The composting process occurs when biodegradable waste is piled together with a structure allowing for oxygen diffusion and with a dry matter content suiting microbial growth. The temperature of the biomass increases due to the microbial activity and the insulation properties of the piled material. The temperature often reaches 650C to 750C within a few days and then declines slowly. This high temperature hastens the elimination of pathogens and weed seeds.
A typical composting plant consist of some or all of the following technical units: bag openers, magnetic and/or ballistic separators, sieves, shredders, mixing and homogenization equipment, turning equipment, aeration systems, bio-filters, scrubbers, control systems etc.
Composting costs include site acquisition and development, regulatory compliance, facility operations, and marketing of the finished product. Additional requirements may include land for buffers around the compost facility, site preparation, and handling equipment such as shredders, screens, conveyors, and turners. Facilities and practice to control odors, leachate, and runoff are a critical part of any compost operation.
The cost of constructing and operating a windrow composting facility will vary from one location to another. The operating costs depend on the volume of material processed. The use of additional feed materials, such as paper and mixed municipal solid waste, will require additional capital investment and materials processing labor.
The capital costs of windrow or aerated piles are lower than in-vessel composting configuration. However, costs increase markedly when cover is required to control odors. In general, costs of windrow systems are the lowest compared to the other two techniques. The in-vessel system is more costly than other methods, mainly with respect to capital expenditures. In addition, it is more mechanized and more equipment maintenance is necessary; however, it tends to be less labor-intensive.