Research shows that Spring supplies are most likely to contain Faecal Coliforms (E.coli) within 3 days of heavy rainfall. The level of contamination is much higher after a long dry spell as the rainfall collects layers of detritus matter that has built up over the summer months. This means that clean filters can become blocked very quickly. The guiding principle is, if the inside of your filter is discoloured, then it is time to change the filter. This will avoid ‘breakthrough’ whereby contaminants overwhelm the filter leading to discolouration of water and the blinding of quartz sleeves.