Greywater Pilot System

The Department of Defense (DoD) has prioritized the need to develop and implement wastewater and water reuse treatment systems that can be operated energy-neutral and be readily deployed. However, current water reuse systems are not optimized for energy efficiency when coupled to stringent regulations on treatment effectiveness and byproduct formation. Showers and laundry comprise the bulk of potable water demand at many military installations. Sustainable reuse of the graywater resulting from these operations would substantially reduce the quantity of new water consumed. 

The goal of this project was to demonstrate the sustainable use of stable electrocatalytic anode materials to treat graywater for reuse in showers or laundry. Successful demonstration of this objective would inform the design of a pilot-scale system providing the DoD with a technically and economically feasible approach for graywater reuse.

A pilot system was constructed, and its piping was tied into the shower discharge manifold of a barracks shower room. The graywater was treated in 30-gallon batches, lasting between 5 and 8 hours. The treatment system consisted of pre-filtration, electrochemical (EC) treatment, chlorine reduction, and pH adjustment. Physical graywater sampling occurred bi-monthly. 

A power supply provides electric current to an EC cell. The EC cell was in a divided configuration with a total cathode area of 1000 cm2 and total anode area of 1000 cm2. Boron-doped diamond electrodes served as cathodes, and Ti/IrO2-RuO2 electrodes served as anodes. Nafion 117 membranes divided each cell. Graywater sampling was conducted to assess the overall performance of the system.

Scale accumulation resulted in the applied voltage increasing, ultimately to the point where the desired current density could not be applied. Overall, this resulted in decreased treatment effectiveness and increased energy demands. To address the scaling, water softening (via ion exchange resin) was applied to the water upgradient of the electrochemical cell. This prevented several scaling issues that were encountered prior to the addition of water softening. Post-demonstration breakdown of the system did reveal some scale accumulation (albeit much less than previously observed), suggesting the need for more aggressive polarity reversal and/or automated acid rinsing than what had been implemented during this demonstration.

 The pH readings were routinely unreliable, which made remote monitoring of system performance problematic. It is unclear as to why the pH electrodes/meter used were problematic. The greywater and electrochemically oxidized water matrix may have substantially shortened the electrode life. It is thus recommended that selected pH measuring instruments be tested for longevity and accuracy at the bench scale prior to field implementation. 

Electrochemically generated perchlorate levels typically ranged between 0.1 and 2 mg/L, which are orders of magnitude above California drinking water levels. While trace (<0.05 mg/L) perchlorate levels were anticipated, the levels generated by the mixed metal oxide anodes in the demonstration system were not anticipated. These results serve as a caution that mixed metal oxide anodes, if used at elevated current density for relatively long time periods, can result in substantial perchlorate formation.

Performance objectives of the pilot system were partially met, and demonstrated the need for continued research in the areas where performance objectives were not met, most notably for scale-up requirements for commercial development. The estimated cost of treated water included all capital and operations and maintenance (O&M) costs. Using the O&M cost alone did not appreciably change the cost per gallon of treated water. The estimated cost of EC treated water was $1.25 gal. (Project Completion - 2022)