Question 2 of 5: Ion-Exchange Softening and the Significance of Selected Water-Quality Parameters

Question 2: Ion-Exchange Softening and the Significance of Selected Water-Quality Parameters (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.

Part (a) — The ion-exchange process for water softening (10 marks)

Ion exchange softens water by swapping the hardness cations for a cation that does not precipitate soap or scale. The exchanger is an insoluble solid — historically the natural zeolite glauconite, today a sulfonated polystyrene–divinylbenzene bead resin — carrying fixed anionic sites with mobile counter-ions. Writing the exchanger as $\mathrm{Na_2R}$, where $\mathrm{R}$ denotes the divalent resin radical, the process has two phases that alternate in the same vessel: a service run and a regeneration.

During the service run the raw water percolates downward through the bed. Calcium and magnesium have a higher affinity for the resin than sodium, so they displace it, ion for ion on an equivalent basis:

The non-carbonate (permanent) hardness is removed by exactly the same mechanism, the associated anion simply being carried through unchanged:

This is the first important contrast with lime–soda softening: ion exchange removes carbonate and non-carbonate hardness with equal ease and in one step, whereas the lime–soda process needs soda ash as a second reagent for the non-carbonate fraction. It is also a genuine exchange rather than a precipitation, so no sludge is produced during the run and the effluent hardness approaches zero — typically below $5\ \text{mg/L}$ as $\mathrm{CaCO_3}$ — until breakthrough.

The bed has a finite capacity, quoted as equivalents of hardness per unit volume of resin (roughly $1$ to $2\ \text{eq/L}$ for a strong-acid resin). When the leading edge of the exchange zone reaches the underdrain, hardness appears in the product and the run is stopped. Regeneration then drives the equilibrium backwards by mass action, using a strong brine, typically 5 to 10 percent sodium chloride:

The cycle is completed by a backwash to reclassify the bed and remove trapped solids, a slow rinse to displace the spent brine, and a fast rinse to polish the resin before it returns to service. If the resin is operated on the hydrogen cycle instead — the first step of a demineraliser — the exchange releases acid rather than sodium salts, and the carbonic acid formed from the bicarbonate alkalinity is stripped in a downstream degasifier:

Three consequences of the chemistry decide whether ion exchange is the right choice. First, the process gives essentially zero hardness, which is corrosive and unstable, so part of the raw water is normally bypassed around the softener and blended back to a finished hardness of about $80$ to $120\ \text{mg/L}$ as $\mathrm{CaCO_3}$. Second, sodium is added in strict equivalence to the hardness removed — roughly $0.46\ \text{mg}$ of sodium per mg of hardness as $\mathrm{CaCO_3}$ — which matters where sodium-restricted diets or drinking-water aesthetic objectives are a concern. Third, the spent regenerant is a small volume of very concentrated calcium and sodium chloride brine; in Canadian practice its disposal, and the chloride loading it places on the receiving water or on a municipal wastewater plant that cannot remove chloride, is usually the governing constraint on the whole scheme.

Part (b) — Significance of fluorides, nitrates, sulfates, sulfides and hardness (15 marks)

Fluorides. Fluoride is the textbook example of a parameter with an optimum rather than a maximum. At roughly $0.7\ \text{mg/L}$ it is incorporated into tooth enamel as fluorapatite and measurably reduces the incidence of dental caries; below about $0.5\ \text{mg/L}$ that benefit is lost, and above roughly $1.5\ \text{mg/L}$ it produces dental fluorosis, with skeletal fluorosis at sustained higher exposures. Health Canada therefore publishes a maximum acceptable concentration of $1.5\ \text{mg/L}$ in the Guidelines for Canadian Drinking Water Quality alongside an optimum supplementation target near $0.7\ \text{mg/L}$. The engineering significance is that fluoride may have to be added (as hydrofluosilicic acid or sodium silicofluoride, with the tight dose control that a narrow optimum demands) or removed from a naturally high groundwater by activated alumina, bone char or reverse osmosis. Fluoride is also conservative: it is not removed by conventional coagulation, settling or filtration.

