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Summary: An increasing fraction of energy is generated by intermittent sources such as wind and sun. A straightforward solution to keep the electricity grid reliable is the connection of large‐scale electricity storage to this grid. Current battery storage technologies, while providing promising energy and power densities, suffer from a large environmental footprint, safety issues, and technological challenges. In this paper, the acid base flow battery is re‐established as an environmental friendly means of storing electricity using electrolyte consisting of NaCl salt. To achieve a high specific energy, we have performed charge and discharge cycles over the entire pH range (0–14) at several current densities. We demonstrate stable performance at high energy density (2.9 Wh L−1). Main energy dissipation occurs by unwanted proton and hydroxyl ion transport and leads to low coulombic efficiencies (13%–27%). [ABSTRACT FROM AUTHOR]

An abstract of the article "Product Specificity Influenced by Catholyte Conditions during the Microbial Electrosynthesis Process CO2 to Acetate," by S. Bajracharya and colleagues is presented.

Toxic selenium oxyanions and sulfur species are often jointly present in contaminated waters and soils. This study investigated the effect on kinetics and resulting products for bio-reduction of selenium oxyanions in the presence of biologically produced sulfur resulting from bio-oxidation of sulfide in (bio)gas-desulfurization (bio-S 0 ) and of sulfate. Selenite and selenate (~2 mmol L -1 ) bio-reduction was studied in batch up to 28 days at 30  o C and pH 7 using lactic acid and a sulfate-reducing sludge, 'Emmtec'. Bio-S 0 addition increased the selenite removal rate, but initially slightly decreased selenate reduction rates. Selenite reacted with biologically generated sulfide resulting in selenium-sulfur, which upon further bio-reduction creates a sulfur bio-reduction cycle. Sulfate addition increased the bio-reduction rate for both selenite and sulfate. Bio-S 0 or sulfate promoted hexagonal selenium formation, whereas without these, mostly amorphous Se 0 resulted. With another inoculum, 'Eerbeek', bio-S 0 accelerated the selenite reduction rate less than for 'Emmtec' because of lower sulfur and higher selenite bio-reduction rates. Bio-S 0 addition increased the selenate reduction rate slightly and accelerated hexagonal selenium formation. Hexagonal selenium formation is advantageous because it facilitates separation and recovery and is less mobile and toxic than amorphous Se 0 . Insights into the interaction between selenium and sulfur bio-reduction are valuable for understanding environmental pathways and considerations regarding remediation and recovery., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

Recovery of selenium (Se) from wastewater provides a solution for both securing Se supply and preventing Se pollution. Here, we developed a high-rate process for biological selenate reduction to elemental selenium. Distinctive from other studies, we aimed for a process with selenate as the main biological electron sink, with minimal formation of methane or sulfide. A sequencing batch reactor, fed with an influent containing 120 mgSe L -1 selenate and ethanol as electron donor and carbon source, was operated for 495 days. The high rates (419 ± 17 mgSe L -1 day -1 ) were recorded between day 446 and day 495 for a hydraulic retention time of 6 h. The maximum conversion efficiency of selenate amounted to 96% with a volumetric conversion rate of 444 mgSe L -1 day -1 , which is 6 times higher than the rates reported in the literature thus far. At the end of the experiment, a highly enriched selenate reducing biomass had developed, with a specific activity of 856 ± 26 mgSe -1 day -1 g biomass -1 , which was nearly 1000-fold higher than that of the inoculum. No evidence was found for the formation of methane, sulfide, or volatile reduced selenium compounds like dimethyl-selenide or H 2 Se, revealing a high selectivity. Ethanol was incompletely oxidized to acetate. The produced elemental selenium partially accumulated in the reactor as pure (≥80% Se of the total mixture of biomass sludge flocs and flaky aggregates, and ~100% of the specific flaky aggregates) selenium black hexagonal needles, with cluster sizes between 20 and 200 µm. The new process may serve as the basis for a high-rate technology to remove and recover pure selenium from wastewater or process streams with high selectivity., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021. Published by Elsevier Ltd.)

