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.
UV light in the germicidal range (roughly 200–280 nm, peaking near 254 nm) is absorbed by nucleic acids and forms pyrimidine dimers (adjacent thymine/uracil bases covalently bonded) in DNA and RNA. These dimers distort the double helix and block replication and transcription, so the organism cannot reproduce even though it may remain metabolically detectable — this is why UV is described as inactivating rather than killing in the strict sense. It is used because it achieves this without adding oxidant chemicals to the water, avoiding chlorinated disinfection by-products, and because it is highly effective against protozoan cysts (Giardia, Cryptosporidium) that are resistant to chlorine at practical doses.
In practice, water is passed through a reactor chamber containing low- or medium-pressure mercury-vapour (or increasingly LED) lamps in quartz sleeves; UV intensity, water depth/flow geometry, and exposure time are sized to deliver a validated minimum UV dose (mJ/cm²) across the whole flow, accounting for the water's UV transmittance (UVT), so that even the least-exposed water parcel achieves the target log-inactivation.
