Question 3 of 15: On-Farm Biogas Feedstocks — Five Inputs and Regulatory Issues

Question 3: On-Farm Biogas Feedstocks — Five Inputs and Regulatory Issues (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

On-farm anaerobic digesters are almost always fed a blend of feedstocks, because each class of organic input brings a different combination of biogas yield, availability, handling difficulty and regulatory burden. Five distinct inputs are described below.

1. Livestock manure (dairy/swine slurry, poultry litter). Pros: already collected and handled as a waste stream on the farm, provides continuous year-round supply, and its own microbial population helps seed and buffer the digester. Cons: moderate biogas yield per tonne (much of the readily digestible energy has already passed through the animal), and poultry litter with bedding can be high-solids and ammonia-rich enough to inhibit methanogens if not diluted. Regulatory issues: as an Agricultural Source Material (ASM) generated on the farm's own operation, manure feedstock is governed by the farm's existing Nutrient Management Strategy/Plan under O. Reg. 267/03 — no additional off-farm approvals are triggered by using it as digester feed.

2. Energy/silage crops and crop residues (corn silage, grass silage, straw). Pros: high volatile-solids content gives a high biogas yield per tonne, and co-digestion with manure balances an otherwise low C:N feedstock. Cons: requires dedicated cropping, harvest and storage (silage pit/bunker), competes with food/feed production for land, and lignified residues such as straw need mechanical or thermal pretreatment to digest effectively. Regulatory issues: generally minimal if grown on the farm's own land (still ASM); purchased silage/residue from another farm can still be treated as ASM under most provincial nutrient-management frameworks since it remains an agricultural-origin material, but volumes should be tracked against the digestate land-application capacity.

3. Off-farm organic wastes (food processing waste, fats/oils/grease, out-of-date food). Pros: very high biogas yield per tonne (especially FOG), and many generators pay a tipping fee to have it accepted, improving digester economics. Cons: variable and unpredictable quality, risk of physical contaminants (packaging, plastics) and inhibitory substances (high salt, antibiotics, disinfectants). Regulatory issues: the largest of the five — receiving off-farm material makes the input a regulated Non-Agricultural Source Material (NASM) under Ontario's Nutrient Management Act, which requires a NASM plan, source characterization/testing, and category-dependent approvals (O. Reg. 267/03) before receipt and before the resulting digestate can be land-applied.

4. Municipal biosolids / septage. Pros: high, consistent volume from a nearby municipality, often with a tipping fee, and typically pre-characterized (municipalities routinely test biosolids). Cons: potential for pathogens, heavy metals and emerging contaminants (e.g., PFAS) to concentrate in digestate, requiring careful land-application planning; public perception/odour concerns near receiving farms. Regulatory issues: biosolids/septage are NASM Category 2 or 3 depending on treatment and metal content under O. Reg. 267/03 — the higher categories require more extensive testing, buffer distances and government notification than Category 1 NASM or straight ASM.

5. Fruit and vegetable culls / horticultural waste. Pros: readily biodegradable, high moisture content aids pumpability, often available at low or no cost from local packers/processors, seasonal supply can complement manure's steadier baseline. Cons: seasonal/intermittent availability requires either storage or a flexible feed blend; high in easily-fermentable sugars, which can cause rapid VFA (volatile fatty acid) accumulation and digester souring if fed too fast. Regulatory issues: off-farm horticultural waste is NASM (typically a lower-risk category than biosolids), so it still needs NASM plan approval, but the testing burden is usually lighter than for biosolids given the lower contaminant risk profile.