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Implications of excess phosphorus (P) in waste streams obtained from soy‐based protein preparation processes on the environment and their potential utilization as P‐source are two significant understudied areas. Soybean‐based protein ingredients for food products retain comparatively enhanced functional properties and are cheaper than other plant‐based proteins. Soybean protein can be extracted and utilized as a food ingredient primarily by preparing soy protein concentrates (SPC) and soy protein isolates (SPI) from soybean meal/defatted soy flour (DSF). In a typical soybean processing facility, along with the soy products and soy‐protein preparations, the recovery of phosphorus as a coproduct will enhance the economic feasibility of the overall process as the recovered P can be used as fertilizer. In this study, the SPC and SPI were prepared from the DSF following widely used conventional protocols and P flow in these processes was tracked. In SPC production, ~59% of the total P was retained with SPC and ~34% was in the aqueous waste streams. For SPI process ~24% of total P was retained with SPI and ~59% went in the waste solid residue (~40%) and aqueous streams (~19%). About 80%–89% P removal from the waste aqueous streams was achieved by Ca‐phytate precipitation. This work demonstrated that in the process of SPC and SPI preparation the phosphorus from the waste aqueous streams can be precipitated out to avoid subsequent eutrophication and the waste solid residue with ~40% P can be reused as a P‐fertilizer as other applications of this residue are unspecified.

Data limitations often challenge the reliability of water quality models, especially in intensively managed watersheds. While numerous studies report successful hydrological model setup and calibration, few have addressed in detail the data challenges for multisite and multivariable model calibration to an intensively managed watershed. In this study, we address some of these challenges based on our reflective experience calibrating the Soil and Water Assessment Tool (SWAT) to the Upper Sangamon River Watershed in central Illinois based on daily flow, annual crop yield, and monthly sediment, nitrate, and total phosphorus loads. We highlight some challenges in SWAT calibration processes due to data errors and inconsistencies, and insufficient precipitation and water quality observations. Following, we demonstrate the merits of additional weather and water quality observations that could help reduce input uncertainties, and we provide suggestions for selecting appropriate observations for the model calibration. After dealing with the data issues, we show that the SWAT model could be calibrated with acceptable results for the case study watershed.

Efforts to reduce riverine phosphorus (P) loads have not been as fruitful as expected or hoped. One reason for the failure of these efforts appears to be that models used for watershed P management have understated and misrepresented the role of in‐stream processes in shaping watershed P export. Here, we update the latest release of the Soil and Water Assessment Tool (SWAT+), a widely used watershed management model, to better represent in‐stream P retention and remobilization (SWAT+P.R&R). We add new streambed pools where P is stored and tracked, and we incorporate three new processes driving in‐stream P dynamics: (a) deposition and resuspension of sediment‐associated P, (b) diffusion of dissolved P between the water column and streambed, and (c) adsorption and desorption of mineral P. The objective of this modeling work is to provide a diagnostic tool that enables researchers to challenge existing assumptions regarding how watersheds store, transform, and transport P. Here, in a first diagnostic analysis, SWAT+P.R&R helps reconcile in‐stream P retention theory (that P is retained at low flows and remobilized at high flows) and a discordant data set in our validation watershed. SWAT+P.R&R results (a) clarify that the theorized relationship between P retention and flow is only valid (for this point‐source affected testbed, at least) at the temporal scale of a single rising‐or‐falling hydrograph limb and (b) illustrate that hysteresis obscures the relationship at longer temporal scales. Future work using SWAT+P.R&R could further challenge assumptions regarding timescales of in‐stream P legacies and sources of P load variability.

The economic viability of corn biorefineries depends heavily on the sale of coproducts as animal feeds, but elevated phosphorus (P) contents can exacerbate manure management issues. Phosphorus removal from light steep water and thin stillage, two concentrated in‐process aqueous streams at wet milling and dry‐grind corn biorefineries, could simultaneously generate concentrated fertilizer and low‐P animal feeds, but little is known regarding how differences in stream composition affect removal. To address this data gap, we show that the solubility of P in light steep filtrate (LSF) and thin stillage filtrate (TSF) exhibits distinct sensitivity to calcium (Ca) and base addition due to differences in P fractionation and protein abundance. In LSF, P was primarily organic, and near‐complete removal of P (96%) was observed at pH 8 and a Ca/total P (TP) ratio of 2. In TSF, TP removal was lower (81%), and there was more equal distribution of organic and orthophosphate, indicating that the Ca requirements of inorganic P precipitation were a limiting factor. The C/H/N ratio, elemental characterization, and crude protein analysis of the precipitated solids indicated that coprecipitation of amorphous solids containing Ca, Mg, and K with soluble proteins facilitated removal of P, particularly in LSF. Although the removal mechanisms and solubility limits differed, these results highlighted the magnitude (40–70 mM) and efficacy (80–96%) of P recovery from two biorefinery streams.