Monitoring aluminium before and after filtration

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.