Question 1 of 5: General

Question 1: General (25 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.

a) Why pH governs ammonia toxicity (3 marks). Total ammonia nitrogen in water exists in equilibrium between two forms — the un-ionized species NH₃ and the ionized ammonium ion NH₄⁺ — and it is specifically the un-ionized NH₃ that is acutely toxic to fish and other aquatic life, because its lack of charge lets it diffuse freely across gill membranes, whereas the charged NH₄⁺ ion cannot cross the membrane nearly as readily. The equilibrium NH₄⁺ ⇌ NH₃ + H⁺ is pH-dependent: as pH rises, the equilibrium is pushed toward the un-ionized NH₃ form (fewer H⁺ ions available to combine with NH₃), so the SAME total ammonia concentration becomes dramatically more toxic at high pH than at low pH — typically the un-ionized fraction rises roughly ten-fold for every one-unit increase in pH near neutral conditions. This is why every regulatory ammonia criterion is expressed as a function of both pH and temperature rather than as a single fixed total-ammonia number, and why a discharge that is compliant at pH 7 can become acutely toxic if the receiving stream's pH drifts upward (e.g. from algal photosynthesis raising daytime pH).

b) Effect of decreasing temperature on activated sludge design (3 marks). Biological reaction rates follow an Arrhenius-type temperature dependence, so cooling the process slows both substrate-utilization and microbial growth rates. SRT: because nitrifier and heterotroph growth rates fall as temperature drops, a LONGER solids retention time is needed at low temperature to keep the biomass growing faster than it is wasted (avoiding washout) and to hit the same effluent quality — winter design SRT is always set higher than the summer minimum, with a safety factor applied at the coldest expected temperature. Aeration requirement: a longer SRT together with slower kinetics generally increases the required aeration BASIN VOLUME (more biomass inventory needed for the same conversion), while oxygen SATURATION concentration rises as water cools (cold water holds more dissolved oxygen), partially offsetting the slower oxygen-transfer kinetics; net effect is usually a larger blower/aeration system sized for the winter condition. Sludge production: slower endogenous decay at low temperature means proportionally MORE of the synthesized biomass survives to be wasted (less is oxidized away), so net sludge production per unit of BOD removed actually INCREASES in cold weather even though growth itself is slower — a counter-intuitive but well-documented design consideration.

c) Aerobic vs. anaerobic digestion of animal manure (3 marks). Aerobic digestion advantages: simpler, more stable operation with fewer upset risks; produces a well-stabilized, low-odour, pathogen-reduced product without the need for gas-handling infrastructure; effective at moderate ambient temperatures without external heating. Disadvantages: high parasitic energy cost for continuous aeration; captures none of the manure's chemical energy as usable biogas; typically achieves lower volatile-solids destruction than a well-run anaerobic system. Anaerobic digestion advantages: recovers manure's energy content as combustible biogas (heat/electricity/vehicle fuel); no aeration energy cost; produces a nutrient-rich, largely odour-reduced digestate suitable for land application. Disadvantages: requires heated, sealed, mixed tankage with a materially higher capital cost; the process is more sensitive to toxic upset (ammonia inhibition, pH crash, temperature swings) and needs closer operator attention; biogas handling introduces an explosion/safety-management burden absent from aerobic systems.

d) Rationale for velocity limits when pumping liquid manure slurry (3 marks). A minimum velocity (typically on the order of 1–1.5 m/s for a manure slurry) is imposed so that the coarser solids fraction remains suspended in the flow rather than settling out and accumulating inside the pipe — a slurry moving too slowly lets grit, bedding fibre and undigested solids drop out of suspension, progressively narrowing the effective pipe bore until the line plugs entirely. A maximum velocity (typically 2–3 m/s) is imposed to limit pipe wall erosion/abrasion from the suspended solids, to keep frictional head loss (and therefore pumping energy cost and required pump horsepower) within a practical range, and to avoid excessive turbulence/agitation that can generate foam or release odorous gases at the discharge point. Together the two limits define a practical operating "window" — too slow risks blockage, too fast wastes energy and wears the system — and this is the same design rationale applied quantitatively to size the pump discharge line in Question 4(2) below.

