Electrochemically and Bioelectrochemically Induced Ammonium Recovery

Streams such as urine and manure can contain high levels of ammonium, which could be recovered for reuse in agriculture or chemistry. The extraction of ...

Ghent University

Microbial fuel cells can be designed to remove nitrogenous compounds out of wastewater, but their performance is at present limited to 0.33 kg NO(3) (-)-Nm(-3) net cathode compartment (NCC) d(-1). By maintaining the pH in the cathode at 7.2, nitrogen removal was increased from 0.22 to 0.50 kg NO(3) (-)-Nm(-3) NCC d(-1). Bio-electrochemical active microorganisms seem to struggle with the deterioration of their own environment due to slow proton fluxes. Therefore, the results suggest that an appropriate pH adjustment strategy is necessary to allow a sustained and enhanced biological activity in bio-electrochemical systems.

Enhanced nitrogen removal in bio-electrochemical systems by pH control.

Autotrophic nitrous oxide removal in bioelectrochemical systems.

New Activated Sludge (NAS(®)) is a hybrid, floc-based nitrogen removal process without carbon addition, based on the control of sludge retention times (SRT) and dissolved oxygen (DO) levels. The aim of this study was to examine the performance of a retrofitted four-stage NAS(®) plant, including on-line measurements of greenhouse gas emissions (N(2)O and CH(4)). The plant treated anaerobically digested industrial wastewater, containing 264 mg N L(-1), 1154 mg chemical oxygen demand (COD) L(-1) and an inorganic carbon alkalinity of 34 meq L(-1). The batch-fed partial nitritation step received an overall nitrogen loading rate of 0.18-0.22 kg N m(-3) d(-1), thereby oxidized nitrogen to nitrite (45-47%) and some nitrate (13-15%), but also to N(2)O (5.1-6.6%). This was achieved at a SRT of 1.7 d and DO around 1.0 mg O(2) L(-1). Subsequently, anammox, denitrification and nitrification compartments were followed by a final settler, at an overall SRT of 46 d. None of the latter three reactors emitted N(2)O. In the anammox step, 0.26 kg N m(-3) d(-1) was removed, with an estimated contribution of 71% by the genus Kuenenia, which constituted 3.1% of the biomass. Overall, a nitrogen removal efficiency of 95% was obtained, yielding a dischargeable effluent. Retrofitting floc-based nitrification/denitrification with carbon addition to NAS(®) allowed to save 40% of the operational wastewater treatment costs. Yet, a decrease of the N(2)O emissions by about 50% is necessary in order to obtain a CO(2) neutral footprint. The impact of emitted CH(4) was 20 times lower.

Floc-based sequential partial nitritation and anammox at full scale with contrasting N2O emissions.

To decrease the load of pharmaceuticals to the environment, decentralized wastewater treatment has been proposed for important point-sources such as hospitals. In this study, a microbial electrolysis cell (MEC) was used for the dehalogenation of the iodinated X-ray contrast medium diatrizoate. The presence of biogenic palladium nanoparticles (bio-Pd) in the cathode significantly enhanced diatrizoate removal by direct electrochemical reduction and by reductive catalysis using the H(2) gas produced at the cathode of the MEC. Complete deiodination of 3.3 μM (2 mg L(-1)) diatrizoate from a synthetic medium was achieved after 24 h of recirculation at an applied voltage of -0.4 V. An equimolar amount of the deiodinated metabolite 3,5-diacetamidobenzoate (DAB) was detected. Higher cell voltages increased the dehalogenation rates, resulting in a complete removal after 2 h at -0.8 V. At this cell voltage, the MEC was also able to remove 85% of diatrizoate from hospital effluent containing 0.5 μM (292 μg L(-1)), after 24 h of recirculation. Complete removal was obtained when the effluent was continuously fed at a volumetric loading rate of 204 mg diatrizoate m(-3) total cathodic compartment (TCC) day(-1) to the MEC with a hydraulic retention time of 8 h. At -0.8 V, the MEC system could also eliminate 54% of diatrizoate from spiked urine during a 24 h recirculation experiment. The final product DAB was demonstrated to be removable by nitrifying biomass, which suggests that the combination of a MEC and bio-Pd in its cathode offers potential to dehalogenate pharmaceuticals, and to significantly lower the environmental burden of hospital waste streams.

Biogenic palladium enhances diatrizoate removal from hospital wastewater in a microbial electrolysis cell.

