How to Use Aluminum Chlorohydrate (ACH) in Water Treatment

Overview

Aluminum Chlorohydrate (ACH) is the highest-basicity aluminum coagulant available, with a basicity of 83–90% and Al₂O₃ content of 23–24%. Its superior basicity means it introduces far fewer protons per unit of aluminum than conventional alum or PAC, resulting in minimal pH depression of the treated water. This makes it the premium choice for low-alkalinity source waters where buffering capacity is limited, and for membrane pre-treatment where stable, consistent coagulation is essential to protect downstream UF or NF membranes.

ACH is supplied as a clear to slightly hazy liquid (density 1.33–1.37 g/mL), making it easy to dose via metering pumps without any dissolution step. It is also widely used in the personal care industry as an antiperspirant active ingredient, reflecting its high purity standard.

Compared to alum and standard PAC, ACH produces significantly less sludge volume — typically 30–50% less — reducing sludge disposal costs. The floc formed is smaller but denser than alum floc, settling or filtering efficiently at low doses.

Preparation & Dissolution

ACH is supplied as a ready-to-use liquid and does not require dissolution. However, it should be diluted for accurate dosing in small plants:

1. Dilution for metering: For small-capacity dosing pumps, dilute ACH 1:5 to 1:10 with clean water in a day tank. This improves dosing accuracy at low flow rates.
2. No heating required: ACH is fully miscible with water at ambient temperatures. Do not heat — it is stable at ambient conditions.
3. Rinse lines before use: Flush dilution lines with clean water before first use to remove any residual alkaline material that could cause precipitation on contact.
4. Avoid mixing with concentrated caustic or lime: ACH is acidic (pH 3.5–4.5) and will precipitate aluminum hydroxide immediately on contact with strongly alkaline solutions. Keep dosing points separated by at least 2–3 meters of pipe flow.
5. Check stock tank pH regularly: ACH can absorb atmospheric CO₂ over time. If stored pH rises above 5.0, the product may be partially polymerized and performance could be affected.

Dosing Guide

Application	Typical Dose (mg/L as ACH liquid)	pH Range	Notes
Low-turbidity drinking water (< 5 NTU)	2–10 mg/L	6.5–7.8	Excellent at very low dose
Medium-turbidity drinking water (5–50 NTU)	8–25 mg/L	6.5–7.5	Jar test recommended
High-turbidity drinking water (> 50 NTU)	20–60 mg/L	6.0–7.5	Consider pre-sedimentation
UF membrane pre-treatment	3–15 mg/L	6.5–8.0	Lower dose than PAC
NF/RO pre-treatment	2–8 mg/L	6.5–7.5	Minimize sludge to membranes
Pharmaceutical water	2–10 mg/L	6.5–8.0	Verify residual Al spec
Wastewater clarification	20–80 mg/L	6.0–8.0	Less common — PAC preferred

Application Procedure

1. Jar test first: Despite ACH's superior performance, raw water characteristics vary widely. Run jar tests at 3, 5, 8, 12, 20, and 30 mg/L to find the minimum effective dose. ACH often shows a narrower optimal dose window than alum, so accuracy matters.
2. Point of application: Dose ACH at maximum turbulence — at the flash mixer inlet, pump impeller zone, or inline static mixer. The high basicity of ACH means the hydrolysis species are already partially pre-formed, but rapid mixing is still essential within the first 1–2 seconds.
3. Flash mixing parameters: G-value of 200–800 s⁻¹ for 20–60 seconds. ACH floc forms slightly faster than alum floc — shorter flash mix times are acceptable compared to alum.
4. Flocculation: G-value of 15–50 s⁻¹ for 15–25 minutes. ACH produces fine, dense floc that benefits from gentle, extended flocculation compared to alum's larger, lighter floc.
5. Sedimentation: Conventional clarifier settling time 1–3 hours. ACH sludge is denser and settles faster than alum sludge, which can allow higher overflow rates.
6. For UF pre-treatment: Dose ACH in-line before a static mixer, 30–60 seconds before the UF membrane inlet. The small, dense floc is ideal for UF capture. Monitor transmembrane pressure (TMP) weekly to confirm performance.
7. Residual aluminum monitoring: Target < 0.1 mg/L residual Al in finished water for drinking water applications. ACH tends to produce lower residual aluminum than alum at equivalent turbidity removal.

Monitoring & Control

Parameter	Frequency	Target / Action
Raw water turbidity	Continuous	Proportional dose adjustment
Finished water turbidity	Continuous	< 0.3 NTU (drinking water)
Coagulation pH	Every 2 hours	6.5–7.8 (ideal for ACH)
Residual aluminum	Daily	< 0.1 mg/L (finished water)
Raw water alkalinity	Daily	> 30 mg/L CaCO₃; lower impact than alum
Sludge volume index (SVI)	Weekly	ACH sludge SVI typically 30–50% lower than alum
TMP (if UF application)	Continuous	Rising TMP indicates coagulation failure
Jar test	Weekly	Re-optimize dose per season

Common Mistakes

• Using ACH like alum and overdosing: ACH is 2–4x more efficient than alum on a dose-per-turbidity-removal basis. Operators switching from alum often start too high and waste chemical while creating needlessly low pH. Start at 30% of the alum dose and titrate up.

• Neglecting the narrow optimal dose window: ACH has a sharper dose-response curve than alum. A dose 20% above optimal can cause charge reversal and effluent deterioration. Precise metering pumps and accurate dilution are essential — analog pumps with ±10% variability are not adequate for ACH.

• Dosing near alkaline chemical addition points: If lime or caustic soda is also dosed to adjust pH, the ACH injection point must be upstream by at least 2–3 meters. Immediate contact with alkaline material causes localized aluminum hydroxide precipitation and wastes chemical.

• Ignoring cold water performance: Below 5°C, ACH floc forms slower than at room temperature. In winter, increase contact time in the flocculation zone by 20–30% and reduce overflow rate in the clarifier accordingly.

• Storing in vented tanks: ACH absorbs CO₂ from air, which gradually raises the solution pH and promotes partial polymerization. Always store in sealed tanks or IBCs with minimal headspace. Check pH of stored ACH after more than 3 months of storage.

Storage & Handling

• Store in HDPE, polypropylene, or fiberglass tanks. Do not use steel or iron containers — ACH is acidic and will corrode ferrous metals.
• Storage temperature: 5–40°C. Do not allow to freeze — freezing causes irreversible separation.
• Shelf life: 12 months from production date in sealed IBCs or drums.
• Wear chemical-resistant gloves, eye protection, and acid-resistant apron when handling. ACH solution (pH 3.5–4.5) causes eye and skin irritation on contact.
• Provide eyewash station within 10 seconds of any ACH handling area.
• Spill cleanup: dilute with water, neutralize with dilute soda ash solution, then absorb with sand or vermiculite. Do not wash into storm drains untreated.