Microbiology of In-vessel Composting
To maintain maximum microbial activity, particularly when processing organic waste such as ABP (Animal By-Product) Category 3 waste or kitchen and garden waste, process factors need to be optimal, requiring careful management and control. Microbial activity is sensitive to changes in process factors, such as the temperature, pH, nutrient and oxygen availability and moisture content. Physical (oxygen uptake, temperature) and chemical properties (pH, volatile fatty acids and metabolite concentrations) can be used to indicate the state of the composting system.
Most of the micro-organisms present during in-vessel composting are bacteria, actinomycetes and fungi.
Bacteria make up 80 % to 90 % of the myriad of micro-organisms and are responsible for most of the initial decomposition of the readily-degradable organic waste and the heat generation characteristic of in-vessel composting.
Actinomycetes and fungi, however, constitute most of the biomass, particularly in the later stages of composting when they are more prevalent.
Fungi have a competitive advantage over bacteria and actinomycetes because they are able to operate in environments which are too dry, acidic, and low in nitrogen for bacteria and are often found growing extensively on carbonaceous organic material. Their enzymes enable them to decompose complex organics such as cellulose, lignin, chitin, and proteins.
Ultimately, the type of organic material determines microbial population dynamics but, because the biochemical reactions during composting are catalysed by enzymes, temperature is the principal influence on microbial activity.
Under optimal conditions, in-vessel composting typically experiences three distinctive stages: active, cooling and maturation. Microbial populations operate within a specific temperature range. The typical temperature and pH profiles during in-vessel composting are shown in the graph above.
Psychrophiles favour temperatures below 10?C, mesophiles thrive in temperature between about 20?C and 45?C and thermophiles grow well between about 50?C to 70?C. An increase above the maximum temperature in the range leads to the thermal destruction of the cell proteins and the death of the micro-organisms, whilst temperatures below the minimum affect the metabolism of the cell.
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