Electrically-assisted anaerobic digestion under ammonia stress: Facilitating propionate oxidation and activating methanogenesis via direct interspecies electron transfer

Improved anaerobic digestion of waste activated sludge under ammonia stress by nanoscale zero-valent iron/peracetic acid pretreatment and hydrochar regulation: Insights from multi-omics analyses

This study developed a novel strategy combining a nanoscale zero-valent iron (nZVI)/peracetic acid (PAA) pretreatment and hydrochar regulation to enhance anaerobic digestion of waste activated sludge (WAS) under ammonia-stressed conditions. The strategy significantly enhanced methane production at ammonia concentrations below 3000 mg/L, with the regulation groups (AN3000/REG) achieving a 50.1 % increase in cumulative methane yield. Metagenomic analysis demonstrated a 14.2 % enrichment of key functional microorganisms, including syntrophic fatty acid-oxidizing bacteria and hydrogenotrophic methanogens, in the AN3000/REG groups. Some of them promote the conversion of butyrate and valerate to acetate through the upregulation of key genes in the fatty acid β-oxidation pathway, thereby supplying sufficient substrates for acetoclastic methanogenesis. Beyond enhancing acetoclastic methanogenesis, the AN3000/REG groups exhibited significant upregulation of other metabolic pathways, with a 34.2 % increase in syntrophic acetate oxidation-hydrogenotrophic methanogenesis genes and a 17.1 % increase in methanol/methylotrophic methanogenesis-related genes. These findings were further validated by the metatranscriptomic and metaproteomic combination analyses. Furthermore, the AN3000/REG groups exhibited a significant enhancement in direct interspecies electron transfer, with functional microbes (e.g., Geobacter, Methanosarcina, and Methanobacterium), pili, and cytochrome c showing significant increases of 1.38-fold, 12.7-fold, and 5.6-fold, respectively. This might be due to the synergistic effects of nZVI and hydrochar in the regulation groups. Additionally, metabolomic analyses revealed that the regulation strategy improved the microbial adaptability to ammonia stress by modulating metabolic products, such as alkaloids. Our study not only provides a promising strategy for alleviating ammonia inhibition during the anaerobic digestion of WAS but also provides a strong basis for understanding the underlying mechanism under ammonia-stressed conditions.

Treating nitrobenzene (NB) wastewater via an enhanced iron‒manganese oxides electron transfer strategy: Methods and mechanisms

Nitrobenzene (NB) is widely used in the fine chemical, pharmaceutical, and textile industries, but its toxicity and resistance to degradation present challenges, and traditional treatment methods are limited due to costs and a lack of efficiency. In this study, using a three-dimensional biofilm electrode reactor (3D-BER) enhanced with iron-manganese oxide catalysts was explored for the treatment of NB wastewater. Modifying polyurethane (PU) sponge fillers with Fe₃O₄ and Mn₃O₄ improved the conductivity and microbial electron transfer of the 3D-BER, significantly enhancing organic compound degradation, nitrogen and phosphorus removal. At the optimal voltage of 6 V and hydraulic retention time (HRT) of 24 h, the reactor achieved CODCr and ammonia nitrogen removal rates of 93.04 % and 86.25 %, respectively, which were more than 20 % higher than those in the control group. The iron and manganese present facilitated increased microbial activity and electron transfer efficiency, and metagenomic sequencing revealed shifts in the microbial communities and the enrichment of specific functional genes related to NB degradation. This integrated bioelectrochemical approach offers an effective and low-cost solution for treating NB wastewater, with broader implications for organic chemical wastewater management.

In-depth exploration of microbial electrolysis cell coupled with anaerobic digestion (MEC-AD) for methanogenesis in treating protein wastewater at high organic loading rates

High concentrations of protein wastewater often reduce treatment efficiency due to ammonia inhibition and acid accumulation caused by its low carbon-to-nitrogen ratio (C/N) after digestion, as well as its complex structure. This study investigates the performance of a microbial electrolysis cell (MEC) driving a protein digestion system with gradually increasing organic loading rates (OLR) of bovine serum albumin, elucidating microbial changes and methanogenic metabolic pathways on bioelectrodes under high OLR "inhibited steady-state" (ISS) conditions. The results showed that the accumulation of ammonia nitrogen (AN) from protein hydrolysis under high OLR conditions disrupted microbial growth and caused cell death on the electrode surface, hindering the electron transfer rate. Toxic AN reduced protein hydrolysis, led to propionate accumulation, inhibiting the acetoclastic methanogenesis process and favoring the hydrogenotrophic pathway. As OLR increased from 6 to 11 gCOD/L, cumulative methane production increased significantly from 450.24 mL to 738.72 mL, while average methane yield and production rate decreased by 10.51% and 50.28%, from 375.20 mL/gCOD and 75.04 mL/(gCOD·d) to 335.78 mL/gCOD and 37.31 mL/(gCOD·d), respectively. Despite these declines, the system maintained an ISS. Moderate OLR increases can achieve an ISS, boosting protein waste treatment capacity, methane production, and net energy output (NEO), with an OLR of 6 gCOD/L being optimal for maximizing NEO per unit substrate. These findings provide theoretical insights into the methanogenesis pathway of high OLR proteins in MEC-AD systems and offer an effective method for treating high OLR protein wastewater in future practical applications.

