Implications of Changes in Water Use, Population, and Climate on Performance and Planning of Wastewater Systems

The installation of water-efficient indoor appliances and fixtures - such as those meeting standards set by the 1992 Energy Policy Act - has led to a significant decline in residential water use over the past 30 years. Other water conservation strategies, like leak repair and greywater reuse, have also contributed to changes in the amount of potable water used by cities. These changes affect downstream systems like wastewater collection and treatment. During the 1970s drought in California, mandatory water conservation measures led to a reduction in wastewater flows by up to 60%, leading to an increase in pipe corrosion, solids settling, and odor due to lower flow velocities. Performance and design of wastewater systems over time is, however, a complex system affected by external drivers, like climate, population and land use, policy and public changes that influence water use, and water utility practices such as maintenance and green infrastructure installation that can reduce flows into the system. This research demonstrates how the use of active water management, e.g. measures that increase water use efficiency or decrease the amount of infiltration and inflow (I&I), may help offset exogenous factors, preventing or delaying the need for upgrades to existing water infrastructure systems. This is evaluated through a scenario analysis of flows to a wastewater treatment Plant, located in Portland, Oregon. Future wastewater flows were projected to 2040 using potential changes in per capita wastewater production, industrial wastewater production, population growth, and ranges of infiltration and inflow. Results show that with current efficiency trends in Portland, rapid population growth could be offset, leading to stabilization of wastewater flows over the next several decades, and elimination of the need for capital expansion. Climate drivers leading to an increase in stormwater inflows have the potential to outweigh declines in wastewater due to efficiency; yet the use of best management practices may help counteract these influences. The present work illuminates the possibility to adopt system wide adaptive water management strategies as a response to future stressors such as climate change, population shifts, and land use changes.