Question 5 of 16: Anaerobic Digester Influent vs. Effluent

Question 5: Anaerobic Digester Influent vs. Effluent (5 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.

Chemical differences. During digestion, microorganisms mineralize a large share of the organic nitrogen in the influent into ammonium-N (NH₄⁺-N); the effluent (digestate) therefore has a HIGHER proportion of its total N in the readily plant-available ammonium form than the raw influent, even though total N is roughly conserved. Volatile solids/organic carbon are destroyed (typically 40–60% VS destruction), most of that carbon leaving as biogas (CH₄ + CO₂); pH typically rises slightly due to the ammonia/bicarbonate buffering that builds up; and odour is markedly reduced because the readily-degradable, odour-causing organics have already been consumed. Phosphorus and potassium are essentially conserved (not volatilized), so their concentration per unit of remaining solids tends to rise slightly as carbon is lost.

Volume differences. If only manure is digested, influent and effluent liquid volumes are similar (a small mass loss to biogas, mostly gaseous carbon, not liquid volume); however, most on-farm digesters co-digest additional feedstocks (crop silage, off-farm organics, and often dilution/wash water), so the effluent digestate volume requiring storage and land application is usually substantially LARGER than the raw manure volume alone.

Nutrient management program changes. Because digestate N is more plant-available (higher NH₄⁺-N fraction) than raw manure of the same type, the available-N factor used to plan application rates should be revised upward relative to Table 4's raw-manure factors, and a fresh lab analysis of the digestate should replace the raw-manure typical values (Table 3) since digestion changes the manure's own composition. The higher ammonium content also raises the ammonia-volatilization risk if digestate is surface-applied without prompt incorporation, so incorporation timing becomes more critical. Finally, if off-farm feedstocks were co-digested, the extra imported nutrients and NASM regulatory classification must be folded into the farm's Nutrient Management Strategy, since the land base now needs to absorb a larger nutrient load than the farm's own manure alone would produce.