Why feedstock decides everything

Feedstock sets how much organic matter enters the plant, how much methane it can make, how stable the biology stays, and how much saleable gas leaves the gate.

That is why a design basis like “200 tonnes of feedstock per day” can mislead. Two loads that weigh the same on a weighbridge can differ widely in moisture, Total Solids (TS), Volatile Solids (VS), C:N ratio and methane potential.

What is a biogas feedstock?

A feedstock is any organic raw material fed to an anaerobic digester for conversion into biogas. Inside the sealed, oxygen-free tank, microbes break it down in four stages.

Each feedstock moves through these stages differently. Food waste breaks down fast and can turn the digester acidic. Straw and grasses break down slowly because their lignocellulose resists hydrolysis. Manure brings buffering capacity and active microbes but relatively little energy per tonne. Feedstock selection is about matching these behaviours to the plant, not just finding the biggest pile.

The main feedstock families

Most CBG plants draw on five broad groups, each with a distinct profile. Select a family to see its strengths, its watch-outs and where its members sit on solids content.

The four numbers that matter

Total Solids (TS) is what remains after water is removed. Volatile Solids (VS) is the organic part of those solids, the part microbes can actually use. Ash and grit add weight but no gas.

Methane potential is the methane a feedstock can yield per tonne of VS (Nm³ CH₄/t VS), measured by Biochemical Methane Potential (BMP) testing. A BMP value is a ceiling, not a guarantee: full-scale output also depends on digester technology, loading rate, retention time, temperature and operation.

C:N ratio balances energy (carbon) against microbial growth (nitrogen). Literature often cites about 20:1 to 30:1 as favourable, though the optimum depends on the feedstock, ammonia levels and operating conditions.

Why wet tonnes mislead: try it yourself

Two feedstocks can arrive at the same 100 tonnes per day and still deliver very different amounts of digestible matter. The calculator starts with the classic example: Feedstock A at 10% TS and Feedstock B at 30% TS, both with VS at 80% of TS.

The biggest feedstock challenges

Most underperforming biogas plants trace their problems back to feedstock, not equipment. The challenges fall into two groups: what happens inside the digester, and what happens before the material reaches it.

Inside the digester

How plants overcome these challenges

Co-digestion: blend, don’t rely on one feedstock

Co-digestion combines feedstocks so each covers the other’s weakness: moisture, C:N balance, buffering, loading and nutrients. Studies report better methane yield or stability for many combinations, but the benefit depends on the substrates, ratio, digester and operation. Mixing alone does not guarantee improvement.

Pretreatment: unlock tough biomass

The right method is the one whose extra gas outweighs its energy and operating cost.

Feed preparation and contaminant removal

Depackaging, screening, de-gritting and sand traps protect pumps and keep digester volume free. Source segregation of municipal waste is far cheaper than cleaning mixed waste at the plant.

Test before you design

Test representative samples, repeated across seasons and suppliers.

Monitor while you operate

Track pH, the VFA-to-alkalinity ratio, ammonia and gas composition. Change feed gradually, and keep trace-element supplements on hand for mono-digestion of energy crops or food waste.

Secure the supply chain

Run a commercial assessment alongside the technical one. It should cover a month-by-month availability profile, minimum assured daily quantity, storage and inventory days, storage losses, collection radius and transport cost.

Long-term supplier or farmer contracts, a backup feedstock, and proper silage or covered storage turn a seasonal resource into a year-round one. A slightly lower-yield feedstock with reliable supply often makes a stronger business than a high-yield one that arrives for only four months.

The key takeaway

Choosing a biogas feedstock is not about picking the highest number in a methane-yield table. It is about finding a feedstock, or a compatible blend, that delivers enough biodegradable organic matter to the digester, consistently and at a sustainable cost.

Get that right, and it sets the foundation for everything that follows: feed preparation, digester sizing, organic loading, gas yield, upgrading and compression capacity, and ultimately the saleable CBG the plant delivers.