Bioelectrochemical systems (BES) can provide an energy efficient way to recover nutrients from wastewaters. However, the electron donors available in wastewater are often not sufficient to recover the total amount of nutrients. This work investigates hydrogen (H 2 ) as an additional substrate for bioanodes. This hydrogen can be produced in the fermentation of complex organic waste or could be recycled from the cathode. Understanding how to influence the competition of electroactive microorganisms (EAM) with methanogens for H 2 gas from different sources is key to successful application of H 2 as additional electron donor in bioelectrochemical nutrient recovery. Ethanol (EtOH) was used as model compound for complex wastewaters since it is fermented into both acetate and H 2 . EtOH was efficiently converted into electricity (e - ) by a syntrophic biofilm. Total recovered charge from 1 mM EtOH was 20% higher than for the same amount of acetate. This means that H 2 from EtOH fermentation was converted by EAM into electricity. Low EtOH concentrations (1  mM) led to higher conversion efficiencies into electricity than higher concentrations (5 and 10  mM). Thermodynamic calculations show this correlates with a higher energy gain for electrogens compared to methanogens at low H 2 concentrations. Cumulatively adding 1 mM EtOH without medium exchange (14 times in 14 days) resulted in stable conversion of H 2 to e - (67%-77% e - ) rather than methane. With H 2 gas as electron donor, 68 ± 2% H 2 was converted into e - with no carbon source added, and still 53 ± 5% to e - when 50  mM bicarbonate was provided. These results show that under the provided conditions, electrogens can outcompete methanogens for H 2 as additional electron donor in MECs for nutrient recovery., (Copyright © 2019 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Calcium phosphate (CaP) granules were discovered in the anaerobic treatment of vacuum collected black water (BW), using upflow anaerobic sludge blanket (UASB) technology. This allows simultaneous recovery of CaP granules and methane in the UASB reactor. However, the role of BW composition on CaP granulation is not yet understood. Moreover, CaP granulation was not observed in previous research on anaerobic treatment of BW, although similar treatment conditions were applied. Therefore, this study shows specifically the influence of bicarbonate and calcium fluctuations in BW on the phosphorus accumulation in the UASB reactor, which directly affects CaP granulation. Without calcium addition, 5% of the total phosphorus (P) fed was found as CaP granules in the reactor (61 mgP g -1 dried matter), after 260 days of operation. Simultaneously, 65% of the COD in BW was efficiently converted into methane at 25 °C. Variations of bicarbonate and calcium concentrations in raw BW showed a significant influence on phosphorus accumulation in the UASB reactor. Geochemical modelling showed that the increase of soluble calcium from 39 to 54 mg L -1 in BW triggers supersaturation for calcium phosphate precursors (Ca x (PO 4 ) y ). Concurrently, bicarbonate decreased from 2.7 to 1.2 g L -1 , increasing further the ionic activity of calcium. Formation and accumulation of seed particles possibly enhanced CaP granulation. Preliminary results showed that addition of calcium (Ca 2+ /PO 4 3- molar ratio of 3) increased the accumulation of total P in the UASB reactor to more than 85%. This further increases the granulation rate and consequently, the process feasibility., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Microbial reduction of selenium sulfide (SeS 2 ) is a key step in a new treatment process to recover selenium from selenate and selenite streams. In this process, selenate is first reduced to selenite, and subsequently selenite is reduced by sulfide and precipitates from the solution as SeS 2 . The latter is bio-reduced to elemental selenium and sulfide. Two anaerobic granular sludges (Eerbeek and Emmtec) were tested for their efficiency to reduce commercial crystalline SeS 2 . Emmtec sludge had the highest reducing capacity with commercial SeS 2 and was therefore also used for the bioreduction of laboratory synthesized amorphous SeS 2 . Synthesized SeS 2 was formed mixing a sulfide solution and effluent containing selenite. With both SeS 2 solids (commercial and synthesized SeS 2 ), Emmtec sludge produced sulfide and a solid consisting of hexagonal elemental selenium. The crystalline hexagonal structure suggests the absence of biomolecules, which stabilize amorphous selenium bio-particles under comparable process conditions (T=30°C and a pH between 6 and 7). Selenium particles were not attached to the biomass, suggesting an extracellular formation. The results support the feasibility of the bio-reduction process using sulfur for recovering selenium from water., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Heptanoate (or enanthate), a saturated mono-carboxylate with seven carbon atoms, is a commercially produced biochemical building block with versatile applications. Currently, heptanoate is mainly derived from the oxidation of heptaldehyde, which can be obtained after pyrolysis of castor oil. The objective of this investigation was to achieve efficient high rate heptanoate production using a mixed culture chain elongation process based on propionate and ethanol. An efficient high rate heptanoate production using chain elongation could offer an alternative for heptanoate production from castor oil. The investigation was performed in an upflow anaerobic filter with a hydraulic retention time of 17 h. A heptanoate production rate of 4.5 g l(-1) d(-1) was achieved with a heptanoate concentration of 3.2 g l(-1). These results show sufficient potential to consider this approach as an alternative for heptanoate production from castor oil. Future research should make heptanoate production from propionate and ethanol more cost-effective., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

