REDUCTION OF THE CONCENTRATION OF BACTERI A AND COLIPHAGES ALONG THE FLOWING STRETCH OF A TREATED SEWAGE CHANNEL

The efficiency of surface waters to eliminate E. coli, fecal streptoco cci, Salmonella spp., and coliphages was evaluated in a small river wh ich receives treated wastewater and which is rich in submerged macroph ytes. The study took place between April and December, 1994. Total col ony count, BOD5, O-2 concentration and water temperature were determin ed in the river as well. As the river does not receive additional wate r downwards along its 17.2 km course, dilution effects could be ruled out as the cause for the elimination of the microorganisms. The reduct ion is assumed to happen rather due to sedimentation, grazing, and ads orption to the submerged waterplants. Immediately after discharge of t he wastewater, the river water contained about 10(5) cfu/100 mL E. col i and 10(4) cfu/100 mt fecal streptococci, about 1 000 pfu/100 mL coli phages, and, as a rule, was positive for salmonella in 10 mL. The redu ction of E. coli, fecal streptococci, salmonella, clostridia, and coli phages at the end of the course was 1 to 2 orders of magnitude. This r eduction took place mainly within the first 4.7 km, a part in which, d ue to low flowing velocities, suspended solids settle down efficiently . Besides, at the end of this part the submerged water plants are espe cially abundant. The reduction of suspended solids correlated positive ly with that of BOD5, bacteria, and coliphages. The reduction of micro organisms was not sufficient to fulfill the requirements of the Europe an Community guidelines for bathing waters and for surface waters used as drinking water source. The regenerating capacity of surface waters is not sufficient to eliminate pathogens from convenionally treated w astewater. Therefore, tertiary treatment is necessary to keep receivin g waters reasonably free from pathogens.