Common Water Treatment Chemicals and Their Roles

Water treatment chemicals are used in municipal drinking water plants, wastewater treatment plants, industrial systems such as cooling towers and boilers, and sometimes pools or well water. Their jobs: remove suspended solids, eliminate pathogens, adjust water chemistry, manage scale and corrosion, and tackle specific contaminants.

1. Coagulants

Coagulants neutralise the negative surface charge on fine particles (colloids, turbidity, organics, some metals) so they form micro-flocs.

• Aluminium sulphate (alum) – budget-friendly for municipal plants, best at pH ~6–8.
• Polyaluminium chloride (PAC) – wider pH range, lower sludge volumes.
• Ferric chloride / ferric sulphate – effective at pH ~4–11; good for colour, phosphorus and organics; denser flocs.

Role: the essential first step for clarifying surface water.

2. Flocculants (Coagulant Aids)

High molecular weight polymers – cationic, anionic or non-ionic polyacrylamides (PAM) – link micro-flocs into larger, denser flocs that settle or filter easily. Typical dose 0.1–2 mg/L. They speed settling and filtration, reduce residual turbidity and aid sludge dewatering.

3. Disinfectants / Oxidants

• Chlorine (gas, sodium hypochlorite, calcium hypochlorite, TCCA) – the most widely used, with residual protection.
• Chloramines – longer-lasting residual, fewer by-products.
• Chlorine dioxide – strong oxidant, fewer trihalomethanes.
• Ozone – powerful, no residual; excellent for taste and odour.
• Hydrogen peroxide & potassium permanganate – iron, manganese, taste and odour.

4. pH Adjusters / Alkalinity Control

pH control optimises coagulation, disinfection and corrosion control; drinking water typically targets 6.5–8.5.

• Raise pH: lime, caustic soda, soda ash, sodium bicarbonate.
• Lower pH: sulphuric acid, hydrochloric acid, CO₂, sodium bisulphate.

5. Corrosion and Scale Inhibitors

• Phosphates / polyphosphates – protective films, metal sequestration.
• Phosphonates (HEDP, ATMP, PBTC) and polyacrylates – threshold inhibitors that stop CaCO₃ and sulphate crystal growth.
• Silicates, molybdates, zinc or blended programs.

6. Other Specialised Chemicals

• Activated carbon – organics, pesticides, taste and odour.
• Chelating agents / descalers – membrane and boiler cleaning.
• Fluoride compounds – where mandated.
• Oxygen scavengers (e.g. sulphite) – boiler feedwater.
• Antiscalants for RO systems.

Typical Process Sequence in Conventional Treatment

1. Raw water
2. Coagulation + rapid mix
3. Flocculation
4. Sedimentation / clarification
5. Filtration
6. Disinfection
7. pH / corrosion adjustment
8. Distribution with residual disinfectant

Chemical selection and dose depend on raw-water quality, treatment goals, regulations, sludge handling and cost. Jar testing is standard for optimising coagulants. Industrial cooling, boiler and wastewater systems rely more on scale/corrosion inhibitors, biocides and specialised coagulants than municipal plants do.

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