Twenty Questions From the Demineralised Water Plant Floor: Coagulation, Resin and RO

Demineralised water production throws up the same practical questions year after year, and the answers decide whether a pure water plant runs cheaply or expensively. Acid and alkali consumption, coagulation control, resin behaviour and membrane fouling account for most of the difference. What follows works through twenty of them in the order they tend to come up on shift.

Q1. How is acid and alkali consumption actually cut?

Four levers: guarantee the quality of the influent water, guarantee regeneration quality and lengthen the water production cycle, guarantee the purity of the regenerant and control the regeneration procedure strictly, and keep the equipment safe, reliable and in normal service. Most plants attack the third lever first because it is cheapest, then discover the first one was the real constraint.

Q2. Why are colloids stable in water?

The surface carries an electric charge, so like charges repel. The surface also holds a hydration layer that keeps particles apart, and it adsorbs substances that stabilise it further. All three together explain why a colloid can stay suspended for weeks.

Q3. What is a coagulant aid for?

Two jobs: improving floc structure so flocs are larger, tougher and heavier, and adjusting pH and alkalinity so coagulation conditions are optimal. A coagulant aid does not coagulate on its own; it promotes the coagulation of impurities already in the water.

Q4. What happens during coagulation?

Colloidal particles carry a negative charge and are in constant Brownian motion, so the system is stable and particles settle only with difficulty. Add a suitable dose of coagulant and the fines are destabilised, adsorption bridging follows, and they flocculate into flocs that settle quickly.

Q5. Which factors affect coagulation most?

pH first: when PAC is dosed it hydrolyses to Al(OH)3 colloid, whose solubility is lowest between pH 6.5 and 7.5, where coagulation performs best. Alkalinity second: when it is insufficient, the hydrogen ions released by hydrolysis depress pH and performance falls. Temperature third: cold water is more viscous, hydrolysis is slow, and flocs form slowly with a loose structure and fine particles that settle poorly. The composition, properties and concentration of the impurities do the rest.

Q6. How do carbonate species shift with pH?

At pH 4.3 or below only free CO2 is present. Between roughly pH 8.3 and 8.4 more than 98 percent is bicarbonate, present as HCO3-. Above pH 8.4 no CO2 remains. That is why alkalinity control and pH control cannot be treated as separate tasks.

Q7. Why treat boiler water internally?

To stop deposits and corrosion accumulating in the boiler proper and its auxiliaries, in both water and steam side, and to keep heat transfer efficient. The second purpose is steam quality: preventing scaling and corrosion of turbine components while cutting blowdown losses for a given water quality.

Q8. How does a centrifugal pump prime itself?

It does not. Before start-up the casing and suction pipe must be completely full. The impeller then throws water to its outer edge by centrifugal force, the water collects in the casing and leaves through the volute, and the impeller eye is left at partial vacuum so atmospheric pressure pushes fresh water up the suction pipe. Continuous throwing and continuous replenishment give continuous delivery.

Q9. What is resin regeneration?

After a period of softening or demineralisation service a resin loses exchange capacity. Treating it with acid, alkali or brine returns it to the ionic form that can exchange again. Note that regeneration restores capacity, not cleanliness: fouling has to be dealt with separately.

Q10. What sets the working exchange capacity of a resin?

Five things: influent water quality, the control criterion for the end point of the exchange run, the height of the resin bed, water temperature and flow velocity, and the effectiveness of regeneration together with the properties of the resin itself.

Q11. What chemical behaviour should an operator expect from ion exchange resin?

Reversibility first, as in RH + Na+ = RNa + H+. Then acid-base behaviour: the anion resin ROH dissociates to R+ and OH-, the cation resin RH dissociates to R- and H+. Then selectivity, since a resin adsorbs different ions to very different degrees. For cation resin the series runs Fe3+ > Al3+ > Ca2+ > Mg2+ > K+ ≈ NH4+ > Na+; for anion resin SO4 2- > NO3- > Cl- > HCO3- > HSiO3-.

Q12. What fouls a mixed bed?

Suspended solids, mostly on the cation resin, which calls for stronger pre-treatment. Organic fouling, mainly on strongly basic anion resin, best treated by soaking for 24 hours in a mixed solution of 1 to 4 percent NaOH and 5 to 12 percent NaCl. Heavy metal and iron fouling, mostly in the anion resin, controlled by protecting pipework from corrosion and reducing iron in the influent.

Q13. Why does membrane performance decline?

Chemical change in the membrane itself through hydrolysis and oxidative attack by free chlorine; physical change through compaction, which lowers permeability and raises salt rejection; and fouling from scaling, micro-organisms and solid particles blocking the surface or the interior.

Q14. What does a security filter actually catch?

Residual fine suspended particles, colloids and micro-organisms, intercepted on and inside a 5 micron polypropylene cartridge. Trapped solids raise resistance, and when the differential pressure reaches 0.1 MPa the cartridge is replaced. The element is a replaceable bayonet type, which is the whole reason this stage is cheap to run.

Q15. How is membrane scaling prevented?

Pre-treat properly and keep the silt density index under control. Hold a suitable working pressure, since higher pressure raises permeate output but excessive pressure compacts the membrane. Keep the concentrate turbulent to limit concentration polarisation and stop sparingly soluble salts precipitating at the surface. Flush chemically on short outages and protect with a formaldehyde-based storage solution on long ones. When permeate flow falls noticeably or salt passage rises, scaling has already started and cleaning is due.

Q16. Why dose NaHCO3 ahead of the RO train?

To remove or reduce residual chlorine and protect membrane stability. In SINOKLE systems residual chlorine is held below 0.1 mg/L at that point, which is the figure worth writing into the operating procedure rather than leaving to judgement.

Q17. Why fit a motorised slow-opening valve before the membrane module?

To stop the high-pressure pump starting or stopping abruptly. A sudden pressure rise sends a surge into the elements, and the resulting water hammer damages the membrane.

Q18. What is a filtration cycle made of?

The operating time between two backwashes, made up of three stages: filtration, backwash and forward rinse. Backwash removes the dirt accumulated during filtration and restores dirt-holding capacity; forward rinse is what guarantees acceptable filtered water, and only when the rinse water is up to standard does the unit enter the next production cycle.

Q19. How does activated carbon remove chlorine?

By chemical reaction, not adsorption. Free residual chlorine is catalysed at the carbon surface and hydrolyses rapidly to release atomic oxygen, which reacts with carbon to form carbon dioxide; hypochlorous acid converts to CO2 the same way. The overall reaction is C + 2Cl2 + 2H2O = 4HCl + CO2. Because carbon is consumed in proportion to residual chlorine, it has to be topped up each year.

Q20. What is the principle behind reverse osmosis?

A semi-permeable membrane passes water but not salts. Apply pressure on the raw-water side and part of the pure water crosses perpendicular to the membrane while salts and colloidal matter concentrate at the surface; the remaining raw water sweeps that concentrate away parallel to the membrane. The permeate carries only a small amount of salt, and collecting it completes the demineralisation.