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.
Disinfection efficiency (captured by the CT concept, concentration × contact time, per the Chick–Watson framework) is governed by: disinfectant concentration and contact time (higher C and longer t both drive inactivation up); type and concentration of organism (spores and protozoan cysts are far more resistant than vegetative bacteria or most viruses); temperature (disinfection rate roughly doubles for every 10°C rise, per an Arrhenius-type dependence); pH (for chlorine, low pH favours the far more effective hypochlorous acid, HOCl, over the weaker hypochlorite ion, OCl⁻); turbidity and suspended solids (particles shield embedded organisms from contact with the disinfectant, and can consume disinfectant demand); and the presence of interfering/demand-exerting substances (ammonia, organic matter, reduced iron/manganese, which consume disinfectant before it can act on pathogens).
