Biogas or LPG: The Slurry, the Water and the Winter
A family of six in a Terai village keeps three cattle that produce about 30 kg of dung a day. They currently buy one LPG cylinder of Rs 1,900 every 45 days. A 2 cubic metre biogas plant has been quoted at Rs 60,000, of which the government subsidy covers Rs 20,000. Feeding the plant needs the daily dung mixed with roughly the same volume of water, about 30 litres, and the household tap runs for only about 30 minutes a day in the dry season. The dung is presently spread on the family's vegetable plot as farmyard manure, and the grandmother argues that the plant will 'eat' the fertiliser the plot depends on. The family has Rs 40,000 in savings, earns a monthly income of about Rs 22,000, and must decide within a week, before the subsidy list closes.
1. Work out the simple payback period on the plant and say whether the money argument alone justifies going ahead.
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LPG use per year: 365 ÷ 45 ≈ 8.1 cylinders, at Rs 1,900 each ≈ Rs 15,400 a year. (Check: 8.1 × 1,900 = 15,390.)
Cost to the family: Rs 60,000 − Rs 20,000 subsidy = Rs 40,000, which their savings exactly cover, so no borrowing and no interest.
Simple payback: 40,000 ÷ 15,400 ≈ 2.6 years, that is about two years and seven months. After that the cooking gas is effectively free, apart from the daily labour of feeding the plant.
On money alone the case is strong: a well-run plant commonly lasts 15 to 20 years, so the family would recover the cost several times over. The honest caveat is that a simple payback ignores two things - the value of the labour (about 15 minutes a day) and the risk that the plant underperforms, which is exactly what the next two questions examine.
2. Evaluate the two objections: the dry-season water shortage and the grandmother's fertiliser argument.
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Water. A 2 m³ plant fed 30 kg of dung needs roughly 30 litres of water a day, on top of drinking and household use, and a tap that runs 30 minutes a day may not supply it in Jestha. This is a design problem with known fixes: harvest roof rainwater in the monsoon into a tank, store water on the days the tap runs, or reduce and split the feed in the driest weeks - a plant fed less produces less gas but is not ruined. The objection is real but solvable, and it should be solved before construction, not after.
Fertiliser. This argument is largely mistaken, and it is worth saying so politely. What comes out of a biogas plant is bioslurry, not waste: the digestion removes little of the nitrogen, phosphorus and potassium, makes them more available to plants, and kills many weed seeds and pathogens. Raw dung spread on a plot loses nitrogen to the air; slurry that is stored covered and applied to the field usually gives a better yield than the same dung used raw. So the plot does not lose its manure - the family gains gas and keeps, arguably improves, the fertiliser, provided the slurry is handled properly.
3. The plant is installed, but output falls sharply in December and January. Explain scientifically why, and give two practical remedies.
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The methane-forming bacteria in the digester are anaerobic and work best at about 30–35 °C. In winter the slurry temperature in a shallow Terai digester can fall towards 15 °C, and below roughly 20 °C the bacteria slow down sharply, so the same 30 kg of dung yields noticeably less gas - the feed has not changed, the rate of digestion has. Cold also delays the breakdown of the feed, so undigested material accumulates and can turn the slurry acidic, which suppresses gas production further.
Remedies:
- Keep the heat in: bury or earth-mound the plant deeper, cover the dome with a straw or compost layer, or build a simple shed or plastic tunnel over it so the sun's heat is trapped.
- Warm the input: mix the dung with warm water and let the slurry stand in the sun before feeding, and avoid feeding very cold water straight from the tap in the morning.
- Do not overload a cold plant: keep the daily feed steady rather than increasing it to chase the lost gas, and stir or recirculate the slurry so fresh dung does not sit in a cold layer. In the coldest weeks the family should keep a cylinder as backup rather than be surprised.