The raw water from Lake Zegrzynski is difficult to treat and requires a high chemical dosing rate of coagulant for the reduction of organic matter in order to comply with EU standards. The high degree of reduction calls for an efficient coagulant, but also an optimal chemistry such as correct dosing amount and an optimal flocculation-pH. The paper describes different removal efficiencies of organic matter, particularly with respect to pH in flocculation.

During the warm season, the raw water can also suffer from large amounts of plankton that are difficult to remove to a satisfactory degree. In a two-step chemical treatment concept, with Dissolved Air Flotation (DAF) and the pulsator process, the plankton content in the final outlet water is reduced to very low levels. It follows that eventual peaks of plankton in the raw water cannot be detected in the final outlet water.

Ozone is added for removal of taste and odour originating from the presence of plankton. In the pre-treatment step prior to the ozonation, a great deal of the organic matter is removed, which reduces the necessary ozone dosage.

The Municipal Water Company of Warsaw City has decided to consider an additional pre-treatment step to the existing water treatment plant at Wieliszew. For that purpose, and bearing in mind an integrated process design, the company has performed pilot-scale experiments with DAF installed prior to a process line identical with the existing full-scale plant. The experimental flow is about 10 m3/h.

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