Biogas production in rural communities in India involves either of two common anaerobic digesters prevalent for biogas production from cattle manure in India: the "Khadi (Hindi: homespun) & Village Industries Commission (KVIC) Floating Dome" model and the "Janata (Hindi: people) Fixed Dome" model . These digesters are relatively cheap, locally built and quite easy to construct and operate. The capital costs, operation costs and maintenance costs of a 15 m3 capacity digester were estimated at $1910, $746 per annum and $92 per annum respectively in 2004 . This report analyzes the viability of biogas produced from a Janata model as a locally sustainable energy source for rural communities in a broad overview.
|Fig 1: Anaerobic process scheme (after ).|
The basic process of biogas production is as shwon in Fig. 1. Biogas is produced by methane producing micro-organisms which digest cellulose and other organic content in animal and plant wastes in a continuous process that occurs at suitable temperatures of 28-35o C or 50-70o C . Biogas contains roughly 50-70% CH4 and 30-50% CO2  and is has a generally accepted mean calorific value of about 20 MJ/m3. [3-5] It also has a fixed Hydraulic Retention Time or Retention Time(HRT), which is the amount of time the organic matter requires to be sufficiently digested so as to remove most pathogen content and odors. In the case of the Janata Model digester, shown in Fig. 2, the retention time using cattle manure has been estimated at 55 days.  It becomes obvious, therefore, that the amount of gas produced is directly dependent on the size and capacity of the tank and the amount of feed material it can digest.
|Fig 2: Schematic of a Janata model anaerobic digester of 6 m3/day capacity (after ).|
The average yield of Biogas in m3 per m3 capacity of digester for a small 6 m3 digester in tropical Indian conditions has been estimated at 0.35 m3 at STP for a daily slurry feed of 100 kg (50% wet dung + 50% water) by Pal et al. in their 1986 study in Lucknow in India.  It is not clear whether the percentages are by weight or by volume and we will assume they are mixing by weighed proportion.
To check whether these numbers are realistically feasible as an alternative source of energy, we must quantify how many cattle are needed to produce the amount of dung needed to feed the digester continuously. Furthermore, we need to estimate how many people will use the energy produced by the digester. The key assumption here is that only people who own the cows will use the energy.
The estimated dung production for cattle is 4.5 kg/head/day according to Ravindranath, Somashekar et al, of which 60% is recoverable . The number of cattle per person in rural households in India, like in any other place, depends on the fertility of the grazing lands and the wealth of the village inhabitants. We can make an educated guess at the number as between 2 and 3 per household as suggested by selective studies .
Some elementary calculation follows, to obtain the number of Joules per person available from Biogas produced in these the Janata model digesters.
Volume of biogas produced per day by the 6 m3 digester = 3.5 m3/day (approximately corroborated by biogas yield per dry ton organic content suggested by Kaltschmitt, Thran and Smith. )
Weight of Slurry fed per day: 100 kg/day
Weight of wet manure (assumed to be 50% of slurry by weight): 50 kg/day
Number of cattle required to produce 50 kg of manure daily: 50/(4.5 × 0.60) = 18.5 or 19 Cattle (assuming 60% of dung is collected)
Number of households whose cattle will be involved: 19/2.5 = 7.6 households
Assumed number of people per household, considering India has quite a sizeable population: 5 people
Number of people involved: 38 people
Assumed calorific value of biogas: 20 MJ/m3 [3-5]
Joules produced by digester per day: 70 MJ/day
Joules available per household per day: 9.211 MJ/day or 9,211 kJ/day
Joules available per person per day: 1.842 MJ/day (1,842 kJ/day)
To put our calculated results in perspective, let us convert the energy available per household to common units of electricity, the kWh. 9,211MJ/day translates to 2.559 kWh, which is about the electrical energy used to power a 100 W incandescent lamp for 25.59 hours (25 hours, 35 minutes and 24 seconds) or a 1000 W Microwave oven for 2.56 hours (2 hours, 33 minutes and 36 seconds).
This is a significant amount of energy generated from cattle manure alone, without completely sacrificing its subsequent use as an organic fertilizer, a crucial ingredient in agricultural economies. [1,3,8] Biogas, it is apparent, is a valuable source of energy. Moreover, assuming a 15 m3/day digester operates for 320 days a year and lasts 10 years, we can estimate costs per MJ as $0.002 in capital costs and $0.008 in operation and maintenance costs based on the cost data suggested by Purohit and Kandpal.  Thus, we arrive at a total cost of $0.01/MJ for our hypothetical digester in 2004 currency value used by them. 
Let us compare this to the cost of electricity, which was suggested by Purohit and Kandpal as INR 4/kWh ($0.091/kWh) or INR 1.11/MJ ($0.025/MJ) in their suggested 2004 currency values. 
As can be seen from the comparisons made above, biogas is a valuable energy source for relatively underdeveloped regions of the world which is not only quite significant in quantity but also available with existing standard technology at competitive costs of production under suitable climatic conditions.
© Tejo Vihas Pydipati, 2010. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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