Drought Response Feasibility and Engineering Study

Indirect potable reuse (IPR) is the blending of advanced-treated reclaimed water with a natural water source to provide drinking water after further treatment. Following the recent drought of record in the Colorado Basin, the 2014 Austin Water Resource Task Force recommended that the City of Austin evaluate the feasibility of IPR in Lady Bird Lake (LBL) as one of several water supply strategies for consideration in the City’s integrated water resources plan.

Implementing IPR in LBL would involve enhanced treatment at the South Austin Regional Wastewater Treatment Plant to produce high standard reclaimed water (project water). The project water would be diverted to the downstream end of LBL above Longhorn Dam during extreme drought. Concurrently, water from the upstream end of LBL below Tom Miller Dam would be withdrawn and pumped to the Ullrich Water Treatment Plant.

An advanced model was developed to simulate hydrologic and water quality impacts of IPR. The Lady Bird Lake model is a dynamic model that couples the USEPA’s EFDC hydrodynamic model and the USEPA’s WASP advanced eutrophication model. It predicts hourly to seasonal variations in water quality for 124 grid cells in the Lake.

The model accounted for seasonal releases from upstream reservoirs and simulated algal-nutrient interactions. It evaluated water quality standards for the reclaimed water and estimated Lake recovery time after IPR has ceased. Thus, a useful tool was developed for future water supply and treatment planning for the City and the region.