18-Env-B9 Environmental Chemistry and Microbiology · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2017 — 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 (6 questions, 50 marks) and Section 2: Microbiology (12 questions, 50 marks) — eighteen 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, coagulation, sludge chemistry, disinfection); Metcalf & Eddy (Tchobanoglous, Stensel, Tsuchihashi & Burton), Wastewater Engineering: Treatment and Resource Recovery (5th ed.) (chemical phosphorus precipitation, biomass stoichiometry, activated-sludge microbiology, SRT/F:M); Madigan, Martinko, Bender, Buckley & Stahl, Brock Biology of Microorganisms (14th ed.) (bacterial structure, growth kinetics, microbial physiology); Guidelines for Canadian Drinking Water Quality (Health Canada).
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.
3.1 Endemic. The habitual, ongoing presence of a disease or organism within a given population or geographic area at a relatively constant baseline rate, as opposed to an epidemic (a sudden increase above baseline).
3.2 Virus. A sub-microscopic, obligate intracellular pathogen consisting of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid), incapable of independent metabolism or reproduction — it must hijack a host cell's machinery to replicate; of concern in water/wastewater as an enteric pathogen (e.g. norovirus, rotavirus, hepatitis A).
3.3 Virulence. The degree or intensity of pathogenicity of an organism — its relative capacity to cause disease/damage in a host, often expressed via a measure such as the infectious dose required to cause illness in a defined fraction of exposed hosts.
3.4 Enzyme. A biological catalyst, almost always a protein, that accelerates a specific biochemical reaction by lowering its activation energy without itself being consumed — the basis of all microbial metabolism (e.g. extracellular enzymes hydrolyzing particulate/colloidal organics before uptake).
3.5 Binary fission. The asexual reproduction mechanism of most bacteria, in which a single cell replicates its chromosome and divides into two genetically identical daughter cells; the basis of exponential (log-phase) population growth, $N=N_0\cdot2^{t/t_d}$ where $t_d$ is the doubling time.
3.6 pH. The negative base-10 logarithm of the hydrogen-ion activity, $\text{pH}=-\log_{10}[\text{H}^+]$, a 0–14 scale measure of acidity/alkalinity; most bacteria (neutrophiles) grow optimally near pH 6–8, and activated-sludge nitrification is particularly pH-sensitive.
3.7 Thermophilic. Describing organisms that grow optimally at elevated temperatures, typically 45–70 °C, as distinct from mesophiles (~20–45 °C) and psychrophiles (<20 °C) — relevant to thermophilic anaerobic sludge digestion, which runs faster and achieves greater pathogen kill than mesophilic digestion.
3.8 MPN (most probable number). A statistically-based enumeration method for viable organisms (commonly coliforms) using a series of replicate dilution tubes/wells scored positive/negative for growth, with the MPN and its confidence interval read from a standard statistical table — used where colony-counting is impractical (e.g. low-density or turbid samples).
3.9 F/M ratio (food-to-microorganism ratio). The mass loading rate of substrate (BOD or COD) applied per unit mass of biomass (MLVSS) in the aeration tank per day, $F/M=\dfrac{Q\,S_0}{V\,X}$ (kg BOD/kg MLVSS·d); a key activated-sludge design/control parameter governing sludge settleability and effluent quality.
3.10 OUR (oxygen uptake rate). The rate at which a mixed liquor consumes dissolved oxygen, typically measured in mg O₂/(L·h) via a respirometer; used operationally as a real-time proxy for biomass activity/health and organic loading in an aeration basin.
