18-Env-B9 Environmental Chemistry and Microbiology · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 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 (11 questions, 50 marks) and Section 2: Microbiology (14 questions, 50 marks) — twenty-five 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); Guidelines for Canadian Drinking Water Quality (Health Canada); MWH's Water Treatment: Principles and Design (3rd ed.) (chlorine disinfection, contact-tank sizing).
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 ratio, $\text{F/M}=\dfrac{Q\cdot S_0}{V\cdot X}$ (mass of BOD/COD applied per day divided by the mass of active biomass, MLVSS, in the aeration basin), is the principal operating and design control parameter for the activated-sludge process. It expresses how much substrate is available per unit mass of organisms and therefore governs sludge characteristics and effluent quality: a low F/M (extended aeration, long SRT) drives organisms into endogenous respiration, producing a well-stabilized, low-yield sludge with excellent settling but higher aeration energy demand; a high F/M (high-rate, short SRT) produces rapid substrate removal and high sludge yield but a less-stabilized, sometimes poorly settling (filamentous bulking-prone at very low F/M, dispersed-growth-prone at very high F/M) sludge. Operators target an F/M range appropriate to the desired process (e.g. 0.2–0.5 kg BOD/(kg MLVSS·d) for conventional activated sludge) to balance effluent quality, sludge production, and settleability.
