•MES was proposed as a route to convert CO2 to commodities.•Performance indicators of MES are plateauing and far from economic competitiveness.•Effective water electrolysis in microbial-compatible ...electrolytes is challenging.•Severe trade-off between production rate and energy conversion efficiency.•Improvement of MES, applicability and possible directions are discussed.
Microbial electrosynthesis (MES) is an electrochemical process used to drive microbial metabolism for bio-production, such as the reduction of CO2 into industrially relevant organic products as an alternative to current fossil-fuel-derived commodities. After a decade of research on MES from CO2, figures of merit have increased significantly but are plateauing yet far from those expected to allow competitiveness for synthesis of commodity chemicals. Here we discuss the substantial technological shortcomings still associated with MES and evoke possible ways to mitigate them. It appears particularly challenging to obtain both relevant production rates (driven by high current densities) and energy conversion efficiency (i.e. low cell voltage) in microbial-compatible electrolytes. More competitive processes could arise by decoupling effective abiotic electroreductions (e.g. CO2 to CO or ethanol; H2 evolution) with subsequent fermentation processes.
Microbial electrocatalysis relies on microorganisms as catalysts for reactions occurring at electrodes. Microbial fuel cells and microbial electrolysis cells are well known in this context; both use ...microorganisms to oxidize organic or inorganic matter at an anode to generate electrical power or H(2), respectively. The discovery that electrical current can also drive microbial metabolism has recently lead to a plethora of other applications in bioremediation and in the production of fuels and chemicals. Notably, the microbial production of chemicals, called microbial electrosynthesis, provides a highly attractive, novel route for the generation of valuable products from electricity or even wastewater. This Review addresses the principles, challenges and opportunities of microbial electrosynthesis, an exciting new discipline at the nexus of microbiology and electrochemistry.
Phosphorus (P) is an essential element for all life on earth. However, natural P resources (phosphate rock) are depleting. The authors describe the current situation and a forecast for future ...phosphate production and reserves. The current depletion of phosphate reserves and the increasingly stringent discharge regulations have led to the development of various P-recovery techniques from wastewater. Existing full-scale P-recovery techniques from the liquid phase, sludge phase, and sludge ash are reviewed. Although the full-scale P-recovery techniques have been shown to be technologically feasible, the economical feasibility, legislation and national policies are the major reasons why these techniques are not yet operational worldwide.
Throughout the 20th century, the prevailing approach toward nitrogen management in municipal wastewater treatment was to remove ammonium by transforming it into dinitrogen (N2) using biological ...processes such as conventional activated sludge. While this has been a very successful strategy for safeguarding human health and protecting aquatic ecosystems, the conversion of ammonium into its elemental form is incompatible with the developing circular economy of the 21st century. Equally important, the activated sludge process and other emerging ammonium removal pathways have several environmental and technological limitations. Here, we assess that the theoretical energy embedded in ammonium in domestic wastewater represents roughly 38–48% of the embedded chemical energy available in the whole of the discharged bodily waste. The current routes for ammonium removal not only neglect the energy embedded in ammonium, but they can also produce N2O, a very strong greenhouse gas, with such emissions comprising the equivalent of 14–26% of the overall carbon footprint of wastewater treatment plants. N2O emissions often exceed the carbon emissions related to the electricity consumption for the process requirements of WWTPs. Considering these limitations, there is a need to develop alternative ammonium management approaches that center around recovery of ammonium from domestic wastewater rather than deal with its “destruction” into elemental dinitrogen. Current ammonium recovery techniques are applicable only at orders of magnitude above domestic wastewater strength, and so new techniques based on physicochemical adsorption are of particular interest. A new pathway is proposed that allows for mainstream ammonium recovery from wastewater based on physicochemical adsorption through development of polymer-based adsorbents. Provided adequate adsorbents corresponding to characteristics outlined in this paper are designed and brought to industrial production, this adsorption-based approach opens perspectives for mainstream continuous adsorption coupled with side-stream recovery of ammonium with minimal chemical requirements. This proposed pathway can bring forward an effective resource-oriented approach to upgrade the fate of ammonium in urban water management without generating hidden externalized environmental costs.
