Benefits and Challenges of Blending Advanced Treated Water With Source Water Upstream of a Surface Water Treatment Plant for DPR

Water scarcity, climate change, and ever-increasing population are encouraging utilities to expand their water supply portfolio by leveraging all potential water supply sources. Recycled water is increasingly being considered as an important water source that can be treated through Advanced Water Treatment Processes (AWTPs) for supplementing potable water supply. AWTPs have been demonstrated to effectively remove chemicals and pathogens from recycled water, protecting public health. Previous larger scale potable reuse projects typically focused on advanced treatment followed by groundwater replenishment (i.e., indirect potable reuse [IPR]). Direct potable reuse (DPR) provides economic advantages over IPR due to limited additional infrastructure requirements (e.g., injection and extraction wells) other than conveyance of DPR source or advanced treated water (ATW). Blending ATW may help utilities during high demand periods (i.e., summer). Figure 1 presents the general scheme of supplementing drinking water sources with ATW. When blending ATW with the raw water source at a surface water treatment plant (SWTP), the differences in water quality may introduce operational challenges and considerations for achieving treatment goals. It is critical to understand whether the blending compromises pathogen removal at the SWTP or provides additional partial pathogen log reduction value (LRV) credit. The Water Research Foundation (WRF) Project 5049: Public Health Benefits and Challenges for Blending of Advanced Treated Water Upstream of a Surface Water Treatment Plant in DPR intends to determine the effects of ATW blending on SWTP operational efficiency and efficacy and develop operational strategies and approaches for establishing LRV credits under DPR regulatory frameworks for SWTP unit processes. Bench-scale testing was conducted with raw water and ATW collected from the SWTP and Advanced Water Purification Facility (AWPF) of five participating utilities with conventional or direct filtration SWTP. The bench-scale jar testing included various blends of raw water and ATW (i.e., 0% to 50% ATW) to assess the impact to zeta potential and coagulation, filterability, and assessing the effects of pre-ozonation. Table 1 summarizes the bench-scale testing results. In general, blending ATW up to 50 percent did not appear to challenge coagulation and flocculation. A 6-week pilot testing was conducted at Plant B to further characterize the effects of blending ATW with raw water. The pilot-scale system included ozonation, coagulation/flocculation, and filtration in series (Figure 2), simulating a direct filtration facility. Tests were conducted with 0% (i.e., raw water), 10%, and 50% ATW blends. Table 2 presents water quality characteristics for the raw water and ATW used during the pilot testing. The test results confirmed that pre-ozonation (0.3 to 0.7 mg/L O3) of the filter influent effectively generates micro-flocs and helps maintain turbidity less than 0.1 NTU in the filter effluent. Additionally, turbidity removal, headloss across the filter, and filter runtime were not affected by blending ATW with the raw water. The results confirmed that blending ATW does not appreciably affect treatment performance at a SWTP in this specific case. Additional pilot testing will be conducted at Plants A and C from Dec 2021 through Feb 2022 with pilot-scale systems representing conventional filtration to further explore treatment- and site-specific effects of blending ATW and approaches for minimizing the effects, if any. This presentation will discuss the results from the bench-scale and pilot-scale testing. Utilities considering ATW blending for source water augmentation are anticipated to benefit from the discussion.