How do the 500 and 1,000 cubic metre a day dairy effluent biogas plants compare on gas, oil saved, cost and payback, and does the carbon claim add up?

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The 1,000 cubic metre a day dairy effluent plant makes five times the biogas of the 500 cubic metre plant in the Bureau of Energy Efficiency (BEE) Gujarat reports, for about 2.6 times the cost. Size follows how dirty the water is, not litres. Neither report shows the working for its carbon claim. Washing water from a dairy carries milk residue. Chemical oxygen demand (COD) is the usual measure of how dirty that water is: how much oxygen it would take to break the waste down, in milligrams for each litre. Today the plants blow air into big tanks with large motors, which uses power. A biogas digester does the opposite: it keeps air out and lets bacteria eat the waste, giving off a burnable gas that can fire the boiler in place of furnace oil, and it needs fewer motors. 500 cubic metre plant: water 20,800 litres an hour, about 500 cubic metres a day, at 2,000 milligrams of COD a litre. That is 998.4 kilograms of COD a day. 80% removed is 798.7 kilograms, and at 0.5 cubic metres of gas for each kilogram removed it makes 399.4 cubic metres of biogas a day. Oil replaced 70,560 a year. Power saved 40,500 kilowatt hours (kWh). Benefit 25.91 lakh rupees (Rs) a year, where a lakh is 100,000. Cost Rs 96.81 lakh, payback 3.74 years. 1,000 cubic metre plant: water 41,666 litres an hour, 1,000 cubic metres a day, at 5,000 COD. That is 5,000 kilograms of COD a day, 4,000 removed, and 2,000 cubic metres of biogas a day. Oil replaced 352,800 a year. Power saved 54,000 kWh. Benefit Rs 105.82 lakh. Cost Rs 247.56 lakh, payback 2.34 years. Oil is valued at Rs 33 a litre in the 500 report and Rs 29 in the 1,000 report, so the small plant is not penalised on price. Power is Rs 6.49 a kWh in both. The 500 report says kilograms throughout and adds a note that 1 kg equals 1 litre, while pricing the oil per litre. The 1,000 report switches from kilograms to litres. The sums divide by 10,000 kilocalories a litre, so it is litres. That is our reading. The cost per cubic metre of water treated is lower in the small plant: Rs 96.81 lakh / 500 = Rs 0.194 lakh, against Rs 247.56 lakh / 1,000 = Rs 0.248 lakh. But the cost per cubic metre of gas a day is half in the large one: Rs 0.242 lakh against Rs 0.124 lakh. The reason is strength. The large plant treats water at 5,000 milligrams of COD a litre, two and a half times the 2,000 of the small one, on twice the flow. That is five times the COD, so five times the gas. Both reports note milk plant effluent COD in a range of 1,500 to 5,000 milligrams a litre. So the answer to 'which size' is not litres a day. It is kilograms of COD a day, from a sample of your own water. The 500 report says 740 tonnes of carbon dioxide (CO2) a year, 34 from power and 706 from fuel. The 1,000 report says 3,535 tonnes, 46 from power and 3,490 from fuel. The power parts are about 0.85 kilograms per kWh, which is plausible. The fuel parts are harder to follow. The report words its fuel line as '706 tonnes reduction of consumption of fuel and amount of bio gas produced' (and 3490 likewise), so the fuel line also counts the biogas produced. The report gives no working, so the figure cannot be rebuilt from oil alone. On the oil alone, 706 tonnes on 70,560 litres is 10 kilograms a litre, and burning a litre of furnace oil gives about 3. At the 2.97 kilograms a litre Senseible uses for furnace oil and 0.675 per kWh of grid power, the 500 plant saves 70,560 x 2.97 = 209.6 tonnes plus 40,500 x 0.675 = 27.3 tonnes, so 236.9 tonnes. The 1,000 plant saves 352,800 x 2.97 = 1,047.8 tonnes plus 36.5, so 1,084 tonnes. Our reading, not stated in either report: 706 is close to 70,560 litres x 10,000 kilocalories / 1,000,000, which is the oil's heat in millions of kilocalories, read as tonnes. For the larger plant the same sum gives 3,528 against the printed 3,490, close but not equal. We cannot confirm the cause, and the biogas part of the wording may explain some of the gap. Do not copy either figure into a report. Burning biogas gives CO2 of plant origin, which is reported separately. Methane that leaks from the digester or the gas holder is a direct emission of the plant. Its design section still says 'Total wastewater generation: 1,000 cubic metres a day' and 'Bio-Gas Generation: 2,000 cubic metres a day', carried over from the larger report, although its own sentence on flow says 500 and its savings use 399.4 cubic metres. Use the savings table, and ask the builder to put the design basis in writing for your flow and COD.

How do the 500 and 1,000 cubic metre a day dairy effluent biogas plants compare on gas, oil saved, cost and payback, and does the carbon claim add up?

What the project is, in plain words

The two reports side by side

Why the larger plant pays back sooner

The carbon claim does not match the oil figures

One leftover in the 500 report

The 500 cubic metre plant, line by line

What Senseible does here