Aluminum (Al) and water

Aluminum is naturally present in rocks, soils and many waters. Its concentration and chemical form depend on the source geology, pH and other water constituents. Aluminum salts are also used as coagulants in water treatment, so treatment itself can contribute to residual aluminum in finished water.

How aluminum behaves in water

Aluminum speciation is strongly affected by pH and by substances such as dissolved organic matter and fluoride. In acidic water, dissolved cationic forms become more important. Near neutral pH, aluminum commonly hydrolyses and may form sparingly soluble aluminum hydroxide floc. At higher pH, soluble aluminate forms become more important. In pure water, aluminum has its lowest solubility at approximately pH 5.5–6; natural-water chemistry can shift this behavior, so the pH alone does not predict a field concentration.^[1]

Bulk aluminum metal behaves differently from dissolved aluminum compounds. A thin, adherent oxide film protects the metal and prevents rapid reaction with water under ordinary conditions. Strongly acidic or alkaline conditions can attack this film; if the metal then reacts, hydrogen gas may be produced.

How aluminum enters water

Weathering and erosion of aluminum-bearing minerals can add dissolved aluminum and mineral particles to surface water and groundwater. Mining, industrial discharges and runoff can also contribute. Aluminum salts used for coagulation are another possible source of residual aluminum in treated water. The amount measured depends on whether the analysis includes dissolved aluminum, suspended particles or both.

Environmental effects

Aluminum is not an essential element for fish and other aquatic animals. At elevated bioavailable concentrations it can interfere with ion regulation and respiration, including by accumulating on fish gills. Effects depend on water chemistry: pH, hardness and dissolved organic carbon can all change aluminum’s bioavailability and toxicity. For that reason, a single concentration is not a reliable universal toxicity threshold. The U.S. EPA’s freshwater criteria account for site-specific water chemistry and should be interpreted within the relevant jurisdiction and regulatory framework.^[2]

Aluminum in drinking water and human health

WHO has not established a health-based guideline value for aluminum in drinking water. Its guidance instead emphasizes minimizing residual aluminum where aluminum coagulants are used. Food is generally the main source of aluminum intake for the population, while drinking water usually contributes a smaller share.^[1]

The EU Drinking Water Directive lists 200 µg/L (0.2 mg/L) for aluminum as an indicator parameter. This is a regulatory parametric value for water intended for human consumption in the EU; it should not be confused with a WHO health-based guideline or applied as a universal limit outside its legal context.^[3]

Water treatment and aluminum removal

Aluminum salts such as alum are used as coagulants to help remove turbidity, color, natural organic matter and microorganisms. Coagulation produces floc that is then separated by processes such as sedimentation or flotation and filtration. Poorly controlled dosing, pH, mixing or filtration can leave elevated residual aluminum in treated water.

Where an aluminum coagulant is used, residuals are controlled by optimizing pH and dose for the source water, ensuring effective rapid mixing and flocculation, and separating the floc efficiently. Jar testing and monitoring both total and dissolved aluminum can help assess performance. WHO reports that 0.1 mg/L or less is achievable in large treatment facilities under good operating conditions, while 0.2 mg/L or less is a practicable level for small facilities; these are operational guidance values, not a guaranteed treatment result or a universal health limit.^[1] Treatment selection for naturally occurring aluminum depends on the source water and the required finished-water quality; no single process or dose is appropriate for every water.