Small scale Biomethanation (Biogas) in India

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Anaerobic digestion (AD) of kitchen waste to produce biogas and liquid slurry on a small scale has been very successful in India, especially South India, where the region’s temperate weather conditions favor the process yearlong.  Many households have such biogas units installed. Total number of units installed in cities is unknown as there are too many companies offering them and the units being installed in both urban and rural areas, while the numbers are not necessarily recorded. In order to have a closer look at this technology, I identified a private company called Biotech with its office in Thiruvananthapuram, Kerala as a case study for small scale biogas. This company alone installed twenty thousand (20,000) units of small scale biogas in Thiruvanathapuram and Kochi, combined. Biotech can be considered a more successful company in this area, which might help gauging number of units installed by others. These units divert about 40 tons of waste from landfills, which is 3.5% of the organic waste generated in both cities together. It also implies avoidance of 2.6% of collection and transportation costs and resulting GHG emissions.

A small scale (2 kg per day) Biogas unit at Biotech's office in Thiruvananthapuram

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Capacity and Cost

Each unit can handle kitchen waste from a household with 3 – 5 members and can generate one cubic meter of biogas every day. Biogas mainly constitutes methane and carbon dioxide and the unit can be connected directly to a cooking stove. Per capita organic waste generation in Thiruvananthapuram and Kochi is 0.17 kg/day and 0.38 kg/day respectively. A single household in Thiruvananthapuram and Kochi produce 0.51 – 0.85 kg/day and 1.14 – 1.9 kg/day respectively (depending on the number of persons in the house). Thus, the capacity of these biogas units is enough for households in these two cities and each unit occupies only 1.25 sq.m of space. They cost $ 470 (INR 21,000) per unit and almost half of this cost is subsidized in different ways. The remaining cost of the digester is paid back in approximately 3 years in the form of savings on cooking fuel. 

The technology was successfully scaled-up by the company to handle 300 kg of organic wastes every day. Space required per kg of waste treated increases with the scale due to increase in the number of single-units used and piping involved. More than 235 institutional units were installed at different hotels and canteens, hospitals, schools, markets and slaughter houses. These institutions use a generator to convert the biogas into electricity which in turn is generally used for street lighting. 1 cu.m of gas can produce 1.5 KW of electricity.

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Comparison

This decentralized technology is the most environmentally friendly technology to recover energy from organic wastes. It can be successfully deployed in South India where the temperatures favor the process yearlong. Biotech company is researching ways to introduce this into other regions of India which are colder. However looking at the public investment and integrated waste management perspective, it takes many such single units to address organic waste from a single community and the technology would be able to address only 51% of the waste stream in Thiruvanathapuram or Kochi. Also, the public investment into the technology is comparatively much higher (Table 1) and the units produce organic slurry which needs to be properly utilized. Table 1 is a comparison between small scale biogas and WTE incineration as waste to energy solutions to the MSWM crisis in Chennai. The values used in these calculations are generation of 6,464 TPD of MSW (in year 2005), organic waste percentage of 41% and calorific value of 10.9 MJ/kg.

Table: Comparison of small scale biogas and WTE incineration as options for MSWM for Chennai (cost in $)

	Small Scale Biogas	WTE Incineration	Comparison
Capital cost*	623 million	241 million
Operational +transportation cost* (20 yrs)	Negligible	243 million
Total expenses to society	623 million Note: Present Value	484 million Note: Future Value	0.77
Landfilling avoided (%)	41%	90%	2.1
Electric energy produced (MWh/day/ton)	0.75	0.76
Total energy produced in 20 yrs (MWh)	26 million	64 million	2.5
Pollution from transportation avoided	41%	0	41

*Costs calculated for the society as a whole

The difference in total costs is because of the difference in scale of the technologies compared and the difference in total energy produced is because the feed for small scale biogas is only organic waste whereas feed for WTE includes rest of the MSW fraction too which are an extra 40 - 50% and have higher calorific value. Despite these differences, small scale anaerobic digestion would (is more likely) be the most sustainable way to treat source separated organic wastes considering the avoidance of emissions from transportation. Since anaerobic digestion works only for source separated organics as is the case with small scale biogas plants, it is not at all an option for mixed solid wastes. As source separation is not practiced in India, it is difficult to collect separated organic wastes on a large scale. That also explains why large scale biomethanation which could have been an option otherwise is not a part of this report.