Promoting direct interspecies electron transfer and acetoclastic methanogenesis for enhancing anaerobic digestion of butanol octanol wastewater by coupling granular activated carbon and exogenous hydrogen

Insight into the performance and microbial community of anaerobic digestion treating cow manure with a novel iron-functionalized activated biochar

The main objective of this research was to evaluate the impacts of FeCl3-activated biochar (FA-BC) on anaerobic digestion (AD) treating cow manure. The study focused on improving AD performance and understanding microbial community structure with the addition of FA-BC, while comparing FA-BC with other conductive additives, such as pristine biochar (P-BC), NaOH-activated biochar (NA-BC), and magnetite. Key findings indicated that FA-BC significantly enhanced the AD performance, supported by an increase in CH4 yield of 11-16% and a reduction in the lag phase by 51%. The high surface area and electrical conductivity of FA-BC synergistically facilitated direct interspecies electron transfer (DIET), leading to these improvements. On contrast, P-BC and NA-BC were not efficient in enhancing the AD performance due to relatively low electrical conductivity. P-BC also improved the CH4 yield, but less effectively than FA-BC. The effects of NA-BC varied with its dosage, showing inhibition at higher dosages due to excessive surface area. Magnetite, despite its high conductivity, made the limited enhancement in CH4 yield owing to its low surface area. Additionally, the statistical analyses revealed that each additive differently affected specific bacterial and archaeal groups depending on their physical and chemical properties. Thus, these findings suggest that FA-BC would be a highly promising additive for enhancing AD systems, with potential applications in waste management and renewable energy production.

Influence of oxygen partial pressure on homoacetogenesis and promotion of acetic acid accumulation through low pH regulation under microaerobic conditions

Homoacetogenesis is an important pathway for bio-utilization of CO2; however, oxygen is a key environmental influencing factor. This study explored the impact of different initial oxygen partial pressures (OPPs) on homoacetogenesis, while implementing low pH regulation enhanced acetic acid (HAc) accumulation under microaerobic conditions. Results indicated that cumulative HAc production increased by 18.2% in 5% OPP group, whereas decreases of 31.3% and 56.0% were observed in 10% and 20% OPP groups, respectively, compared to the control group. However, hydrogenotrophic methanogens adapted to microaerobic environment and competed with homoacetogens for CO2, thus limiting homoacetogenesis. Controlling influent pH 5.0 per cycle increased cumulative HAc production by 18.3% and 18.2% in 5% and 10% OPP groups, respectively, compared with the control group. Consequently, regulating low pH effectively inhibited methanogenic activity under microaerobic conditions, thus increasing HAc production. This study was expected to expand the practical application of homoacetogenesis in bio-utilization of CO2.

New attempts on acidic anaerobic digestion of poly (butylene adipate-co-terephthalate) wastewater in upflow anaerobic sludge blanket reactor

Poly (butylene adipate-co-terephthalate) (PBAT) wastewater is a highly concentrated, acidic, and toxic wastewater generated from biodegradable plastics production. Large amounts of alkali would be consumed when treating PBAT wastewater by anaerobic digestion due to the low pH value. This study employed acidic anaerobic digestion to treat PBAT wastewater and compared to neutral anaerobic digestion. The results indicated that the COD removal rates in the acidic upflow anaerobic sludge blanket (UASB) reactor were 65.5% and 59.9%, respectively at influent pH 6.0 and 5.0 with the COD concentration of around 11,000 mg L-1, and the methane conversion efficiency were 172.5 and 183.8 mLCH4/gCODr (gCODr: COD removed amount), respectively. Correspondingly, the average COD removal and methane conversion efficiency in the neutral UASB reactor were 63.2% and 188.0 mLCH4/gCODr, respectively. The treatment efficiency of acidic and neutral UASB reactors for PBAT wastewater was similar. The hydrogenotrophic methanogenic activity was further enhanced in the acidic UASB reactor compared to the neutral one. The increase of alkalinity in the acidic UASB reactor (2.4 mmol L-1) was higher than the neutral (2.0 mmol L-1). A higher level of syntrophic acetate oxidation bacteria and hydrogenotrophic methanogen was enriched in the acidic UASB reactor, ensuring efficient treatment and saving costs.

Characteristics of dissolved organic matter in point-source wastewaters at a petrochemical plant: Molecular constituents and contributions to the influent of wastewater treatment plant

