Coagulation and flocculation

The jar test, and the chemistry under it

You set the coagulant dose off the jar test and the chart on the wall, and it works. The exam does not ask about your chart. It asks why coagulation works at all, what flocculation does that coagulation does not, and what makes the dose move. This is the step that turns turbidity from something that floats forever into something that settles.

What coagulation actually does

The fine particles that make raw water turbid, clay and organic matter, carry a negative surface charge. Like charges repel, so the particles stay apart and never settle on their own. A coagulant, usually alum, ferric chloride, or a polyaluminum chloride, cancels that charge so the particles stop repelling and start clumping together. The coagulant goes in at the rapid mix, where fast, high-energy mixing spreads it through the water in seconds before it is used up. Two things happen depending on dose: at lower doses the coagulant simply neutralizes the charge, and at higher doses it forms a gelatinous floc that sweeps particles out as it settles, which operators call sweep coagulation.

Flocculation grows the floc

Coagulation destabilizes the particles. Flocculation grows them. After the rapid mix, slow, gentle mixing lets the destabilized particles bump into each other and build into floc large and dense enough to settle. The mixing energy is the whole art: too vigorous and it tears the floc back apart, too gentle and the floc never grows. Detention is usually twenty to forty-five minutes, often in tapered stages that start with more energy and end with less. A small dose of polymer as a coagulant aid makes the floc tougher and faster-settling.

What moves the dose

Three things change what the jar test tells you. Alkalinity is the big one. The coagulant consumes it and pulls the pH down, so a soft, low-alkalinity source needs a base fed in to keep the pH in the band where coagulation works. Temperature is the second: cold water slows coagulation and settling, so winter takes more coagulant and longer flocculation. And the raw water itself shifts with runoff and season, which is why you jar-test rather than set the dose once.

Where operators lose points

• thinking the coagulant raises pH, when it lowers it and uses up alkalinity
• over-mixing the flocculator and shearing the floc apart, or under-mixing so it never grows
• forgetting that cold water needs more coagulant and more time
• confusing coagulation, which neutralizes charge, with flocculation, which grows the floc

Quick reference

• Coagulant neutralizes the negative charge on particles so they collide and stick.
• Rapid mix disperses the coagulant fast; slow mix grows the floc.
• Alum lowers pH and consumes alkalinity; effective coagulation runs about pH 5.5 to 7.5.
• Cold water slows everything; the jar test sets the dose because raw water changes.

Where this fits

What this floc does next is settle out, in sedimentation and clarification. The dosing math is in the free practice tool, and the program details for where you test on your state's page.