Seepage from Wastewater Treatment Plant Effluent Basins

The District serves a population of approximately 1.4 million and treats an average of 130 million gallons of wastewater per day. Wastewater enters the facility in pipes of up to 12 feet in diameter, and treated effluent is stored in 20 earthen-lined basins with a total area of over 120 acres. The accurate quantification of the basins’ seepage rate became critically important in the context of regulatory compliance challenges and potentially costly infrastructure modifications.

LSCE employed a measurement-supported water balance method to determine the seepage rate. The method uses high-frequency measurements (15-second intervals) of meteorological variables, including air temperature, relative humidity, wind speed, the energy flux from the effluent into the atmosphere, and precise effluent level measurements. The evaporation is computed with a bulk-aerodynamic transfer model, which outperformed several other evaporation models over a wide range of meteorological conditions, including Bowen (1926), Penman (1948), Priestley and Taylor (1972), DeBruin (1978), and eddy covariance (Ham and Baum 2009). The methodology is capable of quantifying daily seepage rates in the sub-millimeter realm.

Uncertainty was computed with consideration of the measurement error (i.e., relating to instrumentation specifications) and environmental conditions during testing using the Taylor Series Method. TSM works directly with the partial derivatives of the random and systematic components of the combined standard uncertainty.

The results of this work averted costly basin retrofits and/or retirements and accompanying disruptions to the facility’s operation.

Significant Project Elements

• Research methodology
• Experimental design and fieldwork
• Implementation of complex modeling techniques 
• Coordination with challenging wastewater treatment plant operations
• Effective communication of difficult technical material