Philipp Kuntke

Ammonium recovery and energy production from urine by a microbial fuel cell

Nitrogen recovery through NH(3) stripping is energy intensive and requires large amounts of chemicals. Therefore, a microbial fuel cell was developed to simultaneously produce energy and recover ammonium. The applied microbial fuel cell used a gas diffusion cathode. The ammonium transport to the cathode occurred due to migration of ammonium and diffusion of ammonia. In the cathode chamber ionic ammonium was converted to volatile ammonia due to the high pH. Ammonia was recovered from the liquid-gas boundary via volatilization and subsequent absorption into an acid solution. An ammonium recovery rate of 3.29 g(N) d(-1) m(-2) (vs. membrane surface area) was achieved at a current density of 0.50 A m(-2) (vs. membrane surface area). The energy balance showed a surplus of energy 3.46 kJ g(N)(-1), which means more energy was produced than needed for the ammonium recovery. Hence, ammonium recovery and simultaneous energy production from urine was proven possible by this novel approach.

Source-separated urine opens golden opportunities for microbial electrochemical technologies

The food security of a booming global population demands a continuous and sustainable supply of fertilisers. Their current once-through use [especially of the macronutrients nitrogen (N), phosphorus (P), and potassium (K)] requires a paradigm shift towards recovery and reuse. In the case of source-separated urine, efficient recovery could supply 20% of current macronutrient usage and remove 50-80% of nutrients present in wastewater. However, suitable technology options are needed to allow nutrients to be separated from urine close to the source. Thus far none of the proposed solutions has been widely implemented due to intrinsic limitations. Microbial electrochemical technologies (METs) have proved to be technically and economically viable for N recovery from urine, opening the path for novel decentralised systems focused on nutrient recovery and reuse.

Bioelectrochemical systems for nitrogen removal and recovery from wastewater

Removal of nitrogen compounds from wastewater is essential to prevent pollution of receiving water bodies (i.e. eutrophication). Conventional nitrogen removal technologies are energy intensive, representing one of the major costs in wastewater treatment plants. For that reason, innovations in nitrogen removal from wastewater focus on the reduction of energy use. Bioelectrochemical systems (BESs) have gained attention as an alternative to treat wastewater while recovering energy and/or chemicals. The combination of electrodes and microorganisms has led to several methods to remove or recover nitrogen from wastewater via oxidation reactions, reduction reactions and/or transport across an ion exchange membrane. In this study, we give an overview of nitrogen removal and recovery mechanisms in BESs based on state-of-the-art research. Moreover, we show an economic and energy analysis of ammonium recovery in BESs and compare it with existing nitrogen removal technologies. We present an estimation of the conditions needed to achieve maximum nitrogen recovery in both a microbial fuel cell (MFC) and a microbial electrolysis cell (MEC). This analysis allows for a better understanding of the limitations and key factors to take into account for the design and operation of MFCs and MECs. Finally, we address the main challenges to overcome in order to scale up and put the technology in practice. Overall, the revenues from removal and recovery of nitrogen, together with the production of electricity in an MFC or hydrogen in an MEC, make ammonium recovery in BESs a promising concept.

Hydrogen Gas Recycling for Energy Efficient Ammonia Recovery in Electrochemical Systems

Recycling of hydrogen gas (H2) produced at the cathode to the anode in an electrochemical system allows for energy efficient TAN (Total Ammonia Nitrogen) recovery. Using a H2 recycling electrochemical system (HRES) we achieved high TAN transport rates at low energy input. At a current density of 20 A m-2, TAN removal rate from the influent was 151 gN m-2 d-1 at an energy demand of 26.1 kJ gN-1. The maximum TAN transport rate of 335 gN m-2 d-1 was achieved at a current density of 50 A m-2 and an energy demand of 56.3 kJ gN-1. High TAN removal efficiency (73-82%) and recovery (60-73%) were reached in all experiments. Therefore, our HRES is a promising alternative for electrochemical and bioelectrochemical TAN recovery. Advantages are the lower energy input and lower risk of chloride oxidation compared to electrochemical technologies and high rates and independence of organic matter compared to bioelectrochemical systems.

(Bio)electrochemical ammonia recovery: progress and perspectives

In recent years, (bio)electrochemical systems (B)ES have emerged as an energy efficient alternative for the recovery of TAN (total ammonia nitrogen, including ammonia and ammonium) from wastewater. In these systems, TAN is removed or concentrated from the wastewater under the influence of an electrical current and transported to the cathode. Subsequently, it can be removed or recovered through stripping, chemisorption, or forward osmosis. A crucial parameter that determines the energy required to recover TAN is the load ratio: the ratio between TAN loading and applied current. For electrochemical TAN recovery, an energy input is required, while in bioelectrochemical recovery, electric energy can be recovered together with TAN. Bioelectrochemical recovery relies on the microbial oxidation of COD for the production of electrons, which drives TAN transport. Here, the state-of-the-art of (bio)electrochemical TAN recovery is described, the performance of (B)ES for TAN recovery is analyzed, the potential of different wastewaters for BES-based TAN recovery is evaluated, the microorganisms found on bioanodes that treat wastewater high in TAN are reported, and the toxic effect of the typical conditions in such systems (e.g., high pH, TAN, and salt concentrations) are described. For future application, toxicity effects for electrochemically active bacteria need better understanding, and the technologies need to be demonstrated on larger scale.

