Ex-situ biogas upgrading in thermophilic trickle bed reactors packed with micro-porous packing materials

Two thermophilic trickle bed reactors (TBRs) were packed with different packing densities with polyurethane foam (PUF) and their performance under different retention times were evaluated during ex-situ biogas upgrading process. The results showed that the TBR more tightly packed i.e. containing more layers of PUF achieved higher H₂ utilization efficiency (>99%) and thus, higher methane content (>95%) in the output gas. The tightly packed micro-porous PUF enhanced biofilm immobilization, gas-liquid mass transfer and biomethanation efficiency. Moreover, applying a continuous high-rate nutrient trickling could lead to liquid overflow resulting in formation of non-homogenous biofilm and severe deduction of biomethanation efficiency. High-throughput 16S rRNA gene sequencing revealed that the liquid media were predominated by hydrogenotrophic methanogens. Moreover, members of Peptococcaceae family and uncultured members of Clostridia class were identified as the most abundant species in the biofilm. The proliferation of hydrogenotrophic methanogens together with syntrophic bacteria showed that H₂ addition resulted in altering the microbial community in biogas upgrading process.

In-situ biogas upgrading assisted by bioaugmentation with hydrogenotrophic methanogens during mesophilic and thermophilic co-digestion

In this study, the effects of bioaugmentation of typically dominant hydrogenotrophic methanogens to CSTR co-digesting cheese whey and manure, under in-situ biomethanation operations were investigated. Reactors working at mesophilic (37 °C) and thermophilic (55 °C) conditions were independently treated and examined in terms of microbial composition and process dynamics. Addition of Methanoculleus bourgensis in the mesophilic reactor led to a stable biomethanation, and an improved microbial metabolism, resulting in 11% increase in CH₄ production rate. 16S rRNA and biochemical analyses revealed an enrichment in syntrophic and acidogenic species abundance. Moreover, nearly total volatile fatty acids conversion was observed. Differently, Methanothermobacter thermautotrophicus addition in the thermophilic reactor did not promote biogas upgrading performance due to incomplete H₂ conversion and inefficient community adaptation to H₂ excess, ultimately favoring acetoclastic methanogenesis. Bioaugmentation constitutes a viable tool to strengthen in-situ upgrading processes and paves the way to the development of more sophisticated and robust microbial inoculants.

Going beyond conventional wastewater treatment plants within circular bioeconomy concept - a sustainability assessment study

Wastewater treatment plants (WWTP) have extensive energy processes that undermine their economic and environmental performance. In this context, the integration of wastewater treatment with other biochemical processes such as co-digestion of sludge with organic wastes, and production of value-added products at their downstream processes will shift conventional WWTPs into biorefinery platforms with better sustainability performance. The sustainability of such a biorefinery platform has been investigated herein using an economic and life cycle assessment approach. This WWTP-based biorefinery treats wastewater from Copenhagen municipality, co-digests the source-sorted organic fraction of municipal solid waste and sludge, and upgrades biogas into biomethane using a hydrogen-assisted upgrading method. Apart from bioenergy, this biorefinery also produces microbial protein (MP) using recovered nutrients from WWTP's reject water. The net environmental savings achieved in two damage categories, i.e., -1.07 × 10^-2 species.yr/FU in ecosystem quality and -1.68 × 10⁶ USD/FU in resource scarcity damage categories along with high potential windows for the further environmental profile improvements make this biorefinery platform so encouraging. Despite being promising in terms of environmental performance, the high capital expenditure and low gross profit have undermined the economic performance of the proposed biorefinery. Technological improvements, process optimization, and encouraging incentives/subsidies are still needed to make this platform economically feasible.