e) Concerns with land-applying animal manure (3 marks). (i) Nutrient over-application and runoff/leaching — applying manure at a rate exceeding crop uptake leaves surplus nitrogen (vulnerable to nitrate leaching to groundwater) and phosphorus (which builds up in soil and can be transported by erosion/runoff to surface water, driving eutrophication and algal blooms). (ii) Pathogen transmission — raw or under-stabilized manure carries zoonotic bacteria (E. coli, Salmonella) and parasites that can contaminate edible crops, groundwater wells, or surface water if setback distances/timing restrictions before harvest are not respected. (iii) Odour and air emissions — land application (especially surface broadcast rather than injection) releases ammonia and odorous volatile compounds that affect neighbouring residents and contribute to regional nitrogen deposition. A fourth concern worth naming is soil structure/compaction and salt or heavy-metal (e.g. Zn, Cu from feed additives) accumulation from long-term repeated application on the same fields.

f) Integrated manure management system (3 marks). Collection/housing. Manure and urine are scraped, flushed, or drop through slatted floors into a collection pit at the barn; function is to remove waste from the animal environment for hygiene and worker safety. Contamination concern: odour and pathogen exposure to workers/animals if collection is infrequent, and any leakage from the collection pit itself. Storage. An engineered tank or lagoon holds manure between collection and the next window when land application is agronomically/weather appropriate (e.g. not on frozen or saturated ground); function is to decouple the year-round production rate from the seasonal application window. Contamination concern: ammonia volatilization from an open storage surface, and structural failure/overflow releasing manure directly to surface water. Treatment (digestion or composting). Anaerobic digestion or composting stabilizes the organic matter, reduces pathogens and odour potential, and (for digestion) recovers energy as biogas; function is volume/mass reduction and product stabilization ahead of land application. Contamination concern: fugitive biogas/methane emissions, and odorous compounds released during compost turning. Land application/utilization. The stabilized manure or digestate is applied to cropland as fertilizer, substituting for synthetic nutrients; function is beneficial nutrient recycling back to agricultural production. Contamination concern: nutrient runoff/leaching and odour at application, as detailed in part (e) above.

g) Odour compounds from anaerobic digestion of swine manure (3 marks). (i) Hydrogen sulfide (H₂S) — produced by sulfate-reducing bacteria under anaerobic conditions; mitigated by biogas scrubbing (iron-sponge or biological H₂S scrubbers) before combustion, and by enclosing/covering any open transfer or storage points so the gas is captured rather than released to ambient air. (ii) Volatile fatty acids (VFAs) (e.g. acetic, butyric, valeric acid) — intermediate anaerobic-digestion products with strong, sour/rancid odours; mitigated by maintaining stable digester operation (adequate HRT, no organic overload) so VFAs are fully converted to methane rather than accumulating, and by covering/enclosing the digestate storage that follows the digester. (iii) Ammonia (NH₃) — released from the digestate's elevated ammonium-nitrogen content, especially at the digestate's typically higher pH; mitigated by subsurface injection rather than surface broadcast at land application, and by covering post-digestion storage to limit the air-liquid contact area available for volatilization.

h) On-farm deadstock disposal for a large poultry operation (4 marks). Approach 1 — on-farm composting. Environmental risks: (1) leachate generation if the compost pad lacks an impermeable base/collection system, (2) incomplete pathogen kill if the pile fails to reach/sustain thermophilic temperature throughout, (3) odour and vector (rodent/scavenger) attraction if the carcass is not fully covered by carbon-rich bulking material. Mitigation: site the composting pad on an impermeable pad with leachate collection, and use an adequate carbon:nitrogen bulking-agent cover with regular temperature monitoring to confirm the thermophilic kill window is met. Approach 2 — on-farm incineration. Environmental risks: (1) air emissions (particulates, odour, and dioxins/furans if combustion is incomplete), (2) ash containing concentrated metals or, if under-combusted, residual pathogens, (3) risk of an air-permit exceedance during high-throughput mortality events (e.g. a disease outbreak). Mitigation: operate with a secondary/afterburner chamber at sufficient temperature and residence time to destroy organic combustion by-products, and dispose of ash only through an approved landfill or permitted land-application pathway.