The retention of protozoa cysts and oocysts at varying depths in slow sand filters is a subject still not completely understood. A better understanding may contribute to an improved assessment of the risk of infections in those populations that receive water treated by slow sand filtration and could also contribute to the improvement of the planning and design of slow sand filters. With these aims, this paper intends to communicate advances in the understanding of the retention of Cryptosporidium oocysts and microbiological indicators of water quality in the filter media. In the study, Cryptosporidium oocysts presented a similar behaviour for the higher filtration rate (6 m3 m−2 ·d) in both flows investigated. For lower filtration rates (3 m3 m−2 ·d), the penetration of these oocysts is higher in the downflow filter. In the upflow filter the retention is limited to the initial 0.45 m of the filter media. There was no accumulation of oocysts inside the filter media as the run progressed, suggesting the existence of some control mechanism. The schmutzdecke seems to play an effective role in the removal of protozoa only when it is well developed. A weak relationship was found between the Cryptosporidium oocysts and both water quality and treatment efficiency indicators.

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