Bioconversion of waste-to-resources (BWR-2021): Valorization of industrial and agro-wastes to fuel, feed, fertilizer, and biobased products

The mitigation of greenhouse gas (GHG) emission is one of the major focuses of The Glasgow Climate Pact, a global agreement that is believed to accelerate climate action. Following the energy sector, industrial and agro-wastes are the major contributors to global GHG emission. With the rapid growth in population, affluence, and urbanization, the GHG emission from waste sector is likely to be further aggravated if timely measures are not taken to address this burning issue. Thus, a significant research and development efforts are being made in shifting conventional waste treatment approach to resource recovery from waste, incorporating a circular bioeconomy concept. There have been significant advances in technologies such as anaerobic digestion, composting, pyrolysis, algae farming, and microbial fuel cell for recovering resources from organic wastes. This virtual special issue (VSI), "Bioconversion of Waste-to-Resources (BWR-2021)", contains 25 manuscripts covering various aspects of wastes and residual biomass valorization to high value products, including development of new technologies, optimization of current technologies for more efficient utilization of wastes and residues. The key findings of each manuscript are briefly summarized here, which can serve as a valuable resource for researchers in the field of resource recovery from wastes.

Enhanced fermentative lactic acid production from source-sorted organic household waste: Focusing on low-pH microbial adaptation and bio-augmentation strategy

Lactic acid (LA) production at low pH could significantly reduce the need for neutralizing agents, leading to reduction of operational costs. In the present study, LA production at acidic conditions was investigated using source-sorted organic household waste (SSOHW). Controlling the pH at low value (i.e. 5.0) and bio-augmenting with Pediococcus acidilactici led to a concentration of 39.3 ± 0.5 g_-LA/L with a yield of 0.75 ± 0.02 g_-LA/g_-sugar. In contrast, secondary fermentation at higher pH level (i.e. 5.5 and 6.0) resulted in complete LA degradation. Subsequently, consecutive batch fermentations were conducted to adapt P. acidilactici to SSOHW and improve the LA production. Results showed that P. acidilactici could successively adapt in the SSOHW reaching a relative abundance above 2.8% at adaptation process. The added P. acidilactici ensured a high concentration of LA at three consecutive generations, achieving an increment above 18% compared to control test (abiotic augmentation). Moreover, adaptation processes (i.e. maintaining pH at 4.0 or stepwise decreasing the pH from 5.0 to 4.0) significantly improved LA concentration and productivity at the pH of 4.0. Overall, the results provide a promising method to reduce the LA production costs using residual resources.

When microbial electrochemistry meets UV: The applicability to high-strength real pharmaceutical industry wastewater

Wastewater from pharmaceutical and related industries contains many residual pharmaceutical components rich in color and high COD contents, which cannot be removed through the traditional wastewater treatment processes. Recently, microbial electrolysis ultraviolet cell (MEUC) process has shown its promising potential to remove recalcitrant organics because of its merits of wide pH range, iron-free, and without complications of iron sludge production. However, its application to the real pharmaceutical-rich industrial wastewater is still unknown. In this study, the MEUC process was validated with real ciprofloxacin-rich (6863.79 ± 2.21 µg L^-1) industrial wastewater (6840 ± 110 mg L^-1 of COD). The MEUC process achieved 100% removal of ciprofloxacin, 100% decolorization, and 99.1% removal of COD within 12, 60 and 30 h, respectively, when it was operated at pH-controlled at 7.8, applied voltage of 0.6 V, UV intensity of 10 mW cm^-2, and cathodic aeration velocity of 0.005 mL min^-1 mL^-1. Moreover, fluorescence analysis showed that protein- and humic-like substances in such wastewater were effectively removed, providing further evidence of its high treatment efficiency. Furthermore, eco-toxicity testing with luminescent bacteria Vibro Feschri confirmed that the treated effluent was utterly non-toxic. The results demonstrated the broad application potential of MEUC technology for treating industrial wastewater.

