The efficacy of a recovered wash water plant in removing cyanobacteria cells and associated organic compounds

The treatment works under investigation uses a recovered wash water plant (RWWP) to remove impurities prior to recycling filter backwash water. Filter backwash water (raw water) is characterized by high quantities of cyanobacteria cells and associated organic compounds; a potential threat when recovered water is recycled. The aim of this study is to identify the cyanobacteria cells and associated organic compounds in the filter backwash water and to subsequently evaluate the effectiveness of the RWWP in removing these organic impurities during the following periods; autumn-winter and spring-summer. Results showed that at least six major phytoplankton groups were present in the filter backwash water with turbidity levels (59 and 46 NTU; autumn-winter and spring-summer, respectively) being much higher than the drinking water productions standard of ≤5 NTU. Cyanobacteria were a dominant group (mean of 80% and above) in the total phytoplankton composition of the raw water and consisted of three genera (Anabaena sp., Microcystis sp. and Oscillatoria sp.), which were effectively removed by the RWWP (up to 99%). However, associated organic compounds such as geosmin, total organic carbon (TOC), dissolved organic carbon (DOC) and microcystin were not effectively removed during the different seasonal periods but were of such low concentrations that they posed no major risk to the drinking water quality, meeting the RWWP water quality standard.