Hygienic quality of public natural swimming pools (NSP)

Natural Swimming Pools (NSP) have become more popular in the past 20 years, both for private and public use. Still the hygienic status is a matter of discussion. Elimination rates in NSP are well defined for E.coli, Enterococci and Pseudomonas but a lack of knowledge exists regarding elimination rates in NSP concerning the parasitic protozoans Giardia and Cryptosporidium. First studies indicate that the in-situ Zooplankton filtration proved to reduce these protozoans efficiently: The in-situ elimination of Cryptosporidium is dependent on the population of zooplankton. In the 50% percentile the elimination rate takes 4 times less time than in the chlorinated pool. The ex-situ elimination of Cryptosporidium in the NSP is approx. 10% faster than in the chlorinated pool. In ex-situ treatment of NSP the elimination rate reached 2 log-steps versus 1 log-step in chlorinated pools. For the further development of NSP concepts towards the best possible hygiene and health status some elementary questions, stated in this article, have to be solved during the next years or decades.