Valencia Trader

TEG Environmental UK

The Role of Composting Today

Composting completes the life cycle of organic material in today’s modern lifestyle. The flow chart illustrates the integral role that composting plant, which may treat sludge from the local sewage works or the community’s green and kitchen waste, plays in an environmentally conscious society.

Composting is a biological process carried out by a variety of micro-organisms such as bacteria and fungi.

Through a series of complex biochemical reactions in predominantly aerobic conditions, micro-organisms convert the organic material to a more stable product. The degree of decomposition and, therefore, the stability of the composted organic material depends on the duration and the temperatures reached during composting.

For more information about composting systems, waste recycling and thermophylic decomposition see TEG Environmental at www.tegenvironmental.co.uk

Microbiology of Composting

Micro-organisms are key to composting

To maintain maximum microbial activity, process factors need to be optimal and this requires a degree of management and process control. Microbial activity is sensitivite 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 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 material and the heat generation characteristic of 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 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.


Temperature and pH profiles during composting


Under optimal conditions, 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 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.

Composting Systems

The wide variety of composting systems can be broadly classified as open and closed or in-vessel.

Open systems include turned piles such as windrows and static piles, either of which can be aerated by natural or forced convection.

In-vessel systems can be vertical, such as tower composters, or horizontal, such as tunnel composters and rotating drums. In-vessel systems range in complexity from the simple box containers, which rely on natural aeration, to the reactor-type vessels with elaborate control systems.

Systems can be further classified as interventionary (mixed, agitated and stirred) and non-interventionary (static and non-agitated) systems.

For more information about composting systems, waste recycling and thermophylic decomposition see TEG Environmental at www.tegenvironmental.co.uk