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.
The food-to-microorganism ratio, $\text{F/M}=\dfrac{Q\,S_0}{V\,X}$ (kg BOD applied per day per kg MLVSS in the aeration tank), expresses how much substrate is being supplied relative to the size of the biomass population available to consume it.
Operators use it as a primary process-control parameter for activated sludge: a high F/M (substrate in excess of biomass) pushes the population toward the log growth phase — fast BOD removal but poorly-flocculated, dispersed-growth sludge that settles badly; a very low F/M (biomass in excess of substrate, extended aeration) drives the population into deep endogenous decay, which can favour filamentous organisms and poor settling from the opposite direction. A moderate F/M (typically ~0.2–0.5 kg BOD/kg MLVSS·d for conventional activated sludge) keeps the biomass near the stationary growth phase (Question 12), producing well-flocculated sludge with good settling (SVI) characteristics. Plants are routinely operated by adjusting the MLSS concentration (via wasting rate) or aeration-tank volume in service to hold F/M within the target design band for the desired effluent quality and sludge settleability.
