Polyaluminum chloride (PAC) is a pre-hydrolyzed aluminum coagulant whose dose must track raw-water turbidity to stay effective: too little leaves particles stable, while too much can restabilize them, waste chemical, and raise residual aluminum in the treated water. Because the dose–turbidity relationship is non-linear and shifts with temperature, alkalinity, and organic load, the reliable approach is to build a plant-specific dose curve from jar tests and adjust it with online feedback, rather than to run one fixed dose year-round.
Why the Dose–Turbidity Relationship Is Not Linear
The coagulation mechanism changes with particle concentration. At low turbidity (below roughly 10 NTU), charge neutralization dominates and there are few particles available to collide, so flocs form slowly and even small overdoses can reverse the particle charge and push settled turbidity up instead of down. At moderate turbidity (roughly 10–100 NTU), sweep flocculation takes over and the acceptable dose window widens. At high turbidity (above roughly 100 NTU), abundant particle collisions actually assist floc growth, so the required dose rises far more slowly than in proportion to turbidity. Doubling turbidity almost never means doubling the dose. The practical consequence is that a dosing table with a single mg/L figure per season is a blunt instrument — a fitted curve is the right tool.
Build a Dose–Turbidity Curve for Your Source
Systematic jar testing across the turbidity range your source actually produces is the foundation. Test during low-, mid-, and high-turbidity episodes over the seasons, find the optimum dose at each condition, and plot optimum dose against raw turbidity on log–log axes. For many surface waters treated with liquid PAC at 10–12% Al₂O₃, the points fall close to a power-law curve of the form Dose = k × Turbidity^n, which can be programmed into dosing control as a feed-forward baseline. Treat the fitted curve as a hypothesis to be re-verified, not a permanent law — sources drift, and last season's coefficients carry no guarantee.
- Capture jar test data during several genuinely different turbidity events, not on one convenient day.
- Hold mixing times and speeds constant between sessions so that optimum doses stay comparable.
- Record temperature, pH, and alkalinity alongside every optimum dose you log.
- Re-test after storms, algal blooms, or any change in the raw-water source.
Correcting for Temperature, Alkalinity, and Organics
Turbidity is not the only driver. Cold water slows floc-formation kinetics markedly, and operators typically find that winter conditions need noticeably higher PAC doses or a flocculant aid such as polyacrylamide to reach the same settled turbidity — one of the advantages of PAC is that it tolerates cold water better than alum, but it is not immune. Elevated natural organic matter, visible as color or a higher UV254 reading, consumes coagulant before turbidity removal starts, so organics-rich episodes typically demand more than the turbidity curve alone predicts. Alkalinity matters less for PAC than for alum, because PAC is pre-hydrolyzed and consumes less of it, but very soft, low-alkalinity waters can still drift in pH at high doses and deserve monitoring.
Liquid PAC or PAC Powder?
The dosing chemistry is the same; the logistics differ. Liquid PAC (around 10–12% Al₂O₃) is dosed as supplied with no make-down step, which suits plants that want simple, continuous feed. PAC powder (around 28–30% Al₂O₃) concentrates more active content into every tonne shipped and stored, which favors remote sites and long supply chains, at the cost of a dissolving step. When comparing doses or prices between the two forms — or against other coagulants — always convert to an Al₂O₃ basis first; comparing product mass per litre across different concentrations is misleading.
Recognizing Overdose and Underdose
- Settled turbidity rising as the dose increases — classic restabilization; back the dose off rather than pushing further.
- Fine pinpoint floc that will not settle — usually underdosing, cold water, or organics consuming the coagulant.
- Elevated aluminum residual in treated water — persistent overdosing or poor floc capture downstream.
- Shortening filter runs and fast head-loss growth — carryover of light floc onto the filters.
- Growing sludge volumes without better clarity — chemical spent making hydroxide rather than removing particles.
Closing the Loop with Instrumentation
A raw-water turbidimeter driving the feed-forward curve, combined with flow-paced dosing, holds the baseline. Feedback trim then comes from settled-water turbidimeters or from a streaming current detector, which senses residual particle charge within seconds of dosing and catches source changes before the clarifier shows them. However automated the loop becomes, the jar test remains the calibration instrument that keeps the curve honest — schedule it, do not wait for an upset to force it.
Verify at the Bench, Then Dose with Confidence
Every recommendation above funnels back to one practice: jar test on your actual water, at the actual temperature, across the real turbidity range your source produces. CHIMI ART supplies liquid PAC, PAC powder, and complementary polyacrylamide flocculants, and our technical team can help design the jar test matrix, fit your dose–turbidity curve, and support the plant trial. Contact us to get started.