Aluminium chlorohydrate (ACH)
Polyaluminium chloride taken to its logical conclusion: the highest aluminium concentration, the highest basicity, almost no alkalinity demand and the smallest sludge volume of any aluminium coagulant. Priced accordingly.
Aluminium chlorohydrate is a highly basic polyaluminium chloride, Al₂(OH)₅Cl, supplied at 23–24% Al₂O₃ with a basicity of 80–85%. It is roughly 2.3 times as concentrated as standard liquid PAC and consumes almost no alkalinity, because the hydroxide it needs is already built into the molecule.
Typical dissolved air flotation doses run 20–120 mg/L as supplied. It is chosen for three situations: low-alkalinity water where other coagulants crash the pH, sites paying by the tonne for sludge disposal, and long freight routes where shipping water is the dominant cost.
Specification
| Parameter | Liquid | Powder |
|---|---|---|
| Al₂O₃ content | 23.0–24.0% | 45.0–47.0% |
| Basicity | 80–85% | 82–86% |
| Chloride | 7.5–8.5% | 16.0–18.0% |
| pH, 1% solution | 3.5–5.0 | 3.5–5.0 |
| Density at 20 °C | 1.33–1.36 kg/L | Bulk 0.6–0.8 kg/L |
| Insolubles | ≤ 0.1% | ≤ 0.5% |
| Appearance | Colourless to pale yellow | White to pale yellow powder |
| Freezing point | −15 °C | n/a |
Against the alternatives
| ACH | PAC liquid | Alum liquid | |
|---|---|---|---|
| Al₂O₃ as supplied | 23–24% | 10–11% | 8.0–8.3% |
| Basicity | 80–85% | 45–75% | 0% |
| Alkalinity destroyed | Negligible | Low | High |
| Effective pH | 5.5–9.5 | 5.5–9.0 | 5.8–7.5 |
| Relative dose for same Al | 1.0 | 2.3 | 2.8 |
| Relative sludge volume | Lowest | Medium | Highest |
| Residual aluminium | Lowest | Low | Highest |
| Cost per tonne | Highest | Medium | Lowest |
| Freight per kg of Al | Lowest | Medium | Highest |
Dose ranges
| Application | ACH dose | Equivalent liquid PAC | Coagulation pH |
|---|---|---|---|
| Potable water, low turbidity | 5–18 mg/L | 10–40 mg/L | 6.5–7.5 |
| Potable water, low alkalinity source | 10–35 mg/L | 25–80 mg/L | 6.0–7.5 |
| Municipal primary DAF | 18–52 mg/L | 40–120 mg/L | 6.5–7.5 |
| Dairy and beverage DAF | 30–110 mg/L | 70–250 mg/L | 6.0–6.8 |
| Paper mill whitewater | 18–65 mg/L | 40–150 mg/L | 6.5–7.5 |
| Cooling tower and pool clarification | 2–10 mg/L | 5–22 mg/L | 7.0–7.8 |
Handling
- Materials: HDPE, polypropylene, PVC, PVDF, FRP with vinyl ester, PTFE. Not mild steel, aluminium, copper or 304 stainless. Less aggressive than PAC because of the lower free chloride, but still acidic at pH 3.5–5.0.
- Storage: 5–35 °C, shaded and vented. Shelf life 12 months, longer than standard PAC because the highly basic structure is more stable.
- Viscosity: noticeably higher than PAC at 30–45 cP. Size dosing lines and pump heads for it — an undersized suction line on ACH will cavitate.
- Protection: splash goggles, gloves, apron. Eyewash within reach of the transfer point.
Frequently asked questions
What is the difference between ACH and PAC?
ACH is a polyaluminium chloride at the extreme end of the basicity range. Standard PAC runs 45–55% basicity at 10–11% Al₂O₃; ACH runs 80–85% basicity at 23–24% Al₂O₃. In practice that means roughly 2.3 times the aluminium per litre, almost no alkalinity consumption, a wider pH tolerance and less sludge — at two to three times the price per tonne.
When is ACH worth the premium over PAC?
Three situations. When raw alkalinity is below about 60 mg/L as CaCO₃ and any other coagulant will crash the pH. When sludge disposal is charged by volume or tonne, because ACH generates the least. And when freight is a large share of landed cost, because you are shipping less water per kilogram of aluminium. Outside those cases, standard PAC is usually the better buy.
Does ACH need pH correction?
Rarely. That is its principal operational advantage. Because the hydroxide is already in the molecule, it consumes almost no alkalinity, so pH moves very little on dosing. On streams where PAC or alum require a lime or caustic dose to hold pH, ACH often removes that dosing system entirely — which should be counted in the cost comparison.
Can ACH be used in drinking water?
Yes, in a certified drinking-water grade, and it is common on low-alkalinity and highly coloured surface waters. It produces the lowest residual aluminium of the common aluminium coagulants, which matters where a 0.2 mg/L standard applies. Verify NSF/ANSI 60 or EN 883 certification on the specific grade rather than assuming it.