MAIN PRINCIPLES OF WATER-QUALITY IMPROVEMENT IN RESERVOIRS

Pollutant behaviour in mixed reservoirs can be described by simple kinetic models. Theoretically it can be shown that water-quality improvement in reservoirs benefits greatly from compartmentalization of the available reservoir volume, especially for 'fast' processes. The Biesbosch reservoirs are pumped storage reservoirs, fed with rather polluted and highly eutrophic water from the River Meuse. Air injection at the reservoir bottom prevents thermal stratification, which would otherwise impair water quality. The reservoirs act as a series of fully-mixed reactors, thus maximizing benefits from the equalization of peak concentrations of pollutants. However, equalizing is not sufficient to keep the reservoir outlet concentrations of the herbicides atrazin and diuron, permanently below the acceptable maximum level of about 0.4 mu g/L. Occasionally it has been necessary to close the water intake when river-water concentrations exceeded 1 mu g/L. Quality control by selective intake is also used under spate conditions to keep away contaminated suspended solids from the reservoir sediments and in case of pollution incidents. The self-purification potential of the Biesbosch reservoirs is described for elimination processes with typical reaction rates: 'slow' processes (ammonium), 'intermediate' processes (Cryptosporidium and Giardia) and 'fast' processes (enteric viruses). The results confirm that compartmentalization of the Biesbosch scheme in three separate reservoirs greatly enhances water-quality improvement. The Biesbosch storage system can be regarded as an integral part of the treatment of polluted river water into safe and wholesome drinking water.