Novel bioaugmentation strategy boosted with biochar to alleviate ammonia toxicity in continuous biomethanation

This study investigated for the first time if ammonia tolerant methanogenic consortia can be stored in gel (biogel) and used in a later time on-demand as bioaugmentation inocula, to efficiently relieve ammonia inhibition in continuous biomethanation systems. Moreover, wood biochar was assessed as a potential enhancer of the novel biogel bioaugmentation process. Three thermophilic (55 °C), continuous stirred-tank reactors (R_Bgel, R_Char and R_Mix), operated at 4.5 g NH₄⁺-N L^-1 were exposed to biogel, biochar and mixture of biogel and biochar, respectively, while a fourth reactor (R_Ctrl) was used as control. The results showed that the methane production yields of R_Mix, R_Char and R_Bgel increased by 28.6%, 20.2% and 10.7%, respectively compared to R_Ctrl. The highest methane yield was achieved by the synergistic interaction between biogel and biochar. Additionally, biogel stimulated a rapid recovery of Methanoculleus thermophilus sp. and syntrophic acetate oxidising bacteria populations.

Self-sustained ammonium recovery from wastewater and upcycling for hydrogen-oxidizing bacteria-based power-to-protein conversion

Microbial protein (MP) production by autotrophic hydrogen-oxidizing bacteria is regarded as a potentially sustainable approach to mitigate food crisis, water pollution, and climate change. Herein, a hybrid biological-inorganic (HBI) system which coupled energy-neutral ammonium recovery and in-situ upcycling for MP production was demonstrated. No energy and acids/bases were needed for ammonium recovery and pH control. The system was tested with different amounts of CO₂ supply and operated at different operational modes (microbial fuel cell or microbial electrolysis cell mode). 0.381 g/L of biomass containing 64.79% of crude protein was produced using the recovered nitrogen and therefore led to 76.8% of ammonium recovery and 84.6% of COD removal from real municipal wastewater. The system although not yet optimal in terms of efficiency has a meaning in alleviating food crisis and environmental issues. Altogether, this study offers insight into developing an energy and resource-efficient power-to-protein process to supplement conventional food production globally.

Advances in microalgal research for valorization of industrial wastewater

This review article focuses on recent updates on remediation of industrial wastewater (IWW) through microalgae cultivation. These include how adding additional supplements of nutrient to some specific IWWs lacking adequate nutrients improving the microalgae growth and remediation simultaneously. Various pretreatments strategy recently employed for IWWs treatment other than dealing with microalgae was discussed. Various nutrient-rich IWW could be utilized directly with additional dilution, supplement of nutrients and without any pretreatment. Recent advances in various approaches and new tools used for cultivation of microalgae on IWW such as two-step cultivation, pre-acclimatization, novel microalgal-bioelectrical systems, integrated catalytic intense pulse-light process, sequencing batch reactor, use of old stabilized algal-bacterial consortium, immobilized microalgae cells, microalgal bacterial membrane photobioreactor, low-intensity magnetic field, BIO_ALGAE simulation tool, etc. are discussed. In addition, biorefinery of microalgal biomass grown on IWW and its end-use applications are reviewed.

Syngas biomethanation: effect of biomass-gas ratio, syngas composition and pH buffer

The concept of syngas biomethanation is attractive, however, it still needs improvement in optimizing the operational conditions. In the present study, syngas fermentations under different carbon monoxide (CO), carbon dioxide (CO₂) and hydrogen (H₂) compositions were conducted under two different biomass-gas ratio (BGR) systems. The results showed that high BGR enhanced the CO consumption rate, achieving a 60% enhancement with CO as the sole substrate. Stoichiometric H₂ addition could successfully convert all the CO and CO₂ to pure methane, however, higher H₂ partial pressure might decline the CO consumption due to pH inhibition from consumption of bicarbonate. Microbial analysis showed different syngas composition could affect the bacteria community, while, archaea community was only slightly affected with Methanothermobacter as the dominant methanogen. This study provided strategy for efficient syngas biomethanation and deeper insight into effect of H₂ addition on CO conversion under different BGR systems.