Anthropogenic nitrous oxide (N(2)O) emissions represent up to 40% of the global N(2)O emission and are constantly increasing. Mitigation of these emissions is warranted since N(2)O is a strong greenhouse gas and important ozone-depleting compound. Until now, only physicochemical technologies have been applied to mitigate point sources of N(2)O, and no biological treatment technology has been developed so far. In this study, a bioelectrochemical system (BES) with an autotrophic denitrifying biocathode was considered for the removal of N(2)O. The high N(2)O removal rates obtained ranged between 0.76 and 1.83 kg N m(-3) net cathodic compartment (NCC) d(-1) and were proportional to the current production, resulting in cathodic coulombic efficiencies near 100%. Furthermore, our experiments suggested the active involvement of microorganisms as the catalyst for the reduction of N(2)O to N(2), and the optimal cathode potential ranged from -200 to 0 mV vs standard hydrogen electrode (SHE) in order to obtain high conversion rates. Successful operation of the system for more than 115 days with N(2)O as the sole cathodic electron acceptor strongly indicated that N(2)O respiration yielded enough energy to maintain the biological process. To our knowledge, this study provides for the first time proof of concept of biocathodic N(2)O removal at long-term without the need for high temperatures and expensive catalysts.

Biocathodic nitrous oxide removal in bioelectrochemical systems.

N2O emissions from the biological treatment of sewage, manure, landfill leachates and industrial effluents have gained considerable interest among policy makers and environmental scientists. Estimated global emission rates from these sources can contribute up to 10% of the anthropogenic N2O emissions. Particularly at the level of a treatment plant, the N2O impact can be very significant and reach up to 80% of the operational CO2 footprint. Imperfect nitritation by an imbalance in the two-step nitritation metabolism of ammonia-oxidizing bacteria is considered as the main contributor to N2O production with hydroxylamine and particularly nitrite as key precursors. Monitoring of these compounds is warranted to understand and abate N2O emissions. Mitigation strategies should also comprise optimizations of the process parameters as well as bio-augmentative approaches empowered to restore the functional capacity and to deal with unwanted accumulation of intermediates. These strategies require validation for their effectiveness and costs at full-scale.

Strategies to mitigate N2O emissions from biological nitrogen removal systems.

The presence of elevated concentrations of nitrates in drinking water has become a serious concern worldwide. The use of autotrophic denitrification in microbial fuel cells (MFCs) for waters with low ionic strengths (i.e., 1000 μS·cm(-1)) has not been considered previously. This study evaluated the feasibility of MFC technology for water denitification and also identified and quantified potential energy losses that result from their usage. The low conductivity (<1600 μS·cm(-1)) of water limited the nitrogen removal efficiency and power production of MFCs and led to the incomplete reduction of nitrate and the nitrous oxide (N(2)O) production (between 4 and 20% of nitrogen removed). Cathodic overpotential was identified as the main energy loss factors (83-90% of total losses). That high overpotential was influenced by denitrification intermediates (NO(2)(-) and N(2)O) and the potential used by microorganisms for growth, activation, and maintenance.

Autotrophic denitrification in microbial fuel cells treating low ionic strength waters.

Ammonia inhibition during anaerobic digestion limits the substrate loading rate and endangers process stability. Furthermore, digestates are interesting feedstocks for nutrient recovery. In this lab-scale study, an electrochemical cell was used to investigate the NH(4)(+) flux from anode to cathode. Under optimal conditions with synthetic wastewater, an NH(4)(+) charge transfer efficiency of 96% and NH(4)(+) flux of 120 g N m(-2) d(-1) could be obtained at a concomitant electricity input of 5 kWh kg(-1) N removed. A more selective NH(4)(+) transfer could be established by maintaining a high concentration of other cations in the cathode compartment. Comparable NH(4)(+) fluxes could be obtained with digestate at an electrical power input of 13 kWh kg(-1) N removed and 41% current efficiency. The ammonium level in the digestate could be lowered from 2.1 to 0.8 - 1.2 g N L(-1). Interestingly, also potassium fluxes of up to 241 g K(+) m(-2) d(-1) could be obtained at 23% current efficiency. As the cathode can be operated at high pH without the need for chemical addition, stripping and absorption of dissolved ammonia could reach 100% efficiency. By valorization of the generated side products, this technology shows economic potential for practical application.

Electrochemical resource recovery from digestate to prevent ammonia toxicity during anaerobic digestion.

