18-Env-B9 Environmental Chemistry and Microbiology · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2017 — 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 (6 questions, 50 marks) and Section 2: Microbiology (12 questions, 50 marks) — eighteen 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, coagulation, sludge chemistry, disinfection); Metcalf & Eddy (Tchobanoglous, Stensel, Tsuchihashi & Burton), Wastewater Engineering: Treatment and Resource Recovery (5th ed.) (chemical phosphorus precipitation, biomass stoichiometry, activated-sludge microbiology, SRT/F:M); Madigan, Martinko, Bender, Buckley & Stahl, Brock Biology of Microorganisms (14th ed.) (bacterial structure, growth kinetics, microbial physiology); Guidelines for Canadian Drinking Water Quality (Health Canada).
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's growth, plotted as $\log(\text{number of viable cells})$ against time, follows four characteristic phases. In the lag phase, cells adapt biochemically to the new medium (synthesizing enzymes, no significant increase in cell numbers) before division begins. The log (exponential) phase follows, where cells divide at their maximum rate under unlimited substrate/nutrient availability, producing a straight-line rise on the semi-log plot with growth rate $\mu=\mu_{max}$. As a limiting substrate/nutrient is depleted or inhibitory by-products accumulate, growth slows and the population enters the stationary phase, where the division rate approximately equals the death rate and net cell number is roughly constant. Finally, in the death (decline) phase, nutrient exhaustion and toxic-product accumulation cause the death rate to exceed the (near-zero) division rate, and viable cell numbers decline, often approximately exponentially.
