The article presents the results of an investigation concerning the influence of a preliminary sedimentation pond with a biological filter on value changes of chosen physicochemical water parameters in the Wierzbiak River, which feeds
a water reservoir in Mściwojów.
A preliminary sedimentation pond with biological filter is part of a pre-liminary pond. It consists of three chambers of different depth, where the central one is planted with reed (Phragmites australis) and functions as a biological filter, where the processes of water purification take place.
The investigation was conducted between June and October 2008. In the samples of water collected every second week, concentration of nitrates and phos-phates was marked. Directly, in the testing ground, the temperature, conductivity and pH were measured. In the samples of bottoms and reed, concentration of ni-trogen, phosphorus and dry matter were marked.
The results of the investigation suggest a significant decrease of nitrates concentration and an increase of temperature in water flowing out the sedimenta-tion pond. In the majority of cases a reduction of phosphates concentration in the sedimentation pond was noticed. The water flow through the pond does not influ-ence pH value.