Dissolved organic matter (DOM) in point-source petrochemical wastewaters (PCWs) from different operating units is closely linked to the efficiency of wastewater treatment plant (WWTP). However, systematic studies on DOM characters of point-source PCWs and their influences on WWTP influents were seldom conducted. In this study, DOM in three low-salinity point-source PCWs and four high-salinity point-source PCWs at a typical petrochemical plant were comprehensively characterized at a molecular level. Orbitrap mass spectrometry results indicated that point-source PCWs had diverse DOM constituents tightly related to the corresponding petrochemical processes. Phenols in oily wastewaters (OW), phenols and N-containing compounds in coal partial oxidation wastewater (POXW), and naphthenic acids (NAs) and aromatic acids in crude oil electric desalting unit wastewater (EDW) were characteristic DOM constituents for low-salinity point-source PCWs. While S-containing compounds (mercaptans, thiophenes) and NAs in spent caustic liquors (SCL), alcohols and esters in butanol-octanol plant wastewater (BOW), high molecular weight aromatic ketones in phenol-acetone plant wastewater (PAW), and oxygenated NAs as well as short chain N-containing compounds in concentrate from reverse osmosis unit (ROC) were characteristic DOM constituents for high-salinity point-source PCWs. Spearman correlation analysis indicated that though with relative low pollutant contents (OW) and discharge volume (EDW), N/O/S-containing compounds of OW and EDW greatly contributed to the polar DOM constituents of low-salinity influent in WWTP (R > 0.5, P < 0.001). While N-containing compounds of ROC mainly contributed to the polar DOM of high-salinity influent (R > 0.5, P < 0.001). Though N-/S-containing species in PAW had low contents, they also posed obvious impacts on DOM constituents of high-salinity influent. Interestingly, some O-/S-containing species were newly formed during the confluent process of high-salinity point-source PCWs. The results strengthened the combined contributions of pollutants contents, discharge emission and DOM constituents of point-source PCWs to the water matrix of WWTP influents, which would provide reference for the management of PCW streams.

Performance of anaerobic digestion of phenol using exogenous hydrogen and granular activated carbon and analysis of microbial community

Anaerobic conversion rate of phenol to methane was low due to its biological toxicity. In this study, the coupling of granular activated carbon (GAC) and exogenous hydrogen (EH) could enhance greatly methane production of phenol anaerobic digestion, and the metagenomic was firstly used to analyze its potential mechanism. The results indicated that a mass of syntrophic acetate-oxidizing bacteria and hydrogen-utilizing methanogens were enriched on the GAC surface, and SAO-HM pathway has become the dominant pathway. The energy transfer analysis implied that the abundance of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH) oxidase increased. Furthermore, direct interspecies electron transfer (DIET) was formed by promoting type IV e-pili between Methanobacterium and Syntrophus, thereby improving the interspecies electron transfer efficiency. The dominant SAO-HM pathway was induced and DIET was formed, which was the internal mechanism of the coupling of GAC and EH to enhance anaerobic biotransformation of phenol.

Strategies for enhancing the efficacy of anaerobic digestion of food industry wastewater: An insight into bioreactor types, challenges, and future scope

Food waste have become a growing concern worldwide with raising population and economic growth. Wastewater discharged from food industries contains many valuable and toxic components that have a negative impact on the ecological system. Large amounts of wastewater are discharged from the food industry, which necessitates the creation of effective technologies. Wastewater from the food industry can be seen as a rich source of energy and a primary source for generating valuable products. Waste disposal and resource recovery are sustainably valued by anaerobic digestion of wastewater from the food sector. The characteristics, composition, and nature of wastewater produced from various food sectors are elaborated upon in this review. An overview of the anaerobic digestion process for wastewater treatment in the food industry is included. Enhancement strategies for the anaerobic digestion process have been discussed in detail. In addition, various types of reactors utilized for performing anaerobic digestion is illustrated. Though anaerobic digestion process possesses advantages, the challenges and future scope are examined for improving the outcome.

Enhanced direct interspecies electron transfer and methane production during anaerobic digestion of fat, oil, and grease by coupling carbon-based conductive materials and exogenous hydrogen

To investigate the combination of carbon-based conductive materials and exogenous hydrogen (EH2) on methane recovery from fat, oil, and grease (FOG), granular activated carbon (GAC) and carbon cloth (CC) were chosen to collaborate with EH2, resulting in increased methane production by 59 % and 84 %, respectively. Further digestion of long chain fatty acids (LCFAs) confirms that enhanced direct interspecies electron transfer (DIET) was achieved in the reactors with GAC/CC + EH2 than those with GAC/CC only. Other evidences (such as increased microbial population and rapid degradation of volatile fatty acids) were found to support the role of GAC/CC + EH2 in promotion of DIET. Significant change of microbial community was observed using GAC/CC + EH2, which was mainly attributed to the enrichment of electrogenic species (such as Spirochaetaceae, Syntrophomonas palmitatica, and Methanosaeta), leading to some changes in metabolic pathways during acidogenesis and methanogenesis. Together, enhanced DIET was achieved by GAC/CC + EH2, thus improving the methane recovery from FOG.

Anaerobic recalcitrance in wastewater treatment: A review

Anaerobic treatment is applied as an alternative to traditional aerobic treatment for recalcitrant compound degradation. This review highlighted the recalcitrant compounds in wastewaters and their pathways under aerobic and anaerobic conditions. Forty-one recalcitrant compounds commonly found in wastewater along with associated anaerobic removal performance were summarized from current research. Anaerobic degradability of wastewater could not be appropriately evaluated by BOD/COD ratio, which should only be suitable for determining aerobic degradability. Recalcitrant wastewaters with a low BOD/COD ratio may be handled by anaerobic treatments after the adaption and provision of sufficient electron donors. Novel indicator characterizing the anaerobic recalcitrance of wastewater is called for, essential for emergent needs to resource recovery from high-strength recalcitrant wastewater for fulfilling appeals of circular bioeconomy of modern societies.

