18-Env-A4 Water and Wastewater Engineering · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2017 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. Question 1 is compulsory; the paper instructs candidates to attempt any three of the remaining four questions — all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — oxygen sag/Streeter-Phelps, MLSS/MLVSS, population equivalent, primary clarifier design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — turbidity, alkalinity chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — coagulation-flocculation, softening, disinfection by-products, pH; 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.
The raw water is moderately turbid (30–50 NTU, far above the <1 NTU finished-water target, so full coagulation-flocculation-sedimentation-filtration is needed, not direct filtration), hard (200–250 mg/L as CaCO3 falls in the “hard” 150–300 mg/L band, warranting a softening step), subject to seasonal taste-and-odour (T&O) episodes (typically algal metabolites such as geosmin/2-MIB, needing oxidation/adsorption ahead of chlorination), and near-neutral to mildly alkaline (pH 7.0–8.7, compatible with conventional alum coagulation without extensive upstream pH correction). One integrated conventional train with a lime-soda softening stage addresses all four characteristics, shown below.
Pre-oxidation. Potassium permanganate or powdered activated carbon is dosed immediately after screening, ahead of any chlorine contact, to oxidize/adsorb the geosmin and 2-MIB responsible for the seasonal taste-and-odour episodes before they can react with chlorine downstream and generate additional disinfection by-products.
Coagulation (rapid mix) and flocculation. Alum (or ferric chloride) plus a polymer coagulant aid is flash-mixed into the flow to destabilize the 30–50 NTU turbidity-causing colloids by charge neutralization, then carried through a tapered-energy flocculation basin (decreasing velocity gradient $G$ across compartments) to grow the destabilized colloids into settleable floc. The near-neutral to mildly alkaline raw pH (7.0–8.7) sits comfortably in alum’s effective coagulation window, so no separate upstream pH-depression step is required.
Sedimentation combined with lime-soda softening. Because the raw hardness (200–250 mg/L as CaCO3) is in the “hard” band, lime (and soda ash where hardness is partly non-carbonate) is dosed into the same clarifier stage, raising pH to roughly 10–11 to precipitate CaCO3 and Mg(OH)2, which settle with the coagulated turbidity floc in one combined basin.
Recarbonation, filtration, disinfection. CO2 is re-injected after softening to bring the elevated pH back down to a non-scaling range before rapid-sand filtration polishes any residual floc/CaCO3 carryover. Chlorine (plus fluoride) is dosed after filtration — not before — so that most of the natural organic matter and T&O precursors have already been removed, minimizing DBP formation while still providing a chlorine residual through the clearwell and into distribution.
