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.
1.1 Mole. The SI base unit of amount of substance: exactly $6.022\times10^{23}$ elementary entities (atoms, molecules, ions) — Avogadro's number. The mass of one mole of a substance, in grams, equals its molecular (or atomic) weight.
1.2 Solubility Product ($K_{sp}$). The equilibrium constant for the dissolution of a sparingly-soluble ionic solid into its constituent ions, e.g. for $\text{CaCO}_3(s)\rightleftharpoons\text{Ca}^{2+}+\text{CO}_3^{2-}$, $K_{sp}=[\text{Ca}^{2+}][\text{CO}_3^{2-}]$. If the ion product exceeds $K_{sp}$ the solid precipitates; below it, more solid dissolves — the basis of lime softening and chemical phosphorus precipitation.
1.3 ORP (Oxidation–Reduction Potential). A voltage (mV), measured against a reference electrode, that indicates the net tendency of a solution to gain or accept electrons (be reduced) versus donate them (be oxidized). Positive ORP indicates oxidizing conditions (e.g. chlorinated, aerobic water); negative ORP indicates reducing conditions (e.g. anaerobic digesters, septic sediments). It is widely used as a real-time surrogate for disinfectant residual and redox state.
1.4 Temporary hardness. The portion of total water hardness (dissolved $\text{Ca}^{2+}$/$\text{Mg}^{2+}$) that is associated with bicarbonate/carbonate alkalinity and can therefore be removed by boiling — heat drives $\text{Ca(HCO}_3)_2\to\text{CaCO}_3\!\downarrow+\text{CO}_2+\text{H}_2\text{O}$, precipitating the calcium as scale. Hardness in excess of the alkalinity (associated with sulfate, chloride, nitrate) is permanent hardness and is unaffected by boiling.
1.5 Equivalent weight. The molecular (or atomic) weight of a substance divided by its valence (the number of $\text{H}^+$, $\text{OH}^-$, or electrons it supplies/accepts per formula unit in the reaction of interest). One equivalent of any substance reacts exactly with one equivalent of any other — the basis of normality ($N=$ equivalents per litre) and of expressing hardness/alkalinity as mg/L as $\text{CaCO}_3$ (equivalent weight 50).
