Impact of wastewater treatment plant discharge on enzyme activity in freshwater sediments.

The biological self-purification processes are a central point for the ecological states of rivers. The degradation efficiency of pollutants is mainly due to the microflora and can be detected with enzyme-activity tests. Extracellular-enzyme activity of freshwater sediments was measured, in a microcosm, versus different pollution levels caused by organic wastewater treatment plant (WTP) discharges. Biochemical evaluation of Vmax values for each exoenzyme appears to be a function of the organic matter content of the WTP effluent (Fig. 2). Glucosidase and peptidase reveal a significant negative correlation (respectively, r = 0.99, with P < 0.02; and r = 0.853, with P < 0.08) of Vmax versus DOC concentration (i.e., an inhibition effect). The same relationships were observed with K(m) values, beta-Glucosidase and aminopeptidase activity are well described by the Michaëlis-Menten equation, but linearization with the Lineweaver-Burk equation does not fit with a simple type of inhibition. Two sediments (sand and silt) have been tested, and the differences in the exoenzyme activity of the two sediments after WTP effluent input could be explained by their physicochemical differences. The effects of WTP effluents on a freshwater sediment indicate that, in the current experiments, the microbial potential exoenzyme activity does not increase: this finding implies that, in rivers, the global hydrolytic potential could remain steady down-stream of a discharge point.