Valorization of municipal organic waste into purified lactic acid

Municipal organic waste (biowaste) consists of food derived starch, protein and sugars, and lignocellulose derived cellulose, hemicellulose, lignin and pectin. Proper management enables nutrient recycling and sustainable production of platform chemicals such as lactic acid (LA). This review gathers the most important information regarding use of biowaste for LA fermentation covering pre-treatment, enzymatic hydrolysis, fermentation and downstream processing to achieve high purity LA. The optimal approach was found to treat the two biowaste fractions separately due to different pre-treatment and enzyme needs for achieving enzymatic hydrolysis and to do continues fermentation to achieve high cell density and high LA productivity up to 12 g/L/h for production of both L and D isomers. The specific productivity was 0.4 to 0.5 h^-1 but with recalcitrant biomass, the enzymatic hydrolysis was rate limiting. Novel purification approaches included reactive distillation and emulsion liquid membrane separation yielding purities sufficient for polylactic acid production.

Ex-situ biogas upgrading in thermophilic up-flow reactors: The effect of different gas diffusers and gas retention times

Four different types of ceramic gas distributors (Al₂O₃ of 1.2 μm and SiC of 0.5, 7 and 14 μm) were evaluated to increase biomethane formation during ex-situ biogas upgrading process. Each type of gas diffuser was tested independently at three different gas retention times of 10, 5 and 2.5 h, at thermophilic conditions. CH₄ production rate increased by increasing input gas flow rate for all type of distributors, whereas CH₄ concentration declined. Reactors equipped with SiC gas distributors effectively improved biomethane content fulfilling natural gas standards. Microbial analysis showed high abundance of hydrogenotrophic methanogens and proliferated syntrophic bacteria, i.e. syntrophic acetate oxidizers and homoacetogens, confirming the effect of H₂ to alternate anaerobic digestion microbiome and enhance hydrogenotrophic methanogenesis. A detailed anaerobic bioconversion model was adapted to simulate the operation of the R₁-R₄ reactors. The model was shown to be effective for the simulation of biogas upgrading process in up-flow reactors.

Multicomponent nanoparticles as means to improve anaerobic digestion performance

Sufficient quantity of trace metals is essential for a well performing anaerobic digestion (AD) process. Among the essential trace elements in active sites of multiple important enzymes for AD are iron and nickel ions. In the present study, iron and nickel in the form of Fe₂O₃ and NiO were coated on TiO₂ nanoparticles to be used in batch and continuous operation mode. The effect of TiO₂, Fe₂O₃-TiO₂, and NiO-TiO₂ nanoparticles on each step of AD process was assessed utilizing simple substrates (i.e. cellulose, glucose, acetic acid, and mixture of H₂-CO₂) as well as complex ones (i.e. municipal biopulp). The hydrolysis rate of cellulose substrate increased with higher dosages of the coated TiO₂ with both metals. For instance, the hydrolysis rate was increased up to 54% at Fe₂O₃-TiO₂ and at a concentration of 23.5 mg/L for NiO-TiO₂ it was increased up to 58%, while higher dosage suppressed the hydrolytic activity. Experimental results revealed that low dosages of NiO-TiO₂ increased the accumulated methane production up to 24% probably by increasing the enzymatic activity of acetoclastic methanogenesis. NiO-TiO₂ showed positive effect on batch and continuous AD of biopulp and improved methane yield up to 8%.

Bioavailability and effect of α-Fe2O3 nanoparticles on growth, fatty acid composition and morphological indices of Chlorella vulgaris

The wide application of α-Fe₂O₃ nanoparticles (NPs) in different fields has resulted in release and accumulation of these materials into the aquatic ecosystem. Therefore, it is important to understand the potential impact of these NPs on aquatic organisms especially primary producers i.e., microalgae. Present study aimed to investigate the bioavailability and the effect of α-Fe₂O₃ NPs on growth of iron deprived cells of Chlorella vulgaris. Results showed that α-Fe₂O₃ NPs are not available as iron source to support the growth of C. vulgaris. Moreover，α-Fe₂O₃ NPs induced stress condition to C. vulgaris, which were reflected in its growth rates, total lipid contents, fatty acid profile and cell morphology. Specifically, low concentrations of α-Fe₂O₃ NPs (0.1, 0.5, 2.5, 5, 10 mg/L) showed similar growth profile and total lipid contents at both exponential and stationary growth phases. At 50 and 100 mg/L α-Fe₂O₃ NPs concentrations biomass reduced by 41.2% and 83.7% whereas total lipid contents increased by 39.7% and 25.5% respectively at exponential growth phase along with reduction in fatty acids. The results illustrated novel insights into the microalgal interaction with nanoparticles, providing fundamental knowledge for the development of future microalgae ecology and cultivation technology.

