Reclaimed Water Disinfection: Challenges and Solutions at Brightwater

APPLICABILITY Owners and operators of water reuse systems with variable customer demand and/or an expansive distribution system face challenges when free chlorine is used for disinfection and distribution system residual. Free chlorine decays relatively quickly, making it difficult to select the correct dosage. Low customer demand can result in non-compliant low residual concentration (high system water age), and high customer demand can result in a detrimentally high residual concentration (low system water age). Process control based on a consistent free chlorine dosage is not suitable for the range of operating conditions. Free chlorine's preferential reactions with inorganic species that can be intermittently present, such as ammonia or nitrite, further complicate process control. This presentation describes a two-track approach that the King County, WA Wastewater Treatment Division (WTD) is pursuing to implement an alternative disinfection process for the Brightwater Reclaimed Water (RW) System. The first track is new ultraviolet (UV) disinfection with new chloramine for distribution residual. The second track is an innovative virus removal crediting via the existing membrane bioreactor (following Water Research Foundation guidelines) with chloramine for additional virus removal credit and distribution residual. Owners, operators, and designers of reuse systems would find this presentation valuable as disinfection is a critical process and disinfection crediting for membrane bioreactor (MBR) systems and maintenance of chlorine and chloramine residual are important topics for non-potable and potable reuse systems. DEMONSTRATED RESULTS AND OUTCOMES The Brightwater RW System produces unrestricted non-potable reuse water. Operation of the system is frequently interrupted from outages due to a non-compliant distribution chlorine residual. The minimum total chlorine concentration required is 0.5 mg/L and the maximum is 4.5 mg/L. An alternatives analysis determined a preferred approach for improving system reliability. The methods and results of the analysis and initial implementation steps are described below. Outage Root Cause Analysis: The unreliability of the RW system frustrated WTD and customers. Analysis determined that the root causes of outages were flow variations leading to a wide range of water ages, the chlorine dose, the metering pump turn-down, ammonia spikes, and treatment plant low flow periods as summarized in Figure 1. Several issues were linked to the free chlorine disinfection system. When the demand was low, the total chlorine residual fell below the minimum. If the demand was high and water moved quickly through the system, the dosed chlorine concentration was too high to decay (and thus exceeded required limits) before reaching the compliance point. Disinfection Alternatives Analysis: The project team conducted a high-level analysis of disinfection process alternatives as summarized in Figure 2. The disinfection process requirement is to achieve 4-log virus removal per Washington regulations. UV for primary disinfection with chloramine for residual, free chlorine primary with chloramine for residual, and chloramine for both primary disinfection and residual were short listed. Ozone, peracetic acid, and pasteurization were eliminated due to implementation cost and less application experience. Free chlorine was eventually eliminated due to process variability caused by ammonia and nitrite spikes. Due to chloramine's limited effectiveness on viruses, the project team pursued further investigation of chloramine disinfection combined with virus removal credits for the upstream MBR system. Full Scale MBR Virus Removal and Chloramination Testing: A protocol for conducting exploratory testing of the MBR system was developed following guidance outlined in WRF report 4997 (Salveson et al, 2021), which provides a validation testing protocol for MBR systems producing potable reuse water. Five sample pairs of the MBR feed and filtrate were collected and analyzed for bacterial coliphage removal. The results demonstrated that a minimum of 3.4-log virus removal occurred. Removal data is summarized in Figure 3. Bench top testing determined chloramination of bacteriophage MS2 could provide the remaining log removal. Outcome: The project team is pursuing both approaches, UV disinfection with chloramination and MBR virus crediting with chloramination, to implement a solution as quickly as possible without forgoing the potential to reduce project cost and simplify the process. Design of UV and chloramination facilities are underway while concurrently planning the Tier 2 testing needed to demonstrate virus removal through the MBR system. If MBR virus removal crediting is successful, the design can accommodate removal of the UV facility. RELEVANCE TO AUDIENCE This presentation would pique the interest of water reuse system owners and owners of wastewater MBR systems. This work details challenges with water age and maintaining distribution residual, while presenting the benefits of chloramination. MBR pathogen removal crediting is a novel approach to effluent disinfection that has the potential to reduce disinfection process requirements thus minimizing chemical and energy use. The MBR validation testing protocol per WRF guidance applied under this project will be of particular interest to engineers and owners pursuing MBR pathogen removal credits.