This research demonstrated the selective production of n-butyrate from mixed culture by applying 2 bar carbon dioxide into the headspace of batch fermenters or by increasing the initial substrate concentration. The effect of increasing initial substrate concentration was investigated at 8, 13.5 and 23 g COD/L with potato processing waste stream. Within 1 week of incubation, n-butyrate fraction selectively increased up to 83% by applying 2 bar hydrogen or 78% by applying carbon dioxide into the headspace whereas it was only 59% in the control reactor. Although the fraction of n-butyrate was elevated, the concentration remained lower than in the control. Both the highest concentration and fraction of n-butyrate were observed under the highest initial substrate concentration without headspace addition. The concentration was 10 g COD/L with 73% fraction. The operational conditions obtained from batch experiments for selective n-butyrate production were validated in a continuous process., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The objective of this investigation was to further increase the medium chain fatty acid (MCFA) production rate by reducing the hydraulic retention time (HRT) in an upflow anaerobic filter. The results showed that the volumetric MCFA production rate was increased to 57.4 g MCFA l(-1) d(-1), more than 3 times higher than previous work. Despite the lower MCFA concentrations at 4h HRT, the MCFA selectivity remained above 80%. Extra carbon dioxide additions and higher yeast extract concentrations were required to increase the MCFA production rate. More research related to substrates and (micro)nutrients in mixed culture continuous reactors needs to be performed to reduce yeast extract use in chain elongation., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Recently, interest has regained for medium chain fatty acids (MCFAs) as a low cost feedstock for bio-based chemical and fuel production processes. To become cost-effective, the volumetric MCFA production rate by chain elongation should increase to comparable rates of other fermentation processes. We investigate the MCFA production process at a hydraulic retention time of 17 h in an upflow anaerobic filter to improve the volumetric MCFA production rate. This approach resulted in a MCFA production with a volumetric production rate of 16.6 g l(-1) d(-1), which is more than seven times higher than the current production rate. Moreover the rate is now in the range of other fermentation processes like methane, butanol and ethanol production. Increasing the ethanol load lead to higher volumetric production rates and a high MCFA selectivity of 91%. During operation, methane percentages lower than 0.1% were detected in the headspace of reactor., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

This study investigated the carboxylate spectrum from mixed culture fermentation of three organic waste streams after supplying 2 bar hydrogen and carbon dioxide or a mixture of these two gases to the headspace. Under any modified headspace, propionate production was ceased and butyrate, caproate and the total carboxylate concentrations were higher than in the reactors with N(2) headspace (control). Production of one major compound was achieved under hydrogen and carbon dioxide mixed headspace after 4 weeks of incubation. Both the highest acetate concentration (17.4 g COD/l) and the highest fraction (87%) were observed in reactors with mixed hydrogen and carbon dioxide headspace independent of the substrate used. In the control reactor, acetate made up maximum 67% of the total products. For other products, the highest concentration and fraction were seldom observed together. Selective butyrate production reaching a 75% fraction was found under the carbon dioxide headspace on the carbohydrate rich waste., (Crown Copyright © 2012. Published by Elsevier Ltd. All rights reserved.)

