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18-Env-B9 Environmental Chemistry and Microbiology · May 2016

Question 11 of 20: Effect of Nitrification on the BOD Test

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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).

Section 1: Chemistry (8 questions, 50 marks)

Question 11: Effect of Nitrification on the BOD Test (3 marks)

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.

A standard BOD bottle exerts oxygen demand in two distinguishable stages when nitrifying organisms are present in (or seeded into) the sample. The first, carbonaceous stage (CBOD) is the oxidation of organic carbon by heterotrophs and follows the classic first-order BOD curve, essentially plateauing by about day 8–10. If autotrophic nitrifiers (Nitrosomonas, Nitrobacter) are present, once their population has built up — typically after a lag of about 6–10 days — they begin oxidizing ammonia to nitrite and then nitrate, exerting a second, nitrogenous demand (NBOD) that appears as a second rising limb on the cumulative oxygen-demand curve well after the carbonaceous curve has flattened.

Time (days)Oxygen demand exertedEffect of nitrification on the BOD testnitrification begins (~day 8)carbonaceous BOD (CBOD) onlyCBOD + nitrogenous (NBOD) — with nitrifiers present05101520
Figure — cumulative oxygen demand vs. time: carbonaceous BOD alone (blue) plateaus by ~day 10; with active nitrifiers present (red), a second rising limb (NBOD) appears once the nitrifier population is established, inflating the apparent BOD if the test is read at 20+ days rather than the standard 5-day point.

This is why the standard test is read at 5 days (BOD5) — before nitrification typically begins — and why a nitrification inhibitor (e.g. TCMP, allylthiourea) is added when a longer incubation or nitrifier-seeded sample is used, to isolate the carbonaceous demand that the test is meant to represent.