On-site biological hand washing water treatment can improve global access to safe hand washing water, but requires a thorough understanding of the chemical composition of the water to be treated, and an effective treatment strategy. This study first presents a detailed characterization of the individual inputs to hand washing water. We demonstrate (i) that soap is likely the most significant input in hand washing water, representing ∼90% of mass loading, and (ii) that inputs to hand washing water have low concentrations of biologically-essential macro- and micro-nutrients (nitrogen, phosphorus, potassium, copper, zinc, molybdenum and cobalt) with respect to carbon, which may impair biological carbon removal. This study next formulates a recipe that recreates a representative composition of hand washing water and develops a procedure to identify and supplement nutrients in which this recipe is estimated to be deficient. Batch testing of the nutrient-supplemented hand washing water with an inoculum of planktonic bacteria demonstrated improved assimilable organic carbon removal (99% vs. 86% removal) and produced lower final dissolved organic carbon concentrations (1.7 mgC/L vs. 3.5 mgC/L) compared to realistic (nutrient-deficient) washing water. Supplementing nutrients did promote cell growth (50x higher final total cell count). Full-scale testing in a biologically activated membrane bioreactor (BAMBi) system treating 75 L/day of nutrient-supplemented hand washing water showed that long-term operation (100 days) can deliver effective carbon removal (95%) without detrimental fouling or other disruptions caused by cell growth. This work demonstrates that biological treatment in a BAMBi system, operated with appropriate nutrient-balancing offers an effective solution for decentralized treatment of light greywater.
Innovative solutions are necessary to enable the decentralized recycling of greywater for applications requiring high-quality water, such as hand washing. While physical barriers such as ultrafiltration membranes effectively prevent the passage of bacteria, and chemical and biological treatments can effectively reduce the carbon content of the treated water, there exists a knowledge gap regarding the application of anti-bacterial strategies to prevent the growth of harmful bacteria following treatment. In this study, the effluent water from a household-scale greywater treatment system was fed to seven parallel experimental post-treatment tanks: three receiving direct chlorination with free chlorine residuals of 0.2, 1 or 5 mg Cl 2 /L, three with chlorine produced through electrolysis at the same residual concentrations, and one control with no chlorine added. For increasing concentrations of direct chlorination, the median total cell count (TCC) values were 9 × 10 4 , 2.9 × 10 4 and 1.8 × 10 3 cells/mL, respectively. Electrolysis treatment produced very similar TCC concentrations, 8.8 × 10 4 , 1.1 × 10 4 and 2.3 × 10 3 cells/mL. The TCC concentrations were lower than the concentration of the water entering each tank (∼3 × 10 5 cells/mL). Intact cell count (ICC) measurements indicated that the viable cell concentrations, were less than 10% of the TCC values. Though electrolysis treatment can produce powerful oxidants, such as hydroxyl radical, there was no evidence that electrolysis in this system provided additional benefits beyond chlorine production for control of total or intact cell counts. Oxidation of bacteria by chlorine was the dominant anti-bacterial mechanism in our system. Monitoring of dissolved organic carbon (DOC) and assimilable organic carbon (AOC) did not suggest that carbon-limitation significantly impacted cell counts when chlorination or electrolysis treatment was applied. This work demonstrates that either direct chlorination or electrolysis treatment are able to reduce bacteria concentrations over long-term operation of a hand washing water treatment system. We recommend selecting chlorine residual targets such that a chlorine residual is maintained during periods of challenging operating conditions. We observed that a target residual of 1 mg Cl 2 /L, in our system, maintained the TCC below the concentration found in Zürich drinking water.
Highlight• Bacterial growth in fecally-contaminated water is highly variable and dependent on several factors. • Regrowth occurs after chlorination (low doses, no residual).• Indigenous microbial communities variably impact bacterial growth.• A combination of treatments can both inactivate and inhibit growth.The Blue Diversion AUTARKY Toilet is a urine-diverting toilet with on-site treatment. The toilet is being developed to provide a safe and affordable sanitation technology for people who lack access to sewer-based sanitation. Water used for personal hygiene, hand washing, and flushing to rinse urine-and feces-collection bowls is treated, stored, and recycled for reuse to reduce reliance on external water supplies. The system provides an opportunity to investigate hygiene of water for reuse following treatment. Treatment in the toilet includes a Biologically Activated Membrane Bioreactor (BAMBi) followed by a secondary treatment technology. To identify effective secondary treatment, three options, including granular activated carbon (GAC) only, GAC+chlorine (sodium hypochlorite), and GAC+electrolysis are considered based on the bacterial inactivation and growth inhibition efficiency. Four different hygiene-relevant bacteria are tested: Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa, and Salmonella typhimurium. Our evaluation demonstrates that-despite treatment of water with the BAMBi-E. coli, P. aeruginosa, and S. typhimurium have the potential to grow during storage in the absence of microbial competition. Including the indigenous microbial community influences bacterial growth in different ways: E. coli growth decreases but P. aeruginosa growth increases relative to no competition. The addition of the secondary treatment options considerably improves water quality. A column of GAC after the BAMBi reduces E. coli growth potential by 2 log 10 , likely due to the reduction of carbon sources. Additional treatments including chlorination and electrolysis provide further safety margins, with Nguyen et al. Pathogens in Water for Reuse more than 5 log 10 inactivation of E. coli. However, reactivation and/or regrowth of E. coli and P. aeruginosa occurs under in the absence of residual disinfectant. Treatment including the BAMBi, GAC, and electrolysis appear to be promising technologies to control bacterial growth during storage in water intended for reuse.
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