APPLICATION
PAC at the Municipal Plant: Phosphorus Removal, Polishing and CSO
Municipal plants reach for PAC for one of three reasons: an effluent phosphorus limit that biology alone cannot hold, a polishing stage before discharge, or storm tanks that need fast clarification. All three are good fits for polyaluminium chloride - and all three carry a sludge-volume bill that deserves an early estimate.
Chemical phosphorus removal, sized from the ratio
Aluminium precipitates phosphate as insoluble aluminium phosphates; the theory asks for one mole of Al per mole of phosphorus, and practice asks for more because competing reactions consume some of the dose. Working doses typically land at 1.5 - 2.5 moles of Al per mole of total phosphorus to be removed, with the tighter effluent targets pushing to the upper end.
Translate that to chemical: each gram of phosphorus to remove needs roughly 8 - 13 grams of pure Al2O3-equivalent aluminium under field conditions. From an influent TP of 8 mg/L to an effluent target of 0.5 mg/L, that is 7.5 mg/L of P times the ratio - your jar test then finds the exact number on your own wastewater. Dose point matters as much as quantity: simultaneous precipitation into the aeration basin is steady and self-mixing; post-precipitation before tertiary filtration is dose-efficient and easy to control.
Polishing and storm treatment
As a polishing chemical ahead of sand or disc filters, PAC at modest doses tightens suspended solids and the phosphorus that rides on them, and it stabilises effluent through seasonal load swings. The filtered effluent also carries lower residual aluminium than an unfiltered coagulated stream, which matters where discharge permits watch it.
Storm tanks and CSO storage benefit from a fixed, pre-set dosing package: high flows arrive without operators watching, so a simple duty-dose that works across the plausible range beats fine tuning. PAC's fast floc formation and wide pH window make it forgiving under exactly those unattended conditions.
The sludge bill, estimated before it surprises you
Every kilogram of PAC brought into the stream ends up as solids: the precipitated phosphorus plus the aluminium hydrolysis products plus the extra suspended solids swept out. Chemical P removal commonly raises sludge volume by a noticeable margin - plan digester, dewatering and disposal capacity for it, and put a cationic PAM programme on the dewatering line, where the extra chemical sludge is actually quite filterable.
The other control point is pH: PAC hydrolysis consumes alkalinity, and soft-water catchments can drift below the pH the biology wants. Where that risk exists, dose alkalinity alongside and monitor the aeration basin pH rather than trusting the effluent end of the plant to complain first.
Frequently asked questions
PAC or ferric for phosphorus removal?
Both work; ferric often binds phosphorus slightly more strongly per unit of chemical, while PAC is easier on equipment, keeps the effluent colour clean and behaves over a wider pH band. Many plants run PAC for routine duty and keep a ferric option for tight limits - the jar test on your own wastewater decides economically.
Where should I inject the PAC?
For simultaneous precipitation, into the aeration basin inlet or the return line where mixing is guaranteed; for post-precipitation, after secondary clarification with a flocculation and filtration stage behind it. Avoid raw injection into clarifier feedwells - short mixing gives pin flocs and carries chemical into the sludge without doing its job.
How do I keep residual aluminium low in the effluent?
Keep the dose inside the ratio-based corridor, avoid overshooting when influent TP drops, and filter the coagulated stream where the permit is strict. Rising effluent aluminium is almost always an overdose signal or a pH drift, and both are visible in the daily log before the lab confirms them.