Monitoring and Maintenance of Phosphate Adsorbing Filters

Field installations of two iron-enhanced sand filters (IESFs), designed to remove phosphate and particulates from stormwater runoff, were monitored and maintained for 1–3 years. One application, a traditional IESF in an agricultural watershed, retained over 64% of the influent phosphate load, whereas the second, a pond perimeter IESF in a developing suburban watershed, retained 26%. The measured average effluent event mean concentration (EMC) for the traditional IESF was $56.1\mathrm{\mu g}/\mathrm{L}$. All events exhibited positive removal of phosphate (i.e., effluent $\mathrm{loads}<\mathrm{influent}$ loads). In contrast, the measured percent phosphate retained for the pond perimeter IESF in 2013, 2014, and 2015 was 18, 25, and 45%, respectively. In addition, the average effluent EMC for the 3 years was 64.1, 54.2, and $19.9\mathrm{\mu g}/\mathrm{L}$, respectively. Half of the events (14 of 28) were found to have negative removal (i.e., effluent $\mathrm{loads}>\mathrm{influent}$ loads). Events with negative removal tended to be smaller events with low influent phosphate concentrations ($3.7–39.4\mathrm{\mu g}/\mathrm{L}$). Nonroutine maintenance improved the hydraulic performance of the pond perimeter IESF and, after a rinsing event, also improved phosphate retention rates to an average of 45%. There are believed to be at least two reasons for this difference in performance between the two IESFs: First, the traditional IESF was treating agricultural tile drainage with a low particulate phosphorus concentration, while the pond-perimeter IESF had a degrading mat of filamentous algae transported onto the surface, creating a source of phosphate that was not quantified. Second, the pond-perimeter IESF had treated a relatively large volume of water for its size, resulting in substantial flow-through in the filter within 5 years of operation. This is greater than anticipated for an IESF, and may have partially caused the reduction in performance.