18-Env-A4 Water and Wastewater Engineering · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved calculator permitted. The paper instructs that Question 1 is compulsory and any three of the remaining four questions are required; all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — activated-sludge kinetics, solids/hydraulic retention time, nitrogen and phosphorus forms; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — discrete settling theory, indicator organisms, coagulation chemistry; MWH’s Water Treatment: Principles and Design (3rd ed.) — process selection, softening, rapid and slow sand filtration; 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 is far above a <1 NTU finished-water target, ruling out membrane-free direct filtration and requiring full coagulation/flocculation/sedimentation), hard (200–250 mg/L as CaCO&sub3; sits in the "hard" 150–300 mg/L band, warranting lime-soda softening), subject to seasonal taste-and-odour (T&O) episodes (typically algal geosmin/2-MIB, needing pre-oxidation or adsorption ahead of the point chlorine would otherwise react with those precursors), and mildly acidic-to-neutral (pH 6.5–7.0, squarely inside alum's effective coagulation window so no separate pH adjustment is needed before rapid mix). A conventional coagulation/flocculation/sedimentation train with an added lime-soda softening stage addresses all four characteristics in one integrated process, as shown below.
Intake & screening. Coarse and fine bar (or travelling band) screens remove debris and fish ahead of process, with a raw-water pump station lifting flow to the head of the plant.
Pre-oxidation (T&O control). Potassium permanganate or powdered activated carbon is dosed at the intake to oxidize/adsorb the geosmin/2-MIB responsible for seasonal earthy/musty odour episodes before those organic precursors can meet free chlorine downstream and form taste-active or regulated disinfection by-products.
Rapid mix (coagulation) and flocculation. Alum plus a polymer coagulant aid is flash-mixed in to destabilize the 30–50 NTU turbidity-causing colloids by charge neutralization, then carried through a tapered-energy flocculation basin to grow destabilized colloids into settleable floc. Because the raw pH (6.5–7.0) already sits inside alum's optimal coagulation range, no acid or caustic dosing is needed ahead of rapid mix.
Sedimentation combined with lime-soda softening. Because raw hardness (200–250 mg/L as CaCO&sub3;) falls in the "hard" classification, lime (with soda ash if part of the hardness is non-carbonate) is dosed into the same clarifier stage, raising pH to roughly 10–11 to precipitate $\text{CaCO}_3$ and $\text{Mg(OH)}_2$, which settle alongside the coagulated turbidity floc in one combined basin.
Recarbonation. $\text{CO}_2$ is re-injected after the softening clarifier to bring the elevated pH back down toward the 6.5–7.0 target, preventing continued $\text{CaCO}_3$ precipitation (post-precipitation, scaling) in the filters and distribution system.
Filtration. Rapid (dual-media or sand) gravity filters polish any residual floc or fine carbonate carryover escaping sedimentation, producing low-turbidity (<0.3 NTU) filtered water.
Disinfection and fluoridation. Chlorine is dosed after filtration — not before — because the T&O/organic precursors have already been reduced by pre-oxidation and clarification, minimizing trihalomethane formation while still meeting the CT (concentration × contact time) needed for primary disinfection credit. Fluoride is added at this point per municipal policy.
Clearwell & distribution. A clearwell provides the contact time needed to satisfy CT disinfection requirements and equalizes flow into the distribution pumps.
