Coupled Microbial Electrolysis Cells and Struvite Precipitation for Enhanced Nutrient Recovery from Wastewater

Abstract

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 [1-5]. 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 [5-7]. Fluidized bed reactors (FBRs) are commonly used for crystallized phosphate recovery because the design creates an abundance of reactive surface area and solution turbulence [8], 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 [3]) 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 [4,7,9,10]. 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) [3,11]. 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 [12-14]. 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). This talk will discuss preliminary results from a bench-scale MEC-FBR system (Figure 1) that demonstrate the potential for an MEC to enable economical pH control in an FBR struvite precipitation system. Net alkali generation rates of approximately 2.3 ± 1.5 mmol m- 2 h-1 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). 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. As well, the data provide preliminary indication of the possibility of further cost advantages from the use of pulsed MEC potentials. The authors acknowledge financial support from USEPA Contract No. EP-D-17-006.