phosphate – 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.

Why Recover Nutrients from Anaerobic Digestion?

All of the nitrogen and phosphorus present in an anaerobic digester’s influent can be found in the effluent digestate. However, during the digestion process the organic nitrogen is by and large converted to ammonia, and the organic phosphorus converted to soluble orthophosphate. The ammonia and phosphorus are normally discharged to holding ponds prior to field application. The pH increases in the holding ponds due to the loss of CO₂ resulting in a shift of the ammonium (NH₄⁺) to toxic ammonia gas (NH₃) that is subsequently lost to the atmosphere. Upon land application additional ammonia losses occur and dissolved orthophosphate leached.

Anaerobic digestion does not recover the nutrients as often claimed. Some of the nutrients accompanied by pathogens, hormones, and antibiotics are land applied to improve agricultural yields. The remaining nutrients are lost to the environment. Methods to reclaim ammonia are limited to high temperature ammonia stripping, or ion exchange with acid stripping of NH₃ to form dilute solutions of ammonium sulfate or ammonium nitrate. Those methods require chemical reactants and produce products having little economic value. Other options include recovery of a portion (< 15%) of the nitrogen and a majority of the orthophosphate found in digestate, with the addition of magnesium, as crystalline struvite (MgNH4PO4.6H2O). However, that process is expensive, requires reactants, removes only a portion of the nitrogen and may be inhibited by the presence of calcium in the digestate requiring acidification.

Figure 1. laboratory scale rotating photo bioreactor

What Did We Do?

This work was performed to verify a process for recovering ammonia as a highly concentrated and valued ammonium hydroxide and the orthophosphate as solid hydroxyapatite. Based on both previous pilot investigations, and the work of others, the RPB process was expected to remove and recover 80% to 90% of the ammonia and phosphate without the addition of chemicals, at normal digestate temperatures, and ambient pressures. Products that had a value greater than the cost of recovery were expected to be produced. The process uses a single reactor containing concentrated phototrophic organisms (cyanobacteria) that consume the bicarbonate alkalinity of the substrate for growth and thereby raise the pH, shifting the digestate ammonium to ammonia gas that can be stripped at low temperatures. The high pH and low bicarbonate concentration used in ammonia recovery are also required for the precipitation of orthophosphate as calcium carbonate or hydroxyapatite. The removal and reclamation of both ammonia and phosphate require an elevated pH and low bicarbonate alkalinity produced by the cyanobacteria.

Three laboratory scale rotating photo bioreactors, shown in Figure 1, were constructed to verify the removal and recovery of ammonia as a highly concentrated ammonium hydroxide solution that could be sold as diesel exhaust fluid. The lab scale pilot used cyanobacteria to increase the solution pH and shift the ammonium to ammonia gas that was continuously removed by recycled air flowing over the plates. The bioreactors were operated at 3.5 RPM using different attached growth media, under different lighting (30 – 80 PAR) conditions, stripping gas flow rates, and Hydraulic Retention Times (HRT). Concentrated, turbid, anaerobic centrate, having an ammonia concentration between 1,000 and 2,400 mg/L, was utilized as the substrate.

What We Have Learned?

The laboratory scale pilot bioreactors were able to establish that carbon fiber was the best fixed film media from a variety of inorganic fabrics. The operation further established that the stripping gas flowing over the cyanobacteria growth plates was sufficient to strip essentially all of the ammonia gas and thus eliminate ammonia toxicity to the cyanobacteria. Light intensity controlled the cyanobacteria growth rates, and thus the pH of the solution. The optimum HRT is yet to be determined. The system is currently operating at a 6 hour HRT but the final value may be significantly lower. Concentrated (15%) ammonia is currently being recovered but the final values are expected to be greater.

Future Plans

This study investigated most of the variables associated with stripping and recovering ammonia from a turbid, highly concentrated, digestate using a fixed film autotrophic system. The optimum rotation rate was one of the few variables not thoroughly investigated. The results obtained have established the basis for the design and construction of a pilot facility that will reclaim ammonia as a valued diesel exhaust fluid for Selective Catalytic Reduction (SCR) of combustion NOx thus eliminating, the two primary sources of reactive nitrogen discharged to the environment. Future work will focus on removing and reclaiming sufficient quantities of ammonia as diesel exhaust fluid and testing the fluid in diesel engines that use SCR to remove exhaust NOx.

Author

Dennis A. Burke PE, CEO, Environmental Energy & Engineering Company; waeng@me.com

Nutrient Management aka, Nutrient Reclamation from all organic waste

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. 2015. Title of presentation. Waste to Worth: Spreading Science and Solutions. Seattle, WA. March 31-April 3, 2015. URL of this page. Accessed on: today’s date.