Process Differences by Feedstock type

Choosing a feedstock for a Compressed Biogas (CBG) plant determines what pre-treatment equipment you must buy, what operational risks you manage daily, and which process steps require the most attention. This table compares five feedstock categories on the challenge that most limits their digestion efficiency and the specific process step that addresses it.

Agro-waste — crop residue like paddy straw, corn stalks, and sugarcane tops — contains high concentrations of lignin, a plant structural compound that bacterial enzymes cannot easily break down. Without pre-treatment, much of the carbon in agro-waste passes through undigested. The solution is mechanical shredding (to increase surface area) combined sometimes with steam explosion (high-pressure steam treatment that physically disrupts the lignocellulosic structure). Animal waste is already highly digestible but produces relatively low gas per tonne and carries fine grit from feed contamination — the unique step is a dedicated sand trap or slurry mixing tank to settle out grit before it reaches the reactor and damages pumps.

MSW (Municipal Solid Waste) is the most contamination-prone feedstock — plastics, glass, batteries, and other non-organic materials must be removed before digestion. Heavy mechanical sorting using screens, trommels, and magnetic separators is mandatory, and CPCB authorisation specifies minimum sorting standards. Industrial effluents (food processing waste, distillery slops) are easy to pump but often arrive with high acidity from fermentation metabolites in the source process — dilution and pH adjustment with lime or sodium bicarbonate is the critical pre-treatment. Energy crops — Napier grass, maize silage — produce the highest gas yield per tonne but face a fundamental supply challenge: they grow seasonally and must be ensiled during off-season months to provide year-round feedstock.