Assessing the performance of iron enhanced sand filter
Iron-enhanced sand filters (IESFs) retain solids and associated pollutants by filtering and through adsorption of phosphate (soluble reactive phosphorus) from stormwater. A typical method for assessing the performance of BMPs with underdrains is therefore measuring and comparing pollutant concentrations at the influent to the filter and effluent from the underdrain outlet. Before developing an assessment program, it is important to have well-defined goals so that the effort required to develop and implement the program is focused to achieve the desired results. The need for assessment can be determined by permit, voluntary watershed management goals, TMDL allocation, or protection efforts, among others. Rather than proposing a series of rigid procedures, four assessment options are provided below that can be used in various combinations depending on information needs, budgetary constraints, time frames, and legal requirements. An online manual for assessing BMP treatment performance (Gulliver, et al. 2010) advises on a four-level process to assess the performance of a Best Management Practice.
- Level 1: Visual Inspection. Level 1 inspections consist of a brief site visit to determine whether a stormwater treatment practice is clogged or otherwise impeded. Level 1 inspections can be used to assess hydraulic performance of the IESF but cannot be used to determine phosphorus removal performance. A downloadable checklist can be found at this link.
- Level 2: Capacity Testing. Level 2 testing consists of a series of point measurements to determine infiltration and/or sediment accumulation capacity. For an IESF, this would involve media sampling for iron, phosphate and total phosphorus concentrations (mg/kg). Although there is currently no metric for relating media concentrations to performance, research is currently being performed to develop this metric.
- Level 3: Synthetic Runoff Testing. Level 3 testing consists of application of water from a clean source (e.g., a fire hydrant or water truck) to the practice to simulate a rain event in order to determine drain time. For an IESF, it is recommended to sample for and measure phosphate and total phosphorus concentrations in the water source and the filter effluent at numerous times during the test. If SRP concentration in the water source is below 0.1 mg/L, a phosphate salt, such as potassium phosphate, may need to be added to the source for measurement accuracy.
- Level 4: Monitoring. Level 4 monitoring consists of flow measurements and sampling of water in response to or between natural rainfall events, and can be used to determine peak flow reduction and pollutant removal efficiency. Monitoring of an IESF should include storm volume discharged through the filter, and SRP and total phosphorus concentrations of the filter influent and effluent.
Determination of Appropriate Monitoring Level(s) for a Stormwater Treatment Practice