Bio-electrochemically extracted nitrogen from residual resources for microbial protein production

Upcycling of nutrients from residual resources for producing microbial protein (MP) is an attractive method to valorize residues. In this study, we investigated bio-electrochemical methods to recover ammonia-N, for further production of MP. Reject water and digestate were used for ammonia-N recovery in microbial fuel cell (MFC) system. In one-stage process, ammonia-N recovery was 32 - 42% with 57 - 154 kJ/m³ waste stream of electricity generation. For further enhancing recovery efficiency, a two-stage process was developed, achieving efficiency of 53 - 61%. Subsequently, MP was grown with the extracted ammonia-N, and amino acid concentration was 421 and 272 mg/L under 25 °C and 35 °C, respectively. Similar essential amino acid content of MP (especially under 25 °C) with the one from fish demonstrated the attractiveness of upcycling residues to proteins. Based on simplified economic evaluation, the produced energy performed the potential to catch 1.63 - 6.54 €/m³ waste stream.

Comprehensive evaluation of different strategies to recover methanogenic performance in ammonia-stressed reactors

In this study, strategies for recovery of ammonia-stressed AD reactors were attempted, by addition of preserved bioaugmentation consortium in gel (BioG), fresh consortium in liquid medium (BioL), woodchip biochar (BW), and straw biochar (BS). In comparison to control group with ammonia, effective treatments, i.e., BioG, BioL, BW and BS raised the maximum methane production rate by 77%, 23%, 35%, and 24%, respectively. BW possibly acted as interspecies electrical conduits for Direct Electron Transfer based on conductivity and SEM analysis. BioG facilitated slow release of bioaugmentation inocula from gel into the AD system, which protected them from a direct environmental shock. According to microbial analysis, both BioG, BioL and BW resulted in increased relative abundance of Methanothermobacter thermautotrophicus; and BS induced selective raise of Methanosarcina thermophila. The increase of methanogens via these strategies led to the faster recovery of the AD process.

A novel persulfate-photo-bioelectrochemical hybrid system promoting the degradation of refractory micropollutants at neutral pH

Bio-electro-Fenton is emerging as an alternative technology for the efficient and cost-effective removal of refractory micropollutants. Though promising, there are still several challenges that limit its wide application, including acidic operating conditions (pH at 2-3), the addition of supporting electrolytes (e.g., Na₂SO₄), and the issue of iron sludge generation. To address these challenges, a novel hybrid persulfate-photo-bioelectrochemical (PPBEC) system is proposed to remove model micropollutants (carbamazepine and clorfibric acid), from secondary effluent at low persulfate (PS) dosage and neutral pH. The effect of crucial operating parameters on the process was studied, including input voltage, cathodic aeration velocity, and PS dose. Under optimal conditions (0.6 V, 0.005 mL min^-1 mL^-1 and 1 mM), the PPBEC system achieved approx. 0.56-1.71 times greater micropollutant removal with 93% lower energy consumption when compared to the individual processes (UV/PS and PBEC). The improved performance was attributed to a faster production of sulfate radicals by UV irradiation, hydrogen peroxide activation and single-electron reduction, and hydroxyl radicals generated by UV irradiation. Furthermore, the transformation products of carbamazepine and clorfibric acid were identified and the probable pathways are proposed. Finally, the ecotoxicity of the PPBEC treated effluent was assessed by using Vibrio Fischeri, which exhibited a non-toxic effect.

