Have Urban Drainage System Maintenance Practices Made BMPs Passé in a TMDL Toolbox?

Regular maintenance (cleaning and recovery of particulate matter, PM) can be an inconvenient activity for distributed urban stormwater management systems that extend from source area pavement to receiving water discharges. Despite the cost and inconvenience, maintenance can provide significant economy for recovery of PM and associated phosphorus (P) and nitrogen (N) loads as compared with unit operations (nominally, best management practices, BMPs). In this study more than 400 samples of maintenance-recovered PM from pavement (as street sweeping, SS), catch-basin (CB), and BMP cleaning practices across the State of Florida for 14 municipal separate storm sewer systems (MS4s) were collected and analyzed for P and N. The PM build-up, washoff, and recovery functions were modeled for a range of SS and BMP cleaning maintenance intervals and BMP treatment efficiencies. Based on cost data from the MS4s economic analyses were performed to compare PM, P, and N recovery costs from SS and BMPs. Florida-based metrics associating the recovery of P and N loads were developed. Results demonstrate that incremental SS recovery diminishes over time when the pavement is cleaned more frequently. Results indicate that PM, P, and N recovery is significantly more economical (by one to several orders of magnitude) using SS as compared with using and maintaining BMPs. For example, load recovery of phosphorus cost approximately $10,000/pound while for CB cleaning the cost is $1600/pound and the cost of SS is $250/pound based on Florida data. Optimization of application intervals for maintenance practices that are used in tandem with hydrologic restoration systems can lead to very significant cost savings and higher load control than BMPs while providing the necessary hydrologic control.