Polyaluminium chloride (PAC)
The default coagulant for most industrial effluent, and the one that forgives the most. Wide pH window, fast floc, low alkalinity consumption, and it still works at 10 °C when alum has stopped.
Polyaluminium chloride is a pre-hydrolysed aluminium coagulant, Aln(OH)mCl3n−m, supplied either as a 10–11% Al₂O₃ liquid or a 28–30% Al₂O₃ spray-dried powder. Because the hydrolysis has already happened in the reactor rather than in your tank, it forms floc faster than alum, consumes roughly a fifth of the alkalinity, works between pH 5.5 and 9.0, and is far less temperature-sensitive.
Typical dissolved air flotation doses run 50–300 mg/L as supplied liquid, or 15–90 mg/L as 30% powder. The correct dose for a given stream is set by jar test, not by the industry label.
Specification
PAC is not one product. Basicity — the ratio of hydroxyl to aluminium, expressed as a percentage — is what separates the grades, and it changes the behaviour more than the aluminium content does. Higher basicity means more pre-polymerisation, faster floc, lower alkalinity demand and lower residual aluminium, at a higher price per unit of aluminium.
| Parameter | Liquid, standard | Liquid, high basicity | Powder, 30% | Powder, 28% |
|---|---|---|---|---|
| Al₂O₃ content | 10.0–11.0% | 10.0–11.0% | 29.0–31.0% | 28.0–29.0% |
| Basicity | 45–55% | 65–75% | 40–50% | 70–90% |
| Appearance | Pale yellow to amber liquid | Pale yellow liquid | Yellow to pale yellow powder | Yellow powder |
| pH, 1% solution | 3.5–5.0 | 3.8–5.0 | 3.5–5.0 | 3.5–5.0 |
| Density at 20 °C | 1.19–1.25 kg/L | 1.19–1.24 kg/L | Bulk 0.8–1.0 kg/L | Bulk 0.7–0.9 kg/L |
| Water insolubles | ≤ 0.3% | ≤ 0.3% | ≤ 0.5% | ≤ 1.5% |
| Freezing point | −15 to −20 °C | −15 to −20 °C | n/a | n/a |
| Typical duty | General industrial | Potable, low alkalinity | Industrial effluent | Industrial, long freight |
Why it usually beats alum
| Property | Polyaluminium chloride | Aluminium sulphate | Practical consequence |
|---|---|---|---|
| Effective pH | 5.5–9.0 | 5.8–7.5 | PAC survives a CIP swing that puts alum outside its window |
| Alkalinity destroyed | ~0.1–0.3 mg CaCO₃ per mg | ~0.5 mg CaCO₃ per mg | On low-alkalinity water, alum forces you to buy caustic back |
| Performance at 10–15 °C | Little loss | Marked loss | Matters on borehole feed and highland sites |
| Floc formation time | Fast, pre-hydrolysed | Slower | PAC tolerates a shorter flocculation stage |
| Sludge volume | Lower | Higher | Fewer tankers of float sludge to dispose of |
| Residual aluminium | Lower | Higher | Relevant where the receiving water has an Al consent |
| Cost per tonne of product | Higher | Lower | The only column where alum wins — and it is the wrong column |
Dose ranges by application
| Application | Liquid PAC, 10% Al₂O₃ | Powder PAC, 30% Al₂O₃ | Coagulation pH | Polymer behind it |
|---|---|---|---|---|
| Potable water, low turbidity | 10–40 mg/L | 3–13 mg/L | 6.5–7.5 | Usually none |
| Potable water, high turbidity or algae | 30–90 mg/L | 10–30 mg/L | 6.0–7.5 | Anionic, 0.1–0.5 mg/L |
| Municipal primary, DAF | 40–120 mg/L | 13–40 mg/L | 6.5–7.5 | Anionic, 0.5–2 mg/L |
| Dairy processing | 80–250 mg/L | 25–80 mg/L | 6.0–6.5 | Anionic, 1–3 mg/L |
| Brewery and beverage | 60–200 mg/L | 20–65 mg/L | 6.0–7.0 | Anionic, 1–3 mg/L |
| Abattoir and poultry | 100–300 mg/L | 30–100 mg/L | 6.0–7.0 | Cationic then anionic, 2–6 mg/L |
| Paper mill whitewater | 40–150 mg/L | 13–50 mg/L | 6.5–7.5 | Cationic then anionic, 0.5–2 mg/L |
| Edible oil refinery | 150–400 mg/L | 50–130 mg/L | 4.5–6.0 | Anionic, 2–5 mg/L |
| Textile dyeing | 150–500 mg/L | 50–165 mg/L | 6.5–8.0 | Anionic, 2–5 mg/L |
| Plastics recycling wash water | 80–250 mg/L | 25–80 mg/L | 6.5–7.5 | Anionic, 1–4 mg/L |
Handling, storage and materials
PAC is acidic — pH 3.5–5.0 at 1% — and it attacks mild steel, aluminium, copper, concrete and 304 stainless. Getting the materials wrong is the single most common cause of a leaking dosing installation.
- Suitable: HDPE, polypropylene, PVC, uPVC, PVDF, FRP with vinyl ester or chemically resistant lining, PTFE, EPDM and Viton elastomers, ceramic and rubber-lined pump internals.
- Unsuitable: mild steel, galvanised steel, aluminium, copper and copper alloys, 304 stainless, unprotected concrete, natural rubber, nitrile in prolonged contact.
