Resilience of Secondary Wastewater Treatment Plants: Prior Performance Is Predictive of Future Process Failure and Recovery Time

Abstract

Abstract

Performance of secondary wastewater treatment facilities over a long time period is an important consideration in plant operation and discharge regulation. Two characteristics of sustainable wastewater system performance, resilience and stability, were modeled using a Generalized Linear Model (GLM) and on average 41 months of data from plants in the U.S. Environmental Protection Agency's Integrated Compliance Information System database. Sample sizes were 209 plants for biological oxygen demand (BOD); 211 for total suspended solids (TSS); and 110 for ammonia. Independent variables were the previous month's effluent concentration relative to the permit limit; plant capacity, which ranged from 4 to 1,361,000 m³/day; and capacity utilization, which ranged from 5% to 180% of rated plant capacity. First, a GLM was fit to model relative effluent concentration as a function of the independent variables mentioned earlier and a first-order Markov component. The fitted model had an R² of 0.25, 0.28, and 0.25 for individual measurements of BOD, TSS, and ammonia, respectively, and an R² of 0.91, 0.90, and 0.83 for facility averages. These models were used to generate ensembles of 10-year long sequences, from which statistics of system performance, resilience, and stability were computed. The same was performed for BOD, TSS, and ammonia. The study revealed that when discharge of these constituents exceeded their permit limits, the likelihood of subsequent violations increased significantly, indicating lack of resilience. Recovery time, measured as the duration of monthly violations, ranged from 2 to 5 months, depending on capacity and capacity utilization, with small and overloaded plants more likely to have the longest sequential violations for BOD and ammonia.