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.
A batch bacterial culture, tracked by the mass/log-number of viable organisms against time, passes through four classical phases. Lag phase: organisms are acclimating to the new medium (synthesizing enzymes, no significant increase in numbers/mass). Exponential (log) growth phase: nutrients are non-limiting, organisms divide at their maximum rate, and $\ln(N)$ increases linearly with time. Stationary phase: a limiting substrate (food, oxygen) is exhausted or a toxic metabolite/waste product accumulates, so the growth rate falls to equal the death rate and net mass/numbers plateau. Death (decline/endogenous) phase: substrate is fully depleted, organisms consume their own cellular material for maintenance energy (endogenous respiration) and die faster than they reproduce, so viable mass declines — often at a roughly exponential rate itself.
