18-Env-B9 Environmental Chemistry and Microbiology · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-Env-B9, Environmental Chemistry/Microbiology. 3 hours duration; closed-book exam (approved Casio or Sharp calculator only). The paper has two sections — Section 1: Chemistry (11 questions, 50 marks) and Section 2: Microbiology (14 questions, 50 marks) — twenty-five questions constitute the complete exam and all are answered below. Total examination mark 100.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) (water chemistry, disinfection, water/wastewater microbiology, indicator organisms); Metcalf & Eddy (Tchobanoglous, Stensel, Tsuchihashi & Burton), Wastewater Engineering: Treatment and Resource Recovery (5th ed.) (chemical unit processes, chemical phosphorus precipitation, biomass stoichiometry, activated-sludge microbiology); Guidelines for Canadian Drinking Water Quality (Health Canada); MWH's Water Treatment: Principles and Design (3rd ed.) (chlorine disinfection, contact-tank sizing).
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.
Advantages: (1) inactivates chlorine-resistant protozoan cysts/oocysts (Giardia, Cryptosporidium) very effectively at practical doses; (2) forms essentially no regulated disinfection by-products (no THMs/HAAs, unlike chlorine); (3) requires no chemical storage, handling, or transport of hazardous oxidants, improving worker safety; and (4) has a very short contact time (seconds) and compact footprint compared with chemical contact tanks.
Disadvantages: (1) provides no residual disinfectant, so it offers no protection against regrowth or recontamination in the distribution system; (2) effectiveness depends strongly on water clarity/turbidity and UV transmittance — particles can shield organisms from the dose; (3) lamps require regular cleaning (fouling/scaling reduces output) and periodic replacement, adding operating cost and a maintenance burden; and (4) some organisms exhibit photoreactivation/dark repair, partially reversing UV damage after treatment if exposed to visible light.
