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.
Link to this table

Level	When should I perform this assessment?	Advantages	Requirements/limitations	Recommended frequency	Can this be used to obtain a stormwater credit?
Visual inspection	Recommended for all practices	Quick and cost-effective	Available personnel. Does not necessarily identify causes of poor performance.	≥ 1x / year, at start of rainy season	No
Capacity testing	If there are suspected filtration rate problems with the practice, or to determine if the media has capacity for phosphorus removal	Applicable for practices of all sizes, quickly identify specific areas that require maintenance, less time and expense than monitoring	A Modified Philip-Dunne Infiltrometer is recommended for filtration rate testing.	Every few years	Consult with MPCA (Minnesota Pollution Control Agency) or regulatory agency to determine eligibility
Synthetic runoff testing	If there are suspected problems with filtration or pollutant removal, or to establish a baseline condition or baseline performance level	Controlled method to accurately measure drawdown time and/or pollutant removal efficiency	Testing will be limited by available water supply. Must be able to plug or measure all outflow paths other than filtration. Must be able to continuously measure water surface elevation in practice during test. Recommended for small stormwater treatment practices. May require authorization to conduct testing with targeted pollutants.	Every few years	Consult with MPCA or regulatory agency to determine eligibility
Monitoring	Goals include obtaining stormwater credits, assessing performance results and life of filter media, or complying with a permit or regulatory agency	Most comprehensive assessment technique and measures the response to natural rainfall events	Requires the most time, labor, and equipment of the four assessment techniques. Variations in weather, pollutant loading, and flows require correct and simultaneous operation of all instrumentation.	Continuously from construction of the IESF	Yes