Removal and fate of Cryptosporidium in dissolved air drinking water treatment plants

In the first part of the paper, data from pilot plant studies are used to e valuate Cryptosporidium removal by dissolved air Rotation (DAF) clarificati on and dual media filters under challenge conditions. Oocyst removals were investigated for design detention times and hydraulic loadings for winter a nd spring seasons. Coagulation was optimized for turbidity and removal of n atural organic matter. DAF performance was better for spring water temperat ures achieving 2.5 +/- 0.3 log removal of oocysts compared to 1.7 +/- 0.3 l og removal in the winter. Cumulative log removal across DAF and filtration exceeded 5.4, and was not affected by water temperature. Low turbidities an d particle counts are indicators of good treatment and good removals of Cry ptosporidium. The second part of the paper uses a mathematical model to pre dict the fate of Cryptosporidium through a DAF plant and the impact of filt er backwash recycle on oocyst build-up in the plant influent. Model predict ions show that the fate of Cryptosporidium and the build-up of oocysts in t he plant influent depend on: DAF performance, the percent of filtered water production used for backwashing, and the percent of filter backwash recycl e Row. A DAF plant with 2.5% filtered water production for backwashing and that achieves 1.6 log removal or greater of oocysts by DAF clarification wi ll not have a build-up of oocysts in the plant influent regardless of the r ecycle rate. This is because the oocysts are concentrated in the DAF floate d sludge and not within granular filters.