Water Treatment Chemicals: A Selection Guide for Potable, Process and Cooling Systems

Start with the system, not the chemical

The same drum of sodium hypochlorite can be exactly right for a storage tank and completely wrong for a reverse-osmosis feed line. Before comparing products, be clear which of four systems you are treating: potable water for human consumption, process water feeding production or RO, recirculating cooling water, or waste water heading for discharge or reuse.

Each carries a different constraint. Potable systems are governed by what may contact drinking water and by the residual you must hold at the far end of the network. Process water is judged by what it does to the equipment downstream. Cooling water is a balancing act between three failure modes at once. Waste water is judged at the discharge point. Naming the system first eliminates half the catalogue before you look at a single price.

Coagulation and flocculation: what actually removes turbidity

Suspended solids and colour are removed in two stages. A coagulant neutralises the surface charge holding fine particles apart; a flocculant then bridges the destabilised particles into settleable flocs.

Poly aluminium chloride (PAC) works across a broad pH band, produces less sludge than traditional alum and needs less alkalinity — which is why it dominates in Gulf potable and process plants. Aluminium sulphate (alum) remains the cheapest option per kilo where alkalinity is plentiful and sludge volume is not a constraint. Ferric chloride outperforms both on colour, organics and phosphate removal, and holds up better in cold or high-organic water, at the cost of handling a corrosive iron solution and staining risk.

Polymer choice follows the sludge, not the water. Anionic polyacrylamide is the general-purpose settling aid for mineral solids. Cationic polyelectrolyte is what you want for organic sludge dewatering on a belt press or centrifuge. Charge density and molecular weight matter more than brand, and the only reliable way to pick them is a jar test on your own water.

Disinfection: choosing a chlorine source

Three chlorine donors cover most requirements, and they differ in handling more than in chemistry. Sodium hypochlorite solution is convenient to dose but loses strength in storage, faster in heat — a real consideration in a UAE summer, and a reason to buy smaller, fresher deliveries rather than one large drum.

Calcium hypochlorite granules hold their strength far longer and suit sites that dose intermittently or store for months, but they add hardness and need careful dry storage away from organics. Trichloroisocyanuric acid (TCCA) tablets dissolve slowly and carry a stabiliser that protects the residual against UV, which is why they suit open tanks, pools and remote dosing points — but the stabiliser accumulates, and in a closed process loop it can build up to the point where disinfection efficiency drops.

Whatever the source, disinfection is a product of concentration and contact time. A higher dose does not compensate for a tank that turns over in minutes.

pH control decides everything else

pH is not a separate topic from the chemicals above; it governs whether they work. Coagulants have a pH window in which the floc forms properly — outside it, you are dosing to no effect. Chlorine speciates with pH: as pH climbs, more of the residual sits as the far less active hypochlorite ion rather than hypochlorous acid, so the same measured residual disinfects less well. Corrosion and scaling tendencies both move with pH as well.

An acid dosing solution and a caustic (pH booster) solution are therefore the least glamorous and most useful items on a water treatment order. Specify the strength you can dose safely with the pumps you already own — a 30% acid solution is far easier to handle than concentrated acid, and dilution on site is where most incidents happen.

Scale, corrosion and biofouling in recirculating systems

Cooling towers and closed loops concentrate whatever the make-up water contains. Three chemistries hold the line: an antiscalant or scale inhibitor to keep hardness salts in solution past their normal saturation point; a corrosion inhibitor matched to the metallurgy in the loop; and a biocide programme to stop biofilm, which insulates surfaces, shelters corrosion and harbours organisms.

The common mistake is treating these as three independent purchases. An antiscalant that lets you run higher cycles of concentration raises the corrosion and biofouling load at the same time. Buy them as a programme, and see the dedicated cooling water guide below for how the three interact.

You cannot dose what you do not measure

Every dosing decision above depends on a measurement. At minimum, a treated system needs free and total chlorine by DPD, pH, and total hardness and alkalinity by titration. Conductivity gives you cycles of concentration in a cooling loop. Turbidity tells you whether coagulation is working before the customer complains.

Test reagents are consumables with an expiry date, and out-of-date DPD tablets read low — which is how a system ends up over-dosed. Put them on the same reorder schedule as the chemicals themselves.

What to send us for a water treatment quote

• System type: potable, process, cooling or waste water
• Recent water analysis, or the parameters you already monitor
• System volume and make-up or throughput rate
• Target residual, pH or hardness you need to hold
• Dosing equipment installed, and the concentration it can handle
• Storage available on site, and delivery frequency you prefer
• Documentation required: SDS, CoA, certificate of origin

Frequently asked questions

Do I really need a jar test before choosing a coagulant?

For anything beyond a small, stable, well-characterised supply, yes. Coagulant type and dose depend on turbidity, alkalinity, temperature and organic load, and a jar test on your own water costs far less than a month of over-dosing. We can advise on the test protocol and supply the reagents.

Can I switch from calcium hypochlorite to TCCA tablets?

Often, but check three things: the available chlorine per kilogram differs, so the dose changes; TCCA is stabilised and the stabiliser accumulates in recirculating systems; and TCCA lowers pH as it dissolves while calcium hypochlorite raises it. Plan the pH correction before you switch.