Question 2 of 5

Question 2 (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.

(a) Principle of ion-exchange water softening (10 marks)

Ion-exchange softening removes hardness by passing raw water down through a bed of synthetic cation-exchange resin (typically a sulfonated polystyrene, strong-acid cation resin) that has been pre-loaded with sodium ions. As the water percolates through the resin bed, the divalent hardness ions $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ have a much stronger affinity for the resin's fixed exchange sites than the monovalent sodium does, so they displace $\text{Na}^+$ from the resin and take its place, while an equivalent number of $\text{Na}^+$ ions are released into solution: $\text{Ca}^{2+} + 2\,\text{Na-Resin} \rightarrow \text{Ca-Resin}_2 + 2\,\text{Na}^+$ (and analogously for $\text{Mg}^{2+}$). The finished water leaving the bed is essentially hardness-free, with the equivalent sodium concentration raised in its place.

As softening continues, the resin's exchange sites gradually fill with $\text{Ca}^{2+}$/$\text{Mg}^{2+}$ until the bed is exhausted, at which point hardness begins to "leak" through (breakthrough); the bed is then taken off-line and regenerated by backwashing (to reclassify the bed and remove trapped solids) followed by passing a concentrated sodium chloride brine through it, which reverses the exchange by mass action — the very high $\text{Na}^+$ concentration in the brine displaces the accumulated $\text{Ca}^{2+}$/$\text{Mg}^{2+}$ back off the resin and into the spent-brine waste stream, restoring the bed to the sodium form for the next softening cycle. Because a zero-hardness finished water is aggressive/corrosive and softening removes essentially all hardness rather than reducing it to a target level, municipal plants commonly split-treat (bypass a fraction of raw water around the exchanger and blend it back in) to achieve a target finished hardness rather than treating the full flow to zero. Ion exchange is favoured over lime-soda softening where space is limited or the water is low in suspended solids, since it produces a small volume of brine waste rather than a large lime-sludge stream requiring dewatering and disposal.

(b) Jar test procedure for optimum coagulant dose (10 marks)

The jar test is a bench-scale simulation of the plant's rapid-mix/flocculation/sedimentation train, run on a multi-paddle gang stirrer holding a series of identical beakers (commonly six 1 L jars) filled with the same raw water sample. Each jar receives a different, incrementally increasing dose of the candidate coagulant (e.g., alum or a polyaluminum chloride), with the paddles first set to a rapid-mix speed (≈100–150 rpm) for about 1 minute to flash-mix and disperse the coagulant before it hydrolyzes and begins destabilizing the colloidal turbidity. The paddle speed is then dropped to a slow-mix/flocculation speed (≈20–30 rpm) for 15–20 minutes, gently promoting particle collisions so destabilized colloids aggregate into visible floc without shearing the floc apart, and the operator observes and records the time to first visible floc formation and the qualitative floc size/settling character in each jar.

Mixing is then stopped and the jars are allowed to settle quietly for about 30 minutes, after which a sample is withdrawn from a fixed depth in each jar (avoiding the settled sludge blanket) and its residual turbidity (and often pH, colour, and residual coagulant) is measured. Plotting residual turbidity against applied coagulant dose typically shows turbidity falling steeply as dose increases, reaching a minimum, and then rising again if the dose is pushed into overdosing (charge reversal re-stabilizes the colloids); the optimum dose is read as the lowest dose at or just past the point where residual turbidity reaches its plateau/minimum, balancing chemical cost against treated-water quality. The same procedure is used to screen coagulant type, coagulant-aid polymers, and the effect of pH adjustment by simply varying that parameter across the jar series instead of (or in addition to) the primary coagulant dose.

(c) Residual chlorine: definition and significance (5 marks)

Residual chlorine is the chlorine remaining in the water after the initial chlorine demand (reaction with ammonia, organics, iron, manganese, and other oxidizable/reactive material in the water) has been satisfied; it is reported as free residual (hypochlorous acid $\text{HOCl}$ and hypochlorite ion $\text{OCl}^-$) and combined residual (chloramines, formed when free chlorine reacts with ammonia), with total residual being their sum. Its significance is twofold: first, a maintained residual through the distribution system provides an ongoing disinfecting reserve that continues to inactivate pathogens and suppress biofilm regrowth all the way to the customer's tap, guarding against recontamination from cross-connections, main breaks, or growth in the pipe network; second, the residual is used operationally as a simple, continuously-monitorable surrogate indicator of water safety — a measurable free-chlorine residual (Canadian guidance under the Guidelines for Canadian Drinking Water Quality targets a detectable free residual, commonly ≥0.2 mg/L, maintained to the extremities of the distribution system) gives confidence that adequate CT (concentration × contact time) disinfection has been, and continues to be, achieved without requiring a full microbiological assay at every point in the network.