Effects of different quorum sensing signal molecules on alleviation of ammonia inhibition during biomethanation

Anaerobic digestion (AD) is a promising technology for achieving both organic wastes treatment and energy recovery. However, challenges such as ammonia inhibition still remain. Quorum sensing (QS) system is relevant with the regulation of microbial community behaviors by releasing and sensing signal molecules, which could improve methane production during AD process. Therefore, the current study explored the effects of different quorum sensing signal molecules on alleviation of ammonia inhibition. The results showed that both secretion of N-butyryl-DL-homoserine lactone (C4-HSL) and N-(β-ketocaproyl)-DL-homoserine lactone (3OC6-HSL) could be inhibited by high ammonia stress while stimulation of N-hexanoyl-L-homoserine lactone (C6-HSL) and N-octanoyl-DL-homoserine lactone (C8-HSL) secretion might be triggered by ammonia toxicity. Moreover, the alleviation of ammonia inhibition could be achieved by both introducing 3OC6-HSL (0.5 μM) and combination of 3OC6-HSL (0.1 μM) and biochar (4 g/L). Exogenous 3OC6-HSL could regulate microbial social behaviors and enhance the secretion of extracellular polymeric substances (EPS) to promote anaerobic digestion. In addition, the mitigation of ammonia inhibition through exogenous 3OC6-HSL and biochar were confirmed by microbial community changes (Methanobacterium, Propionicicella and Petrimonas). Critical enzymes involved in both acidification and methanogenic steps were enhanced after adding the combination of 3OC6-HSL and biochar. The combination of low levels of 3OC6-HSL and biochar could promote both direct interspecies electron transfer (DIET) process and communication between different anaerobic microorganisms to mitigate ammonia inhibition. The current study will provide primary insights for conquering ammonia inhibition during biomethanation.

Ethanol-type anaerobic digestion enhanced methanogenic performance by stimulating direct interspecies electron transfer and interspecies hydrogen transfer

Ethanol pre-fermentation of food waste effectively alleviates acidification; however, its effects on interspecies electron transfer remain unknown. This study configured the feed according to COD ratios of ethanol: sodium acetate: sodium propionate: sodium butyrate of 5:2:1.5:1.5 (ethanol-type anaerobic digestion) and 0:5:2.5:2.5 (control), and conducted semi-continuous anaerobic digestion (AD) experiments. The results showed that ethanol-type AD increased maximum tolerable organic loading rate (OLR) to 6.0 gCOD/L/d, and increased the methane production by 1.2-14.8 times compared to the control at OLRs of 1.0-5.0 gCOD/L/d. The abundance of the pilA gene, which was associated with direct interspecies electron transfer (DIET), increased by 5.6 times during ethanol-type AD. Hydrogenase genes related to interspecies hydrogen transfer (IHT), including hydA-B, hoxH-Y, hnd, ech, and ehb, were upregulated during ethanol-type AD. Ethanol-type AD improved methanogenic performance and enhanced microbial metabolism by stimulating DIET and IHT.

Paradigm shift in Nutrient-Energy-Water centered sustainable wastewater treatment system through synergy of bioelectrochemical system and anaerobic digestion

With advancements in research and the necessity of improving the performance of bioelectrochemical system (BES), coupling anaerobic digestion (AD) with BES is crucial for energy gain from wastewater and bioremediation. Hybridization of BES-AD concept opens new avenues for pollutant degradation, carbon capture and nutrient-resource recovery from wastewater. The strength of merging BES-AD lies in synergy, and this approach was employed to differentiate fads from strategies with the potential for full-scale implementation and making it an energy-positive system. The integration of BES and AD system increases the overall performance and complexity of combined system and the cost of operation. From a technical standpoint, the primary determinants of BES-AD feasibility for field applications are the scalability and economic viability. High potential market for such integrated system attract industrial partners for more industrial trials and investment before commercialization. However, BES-AD with high energy efficacy and negative economics demands performance boost.