Insights into microbial community profiles associated with electric energy production in microbial fuel cells fed with food waste hydrolysate

Electrochemical process for petroleum refinery wastewater treatment to produce power and hydrogen using microbial electrolysis cell

This research aims to assess the microbial electrolysis cell (MEC) fed with petroleum refinery wastewater (PRW) to produce power density and bio-electrochemical hydrogen. The MEC produces a maximum bio-electricity of 21.4 mA and a power density of 1200123.90 W/m2 with a loading of chemical oxygen demand (COD) of 17000 mg/L. Due to catalyzed oxidation of complex compounds in PRW with a maintained microbial biofilm growth was observed after 90 d of operation of MEC. Results showed that the oxidation of organic substances in PRW enhanced the size in the growth of microbial film which further increased the generation of electrons leading to current density of 5890 mA/m2. The COD removal efficiency of MEC was found to be 89.9%. The bio-electricity and hydrogen production of the MEC was estimated to be 24.5 mA and 19.2 L respectively when loaded with PRW having a COD of 17500 mg/L after 130 d. Present experiments demonstrate the efficiency of MEC technology efficiency in treating petroleum wastewater with the help of microbial biofilm.

Treatment of Organic and Sulfate/Sulfide Contaminated Wastewater and Bioelectricity Generation by Sulfate-Reducing Bioreactor Coupling with Sulfide-Oxidizing Fuel Cell

A wastewater treatment system has been established based on sulfate-reducing and sulfide-oxidizing processes for treating organic wastewater containing high sulfate/sulfide. The influence of COD/SO42- ratio and hydraulic retention time (HRT) on removal efficiencies of sulfate, COD, sulfide and electricity generation was investigated. The continuous operation of the treatment system was carried out for 63 days with the optimum COD/SO42- ratio and HRT. The result showed that the COD and sulfate removal efficiencies were stable, reaching 94.8 ± 0.6 and 93.0 ± 1.3% during the operation. A power density level of 18.0 ± 1.6 mW/m2 was obtained with a sulfide removal efficiency of 93.0 ± 1.2%. However, the sulfide removal efficiency and power density decreased gradually after 45 days. The results from scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) show that sulfur accumulated on the anode, which could explain the decline in sulfide oxidation and electricity generation. This study provides a promising treatment system to scale up for its actual applications in this type of wastewater.

High concentration of ammonia sensitizes the response of microbial electrolysis cells to tetracycline

Microbial electrolysis cell (MEC) is a promising technology for effective energy conversion of wastewater organics to biogas. Yet, in swine wastewater treatment, the complex contaminants including antibiotics may affect MEC performance, while the high ammonia concentration might increase this risk by increasing cell membrane permeability. In this work, the responses of MECs on tetracycline (TC) with low and high ammonia loadings (80 and 1000 mg L^-1) were fully investigated. The TC of 0 to 1 mg L^-1 slightly improved MEC performance in current production and electrochemical characteristics with low ammonia loading, while TC ≥ 4 mg L^-1 started to show negative effects. Generally, the high ammonia loading sensitized MECs to TC concentration, inducing the current and COD removal of MECs to sharply decline with TC ≥ 0.5 mg L^-1. The positive effect of high ammonia loading on MEC due to conductivity increase was counteracted with TC ≥ 1 mg L^-1. The co-contamination of TC and ammonia significantly decreased the bioactivity and biomass of anode biofilm. Although the high concentration of co-existing TC and ammonia inhibited MEC performance, the reactors still obtained positive energy feedback. The network analyses indicated that the effluent suspension contributed much to antibiotic resistance gene (ARG) transmission, while the microplastics (MPs) in wastewater greatly raised the risks of ARGs spreading. This work systematically examined the synergetic effects of TC and ammonia and the transmission of ARGs in MEC operation, which is conducive to expediting the application of MECs in swine wastewater treatment.

Microbial fuel cells for bioelectricity production from waste as sustainable prospect of future energy sector

Microbial fuel cell (MFC) is lauded for its potentials to solve both energy crisis and environmental pollution. Technologically, it offers the capability to harness electricity from the chemical energy stored in the organic substrate with no intermediate steps, thereby minimizes the entropic loss due to the inter-conversion of energy. The sciences underneath such MFCs include the electron and proton generation from the metabolic decomposition of the substrate by microbes at the anode, followed by the shuttling of these charges to cathode for electricity generation. While its promising prospects were mutually evinced in the past investigations, the upscaling of MFC in sustaining global energy demands and waste treatments is yet to be put into practice. In this context, the current review summarizes the important knowledge and applications of MFCs, concurrently identifies the technological bottlenecks that restricted its vast implementation. In addition, economic analysis was also performed to provide multiangle perspectives to readers. Succinctly, MFCs are mainly hindered by the slow metabolic kinetics, sluggish transfer of charged particles, and low economic competitiveness when compared to conventional technologies. From these hindering factors, insightful strategies for improved practicality of MFCs were formulated, with potential future research direction being identified too. With proper planning, we are delighted to see the industrialization of MFCs in the near future, which would benefit the entire human race with cleaner energy and the environment.

