Bacteriological Purification of the Finnish River Kymi

With its total drainage basin of 37 107 km2 and mean annual discharge of 307 m3 s-1, the River Kymi is one of the largest in Finland. Presently it receives treated domestic wastewaters of about 160 000 inhabitants through eight treatment plants and treated industrial effluents from four paper mills, two cardboard mills and one pulp mill. In the 1960s all the wastewaters were discharged untreated to the river. However, the construction of domestic wastewater treatment plants began in the 1970s and mechanical wastewater treatment became common in industry. By the end of the 1980s practically all the wastewaters discharged to the river were treated in activated sludge processes. The aim of this paper was to investigate the influence of wastewater purification on the bacteriological quality of the river during these three decades. The bacteriological quality of the river was evaluated on the basis of annual medians of fecal streptococci (FS) calculated for 15 sites on the basis of 4804 FS enumerations from the years 1964-1992. The results show that the bacteriological quality improved steadily as the number of treatment plants increased and wastewater treatment became more effective. In the beginning of the 1960s the annual FS medians in the central part of the river were typically 1000-1500 CFU/100 ml, whereas in the 1990s they had decreased to 30-40 CFU/100 ml (CFU = colony forming units). The results indicate that FS input to the river was mainly of domestic origin.