The characterization of particle behavior in addition to the measurement of turbidity is becoming more important in performance evaluation as well as in the operation of the water treatment system, in order to provide supporting information on small-sized microorganisms such as Cryptosporidium. Accordingly, the particle counter has been introduced in the evaluation and operation of the treatment system. However, research results on the relation among turbidity, particle counts and/or protozoa have not been concurrent with each other. Therefore, this study investigated the relation to improve performance evaluation of the sand filter so that the risk of protozoan contamination can be reduced. The study results verified that particle counts provide a better insight into the filtration process than turbidity alone. The counts of 4–7 as well as 7–14 μm, instead of total particle counts, can be used to monitor and operate the rapid sand filter effectively, as it has a better relation with Cryptosporidium. Also, the study results showed that the fine single-medium filter is more useful when targeting better water quality despite its low productivity, since it removes small particles more effectively than the dual-media filter. The latter is more useful when targeting better productivity. However, enough depth of sand layer of dual media filter with adequate filtration rate is required to not compromise water quality and productivity, as a shallow sand layer is highly likely to cause an early breakthrough.

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