Butyrate production and purification by combining dry fermentation of food waste with a microbial fuel cell

This study developed and evaluated a high-purity butyrate producing bioprocess from food waste by combining dry fermentation (DF) with a microbial fuel cell (MFC). Acclimatization of a DF reactor with an enrichment culture resulted in high food waste degradation (VS removed, %) and butyrate production. A high VS degradation of 81%, butyrate concentration of up to 24 gCOD_butyrate/L and butyrate yields of 497 gCOD_butyrate/kg VS_added was obtained in the DF reactor. As a result, butyrate comprised 83% of all short chain fatty acids (SCFA) in the DF broth. Acetate (10%) and propionate (7%) comprised the rest of the SCFA. The butyrate composition was further purified by feeding the DF broth to a multi-electrode MFC enriched with anode respiring bacteria (ARB) such as Geobacter sp. (>55%). The ARB in the MFC removed acetate and propionate while purified butyrate was recovered in the MFC effluent. Butyrate purity in the MFC effluent reached as high as 99% at hydraulic retention time of 72 h. Along with butyrate purification, the MFC produced electric power in a range of 0.1-0.6 Wh/gCODbutyrate_recovered (or 0.01-7.85 kWh/ton of food waste), demonstrating that MFCs can be an energy-positive butyrate purification bioprocess.

A multi-perspective review on microbial electrochemical technologies for food waste valorization

The worldwide generation of food waste (FW) has been increasing enormously due to the growing food industry and population. However, FW contains a large amount of biodegradable organics that can be converted to clean energy, which can potentially minimize the utilization of fossil fuels. Conventional biowaste valorization technologies, such as anaerobic digestion and composting, have been adopted for FW management for recovering useful biogas and compost. However, they are often limited by high capital and operation costs, low recovery efficiency, slow process kinetics, and system instability. On the other hand, microbial electrochemical technologies (METs) have been highly promising for efficiently harvesting bioenergy and high value-added products from FW. Hence, this article critically reviews up-to-date studies on applying various METs regarding their value-added products recovery efficiencies from FW. Moreover, this review lists existing challenges, ways to optimize the system performance and provides perspectives on future research needs.

Synergistic pretreatment of CaO and freezing/thawing to enhance volatile fatty acids recycling and dewaterability of waste activated sludge via anaerobic fermentation

To avoid the generally deteriorated dewaterability of sludge in waste activated sludge (WAS) anaerobic acidogenesis, the combination of varied calcium oxide (CaO) dosage (i.e., 0.01-0.07 g/g TS) and freezing/thawing pretreatment (5 F/T cycles) was investigated for concurrently improving the production of volatile fatty acids (VFAs) and dewatering performance of sludge. The highest release of soluble chemical oxygen demand (SCOD) (1836 ± 96 mg/L) and accumulation of VFAs (448.0 mg COD/g VS) were reached through the co-pretreatment of CaO (0.07 g/g TS) and F/T (50 h at -24 °C) (i.e., 0.07 CaO-F/T). Meanwhile, optimal dewaterability of sludge was also achieved in 0.07 CaO-F/T co-pretreated WAS fermentation, which was reflected by large particle size (98.32 μm), low capillary suction time (41.6 s), decreased specific resistance to filtration (SRF, reduced 47.5% against blank) and high zeta potential (-9.59 mV). Co-pretreatment of CaO and F/T reduced the species number of total microbial population, but improved the abundance of acid-producing bacteria. Increased abundance of Bacteroides, Macellibacteroides, Petrimonas, Prevotella, Clostridium, and Megasphaera was positively relevant to the high yields of VFAs. The economic analysis indicated that CaO-F/T was economically acceptable with considerable estimated net profits, which provided a feasible and efficient alternative for further WAS treatment at large scale.

