Pilot-scale investigation of conductive carbon cloth amendment for enhancing high-solids anaerobic digestion and mitigating antibiotic resistance.

This study examined the effectiveness of introducing conductive carbon cloth into a pilot-scale high-solids anaerobic digestion (HSAD) system. Adding carbon cloth increased methane production by 22 % and improved the maximum methane production rate by 39 %. Microbial community characterization indicated a possible direct interspecies electron transfer-based syntrophic association among microbes. Using carbon cloth also enhanced microbial richness, diversity, and evenness. Carbon cloth effectively reduced the total abundance of antibiotic resistance genes (ARGs) by 44.6 %, mainly by inhibiting horizontal gene transfer, as shown by the significant decrease in the relative abundance of integron genes (particularly intl1). The multivariate analysis further demonstrated strong correlations of intl1 with most of the targeted ARGs. These findings suggest that carbon cloth amendment can promote efficient methane production and attenuate the spread of ARGs in HSAD systems.

Antibiotic resistance genes proliferation under anaerobic degradation of polylactic acid and polyhydroxy butyrate bioplastics.

As the global concern over plastic pollution grows, efforts are underway to find environmentally friendly alternatives to traditional plastics. Bioplastics are being extensively researched and developed as a possible solution. This study compared the impact of two bioplastics, polylactic acid (PLA) and polyhydroxy butyrate (PHB), on the proliferation of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) during anaerobic digestion (AD). Both bioplastics (250-500 particles) could be degraded to a certain extent over 79 days, as indicated by higher methane production than the control without bioplastic particles. The PHB 500 reactor showed the highest methane yield along with the highest biodegradation efficiency (91 %) than other reactors amended with PHB and PLA particles. The highest ARG and MGE abundances were also observed in PLA 500, and the lowest ARG abundance was in PLA 250. Conversely, PHB reactors showed a relatively lower ARG abundance than the control. The correlation analysis suggested that most ARGs were positively correlated with PLA and negatively correlated with PHB (except for tetA, tetB, and tetX). Moreover, a correlation between MGEs and ARGs in PLA and PHB reactors was revealed by correlation analysis. These results show that AD responds differently to the different types/levels of bioplastics, which can ultimately influence the behavior of ARG proliferation. Thus, bioplastics may also pose a potential risk for spreading antibiotic resistance. These findings can be used as a basis for setting environmental standards for bioplastics and creating monitoring and control measures to prevent potential negative impacts on public health.

Granular activated carbon remediates antibiotic resistance propagation and methanogenic inhibition induced by polystyrene nanoplastics in sludge anaerobic digestion.

Nano/microplastics (NPs/MPs) in sewage sludge can induce oxidative stress to the anaerobic digestion (AD) and also proliferate antibiotic resistance genes (ARGs). Recently, granular activated carbon (GAC) has been used as an additive to enhance methane production in AD via direct interspecies electron transfer (DIET); however, its impact on AD exposed to NPs/MPs is yet to be studied. This study examined the effect of GAC (5 and 15 g/L) on sludge AD exposed to 150 µg/L of polystyrene nanoplastics (PsNPs). PsNPs decreased methane yield by 32.3% due to elevated levels of reactive oxygen species. However, GAC addition counteracted this adverse effect and improved methane production, attributed to the potential enrichment of DIET-active microbes and the adsorption of PsNPs by GAC. Moreover, GAC reduced the total abundance of ARGs, which was increased by PsNPs exposure. Thus, GAC can provide dual benefits in mitigating methanogenic inhibition caused by PsNPs and ARG spread.

A multifaceted screening of applied voltages for electro-assisted anaerobic digestion of blackwater: Significance of temperature, hydrolysis/acidogenesis, electrode corrosion, and energy efficiencies.

A microbial electrolysis cell-assisted anaerobic digester (MEC-AD) was operated with vacuum toilet blackwater at different applied voltages (0-1.6 V) at room temperature (R20). A parallel MEC-AD was operated at 35 °C (R35) to provide a kinetics index at mesophilic temperature. Both reactors failed at 1.6 V due to the alkaline pH created by anodic corrosion. In R20, the best performance was observed at 1.2 V, with methane yield, COD removal, hydrolysis and acidogenesis efficiency increased by 59.9%, 27.0%, 52.0%, and 44.9%, respectively, compared to those of 0 V. Enrichment of hydrolytic and syntrophic bacteria (e.g., Clostridium, Bacteroidales, Sedimentibacter, Syntrophomonas) and increased abundance of genes encoding complex organics (e.g., proteins, carbohydrates, lipids) metabolism in R20 at 1.2 V corresponded to the enhanced hydrolysis/acidogenesis processes. R20 at 1.2 V generated 1.16 times more net energy than R35 at the optimum voltage for methane yield (0.8 V), indicating ambient temperature operation of MEC-AD systems would be a more sustainable strategy.

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.

Significance of different mixing conditions on performance and microbial communities in anaerobic digester amended with granular and powdered activated carbon.

