• Effect filtratiesnelheid, temperatuur en korrelgrootte op de verwijdering van micro-organismen door langzame zandfiltratie

      Schijven JF; Colin M; Dullemont Y; Hijnen WAM; Magic-Knezev A; Oorthuizen W; Wubbels G; Bruins J; Rutjes SA; de Roda Husman AM; LZO (Rijksinstituut voor Volksgezondheid en Milieu RIVMWaternetKiwa ResearchHet WaterlaboratoriumDuinwaterbedrijf Zuid-HollandWaterbedrijf Groningen, 2009-04-23)
      A mathematical model was developed to predict removal of microorganisms by slow sand filtration, an important treatment in the production of drinking water, under a variety of conditions. These conditions are filtration rate, temperature, grain size and the Schmutzdecke, a slime layer on top of the sand filters. These conditions may vary dependent on required production capacity, weather conditions and the drinking water utility. Insight into the effects of these conditions matter for the translation of data between drinking water utilities and of literature data to utilities and are therefore also of importance for the legally obligated quantitative microbiological risk assessments. The model was developed using experimental data from previous research on pilot plant scale. The processes that determine removal of microorganisms by slow sand filtrations are attachment and straining. Also the effect of the Schmutzdecke was evaluated. The extent of attachment of microorganisms to sand appeared to be utility-specific. The model was applied to predict removal by slow sand filtration of bacteriophage MS2, E. coli and Cryptosporidium for four drinking water utilities under their relevant process conditions and to use these prediction to formulate research proposals on pilot plant scale in order to validate the model.
    • Verwijdering van micro-organismen door langzame zandfiltratie

      Schijven JF; Colin M; Dullemont Y; Hijnen WAM; Magic-Knezev A; Oorthuizen W; Rutjes SA; de Roda Husman AM; LZO (Rijksinstituut voor Volksgezondheid en Milieu RIVMWaternetKiwa ResearchHet WaterlaboratoriumDuinwaterbedrijf Zuid-Holland, 2008-10-14)
      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.