A comparative study on the evaluation of a Cryptosporidium removal surrogate was conducted using Scenedesmus quadricauda, green algae. Bench-scale direct filtration experiments were carried out at various initial concentrations: 500-5,000 oocysts/ml for C. parvum oocysts and 500-10,000 cells/ml for S. quadricauda. From the results, algal cell or Cryptosporidium oocyst counts of the filtrates (C) increased in proportion to their initial concentrations (C0). However, no significant differences in C/C0 profiles were observed over the examined range of the initial concentration, which implied that the removal efficiencies for S. quadricauda cells and C. parvum oocysts were not related to the initial concentrations. Examination of the deposition in the sand filter showed that a large part of S. quadricauda cells or C. parvum oocyst counts were captured in the upper layer of the sand filter, and the deposition rates were gradually reduced along the filter depth. Total cell or oocysts counts deposited in the sand filter increased commensurate with the initial concentration for both microorganisms. The ratio of the deposited cell or oocyst counts to the deposited amounts of flocs showed quite similar values between S. quadricauda and C. parvum, which meant that these two microorganisms were alike in their removal behavior. From these similar characteristics of removal, S. quadricauda was thought to be a reasonable and reliable surrogate of C. parvum oocysts removal in sand filtration.

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