India has an estimated potential of producing about 4.3 million tonnes of compost each year from MSW, which could help reduce the wide gap between availability and requirement of organic manure for soils in India.
Composting is successful because it is a low cost and low infrastructure set-up and also produces compost, which is a marketable byproduct. In addition to making a positive contribution to agriculture, the sale of organic wastes reduces the amount of waste to be collected and disposed of by municipal authorities.
Composting of MSW is undertaken by either of the two methods – Windrow composting or Vermicomposting. Landfill mining and bioremediation are other ways of extracting compost among other resources from landfills.
History of composting and reasons for initial failures
Ten semi-mechanised composting plants (MBTs) were set up in Ahmedabad, Mumbai, Bangalore, Baroda, Delhi, Kolkata, Jodhpur, Jaipur, Kanpur and Vijayawada in the 1970s. The process included removal of big pieces, pulverization, forced aeration with augers and sieving. Almost all the plants have stopped working as there were many problems, which include:
- Semi-mechanized machinery was imported and a minor mechanical fault usually led to breakdown due to nonavailability of spare parts.
- Mixed nature of waste was a major difficulty. Pulverizers got frequently clogged with pieces of rags, plastic and rubber, etc., and blades of which were broken down due to metal and glass pieces present in the waste. Amount of soil mixed into the waste also caused problem in the process, in addition to the lowering of the quality produced.
- Lack of continuous power supply was another problem.
- The process could not be continued in rainy season.
- The actual capacity turned out to be far less than the designed capacity.
- Lack of market for the finished product was another problem. As a result the enterprise could not become self sustained.
During research it was observed that vermicomposting was employed by towns or small cities generating MSW < 100 TPD, whereas larger cities employed mechanical windrow composting. Mechanical composting facilities optimize MSW processing and minimize manual handling of wastes. These composting plants which use mechanical and biological operations to handle mixed wastes are called Mechanical Biological Treatment plants (MBT). MBT and composting will be used interchangeably because almost all windrow composting plants in India operate as MBTs.
The capital investment for building a composting plant is $4,500 per tonne (INR 200,000) of waste processed and the compost is being sold at $45-50 per tonne (INR 2,000-2,200). Availability of government aid and rising entrepreneurial interest resulted in an upsurge in the number of composting facilities nationwide. Among 74 cities examined for their present waste handling techniques, only 22 cities had composting facilities in 2008, whereas by 2010, the number of cities employing composting grew to 40. At present, there are a total of 70 cities which employ MSW composting and 22 new projects are proposed.
In addition to the reasons cited for the failures of composting facilities, another important but overlooked factor is the contamination of end product by heavy metals, glass and plastic.
Windrow Composting or Mechanical Biological Treatment (MBT)
Windrow composting is the most common method of composting in India. It involves the stabilization of organic solid waste through aerobic decomposition. Windrow composting facilities can efficiently handle large quantities of waste in comparison to vermicomposting. For example, plants in Bengaluru, Pimpri and Nashik handle 100 TPD, 500 TPD and 300 TPD of MSW respectively, as compared to a vermicomposting plant in Suryapet which handles 40 TPD. During the MBT process, recyclables are separated from the mixed wastes, baled and sold to a nearby recycling company at a cost of $78 (INR 3,500) per ton of plastics and $56 (INR 2,500) per tonne of paper.
At MBT facilities, mixed wastes are first dried, shredded and sieved into 70mm and 35mm fractions. Only the -35mm fraction undergoes composting; rest is compost rejects and goes to the landfill. The -35mm material is arranged in rows, 2m tall, 3m wide and 11m long. A bacterial-slurry prepared inside the facility is then sprayed on these windrows to accelerate decomposition of the organic material. The windrows are turned once every week continuously for eight weeks. At the end of the eighth week, the waste is shredded and sieved in multiple stages into +16mm and -16mm fractions. The -16mm is the precursor to compost which should be “cured” for another two to three weeks before being sold. It was observed that the demand for compost was higher than the supply from these facilities.
