Lessons Learned from IRWD – Setting Boundary Conditions at Their WWTP when Estimating GHG Emissions for Their Sustainability Evaluation

As utilities integrate sustainability into their decision making, there is no consensus on how to most appropriately quantify sustainability. There is the industry accepted triple bottom line (TBL) and sustainable return on investment (SROI), but regulations typically focus on the more easily quantifiable parameter, greenhouse gas (GHG) emissions. Although GHG emissions calculations can be relatively straight forward, which emissions scope (e.g., 1, 2, or 3) selected is critical when comparing different alternatives for a wastewater treatment plant (WWTP).This desktop case study at Irvine Ranch Water District (IRWD) considers all 3 scope emissions for optimizing the integration of an on-going plant expansion at their Michelson Water Recycling Plant (MWRP). The expansion at MWRP includes a new membrane bioreactor (MBR) with an ultraviolet disinfection (UV) treatment train that operates in parallel with a conventional activated sludge (CAS) train that has tertiary add-on solids removal technologies. Additionally, a sidestream treatment sequencing batch reactor (SBR) and a thermal dryer are being added to the solids treatment train.IRWD is concerned with prior energy brown-outs that occurred in San Diego, CA in 2011 and the importance of an energy efficient process to ensure operation during high energy demand periods. The evaluation considered the following parameters without impairing the plant's ability to meet discharge/reclamation requirements:Energy Demand – Optimize MWRP based on power required and reliabilityChemical Use – Optimize MWRP based on reduced chemical requirementsGHG Emissions – Optimize MWRP based on reduced GHG emissionsThe study evaluated the impacts of variable flow split to the MBR/UV treatment train with 2.6 million liters per day (ML/d) to the MBR as the baseline, followed by alternatives that considered a reduced flow to the MBR in increments of 0.3 ML/d down to 1.3 ML/d.The MBR was found to be more energy intensive than the CAS process. The unit energy demand for flow splitting 1.3– versus 2.6-ML/d to the MBR/UV train required 780 versus 940 kilowatt-hours per million liters (kWh/ML) treated (20 percent greater). If you consider all 3 scope GHG emissions while comparing flow splitting, the impact of diverting more flow to the MBR is not as pronounced. For example, the GHG emissions for 1.3– versus 2.6-ML/d is 13,300 versus 14,760 carbon dioxide equivalents in metric tonnes per year (CO2 eq mt/year); 11 percent greater. The GHG emissions are more similar than energy demand as it also considers the chemicals and energy required for the CAS treatment train tertiary solids removal processes.This desktop study illustrated that although the MBR technology is more energy intensive than a CAS, it is important to evaluate the plant-wide emissions and GHG impacts from chemicals and hauling (i.e., Scope 3 emissions) while considering optimization.