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.
The food-to-microorganism (F/M) ratio is the mass loading rate of biodegradable substrate (BOD or COD) applied to an aeration tank per unit mass of active biomass (MLVSS) present, per unit time: $$F/M=\frac{Q\,S_0}{V\,X}\quad\left[\frac{\text{kg BOD}}{\text{kg MLVSS}\cdot\text{d}}\right]$$ It is used in environmental engineering as a primary activated-sludge design and operating-control parameter, closely linked to SRT (Questions 11/16): a low F/M (extended aeration, ~0.05–0.15) corresponds to a long SRT, near-complete substrate utilization, and well-settling floc, at the cost of a larger reactor volume and higher aeration energy; a high F/M (high-rate, >0.5) corresponds to short SRT, a compact reactor, but a higher risk of filamentous bulking and incomplete treatment (poorer effluent quality, no nitrification capacity). Operators target a specific F/M range (conventional activated sludge is typically ~0.2–0.5) to balance treatment performance, sludge settleability, and capital/operating cost.
