Question 4 of 5: Air Toxics, Odour Control and Noxious Pollutants

Problem 4: Air Toxics, Odour Control and Noxious Pollutants (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.

Part (i) — air toxics: definition and five examples. Air toxics (hazardous air pollutants) are airborne contaminants known or suspected to cause serious health effects — cancer, reproductive/developmental harm, or other severe chronic effects — at typical ambient exposure levels, distinct from the six broadly-regulated "criteria" pollutants (PM, SO₂, NOₓ, CO, O₃, Pb). Five examples and their sources: 1. Benzene — vehicle exhaust and fuel evaporation, petrochemical refining. 2. Formaldehyde — incomplete combustion (vehicle exhaust, wood smoke) and off-gassing from resins/adhesives in composite wood products. 3. Mercury — coal combustion (trace mercury in coal volatilised and emitted) and municipal/medical waste incineration. 4. Polychlorinated biphenyls (PCBs) — legacy electrical transformer/capacitor fluid, released from historical spills and improperly disposed equipment. 5. Dioxins/furans — unintentional by-products of combustion of chlorine-containing waste (municipal solid waste incineration) and some industrial chlorine chemistry.

Part (ii) — odour-emitting industry, control technology, three design principles. Industry: a municipal wastewater treatment plant, specifically the headworks/screening and sludge-thickening areas, releases hydrogen sulphide and organic reduced-sulphur/amine compounds from anaerobic decomposition of settled solids. Control technology: biofiltration. Odorous air is drawn from the source enclosure and passed through a bed of moist organic media (compost/wood-chip/bark mixture) that hosts a resident microbial population; the microbes biologically oxidise the odorous compounds (H₂S→SO₄²⁻, organic sulphides/amines→CO₂+H₂O+biomass) as the air passes through, at far lower operating cost than thermal oxidation for the large, dilute airflows typical of a WWTP. Three fundamental design principles: (1) sufficient empty-bed residence time (typically 30–60 s) for the microbial reactions to go to completion; (2) uniform gas distribution across the full bed cross-section (a plenum/underdrain system) to avoid short-circuiting through preferential low-resistance channels; (3) maintaining bed moisture content and temperature within the microbial population's viable range (typically 40–60% moisture, mesophilic temperature) via a humidification/irrigation system, since a dried-out or waterlogged bed loses removal efficiency rapidly.

Part (iii) — noxious pollutants: definition and two examples. Noxious pollutants are substances that are harmful or offensive to human health, comfort, or the environment even at low concentration, typically through acute irritant, odorous, or toxic effect rather than long-term chronic exposure. Two examples: 1. Hydrogen sulphide (H₂S) — anaerobic decomposition of organic matter (wastewater treatment, livestock manure lagoons) and sour natural gas / kraft pulp mill processing. 2. Ammonia (NH₃) — intensive livestock operations (manure volatilisation) and industrial refrigeration system leaks.