The impact of zeta potential changes on Ceratium hirundinella cell removal and the ability of cells to restore its natural surface charge during drinking water purification

Abstract

Unit processes of a conventional water purification facility are designed to remove suspended material from source water (both inorganic and organic impurities). Organic substances in source water include phytoplankton species (algae and cyanobacteria) that are generally negatively charged on the surface of the cells. The zeta potential (ZP) of algal cells needs to be destabilized in order to enhance removal thereof during water purification. The aims of this study were to investigate the ZP changes of Ceratium hirundinella (C. hirundinella) cells and the ability of cells to restore their natural ZP during the water purification process. C. hirundinella cells (>500 cells per mL) were collected from the Middle Lake in South Africa (coordinates: 26°10′50.40′′S; 28°17′50.11E). A six paddle jar test apparatus was used to simulate unit processes under laboratory conditions using 3 different coagulant options. The ZP analyser with a built-in calibrated pH meter was used to analyse the ZP and pH of the cells and the filtered source water. The coagulant concentration of 10 mg L⁻¹ Ca(OH)₂ and 10 mg L⁻¹ organic polymer achieved the best coagulation conditions as assessed against an operation ZP window of between −10 mV to +3 mV. This Ca(OH)₂ and organic polymer dosing concentration was used to purify water with increasing cell concentrations (2000 cells per mL to 7000 cells per mL, increments of 1000) applying settling times of 20 minutes, 120 minutes and 240 minutes. Results obtained indicated high percentages of cell removal after 20 minutes (82–88%), 120 minutes (93%) and 240 minutes (95%) respectively. However, after extended settling times (120–240 minutes), more metabolically active cells were observed in the supernatant of samples containing higher cell concentrations. The findings showed that the ZP of C. hirundinella cells changes as a result of adding coagulants to form flocs, but may be restored when water purification facilities employ poor optimization practices and allow extended settling periods or retention times.