Fractionation analysis was used to examine treated water before and after filtration at 17 drinking water plants performing aluminium coagulation. Before the filtering units, soluble Al (s:Al) was ≤150 µg/l, except for the three plants also using lime softening. Particulate Al (p:Al) was completely acid leachable, except for two plants. Concentrations before and after filtering units indicated changes in s:Al for many plants. These changes ranged from +42 to −88 µg/l, affected the Al filtration balance and implied reactions occurring in the filter media. These reactions could also generate residual p:Al. Without s:Al change, only problems of filtration effectiveness were likely to cause residual p:Al. After the filtering units, s:Al remained >150 µg/l when using lime softening. When using only alum or polyaluminium chloride, s:Al was ≤80 µg/l. Decreases in s:Al across filtering units occurred for both coagulants and produced s:Al <40 µg/l. Factors influencing equilibria and rates of treatment processes (e.g. pH, temperature, plant design) could explain the s:Al results. Increases in s:Al or partially leachable p:Al were associated with performance deteriorations producing high residual p:Al at the end of filter runs. Residual p:Al contributed to give concentrations ≥100 µg/l at five plants. The different behaviours constituted supplementary information to optimise plant operation.

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