Linking microbial mechanism with bioelectricity production in sludge matrix-fed microbial fuel cells: Freezing/thawing liquid versus fermentation liquor

This first-attempt study elucidated the microbial mechanism associated with bioelectricity output in microbial fuel cells (MFCs) fed with sludge matrices of freezing/thawing (F/T) liquid versus fermentation liquor, while a novel schematic elucidation for exploring cooperative interactions in anodic microbial consortia of MFCs supplied with such two feeds toward electrogenesis was put forward. Moreover, the F/T liquid cultivated main genera of Azospira, Povalibacter, Thauera, Terrimonas, Alicycliphilus, Dokdonella and Simplicispira for dual organics degradation and electrogenesis with power density of 0.152 mW/m² and electrogenesis efficiency of 1.152 kWh/kg COD, while the fermentation liquor fostered higher diversity and medium evenness with the enrichment of Phenylobacterium, Cellulomonas, Edaphobacter, Burkholderia, Clostridium, Sphingomonas, Leifsonia and Microbacterium in anodic biofilm and causing larger power density of 0.182 mW/m² and 1.418 kWh/kg COD-electrogenesis efficiency. Comparative analysis results indicated that the anodic fermentative bacteria exert considerable influence on concurrent organics degradation and electricity production through the synergistic interactions with exoelectrogens toward stable running of MFCs. Besides, the higher anodic microbial diversity, relatively middling community evenness and larger abundance of functional genes associated with electrogenesis together played contributive roles on more power generation through MFCs for treating WAS matrix. This study was conducive to bring about some new microbial mechanism understanding on maximizing bioenergy recovery via MFCs in future sludge management.

Waste valorization using solid-phase microbial fuel cells (SMFCs): Recent trends and status

This review article discusses the use of solid waste processed in solid-phase microbial fuel cells (SMFCs) as a source of electrical energy. Microbial Fuel Cells (MFCs) are typically operated in the liquid phase because the ion transfer process is efficient in liquid media. Nevertheless, some researchers have considered the potential for MFCs in solid phases (particularly for treating solid waste). This has promise if several important factors are optimized, such as the type and amount of substrate, microorganism community, system configuration, and type and number of electrodes, which increases the amount of electricity generated. The critical factor that affects the SMFC performance is the efficiency of electron and proton transfer through solid media. However, this limitation may be overcome by electrode system enhancements and regular substrate mixing. The integration of SMFCs with other conventional solid waste treatments could be used to produce sustainable green energy. Although SMFCs produce relatively small amounts of energy compared with other waste-to-energy treatments, SMFCs are still promising to achieve zero-emission treatment. Therefore, this article addresses the challenges and fills the gaps in SMFC research and development.

Insights into microbial interaction profiles contributing to volatile fatty acids production via acidogenic fermentation of waste activated sludge assisted by calcium oxide pretreatment

This first-attempted study illustrated the calcium oxide (CaO) agentia-pretreatment for prompting waste activated sludge (WAS) solubilization and enhancing volatile fatty acids (VFAs) bio-production through acidogenic fermentation. The 15-h CaO pretreatment was capable to produce a soluble chemical oxygen demand (SCOD) yield of ca. 153.17 mg COD/g VS and VFAs generation efficiency of 327.8 mg COD/g VS with adding dosage of 0.07 g/g TS. The relative frequencies corresponded to metabolic functions profiling were promoted obviously by CaO pretreatment and contributed to biosolid decomposition/VFAs production in sludge fermentation. Main genera of Azonexus, Arcobacter, Acinetobacter, Thauera, Petrimonas, Clostrium and Macellibacteroides cooperated synergically toward triggering concurrent VFAs generation/biosolid biodegradation. Finally, the CaO-pretreatment displayed positive merits in terms of sludge biosolid decomposition/recoverable resource harvest as compared with other alkali pretreatments. This study might shed lights on enriching intensification strategy for WAS management toward high-efficiency of recoverable resource harvest with lower cost.

Insights into redox mediators-resource harvest/application with power production from waste activated sludge through freezing/thawing-assisted anaerobic acidogenesis coupling microbial fuel cells

This first-attempted study demonstrated endogenous redox-mediators harvest/application from waste activated sludge (WAS) through freezing/thawing (F/T) pretreatment-enhanced anaerobic acidogenesis coupled with microbial fuel cells (MFCs). A total of 2.57 kWh electricity was produced from per kg soluble chemical oxygen demand (SCOD) via MFCs just in 2 d with about 90% organics removal, which contained 1.152 kWh/kg COD from F/T liquid together with 1.418 kWh/kg COD from fermentation liquid. The fermentation liquor-MFCs fostered higher anodic biodiversity and more power output as compared with the F/T liquid-MFCs. Essentially, the completely endogenous redox mediators-like substances with relatively high redox activities could be retained after MFC electrogenesis from F/T liquid and played electron shuttle-roles sufficiently in enlarging bio-energy production of MFCs, which seemed to be an effective option for harvesting endogenous redox mediators from sludge. This study might inspire progressive thinking toward aims of high-efficiency of resource recycle/bioenergy production from WAS.

