Removal Mechanisms of Cryptosporidium Oocysts and Their Effective Treatment by Sand Filtration

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Outbreaks of waterborne cryptosporidiosis have often occurred since 1980s. It is pointed out that the removal rate of turbid matter by coagulation and sand filtration should be improved from 2.5-2.7log to 4.0log, in order to prevent the outbreaks.In the present study the fundamental experiments were conducted to elucidate the removal mechanisms of the oocysts by coagtilation and sand filtration, and fluorescent latex micro-beads were used as a surrogater of the oocysts. The findings of the experiments are as follows :(1) The behavior of zeta-potential of the beads with pH value was almost similar to that of the oocysts and kaolin particles. In addition, the characteristic of treatment of the beads was similar to that of kaolin particles, namely turbid matter.(2) In the filtration experiment, the method was newly developed to recover the beads inside the sand filter and in the effluent. The high recovery rates of 90.5-96.4% for the beads were obtained by using this method and it is effective to elucidate the removal mechanisms of the beads by sand filtration.(3) The filtration experiment demonstrated that many beads easily passed through the filter in the term of the initial breakthrough and that the beads leaked with the turbid matter retained inside the filter due to the acidification of the inlet water and a rise of the filtration rate.(4) In order to remove the oocysts effectively by the rapid sand filtration system, it is important to coagulate the turbid matter sufficiently and not to change the conditions of the inlet water and the filtration run. In addition it is necessary to coat the surface of the filter media with coagulants in order to improve the quality of the effluent throughout the filtration run.