Assessing the potential for up-cycling recovered resources from anaerobic digestion through microbial protein production.

Anaerobic digesters produce biogas, a mixture of predominantly CH4 and CO2 , which is typically incinerated to recover electrical and/or thermal energy. In a context of circular economy, the CH4 and CO2 could be used as chemical feedstock in combination with ammonium from the digestate. Their combination into protein-rich bacterial, used as animal feed additive, could contribute to the ever growing global demand for nutritive protein sources and improve the overall nitrogen efficiency of the current agro- feed/food chain. In this concept, renewable CH4 and H2 can serve as carbon-neutral energy sources for the production of protein-rich cellular biomass, while assimilating and upgrading recovered ammonia from the digestate. This study evaluated the potential of producing sustainable high-quality protein additives in a decentralized way through coupling anaerobic digestion and microbial protein production using methanotrophic and hydrogenotrophic bacteria in an on-farm bioreactor. We show that a practical case digester handling liquid piggery manure, of which the energy content is supplemented for 30% with co-substrates, provides sufficient biogas to allow the subsequent microbial protein as feed production for about 37% of the number of pigs from which the manure was derived. Overall, producing microbial protein on the farm from available methane and ammonia liberated by anaerobic digesters treating manure appears economically and technically feasible within the current range of market prices existing for high-quality protein. The case of producing biomethane for grid injection and upgrading the CO2 with electrolytic hydrogen to microbial protein by means of hydrogen-oxidizing bacteria was also examined but found less attractive at the current production prices of renewable hydrogen. Our calculations show that this route is only of commercial interest if the protein value equals the value of high-value protein additives like fishmeal and if the avoided costs for nutrient removal from the digestate are taken into consideration.

The hydrogen gas bio-based economy and the production of renewable building block chemicals, food and energy.

The carrying capacity of the planet is being exceeded, and there is an urgent need to bring forward revolutionary approaches, particularly in terms of energy supply, carbon emissions and nitrogen inputs into the biosphere. Hydrogen gas, generated by means of renewable energy through water electrolysis, can be a platform molecule to drive the future bioeconomy and electrification in the 21st century. The potential to use hydrogen gas in microbial metabolic processes is highly versatile, and this opens a broad range of opportunities for novel biotechnological developments and applications. A first approach concerns the central role of hydrogen gas in the production of bio-based building block chemicals using the methane route, thus, bypassing the inherent low economic value of methane towards higher-value products. Second, hydrogen gas can serve as a key carbon-neutral source to produce third-generation proteins, i.e. microbial protein for food applications, whilst simultaneously enabling carbon capture and nutrient recovery, directly at their point of emission. Combining both approaches to deal with the intermittent nature of renewable energy sources maximises the ability for efficient use of renewable resources.

Membrane electrolysis for the removal of Mg2+ and Ca2+ from lithium rich brines.

Lithium is today an essential raw material for renewable energy technologies and electric mobility. Continental brines as present in the Lithium Triangle are the most abundant and the easiest to exploit lithium sources. Lithium is present in diluted concentrations together with different ions, and it is imperative to fully remove both magnesium and calcium before lithium carbonate can be precipitated. Here we use membrane electrolysis as a novel method to generate hydroxyl groups in situ in a two-chamber electrochemical cell with a side crystallizer, omitting the need for chemical addition and not leading to substantial loss of lithium rich brine. Batch electrolysis experiments fully removed more than 99.99% of both Mg2+ and Ca2+ for three different native South-American brines treated at current densities ranging from 27 to 350 A m-2 (final concentrations were below ICP detection limit: < 0.05 mg L-1). For a brine containing 3090 mg L-1 of Mg2+ and 685 mg L-1 of Ca2+, 62 kWh m-3 are needed for the full removal of both cations when a current density of 223 A m-2 is employed. Most importantly, the Li+ concentration in the brine is not affected. The removed cations are precipitated as Mg(OH)2 and Ca(OH)2. Our process has the potential to simultaneously recover lithium, magnesium, and calcium compounds, minimizing waste production.

Interfacing anaerobic digestion with (bio)electrochemical systems: Potentials and challenges.

For over a century, anaerobic digestion has been a key technology in stabilizing organic waste streams, while at the same time enabling the recovery of energy. The anticipated transition to a bio-based economy will only increase the quantity and diversity of organic waste streams to be treated, and, at the same time, increase the demand for additional and effective resource recovery schemes for nutrients and organic matter. The performance of anaerobic digestion can be supported and enhanced by (bio)electrochemical systems in a wide variety of hybrid technologies. Here, the possible benefits of combining anaerobic digestion with (bio)electrochemical systems were reviewed in terms of (1) process monitoring, control, and stabilization, (2) nutrient recovery, (3) effluent polishing, and (4) biogas upgrading. The interaction between microorganisms and electrodes with respect to niche creation is discussed, and the potential impact of this interaction on process performance is evaluated. The strength of combining anaerobic digestion with (bio)electrochemical technologies resides in the complementary character of both technologies, and this perspective was used to distinguish transient trends from schemes with potential for full-scale application. This is supported by an operational costs assessment, showing that the economic potential of combining anaerobic digestion with a (bio)electrochemical system is highly case-specific, and strongly depends on engineering challenges with respect to full-scale applications.

The type of ion selective membrane determines stability and production levels of microbial electrosynthesis.

Microbial electrosynthesis (MES) can enable electricity-driven bioproduction from CO2. Several membrane types such as anion exchange, cation exchange, and bipolar membranes (AEM/CEM/BPM) can be used to separate the anodic oxidation from the biocathodic reduction. The impact of the membrane type on MES has not yet been studied. Therefore we compared the three membranes for MES of acetic acid. The reactor with AEM enabled in situ recovery of acetic acid. This extraction led to a 32% higher production rate and efficiency compared to the systems that did not include product recovery, as product inhibition was likely occurring. Besides H+/OH-, mainly HCO3- contributed to charge balancing. Due to water displacement across the membrane, the product concentration in the AEM reactor (9g/L) did not exceed the concentration in the CEM reactor (10.5g/L). Overall this comparison shows that the membrane type in MES can be critical towards a stable and efficient process.