Water & Wastewater

A coagulant neutralises the negative surface charge that keeps fine particles and colloids apart, so they gather into small micro-flocs; alum, PAC, ferric chloride and ferric sulphate are coagulants. A flocculant is a high-molecular-weight polymer, such as an anionic or cationic polyacrylamide, that bridges those micro-flocs into large, fast-settling flocs. Most clarifiers use both, dosed in that order, with products and doses chosen by jar testing on the actual water.

Cycles of concentration (COC) compare the dissolved solids in the circulating water with those in the make-up water, usually measured by conductivity or chloride. Evaporation removes pure water, so salts build up until blowdown bleeds them off. Running more cycles saves make-up water and chemicals but raises the scaling and corrosion tendency, so anti-scalant, dispersant and sometimes acid dosing must be matched to the target COC and the make-up water analysis.

Caustic soda reacts fast, dissolves fully and adds no sludge of its own, but it can overshoot to a very high pH and needs careful handling. Hydrated lime is widely used, precipitates metals and fluoride, and forms gypsum sludge with sulphate-rich acids. Magnesium hydroxide reacts more slowly, buffers at around pH 9 even when overdosed and gives a denser sludge, which suits biological plants. The choice depends on the acid, the metals present, sludge handling and discharge limits.

In theory about 1.3 to 1.4 mg/L of sodium metabisulphite removes 1 mg/L of free chlorine, because it dissolves to bisulphite, which reduces chlorine to chloride. Plants dose a margin above this to allow for mixing and flow changes, and use an ORP or free-chlorine monitor to confirm that no chlorine reaches the membranes. Heavy overdosing wastes chemical and can encourage biological growth, so make up solutions fresh rather than storing them diluted for long.