Pure Water Treatment: Twenty Practical Questions on Demineralisation, Resin and RO

Demineralised water production raises a recurring set of practical questions — acid and alkali consumption, coagulation control, resin behaviour, RO fouling — that decide whether a pure water plant runs cheaply or expensively. The twenty questions below cover the points operators ask most often.

Q1. What are the main ways to cut acid and alkali consumption?

(1) Guarantee the quality of the influent water. (2) Guarantee regeneration quality and lengthen the water production cycle. (3) Guarantee the quality and purity of the regenerant and control the regeneration procedure strictly. (4) Keep the equipment safe, reliable and in normal operation.

Q2. Why are colloids stable in water?

(1) The colloid surface carries an electric charge. (2) The colloid surface has a hydration layer. (3) The colloid surface adsorbs substances that stabilise it.

Q3. What is the purpose of a coagulant aid?

(1) To improve floc structure, making the flocs larger, tougher and heavier. (2) To adjust the pH and alkalinity of the treated water so that conditions are optimal for coagulation and coagulation performance improves. A coagulant aid does not itself coagulate, but it promotes the coagulation of impurities in the water.

Q4. What is coagulation?

Colloidal particles in water carry a negative charge; like charges repel, and the particles are also in constant Brownian motion, so the system is extremely stable and the particles settle only with difficulty. When a suitable dose of coagulant is added, the fine colloidal particles are destabilised, adsorption bridging occurs, and they flocculate into flocs that settle quickly. This process is coagulation.

Q5. What are the main factors affecting coagulation?

(1) pH: when PAC is dosed it hydrolyses to Al(OH)3 colloid, whose solubility is lowest at pH 6.5 to 7.5, where coagulation performs best. (2) Alkalinity: when alkalinity is insufficient, the hydrogen ions released as the coagulant hydrolyses depress pH and coagulation performance falls. (3) Temperature: at low temperature the water is more viscous, hydrolysis is slow, flocs form slowly with a loose structure and fine particles that settle poorly. (4) The composition, properties and concentration of the impurities in the water all strongly affect coagulation.

Q6. How do carbonate species in water depend on pH?

(1) At pH 4.3 or below, only free CO2 is present. (2) Between roughly pH 8.3 and 8.4, more than 98 percent is bicarbonate (HCO3-). (3) Above pH 8.4, no CO2 remains.

Q7. What is the purpose of internal boiler water treatment?

(1) To prevent deposits and corrosion accumulating in the boiler proper and its auxiliary systems, in both water and steam, and to improve heat transfer efficiency. (2) To guarantee steam quality and prevent scaling and corrosion of turbine components, while reducing blowdown losses and improving economics for a given water quality.

Q8. How does a centrifugal pump work?

A centrifugal pump uses the rotation of an impeller to generate centrifugal force in the water. Before start-up the casing and suction pipe must be completely filled with water. The motor then drives the shaft, and impeller and water rotate at high speed; centrifugal force throws the water to the outer edge of the impeller, from where it collects in the casing and passes through the volute into the discharge pipe. Meanwhile the centre of the impeller is left at a partial vacuum by the water thrown out, so water from the suction basin is drawn through the suction pipe into the impeller by atmospheric pressure. As the impeller keeps turning, water is continuously thrown out and continuously replenished, which gives the centrifugal pump its continuous delivery.

Q9. What is resin regeneration?

After a period of softening or demineralisation service, a resin loses its ion exchange capacity. It can be restored by treating it with acid, alkali or brine, which returns it to the ionic form that can exchange again. This restoration process is called regeneration.

Q10. What mainly determines the working exchange capacity of a resin?

(1) The quality of the influent water. (2) The control criterion for the end point of the exchange run. (3) The height of the resin bed. (4) Water temperature and flow velocity. (5) The effectiveness of regeneration and the properties of the resin itself.

Q11. What chemical properties does an ion exchange resin have?

