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Decimal Elimination Capacity (DEC) of the slow sand filters of the Dutch drinking waterCompanies was assessed in this study, first by a literature review followed by evaluation of the removal ofenvironmental spores of sulphite-reducing clostridia (SSRC) and small-sized centric diatoms(SSCD) as surrogates. Because these data were not conclusive, additional dosing experimentswere performed on laboratory and pilot plant scale. These experiments confirmed the high DEC-valuesfor Cryptosporidium (4.7 log or more) described in literature. The results also showed thatthe DEC of these filters for persistent organisms can be influenced by long term accumulationand delayed breakthrough. Assessment of the spatial distribution of retained oocysts in the pilotfilter showed a significant reduction of oocyst concentration over time. The sand contained ahigh concentration of zooplankton and on the basis of the literature data it was concluded that theoocyst reduction was most likely caused by predation. Predation will be subject for furtherresearch. In conclusion, slow sand filters have a high DEC for Cryptosporidium and most likelythe risk of oocyst accumulation and breakthrough in biologically active and well constructed andwell operated slow sand filters is low. Additionally, SSRC and SSCD are no good surrogates toassess the capacity of these filters to remove protozoan oocysts. Includes 23 references, tables, figures.

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Edition: Vol. - No. Published: 11/01/2006 Number of Pages: 11 File Size: 1 file , 360 KB