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., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

In order to meet environmental quality criteria, grey water was treated in four different ways: 1) aerobic 2) anaerobic+aerobic 3) aerobic+activated carbon 4) aerobic+ozone. Since each treatment has its own specific advantages and disadvantages, the aim of this study was to compare the ecotoxicity of differently treated grey water using Chironomus riparius (96 h test) and Daphnia magna (48 h and 21d test) as test organisms. Grey water exhibited acute toxicity to both test organisms. The aerobic and combined anaerobic+aerobic treatment eliminated mortality in the acute tests, but growth of C. riparius was still affected by these two effluents. Post-treatment by ozone and activated carbon completely removed the acute toxicity from grey water. In the chronic toxicity test the combined anaerobic+aerobic treatment strongly affected D. magna population growth rate (47%), while the aerobic treatment had a small (9%) but significant effect. Hence, aerobic treatment is the best option for biological treatment of grey water, removing most of the toxic effects of grey water. If advanced treatment is required, the treatment with either ozone or GAC were shown to be very effective in complete removal of toxicity from grey water., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

In a Plant-Microbial Fuel Cell anode-conditions must be created that are favorable for plant growth and electricity production. One of the major aspects in this is the composition of the plant-growth medium. Hoagland medium has been used until now, with added phosphate buffer to reduce potential losses over the membrane because of differences in pH between anode and cathode. We developed a new, improved plant-growth medium that improves current production, while the plant keeps growing. This medium is a nitrate-less, ammonium-rich medium that contains all macro- and micro-nutrients necessary for plant growth, with a balanced amount of bicarbonate buffer. Sulphate presence in the plant-growth medium helps to keep a low anode-potential. With the new plant-growth medium the maximum current production of the Plant-Microbial Fuel Cell increased from 186 mA/m(2) to 469 mA/m(2)., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Microalgal biofilms have so far received little attention as post-treatment for municipal wastewater treatment plants, with the result that the removal capacity of microalgal biofilms in post-treatment systems is unknown. This study investigates the capacity of microalgal biofilms as a post-treatment step for the effluent of municipal wastewater treatment plants. Microalgal biofilms were grown in flow cells with different nutrient loads under continuous lighting of 230 μmol/m(2)/s (PAR photons, 400-700 nm). It was found that the maximum uptake capacity of the microalgal biofilm was reached at loading rates of 1.0 g/m(2)/day nitrogen and 0.13 g/m(2)/day phosphorus. These maximum uptake capacities were the highest loads at which the target effluent values of 2.2 mg/L nitrogen and 0.15 mg/L phosphorus were still achieved. Microalgal biomass analysis revealed an increasing nitrogen and phosphorus content with increasing loading rates until the maximum uptake capacities. The internal nitrogen to phosphorus ratio decreased from 23:1 to 11:1 when increasing the loading rate. This combination of findings demonstrates that microalgal biofilms can be used for removing both nitrogen and phosphorus from municipal wastewater effluent., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Ozonation and adsorption onto activated carbon were tested for the removal micropollutants of personal care products from aerobically treated grey water. MilliQ water spiked with micropollutants (100-1600 μgL(-1)) was ozonated at a dosing rate of 1.22. In 45 min, this effectively removed (>99%): Four parabens, bisphenol-A, hexylcinnamic aldehyde, 4-methylbenzylidene-camphor (4MBC), benzophenone-3 (BP3), triclosan, galaxolide and ethylhexyl methoxycinnamate. After 60 min, the removal efficiency of benzalkonium chloride was 98%, tonalide and nonylphenol 95%, octocrylene 92% and 2-phenyl-5-benzimidazolesulfonic acid (PBSA) 84%. Ozonation of aerobically treated grey water at an applied ozone dose of 15 mgL(-1), reduced the concentrations of octocrylene, nonylphenol, triclosan, galaxolide, tonalide and 4-methylbenzylidene-camphor to below limits of quantification, with removal efficiencies of at least 79%. Complete adsorption of all studied micropollutants onto powdered activated carbon (PAC) was observed in batch tests with milliQ water spiked with 