Effects of organic loading rate and hydraulic retention time on bioaugmentation performance to tackle ammonia inhibition in anaerobic digestion

Three continuously stirred-tank reactors fed with manure operating under high ammonia levels (5.0 g NH3-N L-1) and with increased organic loading rate (OLR), (2.09 R1, 3.02 R2 and 4.0 R3 g VS L-1 d-1), achieved through glucose amendment in R2 and R3, were inoculated with an ammonia-acclimatized microbial culture. Successful bioaugmentation was endured only in R2 and R3, both reactors characterized by high OLR, resulting in 19.6 and 24.5% increase in methane production, respectively. The high OLRs in these reactors favored the co-occurrence of the hydrogenotrophic (Methanobacteriaceae), methylotrophic (Methanomethylophilaceae) and aceticlastic methanogenic pathways. The latter was supported by the successful establishment of ammonium-tolerant Methanosarcina, prevailing in the inoculum. Oppositely in R1, the low OLR prevented the establishment of Methanosarcina, leading to an exclusive hydrogenotrophic methanogenesis and reduced methane production. HRT shortening resulted in limited effect on biomethane performance, indicating a well establishment of the introduced bioaugmentation culture in the reactors.

Microbial dynamics in biogas digesters treating lipid-rich substrates via genome-centric metagenomics

Co-digestion with lipid-rich substrates is a likely strategy in biogas plants, due to their high energy content. However, the process stability is vulnerable to inhibition due to the sudden increase of fatty-acid concentration. Therefore, techniques that promote the adaptation of the microorganisms to the presence of lipids have been proposed. In this frame, the initial hypothesis of the work was that a gradual change in feedstock composition would enable us to elucidate the microbial organisation as a result of deterministic (i.e. chemical composition of influent) and stochastic (e.g. interspecies interactions) factors. This study investigates the response of the biogas microbiome to gradual increment of the Organic Loading Rate by supplementing the influent feedstock with Na-Oleate. The results showed that as a response to the feedstock shifts three clusters describing microbes behaviours were formed. The dynamics and the functional role of the formed microbial clusters were unveiled, providing explanations for their abundance and behavior. Process monitoring indicated that the reactors responded immediately to lipid supplementation and they managed to stabilize their performance in a short period of time. The dominance of Candidatus Methanoculleus thermohydrogenotrophicum in the biogas reactors fed exclusively with cattle manure indicated that the predominant methanogenic pathway was hydrogenotrophic. Additionally, the abundance of this methanogen was further enhanced upon lipid supplementation and its growth was supported by syntrophic bacteria capable to metabolize fatty acids. However, with the shift back to the original feedstock (i.e. solely cattle manure), the microbial dynamicity significantly altered with a remarkable increment in the abundance of a propionate degrader affiliated to the order of Bacteroidales, which became the predominant microorganism of the consortium.

Genome-Centric Metatranscriptomics Analysis Reveals the Role of Hydrochar in Anaerobic Digestion of Waste Activated Sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) has been widely used, while it poses problems including low methane yield and production rate. Hydrochar is produced by hydrothermal liquefaction of biomass; however, little is known about the role of hydrochar in promoting AD of WAS. The present study showed that hydrochar increased the methane production rate by 30.8% and yield by 31.4% of hydrothermal pretreated dewatered WAS. Hydrochar increased the methane production rate and yield by enhancing the acidification and methanogenesis processes. Genomic-centric metatranscriptomics were used to identify the metabolic activities and transcriptomic response of individual metagenome-assembled genomes that were enriched by hydrochar. Although Methanosarcina sp. FDU0106 had been shown unable to used H₂, it had the complete pathway for the reduction of CO₂ to methane. Syntrophomonas sp. FDU0164 expressed genes for extracellular electron transfer via electrically pili, suggesting that Syntrophomonas sp. FDU0164 and Methanosarcina sp. FDU0106 were exchanging electrons via direct interspecies electron transfer. The expression of pili was decreased, indicating that hydrochar could replace its roles. Additionally, Firmicutes sp. FDU0048, Proteiniphilum sp. FDU0082, and Aminobacterium mobile FDU0089 were related to the degradation of organics, which could be related to the enhanced methane yield.

