Extracellular enzyme activities during slow sand filtration in a water recharge plant

Activities of the extracellular enzymes β-glucosidase and phosphatase and bacterial densities were investigated during the filtration process at several sites in a groundwater recharge plant at the Ruhr river (Hengsen recharge plant in Schwerte, Germany). Low numbers of microorganisms and low levels of activity in this type of habitat, compared to most surface waters, caused methodological problems when determining microbial activity. In this study, fluorigenic model substrates, which enable hydrolytic rates as low as 1 nmol (L×h)−1 to be measured, were used to determine extracellular enzyme activities. Highest activities were determined in surface water (107 nmol (L×h)−1 for β-glucosidase and 252 nmol (L×h)−1 for phosphatase), which decreased during the filtration process in the gravel prefilter and the main sand filter until the end of subsurface flow (1.6 nmol (L×h)−1 and 6.8 nmol (L×h)−1, respectively). Similarly, bacterial numbers decreased from 3.4×106 to 0.29×106 cells mL−1. These data showed that microbial activity within the prefilter and the shallow layers of the sand filter had the greatest impact on water quality. In addition to its involvement in the continuous purification of surface water, the microbial community in the sand filter probably acts as a biological buffer against ephemeral increases in the loads of organic matter and nutrients in the recharge plant.