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.
Conventional (media) filtration removes pathogens primarily by physical straining and by chemical/electrostatic attachment to the filter media, and its removal efficiency depends strongly on how well the upstream coagulation/flocculation step has aggregated particles to a filterable size — well-designed and well-operated filtration typically achieves a useful but incomplete log-removal of bacteria (commonly cited around 2-log/99% for conventional filtration credit), not the essentially complete inactivation a properly dosed disinfectant provides. It is only partially effective because: individual bacteria and especially viruses can be smaller than the effective pore/media capture size and can pass through, particularly if coagulation is under-dosed or a filter is not backwashed/maintained properly; some organisms can pass through minor filter imperfections or short-circuit through the media bed; and, critically, filtration provides no residual protection — any organism that survives filtration, or that enters the water after filtration (a distribution-system breach, cross-connection, or main break), is not subsequently inactivated, because there is no disinfectant residual present to act on it.
