Hierarchical Modeling Approach to Evaluate Spatial and Temporal Variability of Wastewater Treatment Compliance with Biochemical Oxygen Demand,Total Suspended Solids,and Ammonia Limits in the United States

Abstract

Abstract

A hierarchical modeling approach was used to diagnose the variability of U.S. wastewater treatment facility compliance with existing discharge permit limits. The first level of hierarchy modeled monthly variability in treated water quality over a 4-year period using a Generalized Linear Model, which relates compliance to plant characteristics such as flow rate, capacity utilization, seasonality, and compliance history. Residuals from this model were subjected to a spatial analysis using Kriging in the second level of hierarchy to model spatial covariability. This hierarchical approach was used for three effluent constituents: biochemical oxygen demand (BOD), total suspended solids (TSS), and ammonia, in discharge monthly report data for the period 2004–2008 from a random sample of more than 100 municipal plants. The first level of hierarchy captured most of the total variance in compliance data for all three pollutants. Seasonality was seen to affect effluent concentrations of BOD and TSS, which have uniform national discharge standards, but not ammonia, for which permit limits are typically seasonal. Levels of compliance for BOD, TSS, and ammonia all were significantly affected by the respective effluent concentration in the previous month. The second level of hierarchy identified geographic regions of anomalous compliance and improved the model's predictive performance for a subset of plants with discharges closer to permit limits.