18-Env-B9 Environmental Chemistry and Microbiology · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2016 — 04-Env-B9, Environmental Chemistry/Microbiology. 3 hours duration; closed-book exam (one 8.5×11" aid sheet, both sides, permitted; any non-communicating calculator permitted). The paper has two sections — Section 1: Chemistry (8 questions, 50 marks) and Section 2: Microbiology (12 questions, 50 marks) — twenty 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, BOD/SRT/F–M); Guidelines for Canadian Drinking Water Quality (Health Canada); MWH's Water Treatment: Principles and Design (3rd ed.) (advanced treatment, UV disinfection, potable reuse).
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.
Plotted as log(viable cell number) against time in a closed (batch) culture with a fixed substrate charge, a bacterial population passes through four characteristic phases.
1. Lag phase. Cells acclimate to the new substrate/environment (synthesizing the enzymes needed to metabolize it) with little or no increase in numbers.
2. Log (exponential) growth phase. Substrate and nutrients are non-limiting, so cells divide by binary fission at their maximum rate; $dX/dt=\mu_{\max}X$, giving a straight line on the log-linear plot — this is the phase an activated-sludge process deliberately avoids operating in, since it corresponds to a poorly-settling, dispersed floc.
3. Stationary phase. Substrate/nutrient depletion (or accumulation of inhibitory by-products) brings the growth rate down to balance the death rate, so net viable numbers plateau — the phase in which activated sludge is normally operated, because it produces well-flocculated, good-settling biomass.
4. Death (endogenous decay/decline) phase. With substrate exhausted, cells consume their own stored/cellular material for maintenance energy (endogenous respiration) and the viable population declines, typically following first-order decay.
