Question 6 of 7: Photochemical Smog, Noxious Pollutants and Odour Control

Problem 6: Photochemical Smog, Noxious Pollutants 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

Physical atmospheric conditions. Photochemical smog forms most readily on warm, sunny days with strong UV radiation, light winds and a temperature inversion (for example a subsidence inversion under a stationary high-pressure system, or a valley or coastal basin). The inversion caps vertical mixing, so NO_x and hydrocarbons emitted during the morning traffic peak stay trapped in a shallow layer near the ground. Sunlight then drives the reactions, and ozone typically peaks in the early-to-mid afternoon, often downwind of the city.

Chemical reactions. Photochemical smog begins with NO_x (mostly NO from combustion) reacting with atmospheric oxygen to form NO₂. Sunlight (UV) then photolyzes NO₂ into NO and a free oxygen atom (NO₂+hν→NO+O), and the free oxygen atom combines with molecular O₂ to form ozone (O+O₂+M→O₃+M). In a "clean" NO_x-only atmosphere, the ozone formed would be rapidly consumed again by reaction with the NO produced in the same photolysis step (O₃+NO→NO₂+O₂), holding ozone at a low steady-state level; the presence of reactive hydrocarbons (VOCs) breaks this cycle by oxidizing NO to NO₂ through a separate radical (peroxy-radical) pathway that does not consume ozone, so NO is regenerated into NO₂ without ozone being destroyed, allowing ozone (and secondary products such as PAN, peroxyacetyl nitrate) to accumulate through the day as sunlight intensity and hydrocarbon loading increase, producing the characteristic urban smog/haze.

(ii) Traffic-related noxious pollutants and an engineering solution

The three solutions target the pollutants at the point where they are actually formed — combustion chemistry and temperature for CO/NO_x, and physical soot capture for PM — which is why a single "catalytic converter" alone is not sufficient for a diesel vehicle: NO_x control and PM control are frequently engineered as separate exhaust after-treatment stages (SCR/EGR for NO_x, DPF for PM) rather than one universal device.

(iii) Biochemical odour control technology for VOC emissions at a power plant

A biofilter is an effective biochemical engineered technology for the dilute, odorous VOC streams found at a power plant (for example fuel-oil tank vents, coal-yard and fuel-handling building exhaust, or wastewater-pond and lube-oil area ventilation). A fan collects the odorous air, which is humidified (and cooled if needed) and then blown through a bed of moist organic or engineered media (wood chips, bark, compost, or inert synthetic carriers). A biofilm of acclimated bacteria and fungi grows on the media. The VOCs dissolve into this biofilm and are oxidized aerobically to CO₂, water and new biomass, and the treated air is released from the bed surface or a short stack.

Design and operating principles for effective, efficient performance:

1. Empty-bed residence time (EBRT) and loading. The bed is sized for an EBRT of typically 30–60 s (up to about 2 min for less soluble VOCs), a media depth of about 1–1.5 m, and a VOC loading below the media's elimination capacity. The inlet air is distributed evenly so the bed does not channel. Removal is confirmed by periodic inlet/outlet VOC and odour measurements, with a design target of about 90% or better.
2. Moisture control. The inlet air is humidified to near saturation and the bed is irrigated to keep media moisture at about 40–60%. A dry bed cracks and loses microbial activity; a waterlogged bed goes anaerobic and raises pressure drop. Pressure drop and moisture are monitored routinely.
3. Temperature, pH and nutrients. The bed is held at about 20–40°C and near-neutral pH (buffering added if acidic by-products build up), with nutrients added if the media is inert.
4. Media life and maintenance. Organic media compacts and degrades over roughly 2–5 years and is turned or replaced when pressure drop rises. Biofilters handle steady, dilute, biodegradable streams well. Short spikes can be smoothed with a small activated-carbon buffer, and high-concentration or poorly biodegradable VOCs are better sent to a biotrickling filter or thermal oxidizer.

A biofilter uses no fuel and little energy (mainly fan power), and produces no NO_x or combustion by-products. That gives it a low operating cost and a small carbon footprint compared with thermal oxidation, which is why it is the preferred biochemical choice for large volumes of dilute odorous air.