Nitrates. Nitrate is the fully oxidised, mobile end product of the nitrogen cycle, and its significance is threefold. As a health parameter it is the direct cause of methaemoglobinaemia in bottle-fed infants, whose gut flora reduce it to nitrite which then oxidises haemoglobin; the Canadian maximum acceptable concentration is $45\ \text{mg/L}$ as nitrate, equivalently $10\ \text{mg/L}$ as nitrate nitrogen. As an indicator it fingerprints the source of contamination, since elevated nitrate in a rural well almost always points to septic-field or manure influence. As an environmental parameter it is a nutrient that, with phosphorus, drives eutrophication — and in marine and estuarine receiving waters it is usually nitrogen, not phosphorus, that limits algal growth. Because nitrate is an anion and highly soluble it is not removed by conventional treatment; the practical options are anion exchange, reverse osmosis, biological denitrification, or blending with a cleaner source.

Sulfates. Sulfate is chiefly an aesthetic and operational parameter rather than a toxic one. Above roughly $500\ \text{mg/L}$ it imparts a bitter taste and acts as a laxative on unacclimatised consumers, and it contributes to the total dissolved solids and to scale in boilers as calcium sulfate. Its more serious significance is in wastewater and in construction. In a sewer or an anaerobic digester, sulfate is the electron acceptor that sulfate-reducing bacteria use to generate hydrogen sulfide, so a high-sulfate water supply guarantees an odour and corrosion problem downstream. In groundwater and in industrial effluent, sulfate attacks Portland-cement concrete by forming expansive ettringite, which is why the Canadian concrete standard grades sulfate exposure classes and calls for sulfate-resisting cement. Sulfate itself is removed only by membranes, ion exchange or chemical precipitation, all of them costly.

Sulfides. Hydrogen sulfide is the reduced counterpart of sulfate and is the single most consequential nuisance parameter in a collection system. It is detectable by odour at a few parts per billion, causes taste and odour complaints in supply at about $0.05\ \text{mg/L}$, and is acutely toxic: at low concentrations it paralyses the sense of smell, and above roughly $700\ \text{ppm}$ in air it is rapidly fatal, which makes it the classic confined-space hazard governed by provincial occupational health and safety regulation. It also drives crown corrosion: $\mathrm{H_2S}$ released into the sewer atmosphere is oxidised by Thiobacillus on the moist pipe soffit to sulfuric acid, which destroys the cement matrix of concrete sewers. In treatment it exerts an immediate chlorine demand, blackens the water by forming ferrous sulfide, and interferes with the BOD test. Control is by ventilation, by maintaining aerobic conditions and velocity in the sewer, by chemical dosing with oxygen, nitrate or iron salts, and by protective linings.

Hardness. Hardness is the sum of the polyvalent metal cations, in practice calcium and magnesium, expressed as mg/L of $\mathrm{CaCO_3}$. It carries no recognised health risk — if anything the epidemiological association with cardiovascular health is mildly favourable — so its significance is entirely economic and operational. Hard water consumes soap by precipitating it as an insoluble scum, and it deposits calcium carbonate scale in water heaters, boilers, kettles and distribution mains, reducing hydraulic capacity and heat-transfer efficiency. Waters below about $75\ \text{mg/L}$ are called soft and above $150\ \text{mg/L}$ hard; municipal softening is normally considered above roughly $150$ to $200\ \text{mg/L}$, and the finished target is a blend near $80$ to $120\ \text{mg/L}$ rather than zero, because a water with no hardness and no alkalinity is aggressive and will dissolve the cement lining and the lead and copper of the distribution system. Hardness is thus the clearest illustration of the general principle that a water-quality parameter is optimised, not minimised.