Abstract
Greywater reuse is a feasible solution for decreasing raw water extraction in urban and rural settings. However, pathogen-specific performance guidelines and regulations have only recently been recommended; practical means to assess performance are missing. Here we examine the efficacy of Staphylococcus spp. as an endogenous surrogate for greywater pathogen reduction performance testing, by evaluating UV-C irradiation of hand-rinse greywater, and the variability in UV resistance between different wild Staphylococcus species. Hand-rinse greywater samples were collected from five participants, and a collimated UV-C beam (256 nm) was used to assess log10 reductions. Assays of colony-forming units on tryptic soy agar (TSA) were compared to mannitol salt agar (MSA) using LysostaphinTM to confirm Staphylococcus spp. After irradiating raw hand-rinse samples to a dose of 220 mJ·cm−2, log10 reductions of Staphylococcus spp. were similar (2.1 and 2.2, respectively, p = 0.112). The similarity of the reduction based on TSA and Staphylococcus-specific culture assays following UV irradiation and the dominating presence of Staphylococcus spp. suggests that Staphylococcus spp. could be used as an endogenous performance surrogate group for greywater treatment testing. Suspended wild Staphylococcus isolates were irradiated with 256 nm UV-C to compare the variability of different Staphylococcus species. Staphylococcus isolates exhibited significant variance in log10 reduction values when exposed to 11 mJ·cm−2 of UV-C. Staphylococcus hominis subsp. hominis exhibited surprising resistance to UV-C, with only a 1.6-log10 reduction when exposed to 11 mJ·cm−2 of UV-C (most other isolates exhibited > 5-log10 reduction). The efficacy of UV-C was also significantly reduced when the sunscreen oxybenzone was present at a possible endogenous greywater concentration.
Highlights
According to UN Water, approximately 2 billion people globally live in areas of water scarcity and another 1.6 billion face economic water shortage [1]
A paired t-test was performed comparing the log10 reduction from UV-C exposure within the T-tryptic soy agar (TSA) and TS-mannitol salt agar (MSA) assays from the five participants; the log10 reductions of total TSA CFU counts (T-TSA) and total staphylococci on MSA (TS-MSA) were not found to be significantly different (p = 0.112)
The similarity in log10 reduction of the T-TSA and TS-MSA assays suggests that TS-MSA was representative of bacterial reduction by UV-C irradiation of the hand-rinse water from the five participants
Summary
According to UN Water, approximately 2 billion people globally live in areas of water scarcity and another 1.6 billion face economic water shortage (lacking the necessary infrastructure for water transportation) [1]. This paper is a follow-up to a 2017 study by Shoults and Ashbolt [7] to better understand how to assess ultraviolet (UV) irradiation performance testing without the need for externally spiked surrogates. S. epidermidis is among the most prevalent species on human skin [19], we proposed total staphylococci be considered as an endogenous surrogate to represent performance testing in greywater treatment systems, given the inherent problems with using FIB [7]. There is a collective need for the scientific community and regulatory bodies to better understand the efficacy of prospective surrogates to assess greywater treatment performance. The experiments in the current study explored an array of considerations to determine the efficacy of total staphylococci as an endogenous performance surrogate. Given the high reported concentrations of the sunscreen benzophenone (BP3), commonly referred to as oxybenzone [21], the effects of oxybenzone on the efficacy of UV irradiation for staphylococci was studied
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