Conductive materials amendment in anaerobic digestion (AD) is a promising strategy for boosting the methanogenesis process. Despite mixing is a critical parameter, the behavior of digesters amended with conductive additives upon different mixing conditions has rarely been investigated. This study investigated continuous mixing, intermittent mixing (10 min in every 12 h), and non-mixing conditions for digesters amended with granular activated carbon (GAC) and powdered activated carbon (PAC). The non-mixed GAC digester provided the highest methane yield (318 ± 28 mL/g COD) from synthetic blackwater, while intermittently mixed GAC and control exhibited similar methane yields (290-294 mL/g COD). For non-mixed systems, microbial richness and diversity increased with GAC and PAC amendment. In contrast, continuous and intermittent mixing increased microbial diversity and richness in control reactors while reduced the same in GAC and PAC amended reactors. Overall, various mixing conditions distinctly changed the degree of enrichment/retention of microbes and consequently influenced methane recovery.

High-solids anaerobic digestion of organic fraction of municipal solid waste: Effects of feedstock to inoculum ratio and percolate recirculation time.

The effects of feedstock to inoculum (F:I) ratio and percolate recirculation time (PRT) were studied for the high-solids anaerobic digestion (HSAD) of the organic fraction of municipal solid waste (OFMSW). Six mesophilic HSAD systems were operated at different F:I ratios (1 to 3 kg VS/kg VS; PRT = 2.5 h/d) and PRTs (1.5 to 3.5 h/d; F:I = 2 kg VS/kg VS). The F:I ratio of 1 provided up to 86% of the theoretical methane potential of OFMSW. In contrast, F:I ratio of 3 provided only 34% methane recovery due to volatile fatty acids (VFAs) accumulation and pH drop. Despite F:I ratio of 2 could provide 70% methane recovery, it could enable almost 45% higher organics processing capacity (VS basis) and lower solids washout during percolate recirculation, as compared to the F:I ratio of 1. However, different examined PRTs showed marginal impacts on methane yields with comparable changes in profiles of percolate characteristics.

High-rate blackwater anaerobic digestion under septic tank conditions with the amendment of biosolids-derived biochar synthesized at different temperatures.

Septic tanks have been widely used for blackwater treatment in developing countries, while high-rate septic tanks with improved methane recovery are yet to be achieved. This study investigated biosolids-derived biochar (synthesized at 300℃, 425℃, and 550℃) as an additive for developing high-rate septic tanks. The experiments were conducted with anaerobic bioreactors operated with synthetic blackwater under septic tank conditions. All biochar amended reactors demonstrated a steady increase in daily methane production for increasing OLR from 0.08 to 3 g COD/L/d. The control reactor showed significant process disturbances at OLRs ≥ 2 g COD/L/d with an accumulation of volatile fatty acids followed by pH drop. At OLR of 3 g COD/L/d, the daily methane production from biochar amended reactors was ~ 4.3 times higher than the control (300 vs. 70 mL per day). Biochar addition established a robust microbiome consisted of a higher abundance of hydrogenotrophic and acetoclastic methanogens and hydrogen-producing fermentative bacteria.

A review of mechanisms underlying the impacts of (nano)microplastics on anaerobic digestion.

The presence of (nano)microplastics in domestic wastewater and their subsequent release to the aquatic environment via the discharge of treated sewage has raised significant concerns. Previous studies have also identified their excessive accumulation in sewage sludge. Anaerobic digestion is one of the most used sludge stabilization methods in wastewater treatment plants. Therefore, understanding the potential effects of (nano)microplastics on anaerobic digestion has been receiving increasing attention from researchers. This article provides a comprehensive review of mechanisms underlying the impacts of (nano)microplastics on anaerobic digestion. Notably, this review covers mechanisms of inhibition/enhancement of anaerobic digestion by (nano)microplastics and their potential impacts on biochemical pathways, key enzymes, functional genes, and microbial communities investigated to date. Moreover, potential environmental risks of biosolids contaminated with (nano)microplastics were highlighted. Finally, knowledge gaps and future research needs were outlined. This review will guide more standardized studies in the future, covering both fundamental and engineering aspects.

An intermittent power supply scheme to minimize electrical energy input in a microbial electrolysis cell assisted anaerobic digester.

From the perspective of energy saving in the operation of microbial electrolysis cell assisted anaerobic digester (MEC-AD), this study focused on developing an intermittent power supply scheme. The applied potential was switched off for 12 and 6 hours/day during the operation of a laboratory-scale MEC-AD system fed with glucose. The results from the operation under continuous applied potential served as the control. The overall biomethane generation and net energy income from the process were unaffected when the applied potential turned off for 6 hours/day. Both quantitative and qualitative analyses of microbial communities suggested that a balanced microbiome could be maintained under short-term switching-off the applied potential. However, performance substantially deteriorated when the applied potential turned off for 12 hours/day. Overall, the results of this study suggest that MEC-AD operation does not need a continuous power supply, and higher energy efficiency can be effectively achieved by intermittently powering the reactor.