Strategies of coagulant optimisation to improve the removal of turbidity and Ceratium hirundinella cells during conventional drinking water purification

Abstract

The dinoflagellate, Ceratium hirundinella (C. hirundinella) cells are known to cause many problems when source water due for purification contains relatively high concentrations. The objective of this study was to investigate strategies for the effective and simultaneous removal of turbidity and C. hirundinella cells using turbidity and total photosynthetic pigments (TPP) as indicators of appropriate coagulant dosages during conventional drinking water purification. Source water samples with low turbidity, and high number of C. hirundinella cells were collected. A laboratory-scale conventional water purification plant was used to simulate coagulation, flocculation, sedimentation and sand filtration. Various coagulant options were dosed as part of conventional coagulation. The coagulant option Ca(OH)$_2$-organic polymer achieved the best removal of both turbidity (50%) and C. hirundinella cells (75-82%) after sedimentation using TPP as an indicator. Ca(OH)$_2$-SiO$_2$ and organic polymer alone achieved better removal of C. hirundinella (57-75%) and turbidity (33-50%) respectively when TPP was used as an indicator rather than turbidity. Sand filtration removed the remaining turbidity and C. hirundinella cells from the supernatant completely. Implementing new purification strategies may increase treatment costs, but the focus of drinking water purification utilities should always be primarily the production of safe and aesthetically acceptable drinking water.