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18-Env-B9 Environmental Chemistry and Microbiology · May 2017

Question 16 of 18: SRT and Its Use in Environmental Engineering

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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).

Section 1: Chemistry (6 questions, 50 marks)

Question 16: SRT and Its Use in Environmental Engineering (3 marks)

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.

Check: this question is essentially identical to Question 11 (the same section's earlier "What is SRT, and how is it used in wastewater engineering?"), just re-asked with "environmental engineering" in place of "wastewater engineering." The full working is reproduced here rather than a "see Question 11" redirect.

Solids retention time (SRT), or mean cell residence time, is the average time microbial (biomass) solids spend in a biological treatment system before removal, $$\text{SRT}=\frac{V\,X}{Q_w X_w + Q_e X_e}$$ i.e. the total mass of biomass held in the system divided by the rate at which that mass is removed (via wasting and any biomass leaving in the effluent). In environmental engineering more broadly — beyond conventional activated sludge — the same SRT concept governs the design of any suspended- or attached-growth biological process (aerated lagoons, membrane bioreactors, biofilm reactors expressed as an equivalent SRT) whenever the design question is "how long must this microbial population be retained to accomplish a given biological transformation" (carbonaceous BOD removal, nitrification, or anaerobic digestion, each with its own characteristic minimum SRT set by the slowest-growing organism required for that transformation).