(1) Reversibility of the ion exchange reaction, for example RH + Na+ = RNa + H+. (2) Acid-base behaviour: the anion resin ROH dissociates to R+ and OH-, the cation resin RH dissociates to R- and H+. (3) Selectivity: a resin adsorbs different ions to very different degrees. (4) The magnitude of exchange capacity, which follows the selectivity series. For cation resin: Fe3+ > Al3+ > Ca2+ > Mg2+ > K+ ≈ NH4+ > Na+. For anion resin: SO4 2- > NO3- > Cl- > HCO3- > HSiO3-.

Q12. What kinds of fouling affect mixed-bed resin?

(1) Suspended solids fouling, which mostly appears on the cation resin; strengthen pre-treatment of the raw water. (2) Organic fouling, which occurs mainly on strongly basic anion resin; the standard remedy is to soak the resin for 24 hours in a mixed solution of 1 to 4 percent NaOH and 5 to 12 percent NaCl. (3) Heavy metal and iron fouling, which forms mostly in the anion resin; control corrosion of pipework and equipment, reduce the iron content of the influent and add iron removal measures.

Q13. What makes RO membrane performance decline?

(1) Chemical change in the membrane itself: hydrolysis, and oxidative attack by free chlorine and active chlorine. (2) Physical change: compaction of the membrane, which lowers water permeability and raises salt rejection. (3) Fouling: scaling, micro-organisms and solid particles blocking the membrane surface or its interior.

Q14. How does a security filter work?

It is mechanical filtration through a 5 micron polypropylene cartridge: residual fine suspended particles, colloids and micro-organisms are intercepted or adsorbed on the cartridge surface and in its pores. As water production continues, the trapped solids raise the resistance, and when the pressure difference between inlet and outlet reaches 0.1 MPa the cartridge must be replaced. The filter element is a replaceable bayonet cartridge.

Q15. How is scaling of an RO membrane prevented?

(1) Pre-treat the raw water properly and keep the silt density index under control. (2) Maintain a suitable working pressure: higher pressure raises permeate output, but excessive pressure compacts the membrane. (3) Keep the concentrate turbulent during operation to reduce concentration polarisation at the membrane surface and prevent sparingly soluble salts from precipitating there. (4) When the RO unit is shut down, flush it with chemicals for short outages and protect it with a formaldehyde-based storage solution for long ones. (5) When permeate flow falls noticeably or the salt content rises, scaling or fouling has occurred and chemical cleaning is required.

Q16. Why is NaHCO3 dosed in RO demineralisation?

To eliminate or reduce residual chlorine in the water and so protect the stability of the RO elements. In SINOKLE systems residual chlorine is held below 0.1 mg/L.

Q17. Why fit a motorised slow-opening valve ahead of the RO membrane module?

To prevent the high-pressure pump from starting and stopping abruptly during RO operation: a sudden pressure rise sends a high-pressure surge into the elements and the resulting water hammer damages the membrane.

Q18. What is a filtration cycle, and what are its stages?

A filtration cycle is the actual operating time between two backwashes. It comprises three stages: filtration, backwash and forward rinse. Backwash removes the dirt accumulated during filtration and restores the dirt-holding capacity of the medium. Forward rinse is the step that guarantees acceptable filtered water; only when the rinse water is up to standard does the unit enter the next production cycle.

Q19. How does activated carbon remove chlorine?

Activated carbon removes residual chlorine by chemical reaction, not by physical adsorption. As free residual chlorine passes through the carbon it is catalysed at the surface, hydrolysing rapidly to release atomic oxygen, which reacts with carbon atoms to form carbon dioxide; hypochlorous acid in the raw water is likewise converted rapidly to CO2 gas. The overall reaction is: C + 2Cl2 + 2H2O = 4HCl + CO2. Because the carbon is consumed in proportion to the residual chlorine in the raw water, it should be topped up each year as appropriate.

Q20. What is the principle of reverse osmosis?

RO exploits the fact that a semi-permeable membrane passes water but not salts, removing most of the salt from the water. Pressure is applied on the raw-water side, so that part of the pure water passes through the membrane perpendicular to it while salts and colloidal matter are concentrated at the membrane surface; the remaining raw water sweeps the concentrated material away parallel to the membrane. The permeate contains only a small amount of salt, and collecting it completes the demineralisation.