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.
Solids retention time (SRT), also called mean cell residence time, is the average length of time microbial solids (biomass) remain within a biological treatment system before being removed (wasted), $\text{SRT}=\dfrac{V\,X}{Q_w X_w + Q_e X_e}$ (mass of biomass in the system divided by the daily mass wasting rate). In wastewater engineering it is the single most important control parameter for an activated-sludge process: it is set independently of the hydraulic retention time (via recycle and controlled sludge wasting) and directly determines whether slow-growing organisms such as nitrifiers can be retained and sustained (SRT must exceed their minimum doubling time, with a safety factor) — too short an SRT washes nitrifiers out and loses nitrification; too long an SRT produces excess endogenous decay, poor settleability, and larger aeration/oxygen demand.
