Investigation of pathogen disinfection and regrowth in a simple graywater recycling system for toilet flushing

Abstract

A B S T R A C TGraywater treatment systems must inactivate pathogens, prevent regrowth, be low cost, and be simple to operate to support their widespread adoption for alleviating water stress. A treatment system comprised only of filtration and disinfection could meet these constraints. To investigate pathogen disinfection and regrowth in such a system with minimal organic matter removal, herein three disinfectants (chlorine, ultraviolet irradiation, and ozone) were tested in combination with three filter types (coarse, sand, and cartridge) for inactivation of pathogens in graywater from the showers and hand washbasins of 14 student residences. Graywater was spiked with bacterial and viral pathogens or surrogates post-filtration. Chlorination post-filtration achieved log reductions greater than 7.1, 8.0, and 7.4 for Escherichia coli, Salmonella enterica, and Pseudomonas aeruginosa, respectively, and 3.8 for MS2 bacteriophage. UV was similarly effective, but would not prevent regrowth without a disinfectant residual. Ozonation generally was ineffective at the doses tested, with the exception that MS2 log removal was 3.7. Pathogen regrowth could be prevented for 4 d with a chlorine residual of 2.75 mg/L even for a simulated high-contamination event (6 log each pathogen). When chlorine residual was maintained, regrowth of indicators and pathogens was prevented for the light graywater investigated.

Constructed wetlands for greywater recycle and reuse: A review

“…In contrast, Ekeren et al (2016), who specifically tested high organic waters (Turbidity 26-37 NTU), found full inactivation of E. coli and S. enterica at CTs of 180-474 mg/L-min. Although they applied greater CTs, Ekeren et al (2016) attributed the observed disinfection efficiency to a phenomenon observed by others (Dietrich et al, 2003;Winward et al, 2008b) in which water with greater organic content allows for greater initial chlorine dosages while not exceeding residual limits (a residual greater than 4 mg/L will damage metal fixtures), and greater initial chlorine dosages increase chlorine particle penetration to inactivate particleassociated microorganisms.…”

“…If we exclude the trials from Ekeren et al (2016), which intentionally tested water of high organic carbon content, all disinfection trials treated water with turbidity values less than 10 NTU. If we compare these trials to the effluent produced by wetlands in Table 3, we see that not all wetlands were able to produce water with these low turbidity values.…”

“…Figures 4 and 5 show the results of the reviewed disinfection trials for chlorination and UV (see Figure S1 for analogous ozone figures), plotting LR against dose. Although we included ozone trials in Table 4, we will only discuss in detail chlorine and UV as i) chlorine and UV have been more widely tested for GW applications (Winward et al, 2008b;Winward et al, 2008c), ii) ozone tends not to perform as well as chlorine and UV (Ekeren et al, 2016) and iii) life cycle costs for ozone disinfection systems are greater than chlorine and UV (Beck et al, 2013). Importantly, most trials reported full inactivation, meaning that the potentially achievable LR at that particular dose and under those particular conditions was likely not realized.…”

“…MS2 coliphage data come from Beck et al (2013) and Ekeren et al (2016), who tested relatively low organic and high organic GW, respectively. Whereas Ekeren et al (2016) generally achieved more effective inactivation of bacteria species discussed above, they were only able to demonstrate LR of MS2 coliphage of 3.8 at a dosage of 474 mg/L-min compared to LR of >5-7 at dosages of 200-450 mg/L-min reported by Beck et al (2013).…”

“…MS2 coliphage data come from Beck et al (2013) and Ekeren et al (2016), who tested relatively low organic and high organic GW, respectively. Whereas Ekeren et al (2016) generally achieved more effective inactivation of bacteria species discussed above, they were only able to demonstrate LR of MS2 coliphage of 3.8 at a dosage of 474 mg/L-min compared to LR of >5-7 at dosages of 200-450 mg/L-min reported by Beck et al (2013). Moreover, the one trial reported by Beck et al (2013) in which full inactivation was not achieved was performed at a dosage of 90 mg/L-min and resulted in an LR of 4.4, greater than the 3.8 reported by Ekeren et al (2016) at a dosage of 474 mg/L-min.…”