Poly Aluminium Chloride in Water Treatment: Dosing & Control

What poly aluminium chloride solves in water treatment

PAC is used as a primary coagulant to destabilize colloids and fine particles so they can collide, grow into flocs, and be removed by sedimentation, flotation, or filtration.

Common treatment goals where PAC is a strong fit

• Rapid turbidity reduction (including high-peak storm events)
• Color and natural organic matter (NOM) reduction (often improves downstream filtration)
• Improved cold-water coagulation compared with some conventional coagulants
• Phosphorus removal in many wastewater and tertiary polishing steps (site-specific)
• Lower sludge volume than some alternatives (depends on dose and solids load)

A practical rule: if your plant sees frequent raw-water swings and you need consistent clarification without aggressive pH correction, PAC is often the simplest operational upgrade.

How PAC works and why it can be more stable than alum

PAC is a pre-hydrolyzed aluminium coagulant. Compared with alum, part of the neutralization is “built in” (often described by basicity), so it tends to create effective polymeric aluminium species quickly and with less pH shock—especially helpful when alkalinity is limited.

What you should watch (operationally)

• Charge neutralization happens fast, so rapid mix intensity and time strongly affect results.
• Overdosing can re-stabilize particles (“charge reversal”), creating pin floc and higher residual turbidity.
• Most applications coagulate well in a broad pH band; performance is commonly strongest around near-neutral conditions (verify by jar test).

If you get “good floc but poor settling,” the cause is often hydraulics (mixing/floc time) or dose control, not the chemistry alone.

Choosing a PAC grade that matches your process

“PAC” isn’t one fixed formulation. Vendors supply different aluminium strengths, basicities, and impurity profiles. Selecting on paper first prevents months of chasing inconsistent coagulation.

When comparing bids, normalize cost to the same basis (for example, cost per kg of Al₂O₃ delivered), not cost per gallon/ton of product.

Dosing PAC: jar testing, conversions, and a worked example

The most reliable dosing method is a jar test sweep that spans “underdose → optimum → overdose,” paired with pH and settled turbidity measurements. Start wide, then narrow.

A practical jar test workflow

1. Measure raw water: turbidity, pH, temperature, alkalinity (if available), and color/UV254 if you track organics.
2. Choose 6–8 PAC doses across a wide range (example: 0, low, medium-low, medium, medium-high, high, very high).
3. Rapid mix: short, high energy (simulate your flash mixer).
4. Flocculate: slower mixing for 10–20 minutes (simulate your floc basins).
5. Settle: 10–30 minutes (or match clarifier detention time).
6. Record settled turbidity and pH; note floc size, density, and settling rate.
7. Pick the dose that meets the goal with the best “settling/filtration feel,” then validate with a smaller refinement sweep around that point.

Simple conversion you can use for planning

If your PAC product is labeled as X% Al₂O₃, then: mg/L as Al₂O₃ ≈ mg/L product × (X/100). This helps compare products and track dose against performance across seasons.

Use this kind of table to brief operators: it makes the “why” behind a target dose visible and repeatable.

pH and alkalinity control: preventing residual aluminium and poor floc

Even though PAC is generally gentler on pH than alum, it can still depress pH and consume alkalinity—especially at high doses or in low-alkalinity source waters. This matters because coagulation windows are pH-sensitive and residual dissolved aluminium can increase when pH drifts away from the sweet spot.

A planning estimate for alkalinity demand

For a rough check, stoichiometric aluminium hydrolysis demand is about 2.94 mg/L as CaCO₃ per 1 mg/L as Al₂O₃. PAC basicity reduces this demand because part of the neutralization is already included.

A simple approximation you can use for screening is: Estimated alkalinity consumed ≈ 2.94 × (1 − basicity fraction) × (mg/L as Al₂O₃). Confirm the real impact by tracking online pH and alkalinity trends.

Operational safeguards

• Trend pH before/after rapid mix; sudden dips usually indicate under-alkalinity or overdosing.
• If alkalinity is low, pre-add alkalinity (lime/soda ash) or choose a higher-basicity PAC grade.
• If residual aluminium is a concern, use jar tests that include pH measurement and (if available) dissolved Al checks after filtration.

Process integration: where PAC dosing point and mixing matter most

PAC is sensitive to how quickly and uniformly it contacts the water. Many “chemical problems” are actually contact and energy problems.

Practical setup checks

• Dose into a true rapid-mix zone (or install an in-line static mixer) to avoid streaking and micro-overdose zones.
• Verify rapid-mix detention time is long enough for dispersion (even seconds can matter at high flow).
• Tune flocculation energy: too aggressive breaks floc; too gentle leaves it small and slow to settle.
• If using a polymer aid, add it after PAC has dispersed (often downstream of rapid mix) to avoid “instant gelling.”

A quick diagnostic: if jar tests look great but the plant looks poor, focus on injection location, mixing energy, and hydraulic short-circuiting.

Troubleshooting guide for common PAC issues

Symptoms, likely causes, and fixes

• Pin floc / haze: often overdosing or poor dispersion → reduce dose slightly, improve rapid mix, widen jar-test sweep to find overdose onset.
• Floc forms but won’t settle: floc too light or sheared → lower flocculator energy, adjust polymer aid, check for short-circuiting.
• Higher residual aluminium: pH out of effective window or excessive dose → stabilize pH/alkalinity, avoid “just-in-case” overdosing, confirm grade purity.
• Rapid filter headloss increase: carryover floc or sticky fines → optimize settling/DAF, adjust polymer, check coagulant dose and mix.
• Seasonal deterioration (cold water): slower kinetics → increase rapid-mix effectiveness, refine dose by winter jar tests, consider polymer support.

Troubleshooting is faster when you log three numbers together: PAC dose, pH after rapid mix, and settled turbidity. Those three often reveal the real failure mode.

Storage, handling, and dosing equipment essentials

PAC is acidic/corrosive and can attack incompatible metals and elastomers. Stable feed is a mechanical design problem as much as a chemistry problem.

Practical handling checklist

• Use compatible wetted materials (confirm with supplier): many plants use plastics (HDPE, PVC, PP) and appropriate gaskets.
• Prevent dilution mistakes: label strengths clearly and standardize operator calculations.
• Maintain consistent agitation where recommended (some products benefit from gentle mixing; avoid aeration unless advised).
• Calibrate chemical pumps regularly; PAC performance degrades quickly with feed-rate drift.

For safety, follow your site chemical hygiene plan: PPE, spill containment, eyewash access, and compatibility checks with other chemicals in the room.

Bottom line: how to get consistent results with PAC

Use jar testing to set the dose, validate pH/alkalinity under real conditions, and optimize mixing before changing chemistry. Most PAC “underperformance” is solved by tightening dose control and improving dispersion and flocculation energy.

If you implement only one improvement, make it this: run a seasonal jar-test program (warm/cold water and storm peaks) and convert all results to a consistent basis (mg/L as product and mg/L as Al₂O₃). That single discipline turns PAC into a predictable, high-confidence tool in water treatment operations.