Question 1 of 7: Engineered air pollution control methods, and nitrogen/phosphorus management in sewage effluent

Question 1: Engineered air pollution control methods, and nitrogen/phosphorus management in sewage effluent (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) Two engineered air pollution control methods for noxious gas emissions

Method 1 — wet chemical (packed-tower) absorption/scrubbing. The noxious gas stream (e.g. H₂S, SO₂, NH₃) is contacted counter-currently with a liquid, often a reactive alkaline or acidic solution, in a packed or tray column; the pollutant transfers across the gas–liquid interface driven by the Henry's-law concentration gradient and is captured (or neutralized) in the liquid phase. (a) Design principles: (1) size the packing height/type and liquid-to-gas (L/G) ratio to give sufficient interfacial area and contact time for the target removal efficiency (an NTU/HTU or number-of-theoretical-stages calculation, not just a rule-of-thumb column height); (2) select a scrubbing liquid chemistry (pH, oxidant, reagent) matched to the specific contaminant's solubility/reactivity, since a generic water scrub removes only highly soluble gases. (b) Operation/maintenance considerations: (1) continuous monitoring and trim-dosing of the scrubbing liquid pH/oxidant level as the pollutant load varies, with programmed liquor blowdown/makeup to prevent dissolved-solids buildup that would reduce absorption capacity; (2) periodic inspection and cleaning of the packing media for scaling, biological fouling or channelling, which silently reduces the effective interfacial area and removal efficiency long before it becomes visible in stack readings.

Method 2 — thermal oxidation (regenerative or recuperative). The gas stream is heated to a temperature (typically 700–900 °C) and held for a design residence time (≥0.5–1 s) sufficient to oxidize combustible noxious compounds (VOCs, odorous organics) to CO₂ and H₂O, destroying rather than transferring the pollutant. (a) Design principles: (1) size the combustion chamber temperature and residence time from the specific compound's auto-ignition/destruction kinetics (harder-to-oxidize compounds need higher T or longer τ), verified against trial-burn or published destruction-efficiency data; (2) incorporate heat recovery (recuperative or regenerative heat exchange) to preheat incoming gas with the hot exhaust, since sustaining combustion temperature on a dilute waste-gas stream is otherwise fuel-intensive. (b) Operation/maintenance considerations: (1) continuous temperature monitoring with an automatic feed-diversion interlock, because destruction efficiency collapses rapidly if chamber temperature drops below the validated setpoint; (2) periodic inspection/cleaning of heat-exchanger surfaces and burner nozzles, since fouling both reduces heat-recovery efficiency and can create localized cold zones where destruction efficiency falls locally below spec.

(ii) Nitrogen and phosphorus: environmental impacts, treatment methods, and beneficial recovery

Nitrogen (N). (a) Environmental impacts: (1) ammonia toxicity to fish and aquatic life at the un-ionized (NH₃) fraction, which rises sharply with pH and temperature; (2) nitrate loading contributing to eutrophication of nitrogen-limited receiving waters (typically estuarine/marine) and posing a methemoglobinemia (“blue-baby syndrome”) risk if it reaches drinking-water wells. (b) Treatment methods: (1) biological nitrification–denitrification (an aerobic zone converts NH₃–N to NO₃⁻, then an anoxic zone with an organic carbon source reduces NO₃⁻ to N₂ gas, which escapes to atmosphere); (2) breakpoint chlorination or air/steam stripping of ammonia as a physical-chemical alternative where biological treatment is impractical. (c) Beneficial recovery: struvite (magnesium ammonium phosphate, MgNH₄PO₄·6H₂O) crystallization recovers both ammonium and phosphate simultaneously from high-strength sidestreams (e.g. digester dewatering liquor) as a slow-release fertilizer product, while also preventing uncontrolled struvite scaling in plant piping.

Phosphorus (P). (a) Environmental impacts: (1) phosphorus is typically the limiting nutrient in freshwater lakes and rivers, so even small loadings drive algal blooms and eutrophication, leading to hypoxia/fish kills as the bloom decays; (2) some cyanobacterial (blue-green algae) blooms triggered by excess P produce toxins (microcystins) that threaten recreational use and downstream drinking-water intakes. (b) Treatment methods: (1) chemical precipitation with alum or ferric chloride, forming insoluble AlPO₄/FePO₄ that is removed by clarification/filtration; (2) enhanced biological phosphorus removal (EBPR), using an anaerobic–aerobic sequence so phosphorus-accumulating organisms take up phosphorus in excess of metabolic need (“luxury uptake”), removing it in the wasted biomass. (c) Beneficial recovery: the same struvite crystallization process recovers phosphorus as a marketable slow-release fertilizer, and where EBPR is used the P-enriched waste activated sludge itself can be land-applied as a biosolids fertilizer (subject to metals/pathogen regulation).