Investigation of pathogen disinfection and regrowth for a low cost graywater reuse treatment system for toilet flushing
Date
2013
Authors
Wiles, Kristen, author
De Long, Susan, advisor
Sharvelle, Sybil, advisor
Goodridge, Lawrence, committee member
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Abstract
Population growth in arid regions is causing water supplies to become increasingly stressed. Water conservation measures such as low-flow fixtures provide some relief, but water savings are limited and relatively small. Graywater reuse is gaining attention as a way to ease the water stress. Graywater is ideal for reuse because it is constantly available, generated on site and requires less treatment than wastewater. Reusing graywater for toilet flushing could reduce total household potable water demands by ~25%. To promote widespread adoption and therefore maximize water savings, graywater treatment technologies must be effective, low-cost, and simple to operate without compromising public health. A treatment system comprised only of filtration and disinfection could meet these constraints; however, because such a system involves minimal organics removal, research is needed to develop a treatment system that effectively inactivates pathogens and prevents regrowth. To develop a treatment system, three filter types (coarse, sand and cartridge) were tested in combination with three disinfectants (chlorine, ultraviolet radiation, and ozone). Raw graywater from the showers and hand basins of 14 student dorms was filtered and then spiked with Escherichia coli, Salmonella enterica, Pseudomonas aeruginosa or the bacteriophage MS2 (virus surrogate). The log-reduction was quantified for each filter and disinfectant combination. Chlorine provided consistent log reductions of all bacteria and viruses. Chlorination post-sand filtration resulted in log-reductions of 6.5, >7.8 and >7.4 for E. coli, S. enterica, and P. aeruginosa, respectively. UV radiation post-sand filtration provided 5.5, >8.3 and >7.1 log-reductions of E. coli, S. enterica and P. aeruginosa, respectively. No significant bacterial disinfection was achieved with ozone post-sand filtration. However, ozone did achieve a log-reduction of 3.7 for MS2. Chlorine post-sand filtration and UV achieved log-reductions of 3.8 and 2.7 for MS2. Disinfection results were found to be generally similar for the coarse, cartridge, and sand filters. Chlorination post-coarse filtration achieved log reductions of >7.1 and >8.0 for E. coli and S. enterica. Chlorination post-cartridge filtration provided log reductions of only 5.2 and >7.8 for E. coli and S. enterica. UV achieved log reductions between 5.5 and 5.7 for E. coli with all filters, and between >7.4 and >8.3 for S. enterica. These batch studies supported the selection of chlorination and a coarse filtration for a demonstration graywater treatment system currently installed in one of the campus residence halls at Colorado State University. Additionally, regrowth studies were conducted on graywater disinfected with chlorine. In these tests, E. coli and total coliforms were monitored for up to seven days. Studies indicate that regrowth of total coliforms and E. coli can be prevented for at least two days with adequate chlorine residual (>2.5 mg/L) and a TOC less than approximately 50 mg/L. Spiked regrowth studies support the results of initial regrowth studies. Graywater spiked with E. coli, P. aeruginosa, and S. enterica was disinfected with chlorine and a residual of 2.75 mg/L total chlorine prevented regrowth of all organisms for four days. Lastly, the demonstration unit was monitored and maintained over the course of the school year. Maintenance activities and observations were recorded for the development of a standard operating procedure (SOP). The SOP allows maintenance and testing to be completed by a non-professional, which was one of the criteria of the demonstration unit.
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Subject
regrowth
disinfection
non-potable water reuse
graywater