Enhanced methane production in anaerobic digestion: A critical review on regulation based on electron transfer

Anaerobic digestion (AD) is a potential bioprocess for waste biomass utilization and energy conservation. Various iron/carbon-based CMs (e.g., magnetite, biochar, granular activated carbon (GAC), graphite and zero valent iron (ZVI)) have been supplemented in anaerobic digestors to improve AD performance. Generally, the supplementation of CMs has shown to improve methane production, shorten lag phase and alleviate environmental stress because they could serve as electron conduits and promote direct interspecies electron transfer (DIET). However, the CMs dosage varied greatly in previous studies and CMs wash out remains a challenge for its application in full-scale plants. Future work is recommended to standardize the CMs dosage and recover/reuse the CMs. Moreover, additional evidence is required to verify the electrotrophs involved in DIET.

Electron shuttles enhanced the removal of antibiotics and antibiotic resistance genes in anaerobic systems: A review

The environmental and epidemiological problems caused by antibiotics and antibiotic resistance genes have attracted a lot of attention. The use of electron shuttles based on enhanced extracellular electron transfer for anaerobic biological treatment to remove widespread antibiotics and antibiotic resistance genes efficiently from wastewater or organic solid waste is a promising technology. This paper reviewed the development of electron shuttles, described the mechanism of action of different electron shuttles and the application of enhanced anaerobic biotreatment with electron shuttles for the removal of antibiotics and related genes. Finally, we discussed the current issues and possible future directions of electron shuttle technology.

Volatile fatty acids production from waste streams by anaerobic digestion: A critical review of the roles and application of enzymes

Volatile fatty acids (VFAs) produced from organic-rich wastewater by anaerobic digestion attract attention due to the increasing volatile fatty acids market, sustainability and environmentally friendly characteristics. This review aims to give an overview of the roles and applications of enzymes, a biocatalyst which plays a significant role in anaerobic digestion, to enhance volatile fatty acids production. This paper systematically overviewed: (i) the enzymatic pathways of VFAs formation, competition, and consumption; (ii) the applications of enzymes in VFAs production; and (iii) feasible measures to boost the enzymatic processes. Furthermore, this review presents a critical evaluation on the major obstacles and feasible future research directions for the better applications of enzymatic processes to promote VFAs production from wastewater.

Applications of Sponge Iron and Effects of Organic Carbon Source on Sulfate-Reducing Ammonium Oxidation Process

The typical characteristics of wastewater produced from seafood, chemical, textile, and paper industries are that it contains ammonia, sulfate, and a certain amount of chemical oxygen demand (COD). The sulfate-reducing ammonium oxidation process is a biochemical reaction that allows both ammonia and sulfate removal, but its low growth rate and harsh reaction conditions limit its practical application. Due to the adsorption properties of the iron sponge and its robust structure, it provides a suitable living environment for microorganisms. To reduce the negative impact on the environment, we employed 4.8 kg of sponge iron in a 2.0 dm3 anaerobic sequencing batch reactor (ASBR). We investigated the effects of the type and concentration of carbon sources on the performance of the sulfate-reducing ammonium oxidation (SRAO) process. The results demonstrated that during a start-up period of 90 days, the average ammonium removal efficiency and the sulfate conversion efficiency of the reactor containing the sponge iron were 4.42% and 8.37% higher than those of the reactor without the sponge iron. The addition of the sponge iron shortens the start-up time of this greenhouse gas-free denitrification process and reduces future costs in practical applications. The removal of total nitrogen (TN) significantly increased after adding organic carbon sources and then declined sharply, while the most considerable reduction of ammonium removal efficiency from 98.4% to 30.5% was observed with adding phenol. The performance of the group employing glucose as the carbon source was recovered on the 28th day, with the average ammonium removal efficiency increasing from 49.03% to 83.5%. The results of this simulation study will help the rapid start-up of SRAO in the water treatment industry and can precisely guide the application of the SRAO process for wastewater containing different organic carbon sources.

Enhancement of methane production by anaerobic digestion of corn straw with hydrogen-nanobubble water

Hydrogen-nanobubble water was proposed to enhance methane production by anaerobic digestion (AD) with corn straw. The effects of H₂-nanobubble water (H₂-NBW) amounts (0%, 20%, 40%, 60%, 80%, and 100%) on methane production characteristics of corn straw were explored. The results showed that the methane yields were increased by 11.54%∼25.29% compared with the control group(CK), and the maximum cumulative methane production reached to 254.36 mL·g-VS^-1 when the H₂-NBW addition was of 60%. Interestingly, the maximum methane concentration increased by 4.37% compared with CK. H₂-NBW addition can destroy the cellulose structure of corn straw, reduce the crystallinity of cellulose, and promote the hydrolysis. The degradation rate of cellulose and hemicellulose were increased by 20%∼33% and 13% ∼25.7% respectively, and the removal rate of TS and VS were increased by 6.82%-27.93% and 8.52%-21.47%, respectively. The modified Gompertz equation fitted the cumulative methane production curves very well, with high correlation coefficients (R² > 0.992).