Carbon and Energy Footprint Analysis of an Advanced Oxidation Process for Removing NDMA in Indirect Potable Water Reuse Operations

The indirect potable reuse system operating in Orange County, CA is currently one of the largest water reuse systems in the world, with average flow treated of 62 MGD, planned for future expansion. The water reuse process is operated by the Orange County Water District (OCWD), using the treated wastewater from the adjacent Orange County Sanitation District (OCSD) Plant 1. The wastewater train includes primary and full secondary treatment with nutrient removal; the water reuse train includes microfiltration (MF), reverse osmosis (RO), a UV/H2O2 advanced oxidation process (AOP), and lime conditioning. N-Nitrosodimethylamine (NDMA), classified as probable human carcinogen by the US EPA, is a contaminant of concern for this system and its effluent concentration is regulated at 10 ng/L. The goal of this research is to show the weight of the AOP on the energy footprint of the water reuse process at OCWD, and on the combined process at OCSD-OCWD. We also show the effects of varying NDMA removal on AOP's carbon and energy footprint for increasingly stringent effluent limits. The AOP's carbon footprint is dominated by the carbon equivalent of the energy footprint. This is amplified during peak periods in comparison to the nighttime, due to the reduced efficiency for power generation that power utilities experience during peak power demand periods. Therefore, even though the consumed energy could be the same during different diurnal periods, carbon footprint could vary significantly. This suggests that flow equalization, when possible, would be a solution to mitigate carbon-equivalent emission, while leaving the energy-footprint and process throughput unaltered.