Water Treatment Coagulant Selection Guide — Alum, PAC, Ferric and Polymer
Direct answer
Coagulant choice is decided by raw water alkalinity, temperature and the sludge you are willing to handle — not by price per tonne. Aluminium sulphate (alum) is the cheapest per kilogram but consumes about 0.5 mg/L of alkalinity as CaCO₃ for every 1 mg/L dosed, so on soft or low-alkalinity water you pay the difference back in caustic soda or soda ash. Poly aluminium chloride (PAC) is pre-hydrolysed, consumes roughly a third of the alkalinity of alum at the same aluminium dose, works far better below 8 °C, and produces a denser, faster-settling floc — which is why most cold-climate plants in Ontario switch to PAC in winter even where alum is adequate in summer. Ferric chloride outperforms both on colour, organics (TOC/DOC) and phosphorus removal and works over a wider pH range, but it is corrosive, stains everything, and produces roughly 30–50 % more sludge by dry mass. Polyacrylamide flocculants are not coagulants: they only bridge floc that a coagulant has already destabilised, and dosing them first wastes the polymer.
Selection by raw water condition
| Condition | First choice | Why |
|---|---|---|
| Low alkalinity (< 40 mg/L as CaCO₃) | PAC | Minimal alkalinity consumption; avoids a second alkali dosing loop |
| Cold water (< 8 °C) | PAC | Retains floc formation rate where alum floc goes pin-like |
| High colour / high TOC | Ferric chloride or ferric sulphate | Superior organics removal, effective at pH 4.5–6 |
| Phosphorus removal to < 1 mg/L | Ferric chloride | Direct precipitation as FePO₄ |
| Sludge volume is the constraint | PAC | Lowest dry solids per unit turbidity removed |
| Cost-driven, warm, well-buffered water | Alum | Lowest delivered cost per mg/L Al |
Dose ranges seen in practice
These are starting points for jar testing, not design values. Surface water at 10–50 NTU typically settles on 20–60 mg/L alum, 10–30 mg/L PAC (as product), or 15–40 mg/L ferric chloride (as 40 % solution). Industrial effluent with emulsified oil or high COD commonly needs 100–400 mg/L. Anionic polyacrylamide is dosed at 0.2–1.5 mg/L into the flocculation zone; cationic polymer for sludge dewatering runs 2–8 kg per dry tonne. Overdosing a coagulant re-stabilises the colloid — the turbidity curve turns back upward, which is the clearest sign you have gone past the optimum.
Jar testing that actually predicts plant behaviour
Run the jar test at plant temperature, not room temperature, and match the rapid mix energy and time. Measure residual turbidity, pH after coagulation, residual aluminium or iron, and settled sludge volume after 30 minutes. A coagulant that gives 0.4 NTU but leaves 0.25 mg/L residual aluminium has failed if your permit limit is 0.2 mg/L. Repeat the winning jar at ±20 % dose to see how flat the optimum is — a sharp optimum means you will need tighter dose control online.
Handling and storage
Alum and PAC solutions are acidic and attack carbon steel and concrete; use FRP, HDPE or lined tanks with EPDM or Viton elastomers. Ferric chloride requires FRP, HDPE or rubber-lined steel — never stainless 304/316, which pits rapidly. Store PAC above 0 °C; it gels irreversibly if frozen. Dry polymer must be aged in solution for 30–60 minutes before dosing or it will not develop its full chain length. Never share a dosing line between a ferric and an alkali stream.
What to send with your enquiry
Raw water or effluent analysis (turbidity, alkalinity, pH, TOC, temperature range), target treated quality, flow in m³/h, existing dosing equipment and materials of construction, delivery form (IBC, drum, bulk tanker), and whether NSF/ANSI 60 certification is required for potable duty. Without alkalinity and temperature, any dose recommendation is a guess.