Coagulation / Flocculation

Coagulation is the rapid addition and mixing of a chemical coagulant into water to destabilise the negative surface charge of fine suspended particles, colloids and natural organic matter, allowing them to stick together. Flocculation follows, using gentle mixing over a longer period to promote the collision and growth of the destabilised particles into larger, visible flocs. The resulting flocs are dense enough to be removed by sedimentation, dissolved air flotation or filtration in the downstream separation stage. Together, coagulation and flocculation are the primary mechanism for removing turbidity, colour, natural organic matter and fine suspended solids from drinking water and wastewater.

IWTE designs systems for a range of coagulants selected based on the source water chemistry, treatment objectives and operating conditions. Ferric sulphate and ferric chloride are widely used for surface water treatment, providing effective turbidity and natural organic matter removal over a broad pH range. Aluminium sulphate (alum) is a well-established coagulant for drinking water treatment. Polyaluminium chloride (PAC) offers improved coagulation performance at lower temperatures and over a wider pH range than alum, with lower residual aluminium in the treated water. Organic polymer coagulant aids are used to enhance floc formation and reduce primary coagulant dose.

Coagulant dose is determined by jar testing, a laboratory procedure in which small doses of coagulant are applied to samples of the source water under controlled mixing conditions and the resulting floc formation and settled water quality are evaluated. Jar testing identifies the optimum coagulant type, dose and pH for the specific source water and allows the design dose range to be established. IWTE provides jar testing support during the system design phase and can incorporate online streaming current monitors or turbidimeter-based feedback control to adjust the coagulant dose automatically in response to changes in source water quality.

Rapid mix, also called flash mixing or coagulation mixing, is the first stage where the coagulant is introduced and dispersed uniformly throughout the water in a very short contact time, typically 15 to 60 seconds, under high-intensity turbulence. The purpose is to ensure rapid and complete coagulant distribution before the destabilised particles begin to aggregate. Flocculation is the subsequent stage where gentle, tapered mixing over a longer detention time of 10 to 30 minutes promotes particle collision and floc growth to the size and density required for effective downstream separation. The two stages require different mixing intensities and detention times and are designed and controlled independently.

Yes. Enhanced coagulation, using higher coagulant doses and lower pH than standard turbidity removal, is an established and regulatory-recognised approach for reducing natural organic matter and its associated disinfection byproduct precursors from drinking water sources. Enhanced coagulation typically achieves 40 to 70 percent removal of dissolved organic carbon, reducing the formation potential of trihalomethanes and haloacetic acids when the treated water is subsequently disinfected with chlorine. IWTE designs enhanced coagulation systems for water treatment plants where DBP formation control is a regulatory or operational requirement.

The flocculated water can be directed to several downstream separation processes depending on the water quality, required effluent quality and site constraints. Conventional horizontal flow or inclined plate sedimentation removes settled floc by gravity. Dissolved air flotation (DAF) is used where the floc is light, including for algae-laden water, warm water with poorly settling floc and industrial effluents where flotation is more effective than sedimentation. Rapid gravity or pressure filtration follows sedimentation or DAF to remove residual floc and turbidity. IWTE designs the coagulation and flocculation system in conjunction with the downstream separation process to ensure compatibility and optimal overall performance.

Coagulant dosing is typically controlled by flow-proportional pacing, where the dose rate is automatically adjusted in proportion to the instantaneous flow rate to maintain a constant coagulant-to-water ratio. Additional feedback control using online turbidity measurement of the settled or filtered water allows the dose to be trimmed upward or downward in response to changes in source water quality. For systems treating highly variable source water, streaming current monitors or UV absorbance sensors provide more rapid feedback on changes in coagulation demand, allowing earlier dose adjustment before settled water quality is affected.

Yes. Coagulation and flocculation systems are included as an integral part of every conventional water treatment plant that IWTE designs and supplies for surface water or high-turbidity groundwater sources. The system is engineered in conjunction with the downstream separation process and the overall treatment objectives, with unified PLC and SCADA control. IWTE also supplies standalone coagulation and flocculation packages for integration into existing plants, including upgrade of ageing or undersized clarification stages.