Abstract
The presence of cyanobacteria and their toxins (cyanotoxins) in processed drinking water may pose a health risk to humans and animals. The efficiency of conventional drinking water treatment processes (coagulation, flocculation, rapid sand filtration and disinfection) in removing cyanobacteria and cyanotoxins varies across different countries and depends on the composition of cyanobacteria and cyanotoxins prevailing in the water source. Most treatment studies have primarily been on the removal efficiency for unicellular Microcystis spp., with little information about the removal efficiency for filamentous cyanobacteria. This study investigates the efficiency of conventional drinking water treatment processes for the removal of the filamentous cyanobacterium, Oscillatoria limnetica , dominating the source water (Nile River) phytoplankton in seven Egyptian drinking water treatment plants (DWTPs). The study was conducted in May 2013. The filamentous O. limnetica was present at high cell densities (660–1 877 cells/mL) and produced microcystin (MC) cyanotoxin concentrations of up to 877 μg∙g -1 , as determined by enzyme-linked immunosorbent assay (ELISA). Results also showed that conventional treatment methods removed most phytoplankton cells, but were ineffective for complete removal of O. limnetic a. Furthermore, coagulation led to cell lysis and subsequent microcystin release. Microcystins were not effectively removed and remained at high concentrations (0.37–3.8 μg∙L -1 ) in final treated water, exceeding the WHO limit of 1 μg∙L -1 . This study recommends regular monitoring and proper treatment optimization for removing cyanobacteria and their cyanotoxins in DWTPs using conventional methods. Keywords: cyanobacteria, removal, conventional drinking water treatment
Highlights
Frequent occurrence of extensive overgrowth of cyanobacteria in water bodies worldwide has been attributed to human activities (Huber et al, 2012; Mohamed and Al-Shehri 2015)
Most studies on the removal of cyanobacteria at drinking water treatment plants were conducted with Microcystis, being the most common species in temperate and tropical freshwater sources (Mohamed and Carmichael, 2000; Zamyadi et al, 2012)
Water samples were collected from the intake of 7 drinking water treatment plants (DWTPs) along the Nile River, Egypt, and after each step of treatment processes including coagulation/f locculation/sedimentation (C/F/S), rapid sand filtration and final treated water
Summary
Frequent occurrence of extensive overgrowth of cyanobacteria in water bodies worldwide has been attributed to human activities (Huber et al, 2012; Mohamed and Al-Shehri 2015). Microcystins (MCs) are the most common cyanotoxins in freshwater sources worldwide, with more than 100 variants identified far (Pantelic et al, 2013). They are produced by some cyanobacteria, such as Microcystis, Planktothrix, Oscillatoria, Nostoc, Anabaena and Gloeotrichia (Hisbergues et al, 2003). Most studies on the removal of cyanobacteria at drinking water treatment plants were conducted with Microcystis, being the most common species in temperate and tropical freshwater sources (Mohamed and Carmichael, 2000; Zamyadi et al, 2012). Filamentous cyanobacteria, Oscillatoria spp., can dominate both benthic and planktonic cyanobacterial populations in water sources, and some of its species can produce MCs (Mohamed et al, 2006). Microcystin concentrations were monitored in both raw and treated waters to evaluate the release and removal of these toxins during different treatment processes
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