New insights into inhibition of high Fe(III) content on anaerobic digestion of waste-activated sludge

Optimization of synergistic microwave and zero-valent iron co-pretreatment for anaerobic digestion of waste activated sludge

This study optimized co-pretreatment of microwave temperature (TMW) and zero-valent iron dosage ([ZVI]) to enhance anaerobic digestion (AD) of waste activated sludge (WAS). WAS was pretreated at TMW = 100, 150, or 200 °C and [ZVI] = 1, 3, or 5 g/L using a central composite design. Optimal co-pretreatment (TMW = 168 °C and [ZVI] = 5 g/L) reduced the ratio of volatile solids (VS) to total solids by 21.5 %, increased the solubilization ratio seven-fold, removed 53.5 % of phosphate compared to WAS partly because of lignin fragmentation. Biochemical methane potential identified optimal conditions (TMW = 164 °C and [ZVI] = 4.8 g/L), enhancing VS removal by 70.9 %, methane yield by 60 %, and reducing hydrogen sulfide by 82.4 % compared to Control. Kinetic analysis indicated 61 - 108 % increase in maximum methane production rate. Microbial analysis revealed increased acetoclastic methanogens and decreased hydrogenotrophic methanogens. Thus, microwave-ZVI co-pretreatment enhanced WAS biodegradability and AD efficiency.

Intermittent Microaeration Enhanced Anaerobic Digestion: The Key Role of Fe(III)/Fe(II) Cycle and Reactive Oxygen Species

Microaeration has been reported to improve anaerobic digestion, which is generally attributed to increased microbial diversity, but a thorough explanation is lacking. In this study, it was found that intermittent microaeration (IMA) supplied to an anaerobic digester could oxidize Fe(II) produced by dissimilatory iron reduction (DIR) to form a Fe(III)/Fe(II) cycle and generate extracellular reactive oxygen species (ROS) to improve anaerobic treatment of phenol-containing wastewater. The results showed that compared to the control group without IMA, the removal rates of COD and phenol increased by 29.54 and 49.68 percentage points, respectively, and daily average methane production increased by 85.44%. The Fe(III)/Fe(II) cycle slowed down the loss of iron (13.97%) released from sludge due to the lower solubility of Fe(III) and facilitated •OH generation (1.22 ± 0.04 μM) via Fenton-like reactions. The DIR and the generation of •OH accelerated phenol degradation. Metagenomic analysis revealed that the abundance of methanogens and antioxidant enzymes-encoding genes in response to oxidative stress significantly increased in the IMA group compared to the control, enabling methanogenesis to proceed smoothly under microaeration. This study investigated the extracellular ROS generation induced by microaeration during anaerobic digestion and their roles in promoting anaerobic performance, thereby providing a new perspective for optimizing anaerobic systems with microaeration.

Enhancing Methane Production in Anaerobic Digestion of Food Waste Using Co-Pyrolysis Biochar Derived from Digestate and Rice Straw

Anaerobic digestion (AD) is a preferred method for food waste (FW) treatment due to its sustainability and potential for production of renewable bioenergy. However, the accumulation of volatile fatty acids (VFAs) and ammonia often destabilizes the AD process, and managing the digestate byproduct poses additional challenges. This study investigates the use of co-pyrolysis biochar synthesized from digestate and rice straw (DRB) to enhance methane production and AD efficiency. DRB addition increased cumulative methane yield by 37.1%, improved VFA conversion efficiency, and achieved a 42.3% higher NH3-N-removal rate compared to the control group. The COD-removal rate was 68.7% throughout the process. Microbial analysis revealed that DRB selectively enriched Fastidiosipila and Methanosarcina, promoting direct interspecies electron transfer (DIET) and methane yield. These findings highlight DRB's potential to enhance AD efficiency and support closed-loop resource utilization.

Activated carbon and anthraquinone-2,6-disulfonate as electron shuttles for enhancing carbon and nitrogen removal from simultaneous methanogenesis, Feammox and denitrification system

Anthraquinone-2,6-disulfonate (AQDS) and activated carbon (AC) were employed as exogenous electron shuttles (ESs) for enhancing the performance of an integrated simultaneous methanogenesis, Feammox, and denitrification (SMFD) system treating fish sludge. The addition of AQDS and AC led to an increased total nitrogen removal efficiency by 30.2 % and 66.5 %, an increased total chemical oxygen demand removal efficiency by 9.5 % and 24.5 %, and an improved methane yield by 5.2 % and 12.6 %, respectively. Regarding nitrogen removal, AQDS mainly facilitated NH4+-N oxidation into NO3--N via Feammox, while AC facilitated both Feammox and denitrification. Regarding carbon removal, both ESs promoted the hydrolysis-acidification process via stimulating dissimilatory iron reduction and established direct interspecies electron transfer (DIET) between methanogens and syntrophic bacteria. Microbial analysis confirmed the enrichment of iron-reducing bacteria, denitrifiers, DIET-related methanogens and syntrophic partners in the presence of ESs. The study provides an ESs-assisted strategy for enhancing simultaneous nitrogen and carbon removal from high-strength wastewater.

