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.
Outdoors, airborne pathogens carried on respiratory droplets/aerosols are rapidly diluted and dispersed by essentially unlimited ambient air volume and natural wind/turbulence, so the concentration any nearby person is exposed to falls off quickly with distance and time. Direct sunlight also inactivates many pathogens via UV exposure and desiccation. Indoors, by contrast, air is confined to a small, often poorly ventilated volume, so exhaled aerosols accumulate and recirculate (especially with mechanical HVAC systems that recirculate air rather than fully exhausting it), keeping pathogen concentrations far higher for far longer and at close, sustained proximity to other occupants — all of which raises the effective inhaled dose and infection probability.
