Question 6 of 7: Photochemical Smog, Halogenated Hydrocarbons and Odour Control

Problem 6: Photochemical Smog, Halogenated Hydrocarbons 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 formation

Smog forms under stagnant, sunny conditions — light wind, strong UV radiation, and often a temperature inversion that traps pollutants near ground level (the classic Los Angeles basin setting: surrounding hills plus a subsidence inversion). The chemistry is a photocatalytic chain: NO2 absorbs UV and photolyses, $NO_2+h\nu\rightarrow NO+O$, the atomic oxygen forms ozone, $O+O_2+M\rightarrow O_3+M$. In a "clean" NOx-only atmosphere this ozone would simply react back with NO ($O_3+NO\rightarrow NO_2+O_2$), reaching a steady low level. Reactive VOCs break that cycle: an OH-radical-initiated VOC oxidation chain converts NO back to NO2 without consuming ozone, so O3 (and secondary products such as peroxyacetyl nitrate, PAN) accumulate through the day, trapped under the inversion, producing the characteristic brown haze and respiratory irritants.

(ii) 99.9% destruction of halogenated hydrocarbons

A thermal oxidizer (afterburner) is designed around the classic "3 T's" — Temperature, (residence) Time, Turbulence — sized more conservatively than for ordinary VOCs because C–Cl and C–F bonds need a higher activation energy to fully mineralise:

1. Combustion temperature. Operate the combustion chamber at 980–1,100 °C (vs. ∼760 °C typical for non-halogenated VOC oxidation) — the higher temperature is required to drive the C–Cl/C–F bond cleavage to completion rather than stalling at partially chlorinated/fluorinated intermediates.
2. Residence time and turbulence. Size the chamber for ≥1–2 s gas residence time at the design temperature with turbulent (high-Reynolds-number) mixing, so every parcel of gas actually sees the full temperature/time combination — a destruction and removal efficiency (DRE) target of 99.9% demands that essentially no gas bypasses the hot zone via short-circuiting.
3. Downstream acid-gas scrubbing. Combustion of halogenated compounds releases HCl/Cl2 (or HF); a wet caustic quench/scrubber downstream of the oxidizer is mandatory to neutralise these acid gases before atmospheric release — without it, the process would simply exchange one air-toxics problem for an acid-gas emission.

Assumption (stated per the exam's own instruction to flag assumptions): a 99.9% DRE is achievable by 3-T sizing alone for this waste-gas stream (no unusually refractory perfluorinated species that would need a higher-temperature/longer-residence design, e.g. a catalytic or plasma-assisted step) — consistent with standard EPA/industry guidance for chlorinated-solvent off-gas incineration.

(iii) Odour control at the screening/pre-treatment area

Biofiltration is an effective, low-operating-cost technology for this application: the screening/headworks area is enclosed under a cover with negative-pressure ventilation, and the captured air stream is ducted to an outdoor biofilter bed. Main design principle: the odorous compounds (H2S, mercaptans, amines typical of raw-sewage screenings) partition into a moist biofilm on an organic media bed and are oxidised biologically, with the bed sized from the empty-bed residence time needed for the target compound's removal. Most important operational/maintenance requirement: maintaining media moisture (irrigation control, typically 40–60%) — a bed that dries out loses essentially all removal capacity and can also crack, creating short-circuit air channels that bypass treatment entirely.