Summary
Our understanding of the complex interconnected processes performed by microbial communities is hindered by our inability to culture the vast majority of microorganisms. Metagenomics provides ...a way to bypass this cultivation bottleneck and recent advances in this field now allow us to recover a growing number of genomes representing previously uncultured populations from increasingly complex environments. In this study, a temporal genome‐centric metagenomic analysis was performed of lab‐scale anaerobic digesters that host complex microbial communities fulfilling a series of interlinked metabolic processes to enable the conversion of cellulose to methane. In total, 101 population genomes that were moderate to near‐complete were recovered based primarily on differential coverage binning. These populations span 19 phyla, represent mostly novel species and expand the genomic coverage of several rare phyla. Classification into functional guilds based on their metabolic potential revealed metabolic networks with a high level of functional redundancy as well as niche specialization, and allowed us to identify potential roles such as hydrolytic specialists for several rare, uncultured populations. Genome‐centric analyses of complex microbial communities across diverse environments provide the key to understanding the phylogenetic and metabolic diversity of these interactive communities.
Research highlights ► Microbial electrosynthesis concerns electricity driven bioproduction processes. ► We evaluate the challenges and opportunities for microbial electrosynthesis. ► There is no ...apparent direct financial benefit of electricity over sugar driven production. ► The low energy yield of homoacetogenesis will likely lead to low product yields. ► Land independent bioproduction is a key advantage of microbial electrosynthesis.
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 NH4 + flux from anode to cathode. Under optimal conditions with synthetic wastewater, an NH4 + charge transfer efficiency of 96% and NH4 + 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 NH4 + transfer could be established by maintaining a high concentration of other cations in the cathode compartment. Comparable NH4 + 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.
The advent of renewable energy conversion systems exacerbates the existing issue of intermittent excess power. Microbial electrosynthesis can use this power to capture CO2 and produce multicarbon ...compounds as a form of energy storage. As catalysts, microbial populations can be used, provided side reactions such as methanogenesis are avoided. Here a simple but effective approach is presented based on enrichment of a robust microbial community via several culture transfers with H2:CO2 conditions. This culture produced acetate at a concentration of 1.29 ± 0.15 g L–1 (maximum up to 1.5 g L–1; 25 mM) from CO2 at a fixed current of −5 Am–2 in fed-batch bioelectrochemical reactors at high N2:CO2 flow rates. Continuous supply of reducing equivalents enabled acetate production at a rate of 19 ± 2 gm–2d–1 (projected cathode area) in several independent experiments. This is a considerably high rate compared with other unmodified carbon-based cathodes. 58 ± 5% of the electrons was recovered in acetate, whereas 30 ± 10% of the electrons was recovered in H2 as a secondary product. The bioproduction was most likely H2 based; however, electrochemical, confocal microscopy, and community analyses of the cathodes suggested the possible involvement of the cathodic biofilm. Together, the enrichment approach and galvanostatic operation enabled instant start-up of the electrosynthesis process and reproducible acetate production profiles.
One of the main challenges for the 21st century is to balance the increasing demand for high-quality proteins while mitigating environmental impacts. In particular, cropland-based production of ...protein-rich animal feed for livestock rearing results in large-scale agricultural land-expansion, nitrogen pollution, and greenhouse gas emissions. Here we propose and analyze the long-term potential of alternative animal feed supply routes based on industrial production of microbial proteins (MP). Our analysis reveals that by 2050, MP can replace, depending on socio-economic development and MP production pathways, between 10–19% of conventional crop-based animal feed protein demand. As a result, global cropland area, global nitrogen losses from croplands and agricultural greenhouse gas emissions can be decreased by 6% (0–13%), 8% (−3–8%), and 7% (−6–9%), respectively. Interestingly, the technology to industrially produce MP at competitive costs is directly accessible for implementation and has the potential to cause a major structural change in the agro-food system.
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