- Storage: shaded and vented, 5–35 °C. Sustained storage above 35 °C accelerates the slow re-polymerisation that produces sediment and a drift in performance. Do not store liquid PAC in direct African sun in a black tank — it is a common and avoidable cause of a plant that worked in month one and did not in month six.
- Bunding: 110% of the largest vessel, in a material compatible with the acid, drained to a controlled point rather than to the storm system.
- Powder: hygroscopic. Keep bags sealed and off the floor. A caked bag can be re-dissolved but the dust and the lumps make a mess of the make-up tank; plan for 2–4% loss on long humid storage.
- Personal protection: chemical splash goggles, face shield when transferring, PVC or neoprene gloves, apron. Eye contact is the injury that matters — an eyewash station within ten seconds of the dosing point is not optional.
Making up powder PAC
Powder is the right economics on a long supply line, but only if the site can make solution reliably. The routine is simple and the failure modes are consistent.
- Fill the make-up tank to about 70% with clean water at ambient temperature. Never add water to powder — always powder to water.
- Start the mixer before adding powder. A slow-speed paddle at 60–120 rpm is right; a high-shear mixer entrains air and produces foam.
- Add powder steadily over 5–10 minutes to a target concentration of 10–20% w/v. Above 20% the solution becomes viscous and dose control suffers.
- Mix for 20–30 minutes after the last powder goes in. Undissolved fines that reach the dosing pump will block the injection point within days.
- Top up to volume and let the batch stand 15 minutes. Any sediment at this point is water insolubles — normal at up to 0.5%, a specification problem above it.
- A 10% w/v make-up from 30% Al₂O₃ powder is roughly equivalent to a 3% Al₂O₃ solution, so multiply your liquid-PAC dose by about 3.3 to get the equivalent volumetric dose.
When PAC is the wrong choice
- Phosphorus consents below about 1 mg/L. Iron coagulants precipitate phosphate more completely and more cheaply. Use ferric chloride or ferric sulphate.
- Sulphide-bearing streams. Tannery beamhouse liquor needs iron, which precipitates sulphide as FeS. Aluminium does nothing for it.
- Very high alkalinity with a hard pH ceiling. Where pH cannot be brought below about 8.5, aluminium hydroxide solubility rises and floc quality collapses. Iron holds up better at high pH.
- Sites with an aluminium discharge consent. Residual aluminium in the treated effluent is real, typically 0.1–0.5 mg/L on a well-run plant and higher when overdosed.
- Where sludge goes to agricultural land. Some jurisdictions restrict aluminium loading on soil in a way they do not restrict iron.
Supply
Frequently asked questions
What is the difference between PAC 10% and PAC 30%?
They are the same chemistry at different concentrations. PAC 10% is the liquid, roughly 10–11% Al₂O₃ and about 89% water, ready to dose. PAC 30% is the spray-dried powder at 28–31% Al₂O₃, which must be made up into solution before use. One kilogram of 30% powder replaces roughly three kilograms of 10% liquid, which is why powder dominates on long freight routes.
What does basicity mean and which should I buy?
Basicity is the degree of pre-hydrolysis — the ratio of hydroxyl groups to aluminium, as a percentage. Higher basicity gives faster floc, lower alkalinity consumption and lower residual aluminium. For general industrial effluent with adequate alkalinity, 45–55% is normal and cheaper. For low-alkalinity water, potable duty, or a stream where pH control is poor, 65–90% is worth the premium.
How much PAC do I need per cubic metre?
Dose in mg/L is the same number as grams per cubic metre. At 150 mg/L, one cubic metre takes 150 g of product as supplied, so 1,000 m³/day takes 150 kg/day. Run a jar test to fix the dose — the ranges published above are starting points that span a factor of three or more within a single industry.
Can PAC be used for drinking water?
Yes, and it is the dominant potable coagulant worldwide, but only in a certified drinking-water grade. Those grades carry tighter limits on iron, manganese, arsenic, lead, cadmium and mercury, and are certified to NSF/ANSI 60 or EN 883. Industrial-grade PAC is not a substitute and should not be dosed into a potable stream.
Does PAC work in cold water?
Better than alum, which is the main reason it displaced alum in temperate plants. Because the hydrolysis step has already been carried out during manufacture, PAC does not depend on the reaction kinetics that slow down as water gets colder. On African sites the relevant version of this is highland and borehole feeds at 12–18 °C rather than genuinely cold water.
Can I mix PAC with other coagulants?
Not in the same storage or dosing line. PAC and ferric products react and drop solids that block injection points. Where a stream genuinely benefits from both aluminium and iron — some tannery and municipal duties do — dose them from separate skids at separate injection points and confirm the sequence by jar test.
Why has my PAC gone cloudy or thrown a sediment?
Slow re-polymerisation, almost always driven by heat. Liquid PAC held above 35 °C — an unshaded tank in the sun will exceed that comfortably — gradually forms insoluble aluminium hydroxide. The product is not immediately unusable but the effective aluminium falls, the dose creeps up and the injection point blocks. Shade and vent the tank, and size storage to turn stock over inside six months.
What HS code does polyaluminium chloride ship under?
Most commonly 2827.32, the heading for aluminium chloride. Some customs authorities classify liquid PAC as a prepared chemical mixture under 3824.99 instead, and rulings differ between jurisdictions. We state the code we are shipping under on the proforma so your clearing agent can confirm it against local practice before the vessel arrives rather than after.