Flocculation and Coagulation (Water Treatment)

Coagulation and flocculation remove fine colloidal particles, dissolved organics, and phosphate from farm water by chemically aggregating them into settleable or filterable flocs. Coagulant dosing uses aluminium sulfate (alum) at 5–50 mg/L, ferric chloride at 2–30 mg/L, or organic polymers (PAC, chitosan-derived flocculants) at 0.1–5 mg/L, added at a rapid-mix point with 30–90 seconds of high turbulence to destabilise particle charges, followed by 10–30 minutes of gentle flocculation where micro-flocs collide and grow. Optimal pH for alum coagulation is 6.5–7.5 and for ferric salts 5.0–6.5; outside these windows dosing must rise sharply while floc quality falls. In aquaculture the process serves three roles: clarifying inlet water from rivers or wells, treating effluent to meet discharge limits on suspended solids and phosphorus, and polishing RAS water to relieve mechanical filters of particles below 10 µm. Sludge volume is a real operational cost, roughly 0.3–1.0 kg dry solids per kg of alum dosed, and must be routed to sedimentation or dewatering. Overdosing can leave residual aluminium or iron in the water, which is toxic to fish at low mg/L levels, so jar tests should precede full-scale dosing, and treated inlet water should pass a media filter before reaching tanks.