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.
Chemical unit processes use a reagent’s chemistry — rather than screening, settling or biological metabolism — to transform or remove a contaminant. Five that appear throughout municipal water and wastewater practice:
1. Coagulation–flocculation. A metal coagulant (alum, ferric chloride) hydrolyses to a positively-charged precipitate that neutralizes the negative surface charge on colloidal turbidity/NOM particles, allowing them to aggregate into settleable flocs ahead of clarification and filtration — the primary particulate-removal step in conventional water treatment.
2. Chemical (oxidative) disinfection. Chlorine, chloramines, ozone or UV-generated radicals oxidize/damage the cell walls, enzymes and nucleic acids of pathogens, inactivating bacteria, viruses and protozoa before distribution or discharge; a residual (e.g. free chlorine) is also carried into the distribution system to guard against re-growth.
3. Chemical precipitation (softening / phosphorus removal). Lime (and soda ash) raise pH to drive $\text{Ca}^{2+}$/$\text{Mg}^{2+}$ or metal-phosphate salts below their $K_{sp}$, precipitating hardness or, with alum/ferric salts, phosphorus as AlPO4/FePO4 (Question 7, below) — removing a dissolved species by converting it to a settleable solid.
4. Ion exchange. Water is passed through a resin bed that swaps an undesirable ion (e.g. $\text{Ca}^{2+}$, $\text{NO}_3^-$) for a benign one held on the resin ($\text{Na}^+$, $\text{Cl}^-$); used for hardness polishing and nitrate removal, and regenerated with a concentrated brine once exhausted.
5. Adsorption (activated carbon). Dissolved organics, taste/odour compounds and some trace contaminants partition onto the enormous internal surface area of granular or powdered activated carbon by van der Waals/hydrophobic attraction, polishing water quality after biological/physical treatment has removed the bulk load.