Mathematical Model-Based Optimization of Trace Metal Dosage in Anaerobic Batch Bioreactors

Anaerobic digestion (AD) is a promising and yet a complex waste-to-energy technology. To optimize such a process, precise modeling is essential. Developing complex, mechanistically inspired AD models can result in an overwhelming number of parameters that require calibration. This study presents a novel approach that considers the role of trace metals (Ca, K, Mg, Na, Co, Cr, Cu, Fe, Ni, Pb, and Zn) in the modeling, numerical simulation, and optimization of the AD process in a batch bioreactor. In this context, BioModel is enhanced by incorporating the influence of metal activities on chemical, biochemical, and physicochemical processes. Trace metal-related parameters are also included in the calibration of all model parameters. The model's reliability is rigorously validated by comparing simulation results with experimental data. The study reveals that perturbations of 5% in model parameter values significantly increase the discrepancy between simulated and experimental results up to threefold. Additionally, the study highlights how precise optimization of metal additives can enhance both the quantity and quality of biogas production. The optimal concentrations of trace metals increased biogas and CH4 production by 5.4% and 13.5%, respectively, while H2, H2S, and NH3 decreased by 28.2%, 43.6%, and 42.5%, respectively.

Carbon flow, energy metabolic intensity and metagenomic characteristics of a Fe (III)-enhanced anerobic digestion system during treating swine wastewater

Deep treatment and bioenergy recovery of swine wastewater (SW) are beneficial for constructing a low-carbon footprint and resource-recycling society. In this study, Fe (III) addition from 0 to 600 mg/L significantly increased the methane (CH4) content of the recovered biogas from 61.4 ± 2.0 to 89.3 ± 2.0 % during SW treatment in an anaerobic membrane digestion system. The specific methane yields (SMY) also increased significantly from 0.20 ± 0.05 to 0.29 ± 0.02 L/g COD. Fe (III) and its bio-transformed products which participated in establishing direct interspecific electron transfer (DIET), upregulated the abundance of e-pili and Nicotinamide adenine dinucleotide (NADH), enriched electroactive bacteria. The increase in cellular adenosine triphosphate (cATP) from 6583 to 14,518 ng/gVSS and electron transport system (ETS) from 1468 to 1968 mg/(g·h) promoted the intensity of energy flow and electron flow during anaerobic digestion of SW. Moreover, Fe (III) promoted the hydrolysis and acidification of organic matters, and strengthened the acetoacetic methanogenesis pathway. This study established an approach for harvesting high quality bioenergy from SW and revealed the effects and mechanisms from the view of carbon flow, energy metabolic intensity and metagenomics.

Comparative investigations on the incorporation of biogenic Fe products into anaerobic granular sludge of different sources: Fe loading capacity, physicochemical properties, microbial community and long-term methanogenesis performance

Anaerobic granular sludge (AGS) has been regarded as the core of lots of advanced anaerobic reactors. Formation of biogenic Fe products and their incorporation into AGS could influence interspecies electron transfer and methanogenesis performance. In this study, with anaerobic granular sludge (AGS) from different sources (brewery, chemical plant, paper mill, citric acid factory, and food factory) as the research targets, the formation of biogenic iron products in AGS through the biologically induced mineralization process was studied. Furthermore, the influences of physicochemical properties and microbial community on methanogenesis were investigated. Results showed that all the AGS of different sources possessed the capacity to form biogenic Fe products through dissimilatory iron-reduction process, and diverse Fe minerals including magnetite (Fe3O4), hematite (Fe2O3), goethite (FeOOH), siderite (FeCO3) and wustite (FeO) were incorporated into AGS. The AGS loaded with Fe minerals (Fe-AGS) showed increased conductivity, magnetism and zeta-potential comparing to the control. Those Fe-AGS of different sources demonstrated different methanogenesis performance during the long-term operation (50 days). Methane production was increased for the Fe-AGS of citric acid (6.99-32.50%), food (8.33-37.46%), chemical (2.81-7.22%) and brewery plants (2.27-2.81%), but decreased for the Fe-AGS of paper mill (54.81-72.2%). The changes of microbial community and microbial correlations in AGS as a response to Fe minerals incorporation were investigated. For the Fe-AGS samples with enhanced methane production capability, it was widely to find the enriched populations of fermentative and dissimilatory iron reducing bacteria Clostridium_sensu_stricto_6, Bacteroidetes_vadinHA17 and acetoclastic methanogens Methanosaeta, and positive correlations between them. This study provides comprehensive understanding on the effects of incorporation biogenic Fe products on AGS from different sources.