In the last six decades the urban population has increased five times to 485.35 million (Indian Census, 2011) increasing the burden on municipal corporation to deal with the solid waste management.
h steam turbines generating power.
It is a noiseless and hygienic method with minimal land requirements. As the incineration plants can be set up in city limits the waste transportation cost can be cut down.
However the establishment, operational and maintenance cost of such plants is very high. The residual ash contains pollutants and toxic substances, alarming environmental concerns. The operation and maintenance of such plants require skilled labour.
Pyrolysis ans Gasification
It is a gasification process where wastes such as biomass and homogeneous organic matter are reacted with controlled amount of oxygen at high temperatures (Middleton, 2005; Marshall & Morris, 2006) producing a synthesis gas called syngas which is a fuel.
Syngas can be converted to electric power and it is more efficient than biogas or liquid fuel.
The drawbacks of the process are high power consumption for pre-processing wastes, supply of pure oxygen as a gasification agent, timely closure of service plants for cleanups.
Decomposition of waste material in oxygen-starved environment to gaseous substances using an electric arc gasifier at high temperatures is plasma pyrolysis. The process intends to generate electricity and reduce the slag sent for landfills.
It is not suitable for wet wastes as electricity cost is high for processing it. Transportation and burning of pyrolysis oil with high viscosity is problematic and handling of gasifier ash with toxic matter and its disposal is difficult.
Pelletization/Production of Refuse Derived Fuel (RDF)
It is a method for processing mixed municipal solid wastes producing RDF an enriched fuel feed for thermal processes like incineration and industrial furnaces (Chantland, 2006).
The RDF pellets can be used as coal substitutes and can be stored or transported to longer distances easily. It is not a feasible process during rainy season or for wet wastes.
If RDF fluff/pellets are contaminated by toxic/hazardous material, the pellets are not safe for burning in the open or for domestic use.
Sanitary Landfills and Landfill Gas Recovery
It is the ultimate method for disposal of all types of wastes such as residual sludge, industrial and commercial waste, inorganic wastes and inert matter which cannot be recycled or reused in the future. The main advantage of this technology is that it is highly cost effective and does not require skilled labour for maintenance.
The landfill gas generated can be utilized for direct thermal applications as domestic fuel. However, transportation of waste to landfill sites, getting land for landfill sites in the cities, pollution of ground water by leacheate pollutants and chances of explosion due to emission of green house gases as carbon dioxide and methane at landfill site are some of its drawbacks.
It is mandatory in India to treat the organic waste before disposal as dumping of it is prohibited by law. Hence in India recovery of landfill gas is minimal.
The Choice of Technology
The choice of technology depends on its economic and technical viability, environmental implications and sustainability keeping in view the financial and physical resources.
The key factors are:
- The quality and origin of waste
- Presence of toxic waste components
- Availability of energy producing outlets
- Commercial markets for sale of compost produced
- Land prices and labour costs
- Experience and capabilities of technology providers.
However, compliance of any proposed technology with the Municipal Solid waste (Management and Handling) Rules 2000 issued by the Ministry of Environment and Forests is mandatory.
BOOT, BOO, and DBO Contracts for Treatment and Disposal of Waste
Due to the incapability of municipal authorities in solid waste collection and disposal the relegation of responsibility to private players and their participation is gradually increasing. Build, Own, Operate and Transfer (BOOT) and Build, Own and Operate (BOT) are the popular models.
The municipal authorities provide land on lease for plant set up and garbage at plant sites. According to the BOOT contract the private firm invests money to build, own and operate the plant for 20 to 30 years and then transfer it to the municipal authorities after investment returns.
But in BOT contract the plant is dismantled after the contract period and the land is taken up by municipal authorities. In some cities DBO contracts are adopted where the plant building, ownership and land costs are borne entirely by the municipal authorities with the private firm responsible for only designing, building and operating the plant.
All the technological options are analysed, their salient features and cost implications are taken into consideration. Study of environmental implications and suitability to biophysical environment of India is carried out. The research shows that composting, vermin-composting and biomethanation are the preferred techniques.
Choice of technology depends on the type of waste. Composting and biogas generation is for slaughter house and fish market wastes, Vermi composting for homogeneous wastes such as fruits and vegetable wastes and windrow for heterogeneous wastes. Precaution has to be kept near thermal conversion plants so that the fuel gas emitted does not pollute the environment.
Sanitary land filling is the easiest option. However in India pyrolysis, plasma pyrolysis and pelletization are rare technologies and not in use at large scale. The analysis indicates that no technology is perfect. All of them have merits and demerits. Therefore, the choice of technology has to be done judiciously.