Remediation of suspended solids and turbidity by improved settling tank design in a small‐scale, free‐standing toilet system using recycled blackwater

Abstract

Our research is focused on the development of decentralized waste water treatment technologies enabling onsite water reuse. Accumulation of solids with recycling of treated blackwater increases the energy required for disinfection with an electrochemical process. We hypothesized that improving the preprocess settling of blackwater by increasing the tortuosity of the liquid flow path would reduce this energy demand by reducing particle‐associated chemical oxygen demand (COD). This approach successfully reduced the total suspended solids and turbidity in the process liquid accumulated per user‐day equivalent. A modest reduction in the apparent steady‐state accumulation of COD was also observed, likely because of the retention of COD associated with larger particles in the settling tanks. Interestingly, these improvements did not improve the energy efficiency of the electrochemical disinfection process, as predicted. These observations suggest that improving the energy efficiency of electrochemical disinfection will require remediation of dissolved COD.

A granular activated carbon/electrochemical hybrid system for onsite treatment and reuse of blackwater

“…2 D). As previously discussed ( Hawkins et al., 2018 ), most of the COD present in these tests would be in the soluble or suspended form as most of the particulate COD is removed in settling tanks preceding these tests. These data support the hypothesis that reducing the soluble COD can reduce the disinfection energy required for an electrochemical process.…”

“…Our liquid treatment system utilizes a solids separation mechanism and settling tanks to achieve up to 86% removal of total suspended solids (TSS) and an electrochemical process that presently requires 48–93 kJ/L (13–26 kWh/m 3 ) to achieve complete disinfection ( Hawkins et al., 2017 , Hawkins et al., 2018 , Sellgren et al., 2017 ). We have hypothesized that the high energy requirement for disinfection is likely attributable to the soluble and suspended chemical oxygen demand (COD) in the process liquid ( Hawkins et al., 2018 ).…”

“…Our liquid treatment system utilizes a solids separation mechanism and settling tanks to achieve up to 86% removal of total suspended solids (TSS) and an electrochemical process that presently requires 48–93 kJ/L (13–26 kWh/m 3 ) to achieve complete disinfection ( Hawkins et al., 2017 , Hawkins et al., 2018 , Sellgren et al., 2017 ). We have hypothesized that the high energy requirement for disinfection is likely attributable to the soluble and suspended chemical oxygen demand (COD) in the process liquid ( Hawkins et al., 2018 ). In addition, the treated liquid maintains sufficient color and odor to require a further “polishing” step; surveys from field testing of a prototype unit at CEPT University in Ahmedabad, India confirmed that users were dissatisfied with color and odor of the disinfected liquid recycled for flushing ( Elledge et al., unpublished results .)…”

Improving energy efficiency of electrochemical blackwater disinfection through sequential reduction of suspended solids and chemical oxygen demand

“…Electrochemical disinfection was performed as previously described 3 in an 8-L HDPE tank with a commercially available electrochemical cell (Hayward Salt&Swim 3C) run at 24 VDC. Measurements of water quality parameters and microbial enumeration were performed as previously described in detail [1][2][3][4] . Energy required for disinfection was calculated as previously described 1 and shown in Figure S3.…”

Field testing of an onsite sanitation system on apartment building blackwater using biological treatment and electrochemical disinfection

“…Led by the "Reinvent the Toilet Initiative" sponsored by the Bill & Melinda Gates Foundation, transformative blackwater treatment technologies have been developed to treat toilet effluent onsite and disinfect the liquid for reuse. [8][9][10][11] These emerging technologies aim to produce pathogen-free water for onsite reuse, with the overarching goal of preventing the discharge of untreated wastewater into the environment and improve public health.…”