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.
What it is. Solids retention time (also called mean cell residence time, $\theta_c$) is the average length of time a unit of biomass (MLVSS) remains in the biological process before being wasted, defined as $\theta_c=\dfrac{V X}{Q_w X_w+Q_e X_e}$ — total mass of solids in the system divided by the rate at which solids leave it (mainly the wasting stream).
Why it is used. SRT (not the much shorter hydraulic retention time, HRT) is the parameter that actually governs whether a given organism population can be sustained in the system: an organism is washed out unless its net specific growth rate at least matches $1/\theta_c$. This matters especially for slow-growing autotrophic nitrifiers, whose minimum SRT for survival is far longer than that of ordinary heterotrophs.
How it is used. Designers and operators set SRT (via the sludge-wasting rate) as the primary design/control lever for the biological process: a short SRT (a few days) is used for BOD removal only; a longer SRT (10–20+ days, depending on temperature) is required to reliably retain nitrifiers and achieve nitrification; and an even longer SRT is used in extended aeration to also promote aerobic digestion of the biomass itself, reducing net sludge production. SRT is therefore the parameter operators actively manage — by adjusting the waste activated sludge (WAS) flow rate — to hold the desired treatment objective (carbon removal only vs. full nitrification) as influent conditions vary.
