Question 3 of 7: Particulate air control, ammonia toxicity, and buffer strips

Question 3: Particulate air control, ammonia toxicity, and buffer strips (20 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.

(i) Engineered particulate control methods

Electrostatic precipitator (ESP). Design principle: particles passing between high-voltage discharge electrodes and grounded collection plates acquire a negative charge by field/diffusion charging and migrate under the electric field to the grounded plate, where they are collected and periodically rapped off into a hopper. Best-used where very high collection efficiency (>99%) is needed on a large, continuous, relatively dry flue-gas stream with a favourable particle resistivity — the classic application is coal- or biomass-fired power-plant flue gas, where the huge gas volume makes the ESP's low pressure drop (versus a baghouse) a major operating-cost advantage.

Wet (venturi) scrubber. Design principle: the gas stream is accelerated through a converging–diverging throat where atomized scrubbing liquid is injected; the high relative velocity between gas and droplets drives inertial impaction of particles onto the droplets, which are then removed in a downstream cyclonic or mist-eliminator section. Best-used where the gas stream is hot, sticky, or explosive (so a fabric filter would blind or ignite) and where simultaneous gas-phase pollutant absorption is wanted — e.g. a metallurgical roaster or incinerator off-gas carrying both fine particulate and acid gases (SO₂, HCl).

(ii) Acutely toxic ammonia discharges: causes and treatment

Cause 1 — incomplete nitrification (process upset). Cold temperature, low dissolved oxygen, insufficient SRT, or a toxic shock load (e.g. an industrial slug) can suppress the slow-growing nitrifying bacteria (Nitrosomonas/Nitrobacter), so influent ammonia passes through largely unoxidized. The un-ionized fraction of ammonia (NH₃, which rises sharply with pH and temperature) is directly toxic to fish gill tissue, causing acute mortality even at effluent total-ammonia concentrations that look modest. Treatment: upgrade/restore biological nitrification — increase aeration DO setpoint and SRT (longer than the nitrifier washout SRT at the plant's minimum winter temperature), or add a dedicated nitrifying trickling filter/MBBR polishing stage downstream of the secondary process. Design parameters: minimum aerobic SRT sized to the nitrifier growth rate at the coldest design temperature; DO setpoint (typically ≥2 mg/L) sized to avoid oxygen-limited nitrification kinetics. Operational issues: maintaining adequate DO and SRT through wet-weather flow surges that dilute MLSS and shorten SRT; tracking and rapidly responding to industrial slug loads that could inhibit nitrifiers. Maintenance issues: aeration diffuser fouling/efficiency loss over time reducing achievable DO; regular ammonia (and pH/temperature) monitoring to catch nitrification upset before it reaches the receiving water.

Cause 2 — pH excursion at the outfall (un-ionized fraction spike). Even with good ammonia removal, a pH excursion (e.g. from an industrial batch discharge or algal photosynthesis raising receiving-water pH) shifts the ammonia equilibrium $\text{NH}_4^+ \rightleftharpoons \text{NH}_3+\text{H}^+$ toward the un-ionized, toxic NH₃ form, so a total-ammonia concentration that was previously safe becomes acutely lethal without any change in the mass loading. Treatment: pH control/neutralization ahead of discharge (acid or CO₂ dosing to bring pH back toward neutral, or an equalization basin to dampen pH swings from batch industrial contributors) combined with source control (industrial pretreatment bylaws limiting pH of trade-waste discharges). Design parameters: equalization basin volume sized to the batch discharge duration/pH excursion magnitude; neutralization reagent dose rate sized to the worst-case pH swing. Operational issues: continuous pH monitoring with an automated dosing feedback loop, since manual dosing cannot react fast enough to a batch slug; coordination with upstream industrial dischargers on batch-release timing. Maintenance issues: pH probe calibration/fouling (a drifted probe silently defeats the control loop); reagent feed-pump and storage-tank upkeep.

(iii) Buffer strips on crop land

Benefits: (1) vegetated buffer strips physically filter and slow overland runoff, trapping sediment-bound phosphorus and particulate nitrogen before it reaches the water course, and the root zone takes up dissolved nutrients, measurably reducing nutrient loading and downstream eutrophication risk; (2) they provide co-benefits of streambank stabilization (root reinforcement reduces erosion) and riparian wildlife/fish habitat (shade lowers stream temperature, supports insect drift feeding fish).

Costs: (1) the strip removes productive land from cultivation, a direct opportunity cost to the farm operation that scales with strip width and field size, and may require compensation programs (e.g. CCME/provincial agri-environmental cost-share programs) to secure landowner uptake; (2) buffer strips are only partially effective for dissolved nutrients (especially nitrate, which moves with subsurface tile drainage rather than surface runoff and largely bypasses a surface buffer), and they require ongoing maintenance (mowing/weed control, avoiding trafficking during wet periods) to remain effective, which is an operating cost often under-budgeted relative to the strip's up-front establishment cost.