K-Struvite – Livestock and Poultry Environmental Learning Community

Purpose

Our recent work has focused on developing approaches to nutrient management and recovery, with a particular focus on using electrochemical and membrane technologies to control the chemistry of the recovered nutrient products. We are interested in being able to recover both ammonia and phosphate, and our goal is to create recycled fertilizer products that can allow the agricultural community to control the ratio of nitrogen to phosphorus in the recycled fertilizer products and to control whether those fertilizer products are in liquid form or in solid form. With the electrochemical technology focus, we see benefits that include no required chemical dosing, scalable reactor design, and the ability to couple to renewable energy sources. Our engineering research on nutrient recovery technology is conducted within a team that includes life cycle assessment, economic analysis, agronomic greenhouse and row crop studies, agricultural sector outreach, and the development of a decision-support tool to help farmers understand technology options for water and nutrient management.

What Did We Do?

We have investigated an electrochemical cell design that includes a magnesium metal anode and a stainless-steel cathode. The corrosion of the magnesium anode results in the release of magnesium cations into solution, and these magnesium cations promote the precipitation of struvite, otherwise known as magnesium ammonium phosphate hexahydrate (Figure 1). We have investigated how operating conditions of the electrochemical cell, including voltage, residence time, batch vs flow, and membrane separation of the two electrodes, affect nutrient recovery efficiency and the overall chemistry of the recovered precipitate. Our studies have included control experiments on synthetic wastewater compositions relevant to hog and dairy farm wastewaters, while we have also conducted laboratory-scale studies on natural wastewater samples from both agricultural and municipal sources. To demonstrate initial scale-up of an electrochemical reactor, we have designed a bench-scale reactor (Figure 2) that is capable of producing kilogram-level batches of struvite.

Figure 2. (a) Bench-scale batch reactor demonstration for kg-level struvite precipitation. (b) One engineering challenge is the precipitation of struvite on the electrode surface.

What Have We Learned?

The production of struvite from an electrochemical reactor can be controlled by the applied voltage and residence time of the wastewater in the reactor. Changes in reactor design, including the inclusion of a membrane to separate the anode and cathode and operation in batch vs flow mode, can change the composition of the struvite precipitate and can cause a change in the balance of struvite formed vs hydrogen gas formed from the electrochemical cell. We are also able to produce K-struvite, a potassium-based alternative to conventional struvite, that includes potassium rather than ammonium, and the production of K-struvite allows the recovery of the phosphate in a particulate fertilizer while also allowing the separation and recovery of ammonia in a separate liquid stream. We have learned that one of the primary challenges to the electrochemical reactor operation is fouling of the electrodes by the struvite precipitate (Figure 2), and we have developed a dynamic voltage control approach that enables minimal electrode fouling and therefore increases struvite recovery and decreases energy consumption. Our energy consumption values are similar to that of chemical precipitation processes that have been developed for nutrient recovery.

Future Plans

Future plans include further development and optimization of the dynamic voltage control approach to electrochemical reactor operation, which will allow us to control electrode fouling. We also plan to continue working with natural wastewater samples and further develop flow cell reactor design to understand how to translate our batch reactor studies to a flow reactor environment. Studies on K-struvite will focus on understanding the kinetics of K-struvite precipitation and the competing reactions (e.g., calcium precipitation and struvite precipitation) that might impact K-struvite recovery.

Authors

Lauren F. Greenlee, Associate Professor, Pennsylvania State University

Corresponding author email address

greenlee@psu.edu

Additional authors

Laszlo Kekedy-Nagy, Postdoctoral Fellow, Concordia University

Ruhi Sultana, Graduate Research Assistant, Pennsylvania State University

Amir Akbari, Graduate Research Assistant, Pennsylvania State University

Ivy Wu, Graduate Research Assistant, Colorado School of Mines

Andrew Herring, Professor, Colorado School of Mines

Additional Information

1. Kekedy-Nagy, Z. Anari, M. Abolhassani, B.G. Pollet, L.F. Greenlee. Electrochemical Nutrient Removal from Natural Wastewater Sources and its Impact on Water Quality. Water Research (2022), 210, 118001, DOI: 10.1016/j.watres.2021.118001.
2. Kékedy-Nagy, M. Abolhassani, R. Sultana, Z. Anari, K.R. Brye, B.G. Pollet, L. F. Greenlee. The Effect of Anode Degradation on Energy Demand and Production Efficiency of Electrochemically Precipitated Struvite, Journal of Applied Electrochemistry (2021), DOI: 0.1007/s10800-021-01637-y.
3. Kékedy-Nagy, M. Abolhassani, S.I. Perez Bakovic, J.P. Moore II, B.G. Pollet, L.F. Greenlee. Electroless Production of Fertilizer (Struvite) and Hydrogen from Synthetic Agricultural Wastewaters, Journal of the American Chemical Society (2020), 142(44), 18844-18858. DOI: /10.1021/jacs.0c07916.
4. Wu, A. Teymouri, R. Park, L.F. Greenlee, and A.M. Herring. Simultaneous Electrochemical Nutrient Recovery and Hydrogen Generation from Model Wastewater Using a Sacrificial Magnesium Anode, Journal of the Electrochemical Society (2019), 166(16), E576-E583. DOI: 10.1149/2.0561916jes.

