Question 7 of 7: Photochemical Smog and Odour Control

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) Photochemical smog: reactions/conditions and control methods

Three chemical reactions/conditions drive smog formation. (1) NO2 photolysis: $NO_2+h\nu\rightarrow NO+O$ initiates the cycle under UV/sunlight. (2) Ozone formation: the atomic oxygen immediately combines with ambient O2, $O+O_2+M\rightarrow O_3+M$, forming ozone; in a NOx-only atmosphere this ozone reacts back with NO ($O_3+NO\rightarrow NO_2+O_2$), so O3 stays low. (3) Reactive-VOC/OH-radical chain: volatile organic compounds initiate an OH-radical oxidation chain that regenerates NO2 from NO without consuming ozone, breaking the natural NO–O3 titration and letting O3 (and secondary products such as peroxyacetyl nitrate, PAN) accumulate. These reactions proceed fastest under stagnant, sunny conditions (light wind, strong UV, often a temperature inversion trapping the precursors near ground level).

Three engineering control methods, hard and soft:

1. NOx control (hard engineering). Selective catalytic reduction (SCR) or low-NOx combustion at major stationary sources, and catalytic converters on vehicles, cut the NO2 available to initiate the photolysis cycle.
2. VOC control (hard engineering). Vapour-recovery systems on fuel storage/dispensing, solvent substitution (low-VOC coatings/degreasers), and industrial off-gas capture (carbon adsorption or thermal oxidation) reduce the reactive-VOC pool that breaks the NO–O3 titration.
3. Land-use and transportation demand management (soft engineering). Zoning that separates major NOx/VOC sources from population centres, transit-oriented development, and traffic-demand management (congestion pricing, transit investment) reduce precursor emissions at the source without a physical abatement device, particularly effective in basin/valley airsheds prone to inversion trapping.

(ii) Odorous emissions and control at an industrial facility (rendering plant example)

At an animal-rendering plant, three key odour-causing substances/causes are: (1) hydrogen sulfide (H2S) from anaerobic decomposition of proteinaceous waste, (2) volatile amines and mercaptans released during cooking/rendering of raw material, and (3) fugitive emissions from open storage/handling of raw material and process residuals prior to processing.

Three different control technologies:

1. Wet chemical scrubbing (caustic/hypochlorite scrubber) for the concentrated, high-strength cooker off-gas stream — oxidizes H2S and amines in a packed-bed contactor before release.
2. Biofiltration for lower-concentration, high-volume ventilation air (raw-material receiving/storage area) — odorous compounds partition into a moist biofilm on an organic media bed and are oxidised biologically.
3. Thermal oxidation (regenerative thermal oxidizer) for the highest-strength, driest cooker vent streams where combustion destroys the odorants outright rather than transferring them to another phase.

Operational and maintenance requirements: (1) Source capture/containment integrity — covers, seals and negative-pressure ventilation on storage and process areas must be maintained (gasket/door-seal inspection, fan/duct static-pressure checks) since any breach lets odorous air escape uncaptured, bypassing all downstream treatment entirely. (2) Scrubbant/media performance monitoring — for the wet scrubber, oxidant residual and pH must be tracked and dosed to the varying off-gas load; for the biofilter, media moisture (irrigation control, typically 40–60%) must be maintained, since a dried-out or exhausted bed loses removal capacity and can crack, creating short-circuit channels that bypass treatment.