On the basis of all information collected during this trip, the author estimates that only 6-7% of the input mixed waste (12-15 % of organic waste input) can be recovered as compost. Rest of the MSW, 60% (on wet basis) is landfilled as compost rejects.
Compost Quality and Heavy Metal Contamination
A less observed side effect of improper SWM in India is the introduction of heavy metals into human food chain. Compost from mixed waste composting plants is highly contaminated with heavy metals. Using this compost on agricultural fields will result in contamination of the agricultural soil with heavy metals. Food crops grown on contaminated agricultural soils when consumed will introduce the heavy metals into the food chain and lead to a phenomenon called “biomaginification”.
A study conducted by the Indian Institute of Soil Science (IISS), Bhopal found that compost produced from MSW in India is low grade, with high heavy metal concentrations and low nutrient value.
Compost from only two cities out of 29 passed the statutory guidelines by European countries (except Netherlands) for high quality composts. The two cities are Suryapet and Vijayawada where MSW collection is source separated.
Majority of the samples do not comply with Indian quality control standards for total potassium, total organic carbon, total phosphorus and moisture content; and exceeded the quality control limits for heavy metals contamination by Lead (Pb) and Chromium (Cr).
The study also found that incidence of heavy metals in MSW compost from cities (population < 1 million) is less than half of that from bigger cities; but the compost still doesn’t clear the quality control standards in all instances. If all MSW generated in India in the next decade is composted as mixed waste and used for agriculture, it would introduce 73,000 tonnes of heavy metals into agricultural soils.
Mixed waste composting is therefore not an option for sustainable waste management. In countries like India where more than 91% of MSW is landfilled and there are no other alternatives available, mixed waste composting is widely practiced and considered better (if not the best) than landfilling.
Authorities should make sure they are not ignoring future health costs by choosing economic (cheaper) technologies today and creating a bigger public health crisis in the form of bioaccumulation of heavy metals. Usage of MSW compost for food crops should be regulated; simultaneous research on the risk of bioaccumulation due to usage of MSW compost should be conducted to account for public health, and environmental costs in decision making.
Lack of actual performance data of MSW composting facilities was a major concern during initial research, thus an important finding during research visits is that the compost yield from mixed waste composting facilities (MBTs) is only 6-7%. Rest of the MSW, up to 60% of the input waste (accounting for moisture loss and material loss during decomposition) is discarded as composting rejects and landfilled.
Rejects from composting plants in Bengaluru, Nashik and Pimpri were observed to contain a high percentage of plastics, mainly plastic bags. Composting treats only 11% of dry solids in MSW, the rest of it, i.e., about 90% of waste (on a dry basis, or 60% on a wet basis) ends up in unsanitary landfills in case of no further treatment. However, mixed waste composting still avoids landfilling of MSW and increases the operating life of a landfill by 2.5 years in every 20 years.
Compost rejects at Pimpri composting facility were divided into four distinct fractions, (+) 70mm rejects (overflow from 70mm sieve), (+) 35mm rejects (overflow from 35mm sieve), (+) 16mm rejects (overflow from 16 mm sieve) and (+) 4mm rejects (overflow from 4mm sieve). The number of fractions the rejects are divided into depends upon the facility’s design. Analysis of these rejects showed that overall lower calorific value of composting rejects was 9.5 MJ/kg (2,300 kcal/kg). The lower calorific value of these fractions was found to be as high as 11.6 MJ/kg (2,800 kcal/kg).
The calorific value of input MSW at the Pimpri facility is 7.3 MJ/kg (1,750kcal/kg). It is identical to the average calorific value of urban MSW in India which is also 7.3 MJ/kg. If MSW from all cities is treated in MBT facilities, the calorific value of compost rejects will be different from those from the Pimpri facility. However, since MSW generated in many cities has higher calorific value than the input MSW at Pimpri, we will assume composting rejects from MBT facilities in India have an average lower calorific value of 9.5 MJ/kg.