Verwijdering van micro-organismen door langzame zandfiltratie

Abstract

The removal of waterborne microorganisms by slow sand filtration, regularly applied in Dutch drinking water production as one of the last treatments in drinking water production, was determined. About one per hundred viruses, one per ten thousand bacteria and less than one per hundred thousand parasitic protozoa pass the sand filters. These estimates for removal of pathogens by slow sand filtration constitute some of the critical parameters for the production of safe drinking water as determined by quantitative microbiological risk assessment required by Dutch law. By slow sand filtration, microorganisms are retained because they are not able to pass pores between the sand grains (straining), and by attachment to sand grains. Removal of viruses and bacteria was investigated in pilot plants and that of protozoa in the laboratory with small sand columns. Bacteria and protozoa (1 - 6 mu-m) are strained more effectively than the much smaller viruses (0.02 - 0.2 mu-m). The concentration of the microorganisms in the feeding water of the slow sand filters does not affect removal efficiency. Effects of temperature and the Schmutzdecke were also studied. The Schmutzdecke is a slime layer that gradually forms on top of the sand filter. In operation the Schmutzdecke is scraped off when it clogs the filter too much. At 9 - 12 degrees C, scraping does not affect virus removal, but bacteria are removed a hundred times more when a Schmutzdecke is present than when it is absent. At 14 - 16 degrees C, all microorganisms are removed ten times more than at 9 - 12 degrees C. After scraping off, the efficacy of the Schmutzdecke was restored within 53 days. Finally, it was found that the sand of the two drinking water companies was almost equally effective.