Grantee Research Project Results
Final Report: Electrochemical Nutrient Extraction from Digestate
EPA Contract Number: EPD17006Title: Electrochemical Nutrient Extraction from Digestate
Investigators: Lee, Katherine
Small Business: Faraday Technology, Inc.
EPA Contact: Richards, April
Phase: I
Project Period: November 1, 2016 through April 30, 2017
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2016) RFA Text | Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Water
Description:
Anaerobic digestion is extensively used to recover energy from municipal waste and animal manure. The liquid effluent of anaerobic digestion typically contains high concentrations of ammonium, phosphorus, and divalent cations, which can potentially be recovered by precipitation of sparingly soluble phosphate salts. The majority of processes focus on struvite (magnesium ammonium phosphate) crystallization, as both nitrogen and phosphorus are removed and struvite is salable as slow-release plant fertilizer. The potential economics of recovering renewable sources of nitrogen and phosphorus have led to development of various phosphate salt crystallization systems. Fluidized bed reactors (FBRs) are commonly used for crystallized phosphate recovery because the design creates an abundance of reactive surface area and solution turbulence, enhancing nucleation kinetics and agglomeration. Despite generally high nutrient recovery efficiencies in FBRs, the economic favorability of phosphate salt crystallization systems could be significantly enhanced by increasing the operating pH within the FBR to near the minimum solubility point of struvite (pH = 9). Sodium hydroxide (NaOH) is often used in research-scale investigation of these technologies because it is highly soluble and can rapidly increase the solution pH. However, such chemical addition approaches can drive up phosphorus recovery costs (as high as $3,500/ton-P) significantly beyond the value of nutrients contained in struvite (~$765/ton-P) .
Recently, microbial electrochemical technologies (METs), such as microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) have garnered interest as a chemical-free and energy efficient method of enhancing struvite precipitation. METs consist of an anode, where anaerobic microbes oxidize organic matter and transfer electrons to an external circuit, and a cathode where the electrons and protons catalytically combine by reducing oxygen to water (MFC) or producing hydrogen gas (MEC). In the Phase I program, Faraday and the University of Illinois used a benchtop-scale MEC (1) to evaluate the alkali generation capability of the MEC technology under non-precipitating conditions (viz., the divalent cations needed for precipitation to occur were omitted) , and (2) to confirm the capability of the MEC to facilitate phosphate removal by precipitation. Synthetic digestate containing carbonate, ammonium, phosphate, and optionally magnesium was fed to a benchtop-scale MEC-FBR system, and either a DC or FARADAYIC® pulsed potential was applied. (FARADAYIC® Electrolysis has the potential to enhanced MEC performance, primarily by reducing power consumption; additional benefits from reduced cathode scaling within the MEC are likely.) The pH, MEC current response, and other system parameters were monitored over time as indicators for the MEC performance. A chemical engineering process cost analysis was performed to evaluate the economic favorability of an MEC as compared to caustic soda addition for digestate alkalization.
Summary/Accomplishments (Outputs/Outcomes):
Net alkali generation rates of approximately 2.3 ± 1.5 mmol h‑1 per square meter of projected anode area were observed in both DC and pulsed-potential MEC tests, which are of reasonable magnitude for typical applications. When only the electrical power input is considered, these experimental data translate to a per-mole cost of alkali generated by an MEC (~7¢ mol‑1) that is an order of magnitude lower than the cost of purchased NaOH (~87¢ mol‑1). Data from phosphate precipitation tests indicate both DC and FARADAYIC® electrical waveforms provide satisfactory performance for sparingly soluble salts recovery in the fluidized-bed form factor studied. A preliminary net present value (NPV) analysis of the entire MEC unit operation, including both capital and operating costs, suggests a reasonable rate of return can likely be achieved at relevant installation scales. As well, the data provide preliminary indication of the possibility of further cost advantages from the use of pulsed MEC potentials.
Conclusions:
The MEC technology shows significant technical and economic potential as a means for enhancing phosphate nutrient recovery from anaerobic digestate streams, via maintenance of optimal alkalinity levels for the performance of fluidized-bed precipitation reactors.
Publications
An abstract for the Phase I work has been submitted to the 232nd Meeting of the Electrochemical Society (National Harbor, MD, 1-6 Oct 2017).
Commercialization
The MEC technology investigated in this Phase I program has applications in various wastewater treatment applications utilizing anaerobic digestion as part of their remediation strategy. The technology has the potential to simultaneously reduce/eliminate scaling and/or fouling in such systems from precipitation of sparingly soluble salts, as well as enabling recovery of, e.g., phosphorus and nitrogen nutrient values from the digestate. Target areas of application for the technology include municipal wastewater treatment and manure processing from livestock husbandry operations. The technology is intrinsically scalable, and is able to address digestate stream management in processes ranging from small-scale livestock production to high-density urban wastewater treatment.
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.