Question 5 of 5

Question 5 (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's four flagged characteristics each drive a specific unit process. The 150–200 NTU turbidity is high enough to require the full conventional train — coagulation, flocculation, sedimentation and filtration — rather than direct (in-line) filtration, since a sedimentation step is needed ahead of the filters to avoid rapid filter-run termination at this loading. The pH of 6.5 is on the low side for effective alum/PACl coagulation (which performs best around pH 6–7.5 for aluminum salts, but corrosion-control practice targets a higher finished-water pH), so lime is dosed at rapid mix both to buffer coagulation chemistry and to bring the finished water up toward a non-corrosive, near-neutral-to-slightly-alkaline pH before it reaches the distribution system. The seasonal taste-and-odour episodes (typically algal/geosmin-MIB related in surface sources) are controlled by dosing potassium permanganate ahead of or at rapid mix, which oxidizes the taste-and-odour precursor compounds; this same oxidant conveniently also oxidizes dissolved (reduced) iron $\text{Fe}^{2+}$ to the insoluble $\text{Fe}^{3+}$ (ferric) form, which then co-precipitates with the coagulant floc and is removed in sedimentation and filtration.

The raw water's 100 mg/L hardness is only moderate (roughly the "moderately hard" range) and is not treated with a dedicated softening stage in this train: conventional coagulation with lime for pH control removes only a small, incidental fraction of hardness, so the finished-water hardness leaving the plant is expected to remain close to the raw-water value (≈90 mg/L, allowing for minor incidental removal), which is acceptable for most municipal uses without triggering complaints. If the utility instead required materially softer water, a lime-soda softening stage (with its own sedimentation step for the calcium carbonate/magnesium hydroxide precipitate) would need to be inserted before filtration — this is flagged as a design option rather than included by default, since the raw hardness given is moderate rather than severe. Finally, pathogen control is provided in two complementary ways: physical removal through coagulation/sedimentation/filtration (each barrier removes a fraction of bacteria, protozoa cysts/oocysts and viruses by enmeshment/adsorption onto floc and filter media) and chemical inactivation by chlorine disinfection at the clearwell, sized to deliver adequate CT for the finished water to meet the Guidelines for Canadian Drinking Water Quality's pathogen log-removal/inactivation credits before entering distribution.