23-Chem-B2 Environmental Engineering · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-Chem-B2, Environmental Engineering — May 2016. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.
Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.
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.
One representative technology is selected per contaminant class: a fabric filter for particulates, a packed-bed wet scrubber for toxic gases, and biofiltration for odorous vapours — the three cover the three dominant capture mechanisms (sieving, gas–liquid absorption, biological oxidation) used across industrial air-pollution control.
| Contaminant | Technology | Main design principle | Operational considerations | Limitations |
|---|---|---|---|---|
| (i) Particulates | Fabric filter (baghouse) | Cake filtration: the gas is forced through woven/felted media at a sized face velocity (air-to-cloth ratio); the dust cake that builds up on the media is the true fine-particle filter. | 1) Cleaning-cycle control (pulse-jet/reverse-air) triggered by ΔP, balancing cake retention against blinding. 2) Bag-leak/opacity monitoring per compartment to catch tears early. | 1) Cannot handle sticky, hygroscopic or condensing dusts (blinds the media). 2) Temperature/moisture limits set by the bag fabric; high-temperature gas needs costly specialty media or pre-cooling. |
| (ii) Toxic gases | Packed-bed wet scrubber (absorption) | Gas–liquid mass transfer across the packing surface area; packing height/type set from NTU×HTU for the required removal, with a reactive scrubbant (e.g., NaOH for acid gases such as HCl/SO2/Cl2) to keep the driving-force gradient favourable. | 1) Liquid-to-gas ratio and reagent feed controlled to influent gas concentration (pH trim for acid gases). 2) Monitor pressure drop for packing fouling/flooding. | 1) Generates a liquid waste/spent-scrubbant stream that itself needs treatment or disposal. 2) Poor removal for gases with low aqueous solubility (many VOCs) unless a reactive/specialty liquid is used. |
| (iii) Odorous vapours | Biofiltration | Odorous VOCs (H2S, mercaptans, amines) partition into a moist biofilm on an organic media bed (compost/wood chips/bark) and are oxidised by acclimated microorganisms; bed size set by the empty-bed residence time (EBRT) needed for the target compound. | 1) Maintain media moisture in the 40–60% range (irrigation control). 2) Monitor pressure drop and inspect for channeling/dry zones that short-circuit the airflow. | 1) Slow biofilm start-up/acclimation (weeks) after a media change or upset. 2) Large footprint and poor performance under highly variable or spiking (shock) odour loads. |
