Composting in India

**References in this post need to be updated**

Composting is the biological decomposition of the biodegradable organic fraction of MSW under controlled conditions to a state sufficiently stable for nuisance-free storage and handling and for safe use in land applications [44]. 

Composting is the most widely employed MSWM technique in India, with above 56 composting plants in more than 43 cities [36] [37] [38] [39] [40] [2]; most of these composting facilities handle between 100 – 1000 TPD of MSW [will attach an Appendix]. Composting is successful because it is a low cost and low infrastructure set-up and also produces compost, which is a marketable byproduct. The capital investment for building a composting plant is $ 4,476 per ton (INR 200,000) of waste processed [4] and the compost is being sold at $ 45 – 49 per ton (INR 2,000 – 2,200) [28]. Availability of government aid and rising entrepreneurial interest resulted in an upsurge in the number of composting facilities nationwide. Composting of MSW is undertaken by either of the two methods, a) Windrow composting or b) Vermicomposting. During the trip, I observed that vermicomposting was employed by towns or small cities generating MSW < 100 TPD, whereas larger cities employed mechanical windrow composting, which minimizes manual handling of wastes.

Fig 1: Rejects from the composting plant at Nasik (/Nashik)

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Windrow Composting

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 700 - 1000 TPD, 300 TPD and 700 TPD of MSW respectively [28] as compared to a vermin composting plant in Suryapet which handles 40 TPD.

Fig 2: Compost ready for "curing" at Pimpri Chinchwad composting facility

Mixed MSW that arrives at the facilities is shredded and sieved into 70 mm and 35 mm fractions. Only the -35 mm fraction undergoes composting; rest is compost rejects and goes to the landfill. -35 mm is arranged in rows, 2 m tall, 3 m wide and 11 m 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 8 weeks. At the end of the 8th week, the waste is shredded and sieved again in multiple stages into +16 mm and -16 mm fractions. -16 mm is the precursor to compost which should be “cured” for another 2 - 3 weeks before being sold. 

Fig 3: Compost packed in bags of 50 kg, ready to be sold at Nasik's composting facility

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 research estimates that only 6% of the input mixed waste (12 – 15 % of organic waste input) can be recovered as compost.

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Compost Yield

Lack of actual performance data of MSW composting facilities was a major concern during the research, thus an important finding during this visit was that the compost yield from mixed wastes is only 6% of the input. Rest of the waste is landfilled as “rejects”. Rejects from composting plants in Bengaluru, Nashik and Pimpri were observed to contain a large fraction of plastics, mainly plastic bags, Figure 1. In case of no further treatment of rejects, 92.5% of MSW (accounting moisture loss during composting) will end up in a landfill [28]. However, composting still avoids landfilling of 7.5% of wastes and increases the operating life of a landfill by 1.5 years in every 20 years.

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Heavy Metals in Compost

A study conducted by 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 [24]. Usage of such compost for agriculture of food crops introduces heavy metals from the waste stream into the food chain, which might lead to biologically harmful processes like bioaccumulation [21][22][23]. Figure 4 shows concentration of heavy metals Zinc (Zn), Copper (Cu), Cadmium (Cd), Lead (Pb), Nickel (Ni) and Chromium (Cr) in MSW composts from 29 cities. Majority of the samples do not comply with the standards for lead and chromium as can be observed by the median values being above the Indian quality control standards. Compost from only two cities out of twenty nine 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. 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.

Fig 4: Heavy metal concentration beyond quality control standards in MSW compost from 29 Indian cities, [24]