Investigating bacterial community changes and organic substrate degradation in microbial fuel cells operating on real human urine

The present study investigates the changes in the microbial community and the degradation of organic compounds in microbial fuel cells (MFCs) operated on human urine. An anaerobic community was enriched in “urine-degrading” electroactive microorganisms by stepwise lowering the dilution factor of the anode media from 50 times diluted to undiluted urine. In a duplicated assay a current density of 495 ± 16 mA m−2, a chemical oxygen demand (COD) removal of 75.5 ± 0.7% and a coulombic efficiency (CE) of 26.5 ± 0.7% were obtained during operation on undiluted urine. In a control assay, operated on undiluted urine without the microbial enrichment procedure, a current density of only 81 ± 9 mA m−2 was obtained. The organic compounds commonly found in urine as well as the metabolic products associated with their degradation were analyzed by proton nuclear magnetic resonance (1H-NMR). The main compounds initially identified in the urine were urea, creatinine, glycine, trimethylamine N-oxide and acetate. Most of the organic compounds, except acetate, were depleted within 10 days of operation. The microbial community responsible for urine degradation in the anode of both MFCs was investigated using the Illumina MiSeq platform. Bacteria related with the Firmicutes phyla were enriched in the anodic biofilms compared to the initial anaerobic inoculum, within which Tissierella and Paenibacillus were the dominant genera. Tissierella can metabolize creatinine producing acetate whereas several bacterial species belonging to the Paenibacillus genus demonstrated the ability to function as exoelectrogens. Corynebacterium that comprise urea-hydrolysing bacteria was also amongst the main genera detected in the developed biofilms.

Gas-permeable hydrophobic membranes enable transport of CO2 and NH3 to improve performance of bioelectrochemical systems

Application of bioelectrochemical systems (BESs), for example for the production of hydrogen from organic waste material, is limited by a high internal resistance, especially when ion exchange membranes are used. This leads to a limited current density and thus to large footprint and capital costs. Ion transport between anode and cathode compartment is one of the factors determining the internal resistance. The aim of this study was to reduce the resistance for ion transport in a microbial electrolysis cell (MEC) through the ion exchange membrane by shuttling of CO2 and NH3 between anode and cathode. The transport of these chemical species was enabled through the use of a hydrophobic TransMembraneChemiSorption module (TMCS) that was placed between anolyte and catholyte circulation outside the cell. The driving force for transport was the pH difference between both solutions. The transport of CO2 and NH3 resulted in an increase in current density from 2.1 to 4.1 A m−2 for a cation exchange membrane (CEM) and from 2.5 to 13.0 A m−2 for an anion exchange membrane (AEM) at 1 V applied voltage. The increase in current density was the result of a lower ion transport resistance through the membrane; this resistance was 60% lower for the CEM, as a result of NH3 recycling from cathode to anode, and 82% for the AEM, as a result of CO2 recycling from anode to cathode with TMCS, compared to experiments without TMCS.

Energy-Efficient Ammonia Recovery in an Up-Scaled Hydrogen Gas Recycling Electrochemical System

Nutrient and energy recovery is becoming more important for a sustainable future. Recently, we developed a hydrogen gas recycling electrochemical system (HRES) which combines a cation exchange membrane (CEM) and a gas-permeable hydrophobic membrane for ammonia recovery. This allowed for energy-efficient ammonia recovery, since hydrogen gas produced at the cathode was oxidized at the anode. Here, we successfully up-scaled and optimized this HRES for ammonia recovery. The electrode surface area was increased to 0.04 m2 to treat up to 11.5 L/day (∼46 gN/day) of synthetic urine. The system was operated stably for 108 days at current densities of 20, 50, and 100 A/m2. Compared to our previous prototype, this new cell design reduced the anode overpotential and ionic losses, while the use of an additional membrane reduced the ion transport losses. Overall, this reduced the required energy input from 56.3 kJ/gN (15.6 kW h/kgN) at 50 A/m2 (prototype) to 23.4 kJ/gN (6.5 kW h/kgN) at 100 A/m2 (this work). At 100 A/m2, an average recovery of 58% and a TAN (total ammonia nitrogen) removal rate of 598 gN/(m2 day) were obtained across the CEM. The TAN recovery was limited by TAN transport from the feed to concentrate compartment.