Performance of Field-Scale Phosphorus Removal Structures Utilizing Steel Slag for Treatment of Subsurface Drainage

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 runoff 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−1, 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 runoff, 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.

Utilization of Steel Slag in Blind Inlets for Dissolved Phosphorus Removal

“…The slag used in this study did not clog up, continues to remove dissolved P after three years, and performed as expected based on laboratory flow-through experiments, unlike the slag P removal structure reported by Penn et al [31]. The main difference in those studies is the source water; this study treated surface runoff water instead of tile drainage water, which contains appreciable bicarbonate and dissolved forms of CO 2 compared to surface runoff [31]. The under-performance of slag in their study was attributed to the formation of Ca carbonate instead of Ca phosphate through bicarbonate input, which additionally clogged the structure, besides consuming soluble Ca that would usually precipitate Ca phosphate for P removal.…”

“…While limestone-based blind inlets have been shown to remove little to no dissolved P [4,5], this study demonstrated that a simple update to slag would permit blind inlets to remove dissolved P in addition to sediment, particulate P, and provide obstruction-free drainage of depressions. The slag used in this study did not clog up, continues to remove dissolved P after three years, and performed as expected based on laboratory flow-through experiments, unlike the slag P removal structure reported by Penn et al [31]. The main difference in those studies is the source water; this study treated surface runoff water instead of tile drainage water, which contains appreciable bicarbonate and dissolved forms of CO 2 compared to surface runoff [31].…”

“…This is expected since it has been suggested that for Ca-phosphate precipitation to occur at pH 8, Ca concentrations between 30 and 100 mg L −1 are necessary; at least 50 mg Ca L −1 is required for complete dissolved P removal [29]. For a tile-drain P removal structure utilizing EAF slag, dissolved P removal dramatically decreased when the pH of treated water dropped to 8.5 or less [31]. In addition, Ca phosphate precipitation is also subject to kinetics, and therefore favored by a greater retention time [7].…”

Removal of phosphate from wastewater by steel slag with high calcium oxide column filter system; efficiencies and mechanisms study

“…29 The efficiency of steel slag in removing P from different types of water, surface water as well as wastewater, has been confirmed in the literature. [30][31][32] However, the present study contributes to the literature by explaining the role of aeration at different pH values in enhancing the removal mechanisms.…”

Adsorption Media for the Removal of Soluble Phosphorus from Subsurface Drainage Water

“…Calcium carbonates precipitate on the slag instead of calcium phosphate when bicarbonate and dissolved forms of CO 2 are present in the subsurface drainage, which results from water infiltrating through calcareous soils and microbial respiration [ 41 , 42 ]. This decrease in capacity is due to (1) the bicarbonate and phosphate ions competing to adsorb to the calcium minerals and (2) the decrease in pH due to the formation of calcium carbonate and soluble calcium concentration, which negatively impacts the SFS’s ability to precipitate phosphate ions as calcium phosphate [ 41 , 42 ]. Specifically, the SFS has a decrease in P removal via calcium phosphate precipitation when the pH of the solution is below 8.5 [ 41 , 42 ].…”