Composting of Solid Waste:
- Defined as "biological decomposition of biodegradable organic constituents of waste under controlled conditions to a state sufficiently stable for nuisance free storage & handling and for safe use in land applications"
- About 70% of MSW is decomposable / compostable
- Main organisms involved in composting are: actino-mycetes(filamentous bacteria), fungi and protozoa
Objectives of Composting
- to transform biodegradable OM into biologically stable material thereby reducing the original volume of the waste
- to destroy pathogens, insect eggs and other unwanted organisms and weed seeds that may be present in MSW;
- to retain maximum nutrient content; and
- to produce a product that can be used to support plant growth and as a soil amendment as fertilizer and soil conditioner
Composting time depends on:
- Nature of waste
- Moisture content
- Available nutrients
- Other environmental factors
- Under controlled conditions, can be made more effective and efficient
- Organic matter + O2 + nutrients + microbes ------- New cells + resistant OM + CO2 + H2O + NH3 + SO4 + heat
- Reduces weight/volume of the organic fraction of MSW
- Produces compost ( a humus like material that can be used as a soil conditioner)
- To produce methane - Bio Gas
Steps in Composting:
- PRE PROCESSING
- Sorting/separation: removal of recyclable material, metals, debris, glass etc. -affects the quality of
- Size Reduction/Screening: to provide greater surface area for microbes to attack, to improve insulation, to facilitate homogenizing of initially heterogeneous material, to reduce depth for oxygen diffusion.
- If size is too small, it impedes the diffusion of oxygen and CO2 from the sites being attacked especially during thermophylic stage when Oxygen demand is the highest.
- Typical range for feed material = ½ - 2 inches.
- Adjustment for waste properties (e.gC/N ratio)
- Decomposition and curing/maturation:
- Types are: Static piles and in-Vessel composting
- Preparation and marketing of the final compost product: Grinding, screening, air classification, blending with various additives, granulation, bagging, storage and transportation
- Triangular piles 7 feet high and 14-16 feet wide at the base
- Turning by front end loader or mechanical turner twice per week
- Composting period 2-6 weeks