Photochemical Smog and Odour Control: 23-Chem-B2 Environmental Engineering, May 2018, Q7

Problem 7: Photochemical Smog and Odour Control (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) Causes of photochemical smog and control methods

Cause	Explanation
1. NOx and VOC precursor emissions	Vehicle exhaust and industrial combustion emit NOx and reactive VOCs, the two precursors whose photochemical reaction (driven by UV sunlight) produces ozone and other secondary oxidants — smog cannot form without both precursor classes present together.
2. Strong solar (UV) radiation	Photolysis of NO2 to NO + O (which then forms ozone with O2) is a photochemical reaction requiring UV intensity; smog episodes peak on sunny, high-insolation days for this reason.
3. Temperature inversion / stagnant air (topographic trapping)	An inversion layer caps vertical mixing, trapping precursor emissions and their photochemical products near ground level in a basin/valley airshed (e.g. Los Angeles-type geography), allowing concentrations to build over the course of a still, sunny day rather than dispersing.

Control method	Type	How it reduces smog
Vehicle emission standards / catalytic converters	Hard engineering	Three-way catalysts directly cut tailpipe NOx and unburned-hydrocarbon (VOC) emissions at the source, reducing the precursor loading available for photochemical reaction.
VOC vapour-recovery and control at stationary sources (e.g. the adsorber of Problem 1)	Hard engineering	Captures/destroys VOC emissions from fuel handling, solvent use and industrial processes before they reach the atmosphere, directly lowering the VOC side of the precursor balance.
Land-use/transportation planning and public-transit incentives (soft engineering / policy)	Soft	Reduces total vehicle-kilometres travelled (and hence total precursor mass emitted) by shifting trips to transit/active modes and by siting new emission sources away from inversion-prone airsheds.

(ii) Odour control — animal rendering plant

Selecting the animal rendering plant (chicken slaughterhouse) example:

Key cause of odorous emissions	Explanation
1. Cooking/rendering off-gas (volatile fatty acids, amines, sulfides)	Thermal rendering of animal tissue releases volatile organic sulfur and nitrogen compounds (H2S, mercaptans, trimethylamine) that have extremely low human odour-detection thresholds.
2. Raw-material/carcass handling and holding	Decomposition of blood, offal and holding-bin residues under warm, moist conditions generates continuous fugitive odour even between processing batches.
3. Wastewater treatment / grease-trap and dissolved-air-flotation sludge handling	Anaerobic decomposition of high-strength organic wastewater and skimmed fat/protein solids (e.g. the DAF float of Problem 4) is itself a significant odour source if not promptly processed or covered.

Biofiltration. Odorous process off-gas is ducted through a moist, microbially-active media bed (compost/bark/engineered media); resident bacteria oxidize the volatile sulfur/amine compounds to odourless end products (sulfate, CO2, biomass), routinely achieving >99% odour-unit reduction for well-maintained beds.
Wet chemical scrubbing (multi-stage acid/oxidant scrubber). A staged scrubber (acid stage to neutralize basic amines, followed by an oxidant stage, e.g. NaOCl or ozone, to oxidize sulfur compounds) chemically destroys the odorous species rather than just diluting them, also achieving >99% reduction on properly dosed systems.

Ensuring consistently high performance: for biofiltration, maintain the media bed's moisture content, pH and empty-bed residence time within design range (a dried-out or channeled bed loses microbial activity and short-circuits untreated gas), verified by routine differential-pressure and outlet-odour monitoring; for wet scrubbing, maintain the oxidant/reagent dose via continuous ORP (oxidation-reduction potential) or pH-based feedback control rather than a fixed timer, so the dose tracks the actual (variable) odour load rather than an assumed average.

Topic: Photochemical smog formation chemistry and mitigation, and odour-control technology selection/performance assurance for a rendering facility.

Key relations: Photochemical smog cycle (NOx + VOC + UV → O3/PAN); biofiltration microbial oxidation kinetics; scrubber oxidant demand/ORP control.

Why this works: Smog is fundamentally a three-ingredient photochemical reaction (NOx, VOC, UV) trapped by poor vertical mixing, so effective controls attack one of those ingredients (precursor mass) or the trapping geometry (land-use/siting) rather than trying to intercept the reaction itself once airborne; odour control similarly works by either biologically oxidizing the odorants (biofiltration) or chemically destroying them (scrubbing) rather than simply diluting/dispersing them, which is why both routinely reach >99% removal when properly maintained.

Common pitfall: Treating ozone/smog control as a single-precursor problem (cutting only VOC or only NOx can, in some NOx-saturated urban airsheds, actually increase peak ozone — the VOC/NOx ratio matters); for odour control, assuming a high nominal removal efficiency is self-sustaining without a monitoring/feedback loop to catch media drying or under-dosing before it silently degrades performance.

Source: Cooper & Alley, Air Pollution Control, 4th ed., Ch. 3 (photochemical smog), Ch. 8 (odour control — biofiltration, scrubbing); Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 9 (air quality management).

Question 7 of 7: Photochemical Smog and Odour Control

Problem 7: Photochemical Smog and Odour Control (20 marks)

(i) Causes of photochemical smog and control methods

(ii) Odour control — animal rendering plant