Aluminium speciation was characterized to examine the changes after filtration, in drinking water systems with low (≤56 μg l−1) and high (>100 μg l−1) aluminium concentrations at plant outlet. The treatment chemicals were also examined, and only lime showed significant aluminium contamination. This contamination varied from 0.4 to 2.5 μg mg−1 and was largely water soluble (mean: 85%). The amounts of aluminium in lime added after filtration were equivalent to water concentrations ranging from 8.6 to 34 μg l−1 (mean: 17.5 μg l−1). By using the less contaminated lime supply, the aluminium concentration contributions could have been four times lower (mean: 4.1 μg l−1). When determining the concentration differences between the effluents of one filtering unit and plant outlet, the aluminium changes at the nine low-aluminium plants were associated with lime addition, and were mostly detected in soluble form. The changes at the eight high-aluminium plants paralleled problems occurring at previous stages and were mostly attributed to variations occurring at additional filtering units. These situations prevented detection of other changes (e.g. post-precipitation). The monitoring of leachable particulate aluminium and of various parameters (pH, turbidity, Al–turbidity relationship) was also useful in differentiating particulate matter and demonstrating the effects of undesirable lime properties (dissolution delays, particulate residues).

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