Acknowledgements

The authors acknowledge funding from the USDA NIFA AFRI Water for Food Production Systems program, grant #2018-68011-28691 and funding from the National Science Foundation, grant #1739473.

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2022. Title of presentation. Waste to Worth. Oregon, OH. April 18-22, 2022. URL of this page. Accessed on: today’s date.

Purpose

Intensive animal husbandry produces large volumes of liquid manure with significant amounts of phosphorus, ammonium, and potassium as they pass through the feed of farm animals. As a result, direct land application of manure, the current common approach, causes environmental concerns such as soil over-fertilization and groundwater and surface water contamination, which leads to eutrophication. Manure nutrient management is, therefore, necessary to address these problems. While most engineering options are focused on phosphorus and ammonium recovery, few studies have pursued recovery methods for potassium. In this talk, we present an electrochemical technology using a sacrificial magnesium anode and a stainless-steel cathode for simultaneous recovery of phosphorus and potassium in the form of potassium-magnesium-phosphate (KMgPO₄·xH₂O, K-struvite).

Mg²⁺ + K⁺ + H_nPO₄^n-3 + 6H₂O = KMgPO₄*6H₂O + nH⁺

K-struvite has the potential to be used as a slow-release fertilizer and this technology will add flexibility to the manure management strategies currently available by diversifying the recoverable by-products.

What Did We Do?

Figure 1. Calculated saturation index values as a function of pH. The water matrix contains 3000 mg/L potassium, 1000 mg/L phosphate, and magnesium with Mg:P ratio of 1.4.

To predict the thermodynamic stability of K-struvite, a thermodynamic model was developed based on the average ion concentrations of phosphorus, and potassium measured in real liquid pig manure (Figure 1). According to this model, magnesium phosphate is a possible by-product of K-struvite precipitation. Also, the probable formation of magnesium hydroxide was enhanced with increasing pH value due to the increase in hydroxide ion concentration. As a result, the ideal range for precipitation of K-struvite lies at pH values between 10 and 11.

To understand the role of pH on K-struvite formation, a 50 mM KH₂PO₄ solution was used to perform the preliminary batch electrochemical experiments. A constant voltage of -0.8 V vs. the Ag/AgCl reference electrode was applied to the pure magnesium anode using a potentiostat. One experiment was performed on the natural pH of the initial solution, 4.5, while potassium hydroxide was used to raise the initial pH of the second experiment to 9.5.

What Have We Learned?

Figure 2. The EDS results obtained of the recovered precipitates (a) pH=4.5, (b) pH=9.5 in 50 mM KH₂PO₄.

Energy-dispersive x-ray spectroscopy (EDS) of the recovered precipitates (Figure 2) indicate that by raising the initial pH from 4.5 to 9.5 the amount of potassium is increased in the precipitates. Also, due to the equimolar ratios of K:Mg:P at pH=9.5, the produced precipitates are likely K-struvite, while the pH= 4.5 sample likely contains some amount of magnesium phosphate.

This process also eliminates the disadvantages of the commonly used chemical precipitation methods, including magnesium salt dosing, and adding base to the system for pH control, due to in situ magnesium corrosion and hydroxide production at the magnesium anode surface. These advantages could potentially reduce the operating cost of the system and eliminate the addition of unnecessary salinity to wastewater through magnesium salt dosing.

Future Plans

Further investigation by using multiple characterization techniques (e.g., x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR)) is necessary to identify the exact nature of precipitates. The initial experiments will be repeated at additional pH values to further understand the role of pH on the precipitation of K-struvite in the simplified synthetic wastewater and to further detail the characterization of the composition and morphology of K-struvite precipitates. These experiments are valuable , particularly because there are few literature reports that detail the physical and chemical structure of K-struvite.

Authors

Presenting author

Amir Akbari, Ph.D. Candidate, Department of Chemical and Biomedical Engineering, Pennsylvania State University

Corresponding author

Lauren F. Greenlee, Associate Professor, Department of Chemical and Biomedical Engineering, Pennsylvania State University

Corresponding author email address

greenlee@psu.edu

Additional Information

Once completed, future publications and data repository information will be available at https://sites.psu.edu/greenlee/

Acknowledgements

The authors would like to thank the U.S. Department of Agriculture, NIFA AFRI Water for Food Production Systems (#2018-68011-28691) for providing the funding support of this research through the “Water and Nutrient Recycling: A Decision Tool and Synergistic Innovative Technology” project.