PAC and PAM Dosing Sequence: Coagulant First, Then Polymer

In almost every water and wastewater process, PAC and PAM dosing follows one order: PAC first, PAM second. PAC (poly aluminium chloride) is the coagulant. It neutralises the charge on fine particles so they can come together as small pin floc, and it needs fast, thorough mixing. PAM (polyacrylamide) is the flocculant. Its long chains bridge the pin floc into large, settleable flocs, and it needs gentle mixing that will not tear them apart. Reverse the order, or inject the two too close together, and floc stays small and settling and filtration suffer. This guide is for operators of wastewater plants, water works and industrial effluent systems.

Why PAC goes before PAM

Think of it as two stages:

1. Coagulation: PAC disperses quickly in the rapid mix zone, neutralises particle charge, and small floc begins to form.
2. Flocculation: once pin floc exists, PAM is added and its chains link the small floc into larger flocs that settle well.

If PAM goes in first, the particles are not yet destabilised. The polymer adsorbs onto single particles, cannot bridge, and much of it is wasted. That is why the sequence in PAC and PAM dosing should not be swapped casually.

Injection points for PAC and PAM dosing

PAC injection point

Inject PAC where flow is turbulent: the rapid mix tank inlet, upstream of a static mixer, or at a lift pump discharge. The aim is even dispersion within a short time, so the closer to the mixer the better.

PAM injection point

Inject PAM at the flocculation tank inlet or the start of the flocculation zone, where flow is calmer. After injection, flocs need a period of slow mixing to grow. Fast mixing breaks them.

Spacing between the two

Leave reaction time between the two injection points so PAC can finish coagulating. The right spacing depends on water quality, temperature and tank design, and there is no single number. Most plants find it through jar testing and commissioning. Too short and pin floc has not formed; too long and some of it may settle in the mixing zone.

Solution strength and equipment for PAC and PAM dosing

PAC is usually dosed as a solution with metering pumps. It is corrosive and tends to crystallise, so pump heads and lines should be acid resistant.

PAM must be dissolved and aged first, then dosed as a very dilute solution. Too strong and it becomes too viscous to pump and does not disperse in the water. Ageing takes time, which is why three-tank systems and automatic make-up units are common; see PAM integrated dosing system selection.

Because polymer solution is viscous, choose pump heads and valves suited to viscous liquids, or a progressive cavity pump. The trade-offs are covered in diaphragm vs screw pumps for PAM.

Finding the dose: jar testing

Both doses come from jar testing, then field adjustment. Fill several beakers with the same raw water, add different PAC doses, mix fast for a minute or two, add PAM after the chosen interval, switch to slow mixing, then let them settle. Compare how quickly floc forms, floc size, settling speed and clarity of the supernatant. Repeat when seasons change, raw water shifts or you change chemical supplier. More is not better: excess PAC can raise turbidity again and lower pH, and excess PAM makes floc sticky and blinds filters.

Sludge dewatering is different

Dewatering uses a different approach from clarification. Many dewatering machines take PAM only, or PAC conditioning followed by PAM, depending on sludge and equipment. See sludge dewatering dosing system selection.

Signs the PAC and PAM dosing is not working together

• Fine floc, cloudy clarifier effluent: PAM added too early, or PAC poorly mixed.
• Large but fragile floc: mixing too fast after PAM, or PAM injected into turbulent flow.
• Sticky floc, filters blinding quickly: PAM overdose.
• Low effluent pH: PAC dose high; pH correction may be needed.
• Chemical use rising with no improvement: spacing or injection points wrong.

Safety: PAC solution is an irritant, and spilled PAM makes floors extremely slippery. Wear gloves and eye protection and clean spills promptly.

Common layout mistakes in PAC and PAM dosing

On retrofits we often find both chemicals injected into the same pipe a short distance apart, because that was where the tapping points happened to be. In that case PAC has no time to work before PAM arrives. Another frequent problem is a PAM dosing point placed just upstream of a pump or a sharp valve, which shears the floc as soon as it forms. Moving the polymer point downstream of the turbulent section, or adding a short flocculation pipe, often improves settling without changing either dose. It is worth walking the line and asking, at each injection point, how hard the water is being mixed there.

Automatic control of PAC and PAM dosing

Where flow varies a lot, both pumps can follow a flow signal so the dose ratio stays steady. Packaged PAC and PAM dosing systems are listed under our dosing skids.

Conclusion

Effective PAC and PAM dosing means PAC first at rapid mix for coagulation, PAM second at slow mix for flocculation, with reaction time between them. Set doses by jar testing, age PAM fully and dose it dilute, and choose pumps that suit each chemical. Treat sludge dewatering separately. Send us your flow, water quality and current process, and contact our engineers to configure both dosing lines.

FAQ

Can PAC and PAM be mixed and dosed together?

Not recommended. They act at different stages, and mixing them reduces performance; PAM can also lose effect in strong PAC.

What happens if the PAC and PAM dosing order is reversed?

Particles are not destabilised when PAM arrives, bridging is poor, floc stays small, turbidity stays high and chemical use rises.

Why does PAM need slow mixing?

The large flocs it forms are fragile. Fast mixing breaks them back into fine particles that settle poorly.

Is PAC alone ever enough, or is PAC and PAM dosing always needed?

Sometimes. Whether PAM is needed depends on raw water turbidity, settling time and effluent targets; jar tests will tell you.

Should I use anionic or cationic PAM?

It depends on the water or sludge. Clarification often uses anionic or non-ionic grades, while dewatering usually uses cationic. Test before choosing.