Product

Minimum 90.0% methane by volume; CO2 no more than 4%; CO2 plus N2 plus O2 combined no more than 10%; oxygen no more than 0.5%; total sulphur including H2S no more than 20 mg per cubic metre; moisture no more than 5 mg per cubic metre; free of liquids and particulates; odorised per IS 15319.

Water scrubbing is robust and impurity-tolerant but water-intensive; pressure swing adsorption is dry, compact and efficient but sensitive to inlet quality; membranes are compact and chemical-free and now dominate new Indian plants; amine scrubbing gives the highest purity but carries a thermal operating cost. Historically the global installed split ran roughly 35% water scrubbing, 30% PSA and 23% amine.

Reported slip: amine about 0.05%, membrane about 0.5%, water scrubber about 1.5%, PSA about 1.8-2%. Single-stage membranes can slip more unless multi-staged. Slip is lost revenue and a worse carbon profile at the same time, which is why it belongs in the contract.

PSA delivers roughly 96-98% recovery at 96-99% purity; amine can exceed 99.5% purity at near-zero slip; modern multi-stage membrane and VPSA systems recover 96-99%. Always check a vendor guarantee against your actual gas composition rather than a reference case.

Electricity for CO2 separation runs roughly 0.18-0.30 kWh per cubic metre of raw biogas for PSA, 0.25-0.35 for water scrubbing, 0.2-0.4 for membranes and up to 0.4-0.6 for cryogenic routes. This parasitic load is a real cost line, not a rounding error.

Multi-stage reciprocating or screw compressors, typically four or five stages, raise pressure from about 1 bar to 200-250 bar, with intercooling after each stage to remove heat and condensed moisture. Compression alone consumes about 0.15-0.20 kWh per kg of CBG.

The Wobbe index measures how interchangeable two gases are in a burner. To inject into a CGD or transmission network the gas must match the network's Wobbe and dew-point specification as well as IS 16087 - PNGRB injection rules require hydrocarbon and water dew points no higher than 0 degrees C at pipeline pressure, and entry temperature no higher than 55 degrees C.

Oxygen and nitrogen limits exist because air ingress - during air-dosing desulphurisation, or through membrane operation - dilutes methane and creates both a safety and a pipeline-quality problem. IS 16087 caps oxygen at 0.5%, so air dosing has to be metered precisely rather than set and forgotten.

Cascade delivery trucks compressed gas to a retail outlet and attracts a higher compression charge - 8 rupees per kg to a retail outlet against 2 rupees per kg for pipeline injection. On the ground most producers still prefer cascade and retail sales, because pipeline injection is not yet widely implemented.

Roughly half of raw biogas is carbon dioxide. Captured rather than vented, and then purified and liquefied, it becomes a saleable food-grade or industrial product that displaces fossil-derived CO2. About 0.5 tonnes of CO2 accompanies each tonne of CBG, and most of it is currently vented.

LBG is biomethane cooled to a liquid for higher energy density, useful for long-distance road and marine transport where pipelines do not reach. It is more capital- and energy-intensive than CBG and remains a niche route in India today.

FOM is solid fermented organic manure, LFOM is the liquid fraction, and PROM is phosphate-rich organic manure made by blending FOM with rock phosphate. All three are recognised categories eligible for Market Development Assistance at 1,500 rupees per tonne.

A screw press or decanter centrifuge splits digestate into a fibrous solid cake, which is dried and pelletised into FOM, and a nutrient-rich liquid that becomes LFOM. Separation is the step that turns a bulky effluent into two marketable products.

India amended the Fertiliser (Control) Order in 2025 to add Organic Carbon Enhancer from CBG plants under a new Schedule VIII, notifying FOM and LFOM specifications on 27 March 2025. Labels must state application 15-20 days before sowing.

The FCO requires total N plus P2O5 plus K2O of at least 1.2% with a minimum 14% organic carbon, so FOM is a low-analysis, carbon-rich soil conditioner rather than a concentrated NPK fertiliser. Its value is soil health and carbon - which is a benefit farmers have to be taught to price.

Under the FCO, FOM must show nil pathogens, pH between 6.5 and 8.4, conductivity no more than 4 dS/m, and heavy-metal caps including cadmium at 5, lead at 100, chromium at 300, nickel at 50 and mercury at 0.15 mg/kg. Feedstocks carrying pathogen or metal loads - some manures, sewage sludge - therefore need care at the input end.

FOM frequently misses the nitrogen and potassium minimums, because digestion conserves carbon but not always nutrients, and fibrous material fails the 4 mm sieve test at 70% moisture. Enrichment with N and K - moving toward PROM - fixes the nutrient gap; proper drying and screening fix the physical one.

Uncovered digestate keeps emitting methane and odour, losing carbon value and creating a fugitive-emissions problem that undermines the plant's own climate claim. Covered storage captures the residual gas, which is both extra yield and carbon integrity.

Applied at the right point in the crop cycle, FOM and LFOM nutrients are taken up rather than lost; applied badly they are wasted and can run off. Matching digestate output to the local cropping calendar is part of making it a real revenue line rather than a disposal cost.

Liquid digestate can be drip-fertigated onto the Napier crop that feeds the plant, recycling nutrients and cutting synthetic-fertiliser and water cost. It is central to Napier-based plant economics - and it depends on having leased land and water close enough to make the loop physical rather than notional.