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Development of a cost effective and energy efficient treatment system for graywater reuse for toilet flushing at the multi-residential scale




Hodgson, Brock, author
Sharvelle, Sybil, advisor
Roesner, Larry, committee member
Goemans, Christopher, committee member

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A growing population increases water demand in many metropolitan areas resulting in the need for projects, like graywater reuse, that free up water supply or decrease water consumption. Plumbing for graywater collection from showers and bathroom sinks has been separated from blackwater collection in 14, two-person units at a residence hall at Colorado State University. Treatment technologies were evaluated for the ability to provide safe and cost effective onsite reuse of graywater for toilet flushing. The goal is to develop a system with low use of energy and consumables capable of treating graywater to a quality safe for toilet flushing. The system analyzed filtration utilizing coarse, sand (20-40 microns), or cartridge (100 microns) filtration and the disinfection potential of ultraviolet (UV) with hydrogen peroxide (H2O2), chlorine, UV with chlorine as a residual, or ozonation with chlorine as a residual. Disinfection efficacy was determined by measuring general water chemistry parameters in addition to concentration of E. coli and total coliforms. The influent E. coli averaged 10 2.7±1.1 CFU/100mL and total coliform averaged 10 7.9±1.2 CFU/100mL. Effluent E. coli was reduced to non-detectable concentrations for UV combined with H2O2 and chlorine, but only chlorine measured non-detectable concentrations of total coliform. At the tested doses, ozone combined with chlorine and UV combined with chlorine resulted in limited or no removal of E. coli and total coliforms. Higher doses may prove to provide more efficient disinfection but require more expensive equipment and may impact the projects feasibility. Based on data collected, chlorine appears to be a better approach for disinfection of graywater. None of the disinfectants significantly affected graywater chemistry, but all reduced odors with the exception of UV. There was no significant change of water chemistry as a result of coarse or cartridge filtration. Sand filtration significantly reduced turbidity, total suspended solids (TSS), total organic carbon (TOC) and biochemical oxygen demand (BOD5) by 13±11%, 37±12%, 31±17% and 21±9% respectively. Despite the decrease of TSS and TOC, the sand filter resulted in an increase chlorine demand. As a result, it was concluded that the most effective treatment alternative is incorporation of coarse filtration followed by chlorine disinfection. The health and environmental concerns associated with chlorine disinfection can be minimized by utilizing ammonia in graywater to favor monochloramine formation which results in a smaller dose. Additionally, the influent specific UV absorbance of 1.1±0.6 indicates reduced risk of disinfection by-product formation. The cost, including capital and operation, of implementing various filtration and disinfection approaches along with the total life-cycle project cost at various system sizes were evaluated. At the residence hall scale, the most cost effective disinfection approaches include application of liquid chlorine, ultraviolet with chlorine as a residual, and small-scale ozonation with chlorine as a residual. The cost of the hydrogen peroxide dose rendered its use infeasible. The cost effective filtration approaches were coarse, sand (20-40 microns), and cartridge (100 microns) and the associated capital for each filter did not have a large impact on the life-cycle cost. Graywater reuse for toilet flushing proved financially beneficial particularly in regions with high domestic water costs and at system sizes that reuse ≥1,000 gpd. These projects can be financially feasible and have low payback periods in addition to indoor water use reduction.


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