Hygienic quality of public natural swimming pools (NSP)

Abstract

Natural swimming pools (NSP) have become more popular in the past 20 years, both for private and public use, but their hygienic status remains a matter of discussion. Elimination rates in NSP are well defined for Escherichia 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 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 is four times faster than in the chlorinated pool. The ex-situ elimination of Cryptosporidium in an NSP is approximately 10% faster than in a 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 for the best possible hygiene and health status some elementary questions, stated in this paper, will have to be solved in the next years or decades. This work describes the in-situ (water treatment inside the pool) and ex-situ water treatment (water treatment via external water treatment plants) of NSP, and the influence of zooplankton in existing NSP on the elimination rate of Cryptosporidium and Giardia.

Screening Level Risk Assessment (SLRA) of human health risks from faecal pathogens associated with a Natural Swimming Pond (NSP)

“…Yet very little is known regarding the rate that zooplankton can clear (oo)cysts from the surrounding water, however rates of 22-24 mL•grazer -1 •day -1 and 15-19 mL•grazer -1 •day -1 for Cryptosporidium oocysts and Giardia cysts respectively have been reported ( Connelly et al 2007 ). The filtration capacity of zooplankton reported by ( Eydeler and Spieker 2010 ) cited in ( Bruns and Peppler 2019 ) ranged from 8.5 to 64.8 mL•grazer -1 •day -1 for Rotatoria, Copepoda and Cladocera protozoa. The more pressing issue relates to the poorly documented fate of ingested pathogens within zooplankton and their faecal pellets.…”

“…Five removal barriers were considered in the scoping of the QMRA: a wetland system consisting of zooplankton filtering, a hydro-botanic filter and submerse sand/root filter; a commercial designed Neptune™ surface spray gravel media filter; and UV disinfection ( Figure 1 ). However, there is very limited published data on the performance of these NSP barriers ( Bruns and Pepper, 2019 ), so in combination with related literature estimates we provided reasonable point estimate and plausible ranges in Log 10 removals for each barrier to be applied within the Screening level QMRA model ( Table 4 ). As such, triangular distributions (defined by mode [min and max]) were selected to describe the variability and uncertainty associated with the Log 10 removals.…”

“…Zooplankton filtering: Zooplankton grazing is proposed to provide important in-situ disinfection in NSPs ( Bruns and Peppler, 2019 ), yet requires careful consideration as to how well it may be expected to perform as a barrier for protection of human health. Studies have shown that free-living environmental protozoa can ingest human enteric bacteria and protozoan pathogens including Cryptosporidium oocysts ( Agasild and Nõges 2005 , Connelly et al 2007 , Stott et al 2003 , Trout et al 2002 ).…”

“…Mechanisms of pathogen removal within these filters include adsorption to soil and filter substratum, predation (noting issues identified for zooplankton filtering above), sedimentation, physical filtering of larger organisms and inactivation due to environmental exposure (pH, temperature, sunlight [only for the hydro-botanic filter]). Both filters have been estimated to achieve 10% reduction of E. coli ( Bruns and Peppler 2019 ).…”

“…Neptune filter: The Neptune filter system has a gravel medium depth of 2.00 to 2.20 m; and a retention time of 20 min was assumed by designers to have a higher E. coli removal efficacy of 90% in comparison to the hydro-botanic and submerse filters. Challenge studies with coliphage (assumed to be a reasonable human enteric virus surrogate to mimic their) on similar filters has shown between 95 and 99% reduction ( Bruns and Peppler 2019 ).…”