Microbial indicators for water quality in recirculating shower technology

Recirculating showers save up to 70–80 % of the water and energy use of conventional showers, but water quality in these systems are not studied very well due to the technology's early stages. The aim of this study was to provide an overview of information available on which microorganisms were investigated and at which densities. Based on this platform we further aimed at identifying key microbial indicators and pathogens for monitoring water quality in these systems by integrating data from the top five waterborne outbreaks with findings from studies on (1) microbial investigations in reuse projects involving shower effluents or combined bathroom streams, (2) shower water effluents, and (3) biofilms in conventional showers. Outbreak data were extracted from the National Outbreak Reporting System (NORS) and a systematic literature review, supplemented with additional references found within the papers. Escherichia coli, Total Coliforms, enterococci as faecal indicator bacteria and the heterotrophic plate count were reported in shower effluents at variable, but generally high concentrations, indicating these organisms to be relevant as indication of faecal bacterial contamination and as general microbial densities in recirculating showers. Pseudomonas aeruginosa was abundant in conventional shower biofilms, detectable in shower effluents at concentrations in the range of 2–4 logs CFU/100 mL and ranked among the top five etiological agents of waterborne outbreaks. Consequently, P. aeruginosa is considered as a significant reference pathogen for assessing water quality in recirculating shower systems. A diverse range of Legionella species, particularly Legionella pneumophila, are the primary cause of waterborne outbreaks, and persist in conventional shower biofilms. Therefore, we consider it to be significant as an indicator species in monitoring water quality. Staphylococcus aureus, Mycobacterium spp., and fungi such as Candida albicans could serve as supplementary microbial parameters. Trace organisms such as protozoa (Cryptosporidium spp. and Giardia spp.) and model viruses are also recommended for assessing treatment efficacy. In conclusion, while further research is required to validate these findings, this review critically evaluates relevant pathogens, creating the basis for future microbial water quality assessments in recirculating showers. © 2025 The Authors