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.
Why it is used. Biochemical oxygen demand estimates the amount of dissolved oxygen that heterotrophic microorganisms will consume while biologically degrading the organic matter in a water/wastewater sample. It is used as the standard surrogate measure of the biodegradable organic (pollution) strength of a wastewater or receiving-water sample — for sizing biological treatment processes, tracking treatment-plant performance (influent/effluent BOD removal), and assessing the oxygen-depletion impact a discharge will have on a receiving stream.
How it is conducted. A sample (diluted with aerated, nutrient- and seed-microorganism-amended dilution water so the oxygen demand does not exhaust the bottle — or undiluted for a low-strength sample) is placed in a sealed, airtight BOD bottle with no headspace, and its initial dissolved-oxygen concentration is measured. A parallel set of bottles is incubated in the dark (to exclude photosynthetic oxygen production) at a controlled 20°C for a standard period, most commonly 5 days (BOD5). Dissolved oxygen is measured again at the end of incubation, and the oxygen consumed — corrected for the dilution factor and any oxygen demand contributed by the seed/dilution water itself (a blank) — is reported as the BOD.
As noted in Question 11, incubation is kept dark and is usually stopped at 5 days specifically to measure the carbonaceous demand before significant nitrification begins; a nitrification inhibitor is added when that distinction must be guaranteed on samples likely to already carry an active nitrifier population.