Extracellular electron transfer has, in one decade, emerged from an environmental phenomenon to an industrial process driver. On the one hand, electron transfer towards anodes leads to production of power or chemicals such as hydrogen, caustic soda and hydrogen peroxide. On the other hand, electron transfer from cathodes enables bioremediation and bioproduction. Although the microbiology of extracellular electron transfer is increasingly being understood, bringing the processes to application requires a number of considerations that are both operational and technical. In the present paper, we investigate the key applied aspects related to electricity-driven bioproduction, including biofilm development, reactor and electrode design, substrate fluxes, surface chemistry, hydrodynamics and electrochemistry, and finally end-product removal/toxicity. Each of these aspects will be critical for the full exploitation of the intriguing physiological feat that extracellular electron transfer is today.

Operational and technical considerations for microbial electrosynthesis.

The microbial consumption of nitrous oxide (N2O) has gained great interest since it was revealed that this process could mitigate the greenhouse effect of N2O. The consumption of N2O results from its reduction to dinitrogen gas (N2) as part of the denitrification process. However, there is ongoing debate regarding an alternative pathway, namely reduction of N2O to NH4(+), or assimilatory N2O consumption. To date, this pathway is poorly investigated and lacks unambiguous evidence. Enrichment of denitrifying activated sludge using a mineral nitrogen-free medium rendered a mixed culture capable of anoxic and oxic N2O consumption. Dilution plating, isolation and deoxyribonucleic acid fingerprinting identified a collection of Pseudomonas stutzeri strains as dominant N2O consumers in both anaerobic and aerobic enrichments. A detailed isotope tracing experiment with a Pseudomonas stutzeri isolate showed that consumption of N2O via assimilatory reduction to NH4(+) was absent. Conversely, respiratory N2O reduction was directly coupled to N2 fixation.

Pathway of nitrous oxide consumption in isolated Pseudomonas stutzeri strains under anoxic and oxic conditions.

Short-chain carboxylates such as acetate are easily produced through mixed culture fermentation of many biological waste streams, although routinely digested to biogas and combusted rather than harvested. We developed a pipeline to extract and upgrade short-chain carboxylates to esters via membrane electrolysis and biphasic esterification. Carboxylate-rich broths are electrolyzed in a cathodic chamber from which anions flux across an anion exchange membrane into an anodic chamber, resulting in a clean acid concentrate with neither solids nor biomass. Next, the aqueous carboxylic acid concentrate reacts with added alcohol in a water-excluding phase to generate volatile esters. In a batch extraction, 96 ± 1.6% of the total acetate was extracted in 48 h from biorefinery thin stillage (5 g L(-1) acetate) at 379 g m(-2) d(-1) (36% Coulombic efficiency). With continuously regenerated thin stillage, the anolyte was concentrated to 14 g/L acetic acid, and converted at 2.64 g (acetate) L(-1) h(-1) in the first hour to ethyl acetate by the addition of excess ethanol and heating to 70 °C, with a final total conversion of 58 ± 3%. This processing pipeline enables direct production of fine chemicals following undefined mixed culture fermentation, embedding carbon in industrial chemicals rather than returning them to the atmosphere as carbon dioxide.

Electrolytic membrane extraction enables production of fine chemicals from biorefinery sidestreams.

Organic waste streams can be valorized and reduced in volume with anaerobic digestion (AD). An often-encountered key issue however is the high ammonium (NH4(+)) content of certain waste streams. Ammonia (NH3), in equilibrium with NH4(+), is a toxic compound to the methanogenic community, which limits the organic loading rate and endangers process stability. An electrochemical system (ES) linked to a digester could, besides recovering this nutrient, decrease NH3 toxicity through electrochemical extraction. Therefore, two digesters with and without ES attached in the recirculation loop were operated to test whether the ES could control NH3 toxicity. During periods of high ammonium loading rates, the methane (CH4) production of the ES-coupled reactor was up to 4.5 times higher compared to the control, which could be explained through simultaneous NH4(+) extraction and electrochemical pH control. A nitrogen flux of 47 g N m(-2) membrane d(-1) could be obtained in the ES-coupled reactor, resulting in a current and removal efficiency of 38 ± 5% and 28 ± 2%, respectively, at an electrochemical power input of 17 ± 2 kWh kg(-1) N. The anode also oxidized sulfide, resulting in a significantly lower H2S emission via the biogas. Lastly, limited methanogenic community dynamics pointed to a nonselective influence of the different operational conditions.

Joachim Desloover

Laboratory of Microbial Ecology and Technology (LabMET)

Ghent University

Electrochemically and Bioelectrochemically Induced Ammonium Recovery

Joachim Desloover

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Laboratory of Microbial Ecology and Technology (LabMET)

Ghent University

Electrochemically and Bioelectrochemically Induced Ammonium Recovery

Laboratory of Microbial Ecology and Technology (LabMET)