18-Env-B6 Agricultural Waste Management · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, December 2016 — 04-Env-B6, Agricultural Waste Management (3 hours, open book, all questions to be attempted, 100 marks total).
Reference texts: Rynk et al., On-Farm Composting Handbook (NRAES-54); OMAFRA, Nutrient Management Act, 2002 and O. Reg. 267/03 / Nutrient Management Protocol (NMAN); Metcalf & Eddy, Wastewater Engineering (anaerobic digestion chapter); ASABE Standards (manure storage, land application equipment).
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 rather than a single material, because each class of organic input brings a different combination of biogas yield, availability and handling difficulty.
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.
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.
Off-farm organic wastes (food processing waste, fats/oils/grease, out-of-date food).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), and receiving off-farm material makes the input a regulated Non-Agricultural Source Material (NASM) under Ontario's Nutrient Management Act, adding approvals and testing requirements.
Other possible uses of these materials as byproducts (outside digestion): manure and crop residues can instead be composted (Question 2) for a stabilized soil amendment; separated fibre from manure/digestate can be used as livestock bedding; straw and crop residue can be used directly as bedding or mulch; and FOG/food waste can sometimes be rendered or composted instead of digested. Choosing digestion over these alternatives is primarily an economic decision driven by the value of the biogas (heat/electricity/renewable natural gas) versus the value of the alternative use.
