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 occurrence
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 conventional
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 water,
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 μg L−1), exceeding WHO provisional
guideline value of 1 μg L−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 hazardous
cyanobacterial metabolites