18-Env-B9 Environmental Chemistry and Microbiology · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2016 — 04-Env-B9, Environmental Chemistry/Microbiology. 3 hours duration; closed-book exam (one 8.5×11" aid sheet, both sides, permitted; any non-communicating calculator permitted). The paper has two sections — Section 1: Chemistry (8 questions, 50 marks) and Section 2: Microbiology (12 questions, 50 marks) — twenty 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, BOD/SRT/F–M); Guidelines for Canadian Drinking Water Quality (Health Canada); MWH's Water Treatment: Principles and Design (3rd ed.) (advanced treatment, UV disinfection, potable reuse).
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.
Ultraviolet light in the germicidal range (roughly 200–300 nm, peaking near 254 nm — close to the low-pressure mercury vapour lamp’s output line) is strongly absorbed by nucleic acids (DNA and RNA), which have a strong absorption peak near 260 nm. Photons in this range are absorbed directly by adjacent pyrimidine bases (thymine–thymine, cytosine–thymine) on the same DNA strand, and the absorbed energy drives a photochemical reaction that fuses those adjacent bases together into a covalently-bonded dimer (most commonly a thymine dimer).
These dimers physically distort the DNA double helix at that point, so the organism’s own replication and transcription enzymes can no longer read through the damaged strand correctly. If enough dimers accumulate faster than the cell’s (limited) repair enzymes can excise and fix them, the organism can no longer replicate its genome or synthesize the proteins/enzymes needed for normal metabolism, and is rendered non-viable (inactivated) even though the cell itself may not be immediately lysed or killed outright — UV is therefore properly described as inactivating rather than physically destroying microorganisms.
Unlike chemical disinfectants (chlorine, ozone), UV leaves no residual and produces no disinfection by-products, but it delivers no protection once the water leaves the reactor, so it is often paired with a chemical residual downstream in distributed systems, or used as a final, non-chemical barrier in reuse trains (Question 8).
