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Soil Passage Drinking Water Purification

Soil passage of surface water for drinking water production is effective enough in the removal of viruses. This is one of the conclusions of the research project of Jack Schijven. He hopes to earn his PhD on Monday 2 April at TU Delft. An example of soil passage is dune filtration. “The new law on water facilities states that the chance of a virus infection caused by the consumption of drinking water is to be less than one in ten-thousand people,” says Schijven. “Using the results from the lab and field research, I have developed a mathematical model with which virus removal can be analysed in detail.

About two thirds of the drinking water in the Netherlands comes from ground water. One third is from surface water: rivers and lakes. Surface water is infected with micro-organisms that can cause sickness, this is a result of the dumping of waste water and manure. These germs have to be removed   before the water can be used as drinking water. According to the new law on this matter, not more than one person in ten-thousand can become ill due to virus infections caused by the consumption of drinking water. “That means that no more than 2 viruses per 10 million litres of drinking water,” says Schijven. “That demands a lot from the purification of surface water,. Because the virus concentrations in Dutch surface water are one-hundred-thousand to one million times as high.”

About 40 percent of the water in drinking water production is processed through so-called soil passage. Usually in the form of bank filtration, dune filtration or depth infiltration. From field and laboratory research results, Schijven has concluded that soil passage works well enough to meet the high demands of the new law. “The virus concentrations were reduced 100 million times using dune filtration of 30 metres in a 25 day period, or using a depth infiltration of 38 metres in a period of 40 days,” says Schijven. Using these results he was able to develop mathematical models that accurately describe the attachment and transport of viruses. Schijven: “The viruses attach to sand grains and then die. The most effective virus removal happens in the top few metres of soil passage, because there are (relatively) more attachment opportunities in that area. At greater distances the removal is far less effective.”

Schijven also used these models in a first step towards determining the vulnerability of ground water installations for virus infections. “If we use a worst case scenario: bad attachment to the sand, such as was the case at greater depths in the depth infiltration studies, and a leaking sewer,” says Schijven, “then the water would have to stay in the sand three to seven times longer than the currently standard 60 days to effectively prevent virus infection.” According to Schijven, these numbers are currently not hard facts or data, but they form a good reason to do further research in this area. Currently, Schijven is employed as researcher modeller, at the Microbiological Laboratory for Health Protection (MGB) of the RIVM.

Delft University of Technology. March 2001.

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