Biological or thermal treatment of municipal solid waste can result in recovery of useful products such as compost or energy.
Biological treatment involves using micro-organisms to decompose the biodegradable components of waste. Two types of processes are used, namely:
- Aerobic processes: Windrow composting, aerated static pile composting and in-vessel composting; vermi-culture etc.
- Anaerobic processes: Low-solids anaerobic digestion (wet process), highsolids anaerobic digestion (dry process) and combined processes
In the aerobic process the utilisable product is compost. In the anaerobic process the utilisable product is methane gas (for energy recovery). Both processes have been used for waste processing in different countries – a majority of the biological treatment process adopted world-wide are aerobic composting; the use of anaerobic treatment has been more limited.
Thermal treatment involves conversion of waste into gaseous, liquid and solid conversion products with concurrent or subsequent release of heat energy. Three types of systems can be adopted, namely:
- Combustion systems (Incinerators): Thermal processing with excess amounts of air.
- Pyrolysis systems: Thermal processing in complete absence of oxygen (low temperature).
- Gasification systems: Thermal processing with less amount of air (high temperature).
Combustion system is the most widely adopted thermal treatment process world-wide for MSW. Though pyrolysis is a widely used industrial process, the pyrolysis of municipal solid waste has not been very successful. Similarly, successful results with mass fired gasifiers have not been achieved. However both pyrolysis and gasification can emerge as viable alternatives in the future.
To be viable for energy recovery through thermal processing, the municipal solid waste must possess a relatively high calorific value. In the MSW generated in developed countries, presence of significant quantity of paper and plastics yields a high calorific value of the MSW (typically above 2000 kcal/kg) which makes it suitable for thermal processing.
- New biological and chemical processes which are being developed for resource recovery from MSW are:
- Fluidised bed bio-reactors for cellulose production and ethanol production.
- Hydrolysis processes to recover organic acids.
- Chemical processes to recover oil, gas and cellulose.
GEP ECOTECH offers tailor-made MSW handling and recycling solutions, contact us for details.