Revealing microbial mechanism associated with volatile fatty acids production in anaerobic acidogenesis of waste activated sludge enhanced by freezing/thawing pretreatment

This study investigated the association of volatile fatty acid (VFA) production with microbial mechanism in waste activated sludge (WAS) anaerobic acidogenesis enhanced by freezing/thawing (F/T) pretreatment. WAS solubilization was enhanced with 955.4 ± 10.0 mg/L soluble chemical oxygen demand (SCOD) release by a 50-h F/T pretreatment at -24 °C. The highest level of VFAs (4852 ± 156 mg COD/L) was obtained after a 12-day fermentation. Moreover, phyla of Proteobacteria, Bacteroidetes, Firmicutes, and Ignavibacteriae played vital roles in VFA generation, while high genera abundance of Clostridium, Macellibacteroides, Prevotella, and Megasphaera were positively associated with high yields of short-chain (C2-C5) fatty acids. A schematic diagram was drawn to illustrate the microbial mechanism of enhanced VFA generation by F/T pretreatment during WAS fermentation. This study provides an in-depth exploration of promoting bio-resource recycling from WAS with a low-cost approach (specially in high latitudes) and bring about some new thinking on future WAS management.

Shift of biofilm and suspended bacterial communities with changes in anode potential in a microbial electrolysis cell treating primary sludge

This study, for the first time, documented microbial community shifts in response to the changes in anode potential in a microbial electrolysis cell (MEC) operated with primary sludge. At an anode potential of -0.4 V vs. Ag/AgCl, the MEC showed COD and VSS removal efficiencies of 73 ± 1% and 75 ± 2%, respectively. The volumetric current density and specific hydrogen production rate were 23 ± 1.2 A/m³, and 145 ± 4.1 L/m³-d, respectively. The anodic microbial community was consisted of various fermentative/hydrolytic bacteria (e.g., Bacteroides and Dysgonomonas) and anode-respiring bacteria (Geobacter), while different hydrolytic/fermentative bacteria were abundant in suspension. The MEC showed substantially inferior performance along with a higher accumulation of various volatile fatty acids when the anode potential was switched to more positive values (0 V and +0.4 V). Both biofilms and suspended communities were also shifted when the anode potential was changed. Notably, at +0.4 V, Geobacter genus entirely disappeared from the biofilms, while Paludibacter species (known fermentative bacteria) were selectively enriched in biofilms. Also, the relative abundance of genus Bacteroides (known hydrolytic bacteria) substantially decreased in both biofilms and suspension, which was correlated with the inferior hydrolysis of VSS. Quantitative comparison of biofilms and suspended microbial communities at different anode potentials revealed a sharp decrease in bacterial cell numbers in anode biofilms after changing anode potential from -0.4 V to +0.4 V. By contrast, bacterial cell numbers in suspension were slightly decreased. Collectively, these results provide new insights into the role of anode potential in shaping key microbial players associated with hydrolysis/fermentation and anodic respiration processes when MECs are operated with real biowastes.

Unraveling interactive characteristics of microbial community associated with bioelectric energy production in sludge fermentation fluid-fed microbial fuel cells

This first-attempt study deciphered the interactive characteristics of anodophilic microbial community-associated bioelectricity production in waste activated sludge (WAS) fermentation fluid-fed microbial fuel cells (MFCs). A novel schematic elucidation for illustrating synergistic interactions in anodic microbial consortia towards electrogenesis was proposed. Moreover, the specific genera of Pseudomonas, Desulfovibrio, Phyllobacterium, Desulfuromonas, Chelatococcus and Aminivibrio were dominant in anodic biofilms, leading to an electrogenesis efficiency of 1.254 kWh/kg COD and peak power density of 0.182 W/m² (at feeding level of 1.20 g COD/L). It was apparently higher than those MFCs fed with glucose/acetate. The fermentative species contributed positively in reorganizing microbial community structure in anodic biofilms, positively relating to electrogenesis via interactions with exoeletrogens in MFCs. Finally, a more electrogenesis was positively associated to larger anodic microbial diversity, relatively medium anodic community evenness, together with higher abundance of functional genes related to electrogenesis in functional species in MFCs fed with WAS fermentation fluid.