The use of turbidity and particle number measurements in monitoring filtrate quality in conventional water treatment with polyaluminium chloride

The excessive amount of hydrolysis products of PACl (polyaluminium chloride) results in the formation of a large number of very small non-agglomerated particles (1–2 µm) when reaching and exceeding the isoelectric point. It was found that the residual aluminium occurred mostly in the form of Al₁₃ species. In the study the influence of PACl overdosing on filtrate quality was tested in the aspect of the choice of the best tool to control treatment process. It was reported that although the filtrate turbidity was compliant with drinking water requirements and did not exceed 0.37 NTU, the coagulant hydrolysis products in the form of fine particles were not all retained in the filter bed and were observed in filtrate. Non-agglomerated particles were especially observed in the initial phase of filtration run. There was no direct correlation between the number of fine particles and turbidity in filtrate e.g. if to compare filtrate samples of similar turbidity (sample 2 – filtrate turbidity was 0.21 NTU and sample 5 with turbidity 0.17 NTU). The number of 1 µm particle was 82 1/cm³ and 1184 1/cm³ for sample 2 and 5, respectively. Contrary to turbidity measurement, particle count allows to identify the negative results of PACl overdosing.