEA, 2009).
In addition the EEA report states that the majority of the EU-12 countries still have a high dependency on land-filling and still have high potential for implementing composting technologies (EEA, 2009).
The main markets for which the selected countries and the Flemish region within the EU produce compost are illustrated in Figure 6. The EEA study stresses the importance of markets for the products of biological treatment. While in some cases composting capacity is available, the inadequate quality results in low acceptance of the product in the market (EEA, 2009). Quality standards implemented in the Flemish Region, Germany and Italy seem to have been effective in ensuring that compost quality is sufficient for agricultural use, wholesale and private gardening (EEA, 2009).
In developing countries the future market potential of the technology is large. Being mostly agrarian, organic waste constitutes a large part of the waste handled in the countries. For instance, the Uganda CDM project mentions that around eighty percent of the municipal solid waste in Uganda is of an organic nature.
The technology contributes to socio-economic development and environmental protection in a variety of ways. Discussed in more detail below the contributions of the technology are:
- Reduced waste stream volume which results in a reduced waste volume going into landfills.
- The product, compost, of the composting process can be marketed and sold which generates revenue.
- Rural enterprises and waste handling enterprises are supported.
- Economic development benefits occur in the cases where the costs for other options of waste management are high.
- Rural enterprises have a reduced dependency on fertilizer.
- In the case of replacement of open anaerobic lagoons with aerobic biological treatment methane emissions are prevented and replaced with carbon dioxide emissions.
- The technology is both applicable for small scale and large scale applications. Both will support local employment.
- The leachate from conventional waste management practices in developing countries can be addressed through the implementation of composting technology.
As mentioned, the economy of many developing countries is based on the agrarian sector. Composting supports rural enterprises. When the farm utilizes on-site composting, the need to purchase fertilizers is reduced since the finished product of the composting process can be used as soil amendments. In addition, the farm no longer needs to find a destination for the excess organic waste. Organic waste, such as animal wastes or agricultural waste, can be fed into the composting process on-site. This further reduces cost for rural enterprises.
Composting also provides benefits for waste handling agencies. Composting part of the waste the agency receives, increases the landfill lifetime and provides the waste handling agency with a marketable product in the form of compost.
Furthermore, since composting can also be used for small scale applications, small communities can also initiate local composting programs. Such a program can provide benefits to the local community in the form of increased local employment and reduced costs for waste removal. In the case that there was previously no waste management mechanism present, the initiation of a composting scheme might also improve local health. The establishment of a composting program ensures controlled handling and monitoring of waste streams within the community and therefore mitigating negative side-effects that might occur in the absence of a waste management mechanism.
The initiation of compost waste management schemes result in environmental benefits.
First, the compost is an excellent soil conditioner. Compost adds organic matter to soils, improves moisture retention elevating drought tolerance, improves soil structure, reduces fertilizer requirements, and reduces the potential for soil erosion.
Second, when animal wastes are composted, the composting process reduces the weight, moisture content, odor, and vector-attracting qualities of manure. This is also true for other farm generated organic waste materials. Composting manure also results in the following benefits: a) composting converts the nitrogen contained in manure into a more stable organic form. Although this results in some loss of nitrogen, the nitrogen that remains is less susceptible to leaching and further ammonia losses.; b) Composting high-carbon manure lowers the carbon to nitrogen ration to acceptable levels for land application.; c) the heat generated in the composting process reduces the number of weed seeds contained in the manure, resulting in a significant reduction of weeds over several years of application. In turn, this reduces the need for herbicides.
Third, composting reduces the volume of organic waste that may have previously gone to the landfill. This increases landfill lifetime for waste that actually needs to be land-filled. In addition, it reduces the costs for rural enterprises regarding waste disposal fees.
Fourth, in the case of animal manure, disposal is often a problem which might result in pollution or odor-related nuisance complaints.
Fifth, the composting process results in pathogen destruction. In addition, properly prepared compost has been found to reduce soil-borne plant diseases. These characteristics reduce herbicide, pesticide and fertilizer use.
Like any additional operation, composting requires equipment, labor, and management. While initial investments for a composting operation can be low when existing equipment and facilities are used. This is for instance often the case on small rural enterprises. However, when the volume of material is large, it is likely that existing equipment is not sufficient to meet the demands, or that it requires too much labor. In this case, purchase of dedicated composting equipment is required. Depending on the composting method selected and the characteristics of the waste stream that is to be composted costs can be become high.
On the community level, composting might offer an attractive economic advantage for communities in which the costs of other options are high. The reduced disposal need associated with composting may even be adequate to justify this option.
Aerobic biological treatment projects can be used as CDM projects. Currently, approximately 40 CDM projects use aerobic biological treatment to reduce GHG emissions. This section briefly discusses everal examples from within the current CDM portfolio to illustrate some of the possibilities.
For instance, a CDM project in Brazil  uses aerobic biological treatment to replace anaerobic lagoons. These lagoons are similar to the example in the anaerobic biological treatment description in that they vent the produced methane directly into the atmosphere. The CDM project aerates the wastewater which leads to production of CO2 instead of methane. The lower Global Warming Potential of CO2 than methane leads to a net reduction in greenhouse gas emissions. The CDM methodology recommended for such a project is Avoidance of methane production in wastewater treatment through replacement of anaerobic systems by aerobic systems  AMS-III.I Version 8.
Another example of a composting CDM project  is the Programme of Activities (a CDM project that consists of multiple project activities) in Uganda. This project composts Municipal Solid Waste (MSW) to replace the baseline process of landfilling the MSW. The project generates revenue through the emission reduction credits and from selling the compost. For such a project the recommended methdology is Avoidance of methane emissions through controlled biological treatment of biomass  AMS-III.F Version 8.
A third example of a CDM project is a Bangladesh project  that composts organic waste at landfill sites. The project diverts waste with a high organic content to a composting plant. Under this project methane emissions are avoided. For such a project the recommended methodology is Avoided emissions from organic waste through alternative waste treatment processes AM0025  Version 12.
These methodologies help 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. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html . 
EEA, 2009. Diverting waste from landfill: Effectiveness of waste-management policies in the European Union. European Environment Agency Report No 7/2009. Retrieved 26th of October from: http://www.eea.europa.eu/publications/diverting-waste-from-landfill-effectiveness-of-waste-management-policies-in-the-european-union/?b_start:int=12&-C 
EPA, 1995. Decision Makers' Guide to Solid Waste Management, Volume II. United States Environmental Protection Agency. Retrieved 26th of October from: http://www.p2pays.org/ref/03/02021/02021.pdf 
EPA, 2000. Bio-Solids Technology Fact Sheet: In-Vessel Composting of Biosolids. United States Environmental Protection Agency. Retrieved 26th of October from: http://www.epa.gov/epawaste/conserve/rrr/composting/pubs/index.htm 
United States Environmental Protection Agency (USEPA). General information on composting retrieved 26th of October from:http://www.epa.gov/epawaste/conserve/rrr/composting/index.htm