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, the author 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. These units divert about 40 tons of waste from landfills, which is 7% of the organic waste generated in both cities combined. It also implies avoidance of 4.7% 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
The units are smaller in size, flexible with feed and operation when compared to its counterparts. 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. 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 Thiruvanathapuram and Kochi is 0.17 kg/day and 0.38 kg/day respectively. A single household in Thiruvanathapuram 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.
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 will be helpful in solving MSWM crisis in India sustainably but it takes many single units to address organic waste from a single community. Also, the technology would be able to address only 51% of the waste stream in Thiruvanathapuram or Kochi. The public investment into the technology is comparatively much higher (Table 1). Also, 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
Despite the huge difference in total costs which is because of the difference in scale of the technologies compared, 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.
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