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Evaluating hydrodynamic separators


With the advent of both federal and state storm water management regulations, state and municipal highway departments must consider a broad array of Best Management Practices (BMPs) for meeting storm water treatment objectives for both new road construction and roadway-improvement projects. In recent years, a number of manufacturers have entered the marketplace with a variety of proprietary devices for treating storm water. One of the most common types of devices is the hydrodynamic separator (also referred to as innovative water-quality inlet, particle separator, or swirl concentrator). Evaluating these technologies for application in the highway setting requires consideration of a number of factors relative to these devices’ treatment performance, inspection and maintenance requirements, and installation and operating costs. The Massachusetts Highway Department (MassHighway), under a cooperative agreement with the U.S. Geological Survey, recently conducted a detailed field study of water-quality inlets (WQIs) located on the Southeast Expressway in Boston. That study provided valuable lessons regarding storm water sampling protocols and data analyses used to evaluate hydrodynamic separators. (These products generally consist of refinements in the design of the standard WQI.) This paper discusses the lessons learned and offers recommendations for evaluating the performance of proprietary designs within this class of BMP. A variety of findings came out of the Southeast Expressway (SEE) Study that should be considered when evaluating “hydrodynamic separators.” The study evaluated two separate WQIs, each of which received storm water discharges from deep-sump (four-foot) catch basins. It was found that the one continuously monitored deep-sump catch basin had an annual suspended sediment removal efficiency (SSRE) of 39%, whereas the annual average SSRE for two WQIs was 32% (based on the remaining load after flow through the catch basins). Captured sediments were comprised predominantly of sand-sized particles. Residence time was the primary factor controlling the SSRE. To a lesser degree, antecedent conditions and volume of rain also affected the SSRE. Other findings were that metals and nutrients tend to concentrate on particles smaller than sand and that sediment resuspension occurred in both the catch basin and the WQIs. In addition to the limited suspended sediment removal efficiency of the WQIs, the SEE Study found that the WQIs were ineffective at removing soluble pollutants, fine particles, floatable solids (debris and litter), and oils and grease. Prior to installing hydrodynamic separators, the operators of drainage systems and environmental regulators should obtain scientifically supportable data on the field performance of hydrodynamic separators. Based on the findings and experience obtained over the course of the SEE Study, MassHighway recommends the following key elements for validating the field performance of hydrodynamic separators: • Collect field data that is both representative of the range of rainfall events and that is applicable to the conditions (e.g., ambient particle-size distributions) under which the BMP likely will be installed; • When sampling, differentiate between the effects of “supernatant displacement” and active-particle removal by the separator (i.e., “hydrostatic” versus “hydrodynamic” separation). This requires flow-proportional sampling throughout each storm event; • Account for antecedent conditions, bypass flows, and resuspension when calculating the SSRE; • Sample a sufficient number of storms not only to obtain statistically significant data, but to include the full range of operating conditions to which the device will be subject; • Analyze treatment performance by “Summation of Loads,” which is the preferable method for accuracy and quality control; • Sample storms sequentially, to allow for a mass-balance calculation; • Include measurements of particle-size distribution in the sampling and analysis program to assess the removal efficiency of Total Suspended Solids (or, preferably, Suspended Sediment Concentration), as well as that of other contaminants associated with various particle-size fractions. Hydrodynamic separators should also be evaluated relative to other potential limitations. For example, if these underground structures function to contain fuel spills, then they have the potential to create an explosion hazard. In addition, according to the literature, hydrodynamic separators also may create conditions suitable for breeding mosquitoes and bacteria or conditions that result in liberating nutrients and metals from captured sediments. Based on its evaluation of WQIs and on the literature MassHighway has reviewed to date, further scientifically sound evaluation is necessary to demonstrate the effectiveness of hydrodynamic separators as primary-treatment devices. Although MassHighway has documented the limitations of the WQIs used along the Southeast Expressway (e.g., low overall removal of suspended sediment, particularly fine particles), hydrodynamic separators may be appropriate for pre-treatment and retrofit applications where sand is the target contaminant and where the operator has adequate maintenance capabilities.

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