Question 2 of 6: Water Treatment Process Schematic

Question A2: Water Treatment Process Schematic (25 marks)

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 described is moderately turbid (30–50 NTU is well above the <1 NTU finished-water target, so conventional coagulation/flocculation/sedimentation is required, not membrane-free direct filtration), moderately-to-fully hard (200–250 mg/L as CaCO&sub3; sits in the "hard" range, 150–300 mg/L, so a softening step is warranted), subject to seasonal taste-and-odour (T&O) episodes (typically algal metabolites such as geosmin/2-MIB, needing an oxidation/adsorption step ahead of the point where chlorine would otherwise react with the precursors and form disinfection by-products), and near-neutral to mildly alkaline (pH 7.0–8.7, compatible with conventional alum coagulation without extensive pH correction upstream). A conventional treatment 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 screens (or travelling band screens) remove debris, leaves and fish before the water enters process; a raw-water pump station lifts flow to the head of the plant.

Pre-oxidation (T&O control). Potassium permanganate (KMnO₄) or powdered activated carbon (PAC) is dosed at the intake to oxidize/adsorb the geosmin/2-methylisoborneol responsible for seasonal earthy/musty taste-and-odour episodes (typically algal blooms in warmer months) before those precursors can react with chlorine downstream and form taste-active or regulated 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 pH (7.0–8.7) of this raw water sits comfortably in alum's effective coagulation pH window, so no separate pH-depression step is needed ahead of coagulation.

Sedimentation combined with lime-soda softening. Because the raw hardness (200–250 mg/L as CaCO&sub3;) is in the "hard" classification band, lime (and soda ash if the hardness is partly non-carbonate) is dosed into the same clarifier stage, raising pH to roughly 10–11 to precipitate calcium carbonate and magnesium hydroxide, which settle out alongside the coagulated turbidity floc in one combined sedimentation basin.

Recarbonation. Carbon dioxide is re-injected after the softening clarifier to lower the elevated pH back toward the 7.0–8.7 target range, preventing continued CaCO&sub3; precipitation (post-precipitation, scaling) in the filters and distribution system.

Filtration. Rapid (dual-media or sand) gravity filters polish any residual floc or fine CaCO&sub3; carryover that escapes sedimentation, producing low-turbidity (<0.3 NTU) filtered water.

Disinfection and fluoridation. Chlorine is dosed after filtration — not before — specifically because upstream T&O/organic precursors have already been reduced by pre-oxidation and clarification, minimizing trihalomethane formation while still achieving the CT (concentration × contact time) needed for primary disinfection credit. Fluoride is added here per municipal policy.

Clearwell & distribution. A clearwell provides the contact time needed to satisfy CT disinfection requirements and equalizes flow to the distribution system pumps.