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 three 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). Three examples and their sources: 1. Benzene — vehicle exhaust and fuel evaporation, petrochemical refining (a mobile-source-dominated air toxic). 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.

Part (ii) — odour: definition, units, an odour-emitting industry, and its control technology. (Answered in the coherent order industry→technology, since part (b)'s "this industry" is only introduced in part (c); see the check note.) a) What is odour, and its units. Odour is the sensory perception produced when volatile, olfactorily-active compounds stimulate receptors in the nasal epithelium at a concentration above an individual's detection threshold; because perception is subjective, odour intensity is quantified in odour units (OU/m³) — the dilution factor (with clean air) at which a panel of trained human assessors (dynamic olfactometry, per EN 13725/similar standards) can just detect the sample, so 1 OU/m³ is, by definition, the panel's detection threshold. c) An odour-emitting 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. b) Control technology and three design principles. Biofiltration is a common control technology: 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, which biologically oxidises 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.