Kaolin-assisted polyaluminium chloride coagulation improves picophytoplankton removal from drinking water without raising residual aluminium

ABSTRACT

Two-panel comparison: polyaluminium chloride alone yields 27.6% picophytoplankton removal with increased residual aluminium microflocs; polyaluminium chloride plus 4mg per litre kaolin yields 50-55% removal with reduced residual aluminium.

Picophytoplankton pass through sand filters, breach turbidity targets, and release cyanotoxins on lysis, threatening drinking-water safety. This study examined the mechanistic basis of picophytoplankton removal by polyaluminium chloride coagulation and evaluated kaolin as a low-residual-aluminium enhancement. Jar tests were conducted on artificial raw water dosed with Synechococcus sp. (1.03 × 106 cells/mL) at PAC doses of 0–8 mg as Al/L, alone and combined with kaolin at 0–10 mg/L. Turbidity, zeta potential, size-resolved particle counts, and residual-aluminium microfloc concentrations (acidification particle-counting method) were measured. The optimum polyaluminium chloride dose of 2 mg as Al/L removed 38% of turbidity but only 27.6% of picophytoplankton in the 0.5–1.0 μm fraction was removed, against 57% for larger particles. Acidification particle counting confirmed that aluminium-derived microflocs occupy the 0.5–2.0 μm range, co-resident with target cells. Overdosing the coagulant (4–8 mg as Al/L) significantly increased the residual-aluminium microfloc concentration (ΔN up to +949,330 particles/mL; p < 0.001), whereas the optimum dose did not. Adding 4 mg/L kaolin raised picophytoplankton removal to 50–55% and simultaneously reduced the residual-aluminium microfloc burden (ΔN = −799,163 ± 139,753 particles/mL; p = 0.0007 versus coagulant alone). Kaolin-assisted coagulation is a practical, health-protective retrofit for utilities facing picophytoplankton blooms.