DISTRIBUTION OF NITRIFYING BACTERIA IN A SHALLOW STREAM

A project investigating the dynamics of self-purification processes in a shallow stream is carried out. Effects of the concentration gradien t due to the distance to the pollution source, of hydraulic conditions in the river bed and of storm floods on the distribution of nitrifyin g bacteria were studied with the help of laboratory and field experime nts. Nitrifiers density on the surface of the stream bed increased rap idly up to a distance of 300 m from the WWTP indicating possible compe tition of the nitrifiers with the heterotrophic bacteria close to the WWTP. Afterwards a slight decrease in the downstream direction was obs erved. In vertical profiles, higher bacterial densities were found at sites with rapid infiltration of channel water to the stream bed than at sites with no exchange between channel water and stream bed water o r where stream bed water exfiltrated. A major flood event scoured the nitrifiers nearly totally from the surface of the river bed. Major flo ods belong so to the most dominant processes controlling self-purifica tion in shallow streams. Minor floods, however, don't scour bacteria i n the depth of the stream bed that could then be important for the sel f-purification processes. (C) 1997 IAWQ. Published by Elsevier Science Ltd.