RESEARCH ARTICLE Occurrence of cyanobacteria and microcystin toxins in raw and treated waters of the Nile River, Egypt: implication for water treatment and human health Zakaria A. Mohamed 1 & Mohamed Ali Deyab 2 & Mohamed I. Abou-Dobara 2 & Ahmad K. El-Sayed 2 & Wesam M. El-Raghi 2 Received: 5 November 2014 /Accepted: 20 March 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract Monitoring of cyanobacteria and their associated toxins has intensified in raw water sources of drinking water treatment plants (WTPs) in most countries of the world. However, it is not explored yet for Egyptian WTPs. Therefore, this study was undertaken to investigate the occur- rence of cyanobacteria and their microcystin (MC) toxins in the Nile River source water of Damietta WTP during warm months (April–September 2013) and to evaluate the removal efficiency of both cyanobacterial cells and MCs by conven- tional methods used in this plant as a representative of Egyptian drinking WTPs. The results showed that the source water at the intake of Damietta WTP contained dense cyanobacterial population (1.1–6.6×107 cells L -1 ) dominated by Microcystis aeruginosa. This bloom was found to produce MC-RR and MC-LR. Both cyanobacterial cell density and intracellular MCs in the intake source water increased with the increase in temperature and nutrients during the study period, with maximum values obtained in August. During treatment processes, cyanobacterial cells were incompletely removed by coagulation/flocculation/sedimentation (C/F/S; 91–96.8 %) or sand filtration (93.3–98.9 %). Coagulation/ flocculation induced the release of MCs into the ambient wa- ter, and the toxins were not completely removed or degraded during further treatment stages (filtration and chlorination). MCs in outflow tank water were detected in high concentrations (1.1–3.6 μgL - 1), exceeding WHO provision- al guideline value of 1 μgL - 1 for MC-LR in drinking water. Based on this study, regular monitoring of cyanobacteria and their cyanotoxins in the intake source water and at different stages at all WTPs is necessary to provide safe drinking water to consumers or to prevent exposure of consumers to hazard- ous cyanobacterial metabolites. Keywords Cyanobacteria . Drinking water . Microcystin . Nile River . Treatment plants Introduction Cyanobacteria have many ecological and physiological adap- tations, which make them dominate aquatic environments un- der warmer and nutrient-rich conditions (Carey et al. 2012). They colonize aquatic and terrestrial environments, including extreme habitats such as Antarctic lakes and hot springs (Mohamed 2008). They are found in fresh water lakes and rivers, usually in low concentrations. However, under favor- able conditions including warm temperatures, sunlight irradi- ance, and increased nutrient concentrations, cyanobacterial cells can multiply and form harmful cyanobacteria blooms in shallow inlets and bays and along the shoreline of lakes and rivers (Mohamed and Carmichael 2000; Huber et al. 2012). The presence of cyanobacterial blooms in drinking water sources can deteriorate the water quality and pose a serious threat to animal and human health as some of these cyanobacteria produce potent toxins called cyanotoxins (Dittmann and Wiegand 2006). They include hepatotoxins (microcystins, nodularin, cylindrospermopsin), neurotoxins (anatoxins and saxitoxins), and skin and gastrointestinal irri- tants (Codd et al. 2005). Microcystins (MCs) are the most common cyanotoxins in freshwater sources worldwide, with Responsible editor: Philippe Garrigues * Zakaria A. Mohamed mzakaria_99@yahoo.com 1 Botany Department, Faculty of Science, Sohag University, Sohag 82524, Egypt 2 Botany Department, Faculty of Science, Damietta University, Damietta, Egypt Environ Sci Pollut Res DOI 10.1007/s11356-015-4420-z