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.
Filamentous overgrowth from a low F/M ratio. A sustained low-food, high-biomass condition favours slow-growing filamentous organisms (e.g. Sphaerotilus natans, type 1701, Microthrix parvicella) over floc-forming bacteria; the filaments bridge between flocs and prevent them compacting, giving a high sludge volume index (SVI) — filamentous bulking.
Nutrient deficiency (insufficient nitrogen or phosphorus relative to the BOD load) — floc-forming heterotrophs are more sensitive to nutrient limitation than many filamentous species, again shifting the competitive balance toward filaments.
Low dissolved oxygen in the aeration tank. Filamentous organisms generally have a higher affinity for oxygen at low DO than floc formers, so chronic under-aeration (or poor mixing creating localized low-DO zones) selects for filamentous growth in the same way a low F/M does.
(A related but mechanistically distinct problem, viscous/non-filamentous bulking, arises instead from excess extracellular polymer production under high-F/M, nutrient-imbalanced conditions — it is worth distinguishing from filamentous bulking, since the corrective actions differ.)