Evolution of the microbial community structure in biogas reactors inoculated with seeds from different origin

The aim of this work was to provide solid proofs regarding the achievement of "steady-state conditions", which means that the performance of the anaerobic digester is representative of the applied environmental conditions. For this reason, we investigated how, starting from different inoculum sources (i.e., municipal wastewater treatment, bio-waste treatment, and agricultural waste biogas plant), the microbial community adapted to the operational parameters and led to stable biogas production in thermophilic digesters treating the same influent feedstock. The results revealed that the different system achieved similar process performance and microbial community structure after a period that was equal to four hydraulic retention times, approved by a constant pH of 7.89 ± 0.08, 7.92 ± 0.05 and 7.85 ± 0.08, respectively, and stable TAN concentration of 1500 mg/L. Moreover, it was found that the microbial composition of the inocula was a key factor for the speed of achieving stable process performance; thus, a pre-adapted to the influent feedstock inoculum can shorten the stabilization process. On the contrary, after long term reactor operation, the microbial structure was shaped according to the chemical composition of the influent feedstock. The results of the study can also be used as a guide in future researches on anaerobic degradation, particularly in determining the time interval of an experiment to reflect changes in the microbial community of anaerobic digester.

Degradation of metoprolol from wastewater in a bio-electro-Fenton system

Advanced oxidation processes (AOPs) have been intensely studied for the removal of refractory pollutants because of the strong oxidizing capacity of hydroxyl radical. One of the emerging AOP methods gaining increased attention is bio-electro-Fenton (BEF) which can generate hydroxyl radical in-situ in the cathode chamber using the energy harvested by exoelectrogenic bacteria in the anode. In this study, the feasibility of BEF technology for the removal of metoprolol, a typical micropollutant widely found in the water environment, was for the first time investigated. It was found that applied voltage and working pH had a significant effect on removal efficiency while Fe²⁺ dosage as catalyst showed a little effect. Besides removal by hydroxyl radical, metoprolol might be adsorbed on the surface of the reactor, electrode, and precipitated with iron sludge, especially at neutral pH. In a batch experiment with a supplied voltage of 0.2 V, pH 3, and a Fe²⁺ dose of 0.2 mM, the removal rate of metoprolol in the BEF for the synthetic wastewater and the real effluent from the secondary sediment tank was 66% and 55% within 12 h, respectively. A possible degradation pathway was proposed. Then the removal of metoprolol in a continuous flow BEF system was further studied at different hydraulic retention times (HRTs) of 2, 4, and 6 h, about 77%, 92%, and 95% removal was observed. A toxicity test (less than 20% inhibition on bioluminescence) during treatment and energy cost analysis (5.269 × 10^-3 kWh/order/m³) in treating 10 μg/L of metoprolol containing wastewater effluent at continuous flow mode implied that the proposed BEF has a potential for wastewater treatment.

Innovative air-cathode bioelectrochemical sensor for monitoring of total volatile fatty acids during anaerobic digestion

Bioelectrochemical sensors have proven attractive as simple and low-cost methods with high potential for online monitoring of volatile fatty acids (VFA) in the anaerobic digestion (AD) process. Herein, an innovative dual-chamber air-cathode microbial fuel cell was developed as biosensor for VFA monitoring. The response of the biosensor was nonlinear and increased along with the concentration of VFA mixture increase (2.8-112 mM). Meanwhile, the relationship was linear with low VFA levels (<14 mM) within 2-5 h reaction. High concentrations of bicarbonate decreased the voltage. Stirring speeded up the response and amplified the signal but reduced the saturation concentration (approximately 30 mM) and therefore narrowed the detection range. The applicability of the biosensor was further validated with the effluents from an AD reactor during a start-up period. The VFA concentrations measured by the biosensor were well correlated with the gas chromatographic measurement. The results demonstrate that this biosensor with a novel design could be used for VFA monitoring during the AD process. Based on the 16S rRNA gene sequencing, the dominant microbiomes in the biofilm were identified as Geobacter, Hydrogenophaga, Pelobacter, Chryseobacterium, Oryzomicrobium, and Dysgonomonas.