Performance of field-scale phosphorus removal structures utilizing steel slag for treatment of subsurface drainage

Authors: Penn, Chad; Livingston, Stanley; SHEDKAR, VINAYAK - The Ohio State University; King, Kevin; Williams, Mark

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2020
Publication Date: 2/7/2020
Citation: Penn, C.J., Livingston, S.J., Shedkar, V., King, K.W., Williams, M.R. 2020. Performance of field-scale phosphorus removal structures utilizing steel slag for treatment of subsurface drainage. Water. 12(2). Article 443. https://doi.org/10.3390/w12020443.
DOI: https://doi.org/10.3390/w12020443

Interpretive Summary: Dissolved phosphorus (P) losses to surface waters are considered the main cause of surface water eutrophication, such as Lake Erie. P removal structures are large landscape-scale filters for treating drainage water before reaching a surface water outlet. We constructed the first ever steel slag filter for tile drainage in a bottom-up flow configuration, for removal of dissolved P. A second structure was constructed to treat tile drainage water after discharge to a ditch, also in a bottom-up flow configuration. Dissolved P removal during the first 6 months was outstanding, at nearly 100% removal. However, with continued loading from the tile drain, the slag media began to slowly clog up and simultaneously decrease dramatically in P removal. This was due to the presence of bicarbonate in the inlet water, which is common for tile drainage. While the structure removed a large amount of dissolved P (19.2 kg), it was short-lived. We recommend use of regular un-treated steel slag for treating surface runoff only, due to lower bicarbonate concentrations. Regarding transport of nutrients, it was also observed that dissolved P concentrations were greatest for the largest flow events with the highest flow rates, with the majority of all dissolved P delivered in only a couple of events.

Technical Abstract: Reducing dissolved phosphorus (P) losses from legacy P soils to surface waters is necessary for preventing algal blooms. Phosphorus removal structures containing steel slag have shown success in treating surface runo for dissolved P, but little is known about treating subsurface (tile) drainage. A ditch-style and subsurface P removal structure were constructed using steel slag in a bottom-up flow design for treating tile drainage. Nearly 97% of P was delivered during precipitation-induced flow events (as opposed to baseflow) with inflow P concentrations increasing with flow rate.Structures handled flow rates approximately 12 L/s, and the subsurface and ditch structures removed 19.2 (55%) and 0.9 kg (37%) of the cumulative dissolved P load, respectively. Both structures underperformed relative to laboratory flow-through experiments and exhibited signs of flow inhibition with time. Dissolved P removal decreased dramatically when treated water pH decreased <8.5. Although slag has proven successful for treating surface runo, we hypothesize that underperformance in this case was due to tile drainage bicarbonate consumption of slag calcium through the precipitation of calcium carbonate, thereby filling pore space, decreasing flow and pH, and preventing calcium phosphate precipitation. We do not recommend non-treated steel slag for removing dissolved P from tile drainage unless slag is replaced every 4–6 months.