100-1600 μgL(-1) at a PAC dose of 1.25 gL(-1) and a contact time of 5 min. Three granular activated carbon (GAC) column experiments were operated to treat aerobically treated grey water. The operation of a GAC column with aerobically treated grey water spiked with micropollutants in the range of 0.1-10 μgL(-1) at a flow of 0.5 bed volumes (BV)h(-1) showed micropollutant removal efficiencies higher than 72%. During the operation time of 1728 BV, no breakthrough of TOC or micropollutants was observed. Removal of micropollutants from aerobically treated grey water was tested in a GAC column at a flow of 2 BVh(-1). Bisphenol-A, triclosan, tonalide, BP3, galaxolide, nonylphenol and PBSA were effectively removed even after a stable TOC breakthrough of 65% had been reached. After spiking the aerobically treated effluent with micropollutants to concentrations of 10-100 μgL(-1), efficient removal to below limits of quantification continued for at least 1440 BV. Both ozonation and adsorption are suitable techniques for the removal of micropollutants from aerobically treated grey water., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Ammonium recovery using a two chamber microbial fuel cell (MFC) was investigated at high ammonium concentration. Increasing the ammonium concentration (from 0.07 to 4 g ammonium-nitrogen/L) by addition of ammonium chloride did not affect the performance of the MFC. The obtained current densities by DC-voltammetry were higher than 6A/m(2) for both operated MFCs. Also continuous operation at lower external resistance (250 Ω) showed an increased current density (0.9A/m(2)). Effective ammonium recovery can be achieved by migrational ion flux through the cation exchange membrane to the cathode chamber, driven by the electron production from degradation of organic substrate. The charge transport was proportional to the concentration of ions. Nonetheless, a concentration gradient will influence the charge transport. Furthermore, a charge exchange process can influence the charge transport and therefore the recovery of specific ions., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Vacuum collected black (toilet) water contains hormones and pharmaceuticals in relatively high concentrations (μg/L to mg/L range) and separate specific treatment has the potential of minimizing their discharge to surface waters. In this study, the fate of estrogens (natural and synthetical hormones) and pharmaceuticals (paracetamol, metoprolol, propranolol, cetirizine, doxycycline, tetracycline, ciprofloxacin, trimethoprim, carbamazepine, ibuprofen and diclofenac) in the anaerobic treatment of vacuum collected black water followed by nitrogen removal by partial nitritation-anammox was investigated. A new analytical method was developed to detect the presence of several compounds in the complex matrix of concentrated black water. Detected concentrations in black water ranged from 1.1 μg/L for carbamazepine to >1000 μg/L for paracetamol. Anaerobic treatment was only suitable to remove the majority of paracetamol (>90%). Metoprolol was partly removed (67%) during aerobic treatment. Deconjugation could have affected the removal efficiency of ibuprofen as concentrations even increased during anaerobic treatment and only after the anammox treatment 77% of ibuprofen was removed. The presence of persistent micro-pollutants (diclofenac, carbamazepine and cetirizine), which are not susceptible for biodegradation, makes the application of advanced physical and chemical treatment unavoidable., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Black (toilet) water contains half of the organic load in the domestic wastewater, as well as the major fraction of the nutrients nitrogen and phosphorus. When collected with vacuum toilets, the black water is 25 times more concentrated than the total domestic wastewater stream, i.e. including grey water produced by laundry, showers etc. A two-stage nitritation-anammox process was successfully employed and removed 85%-89% of total nitrogen in anaerobically treated black water. The (free) calcium concentration in black water was too low (42 mg/L) to obtain sufficient granulation of anammox biomass. The granulation and retention of the biomass was improved considerably by the addition of 39 mg/L of extra calcium. This resulted in a volumetric nitrogen removal rate of 0.5 gN/L/d, irrespective of the two temperatures of 35 °C and 25 °C at which the anammox reactors were operated. Nitrous oxide, a very strong global warming gas, was produced in situations of an incomplete anammox conversion accompanied by elevated levels of nitrite., (Copyright © 2010 Elsevier Ltd. All rights reserved.)