Evaluating Phosphorus Recovery from Biosolids to Address Eutrophication in Florida

The land application of biosolids has provided nutrients and organics to agricultural land for decades. More than half of the solids generated in the United States during water reclamation are stabilized and beneficially used as soil amendment. The use of biosolids reduces dependence on inorganic fertilizer, increases the organic content of the soil, provides a low-cost and locally produced fertilizer to the agricultural community, reduces greenhouse gas emissions and sequesters carbon. Historically, biosolids application rates have been calculated almost exclusively on the nitrogen (N) content in the biosolids and the manner in which the biosolids were managed, surface applied, or incorporated into the soil. Basing application rate on nitrogen alone can result in applying phosphorus (P) beyond the crop's agronomic requirements. Like all fertilizers without an adequate buffer to surface water and depth to groundwater, a portion of the phosphorus applied may enter the ground or surface water. In an effort to limit the over-application of phosphorus, a number of states, including Florida, are requiring the development of nutrient management plans. These plans require the determination of the agronomic phosphorus application rate along with the one for nitrogen. The plan is for the application rate to be controlled by the more stringent of the two. Some states require soil monitoring and calculating the total cumulative amount of P that can be applied based on the Cationic Exchange Capacity of the soil. Recent regulations in some States have begun to use the percent Water Extractable Phosphorus (WEP) in estimating the agronomic rate for phosphorus application. Some States use the monitored data. The State of Florida uses multiple methods of calculating the agronomic P application rate depending on the percent WEP, above or below 14.5% WEP. The St. Johns River Water Management District, supported by the State of Florida Department of Environmental Protection (FDEP), has initiated a project titled 'Resource Enhancement and Recovery of Domestic Wastewater Residuals'. The objective of this project is to identify and evaluate technologies to recover the phosphorus (P) in biosolids, reducing the amount in the beneficially used material. The project includes: 1. Evaluate existing technologies and assess regional needs. 2. Review empirical data from technology vendors to quantify expected P-recovery performance. 3. Compare feasible technologies based on cost, performance, and capacity. This project was conducted in three separate tasks. This presentation describes the overall project with a focus on the modeling and technology comparison tasks, which were Tasks B and C. Task B included the identification, screening and evaluation of proven and promising P-recovery technologies that may recover P and reduce the content in biosolids. Twenty P-recovery technologies were screened using criteria established by the project team and the District. Following the screening of the preliminary technologies, a total of four were selected for further evaluation. The task also reviewed the impact the composting and thermal drying have on phosphorus availability. Task C included the modeling of four P-recovery technologies at two Water Reclamation Facilities (WRF) in the State. Both facilities use biological nutrient removal processes for their liquid stream treatment. One WRF anaerobically digests the solids generated during treatment. The second uses aerobic holding, which provides a degree of aerobic stabilization. Each of the selected P-recovery technologies was modeled under both treatment and stabilization scenarios. The model output allowed the project team to estimate the comparative phosphorus recovery rates. Another objective of Task C was to understand the opportunities and challenges associated with implementing a P-recovery technology. This included a cost estimate to implement P-recovery at a set capacity and the impact the technologies could have on the phosphorus concentration in the facilities' biosolids. The evaluation also used non-monetary criteria to identify the technologies that can support a wide range of WRRFs throughout Florida. The project included the development of an educational document to inform policymakers and regulators in the State of Florida regarding the benefits of biosolids and the technologies available to support that use, including P-recovery technologies. This presentation provides a summary of the work performed as part of the project and the recommended steps in support of beneficial use of biosolids in the State of Florida.