Contribution to the evaluation of traces of dissolved and intracellular microcystin in raw and treated Moroccan waters
The supply of Casablanca city with drinking water is insured mainly by waters from the storage reservoir Tit Mellil, which is fed by the treated waters coming from the Sidi Mohamed Ben Abdellah dam. These waters are often affected by eutrophisation, with the attendant proliferation of cyanobacteria, which exercise a negative impact of the production of drinking water. In fact, the physico-chemical analyses carried out on both raw and treated waters have allowed us to make a diagnosis. This diagnosis reveals that the characteristics of the raw waters favour the proliferation of cyanobacteria. Many types of the cyanobacteria are identified, including Microcystis and Oscillatoria, which are considered to be the most toxic as well as the producers of microcystins (MC). We used a colorimetric protein phosphatase inhibition assay for an evaluation of dissolved and intracellular microcystin levels in raw and treated water with a detection limit of 1 ng•L-1. Tests done on treated water at the entrance of the storage reservoir reveal that dissolved MC are present in all the samples taken during the summer-autumn periods from 2006 to 2008. Their rates do not exceed 0.075 μg equivalent to MC-LR•L-1. Though these values are positive, they are below the threshold recommended by the World Health Organisation, which is 1 μg equivalent to MC-LR•L-1 of water. The amounts of microcystins in the treated waters are clearly less than those in the raw waters for the same periods. These values reached a maximum of 1.1 μg equivalent to MC-LR•L-1 in October 2006. On the other hand, an evaluation of the amounts of dissolved microcystins at the exit of the treated water from the storage reservoir revealed a clear decrease of microcystins concentration; this is probably due to the effects of the chlorination that had been applied to these waters.