Alum, PAC or ferric? A practical comparison of the three most common coagulants used in East African water treatment plants, with dosing guidance and cost considerations.
Choosing a coagulant is one of the highest-leverage decisions in any water treatment plant. Get it right and you cut chemical spend, reduce sludge handling and produce consistently clear water. Get it wrong and you chase turbidity all day.
The three workhorses
Aluminium sulphate (alum) remains the most widely used coagulant in Kenya. It is inexpensive, well understood and effective on moderate-to-high turbidity raw water. Its weaknesses are a narrow optimum pH window (typically 6.5–7.5), significant alkalinity consumption and comparatively high sludge production.
Poly aluminium chloride (PAC) is a pre-hydrolysed coagulant. Because part of the hydrolysis has already happened during manufacture, PAC works across a wider pH band (roughly 5–9), consumes far less alkalinity and produces noticeably less sludge — often 30–40% less than alum on the same water. It also performs better in cold water, which matters at higher-altitude plants.
Ferric chloride excels on water with high natural organic matter and in phosphate removal duties. It works at lower pH than alum and produces a dense, fast-settling floc. The trade-offs are corrosivity, staining risk and the need for corrosion-resistant dosing equipment.
Let the jar test decide
No specification sheet substitutes for a jar test on your actual raw water. Run each candidate coagulant across a dose range, record settled turbidity at 30 minutes, and note the pH shift. Compare on delivered cost per megalitre treated — not on price per tonne. A coagulant that costs 20% more per tonne but works at 60% of the dose is the cheaper option.
Watch the alkalinity
Alum consumes roughly 0.5 mg/L of alkalinity as CaCO₃ for every 1 mg/L dosed. On soft raw water this quickly drives pH below the coagulation optimum, at which point you are buying soda ash to correct what the alum did. PAC's lower alkalinity demand often eliminates that secondary chemical entirely — a saving that rarely shows up in the initial price comparison.
Seasonal reality
Raw water quality in East Africa swings hard between dry and wet seasons. Turbidity that sits at 15 NTU in January can hit 400 NTU after heavy rain. Plants that fix a single dose rate year-round overdose for eight months and underdose for two. Re-run jar tests at least quarterly, and always after a major catchment event.
