Performance of an Active Phosphorus Removal Filter in an Urban Watershed

Summary This study describes the performance of a constructed surface water filtration system containing three different types of reactive media to remove phosphorus from Lake Cornelia, located in Edina, Minnesota. This paper will summarize phosphorus removal efficiencies observed during the first year of monitoring and outline operational challenges encountered during start-up and the initial monitoring period. Introduction Lake Cornelia is shallow and has a large urban watershed (975 acres) with highly impervious commercial areas. Average total phosphorus concentrations in Lake Cornelia typically exceed the Minnesota state standard of 60 micrograms per liter (µg/L). This has led to significant algal blooms (including cyanobacteria) and poor water clarity. In collaboration with the City of Edina, the Nine Mile Creek Watershed District constructed a unique surface water filtration system at Rosland Park to address external phosphorus loading to Lake Cornelia. The filtration feature consists of a series of constructed concrete basins with an anthracite pre-filter for particulate removal and parallel up-flow reactive media filters for dissolved phosphorus removal. The filter is positioned in a park between a large upstream stormwater pond and Lake Cornelia. Water discharged from the pond is actively pumped to the filtration system for treatment. The advantage of this filter over a conventional passive iron-enhanced sand filter is that it can treat substantially more water while maintaining a relatively small footprint to preserve the park's green space. Construction of the filtration feature was completed in July 2022, and performance monitoring began in August 2022. The performance monitoring period is expected to continue through fall 2024. This paper will present initial phosphorus removal results, operational and maintenance activities, and cost considerations. Design The filter system includes a fish screen, an aeration manhole, a pretreatment anthracite cell (Cell 1), and three parallel up-flow filter cells (Cells 2, 3, and 4). Each up-flow cell contains a different reactive media for dissolved phosphorus removal (listed below) that was selected based on performance results from an onsite column study:

*Cell 2: a granite sand aggregate material mixed with 5% (w/w) of zero-valent iron (ZVI).

*Cell 3: an aggregate calcium carbonate media mixed with 5% (w/w) of ZVI.

*Cell 4: granite sand mixed with 10% (by weight) aluminum/iron oxide composite. The maximum design flow rate of the filter is 400 gpm. The filter was initially operated for 12 hours per day and was adjusted to run 24 hours per day during the 2023 operating season as water levels allowed. Monitoring Performance of the filtration system is monitored at eight locations (the filter influent, after the aeration manhole, the effluents from Cells 1-4, the composite effluent, and the backwash from Cell 1) on a weekly to biweekly basis from April to November. Several field and analytical parameters are collected and will be outlined in the presentation. Results Typical flow rates, totalized flows, and performance monitoring results for phosphorus removal are summarized in Table 1. The overall total phosphorus removal efficiency is 40%, which is approximately 13 pounds of phosphorus removed since start-up. Variations in phosphorus removal efficiencies are primarily attributed to fluctuations in the hydraulic residence times and media contact efficiencies. Operations and Maintenance Typical routine operation and maintenance activities include periodic backwashes of Cell 1 to remove suspended solids and removing debris from the fish screen, both of which are manual operations. During the spring and fall, when algae concentrations are low, backwashes are typically required weekly. When algae concentrations peak in the summer, backwashes are required twice weekly to maintain forward flow. Operational challenges have included the following:

*Air entrainment in the anthracite filter cell during shut-down periods, which causes air-locking and restriction of forward flow;

*Development of mudballs (an agglomeration of solids and media) in the anthracite media; and

*Decay in Cells 2 and 3 performances due to media fusing, resulting in uneven flow distribution and preferential flow paths . This paper will focus on the reactive media filter cells' phosphorus removal performance and address operational challenges and potential solutions at a high level. Project Significance The surface water filtration system is trialing several unique design features aimed to advance stormwater and surface water treatment techniques for urban watersheds, including:

*Employing filtration media with a high hydraulic capacity to maximize the amount of water treated within a small footprint;

*Utilizing an up-flow filter to minimize degradation of performance due to filter clogging; and

*Implementing an anthracite pre-treatment filter cell for particulate management. These Performance monitoring results will benefit the stormwater community by assessing the performance, operation, and maintenance needs of these design features and providing data to evaluate the suitability of this design for other locations. The next steps are to complete the two-year performance monitoring program, which will ultimately inform design improvements and final media selection for long-term operations.