Direct interspecies electron transfer (DIET) has been considered as an effective mechanism to proceed syntrophic methanogenic metabolism. However, up to now, this working mode has been still not widely established in the Geobacter-rare methanogenic digesters. In this study, a strategy that could enrich Geobacter species and stimulate the methanogenic communities to continuously perform DIET was proposed in a two-phase anaerobic digestion (AD) system with the aim to enhance and stabilize the better AD. The results demonstrated that, under the conditions employed, the ethanol-abundant acidogenic products could be produced via ethanol-type fermentation when acidogenic-phase pH was kept at 4.0–4.5. Enrichments in the methanogenic phase continuously stimulated with the ethanol-abundant acidogenic products presented a higher conductivity, as well as more positive response to granular activated carbon (GAC) supplemented, compared with the enrichments without this stimulation, suggesting that DIET might be established. Microbial community analysis showed that Geobacter species were only detected in the methanogenic enrichments stimulated by the ethanol-abundant acidogenic products. Together with the significant increase of Methanosarcina species in these enrichments, the potential DIET between Geobacter and Methanosarcina species might be permanently established in the methanogenic digester to maintain the acidic balance as well as syntrophic metabolism stable.Keywords: Anaerobic digestion; Direct interspecies electron transfer (DIET); Ethanol-type fermentation; Syntrophic metabolism;

•Granular activated carbon was used for the first time to enhance sludge digestion.•Granular activated carbon accelerated direct interspecies electron transfer.•Granular activated carbon accelerated interspecies hydrogen transfer.•Granular activated carbon improved methane production and sludge decomposition.Anaerobic digestion of waste activated sludge is frequently restricted with the slow fermentation rate due to its complex components. Carbon materials have been reported to enhance syntrophic metabolism of anaerobic wastewater treatment, but its function in anaerobic digestion of sludge has yet to be investigated. In this study granular activated carbon (GAC) was added into a batch-mode anaerobic sludge digestion reactor with an attempt to improve the sludge digestion. The results showed that with adding GAC from 0 to 5.0 g, the methane production increased by 17.4%, and the sludge reduction rate increased by 6.1 percent points (from 39.1% to 45.2%). In the 20 days’ digestion, GAC obviously enriched hydrogen-utilizing methanogens, Geobacter, and other methanogens capable of direct interspecies electron transfer, which could enhance the electron exchange between syntrophs and methanogens to accelerate substrate consumption and methane production.Download high-res image (354KB)Download full-size image

•Carbon cloth supplemented could enhance the digesters to resist the acidic impacts.•Anaerobic digesters in the presence of the carbon cloth had a faster recovery.•No H2 was detected in the digesters with the carbon cloth under the acidic impacts.•The dominant working mode for the syntrophic metabolism might shift from IHT to DIET.•DIET enriched with the carbon cloth maintain stable anaerobic methanogenesis.Anaerobic digesters usually become sour during the operation and inhibit methanogenesis. This effect occurs due to the interspecies electron exchange for the syntrophic metabolism of alcohols and volatile fatty acids (VFAs) via interspecies H2 transfer (IHT). Conductive materials can promote direct interspecies electron transfer (DIET) between the syntrophs and methanogens, providing an alternative to IHT. However, the cooperative mechanism of these two working modes during syntrophic metabolism is unknown, especially during acidic impacts. The results of this study demonstrated that anaerobic digesters supplemented with conductive carbon cloth had a higher capacity to resist the acidic impacts. Conversely, the digesters supplemented with non-conductive cotton cloth (control) were nearly stagnant when the H2 partial pressure of the anaerobic systems increased. Further experiments at high H2 partial pressure to inhibit IHT exhibited almost no effect on the syntrophic metabolism in the carbon cloth-supplemented group, in which the methane production approached the stoichiometric production (350 mL CH4/g COD removal), while the methane production in the control group ceased. The microbial community analysis revealed that the surface sludge attached to the carbon cloth had the highest abundance of Geobacter and Methanosaeata species known to participate in DIET. These results suggested that the predominant working mode for the interspecies electron exchange might have shifted from IHT to DIET in the presence of the conductive carbon cloth during acidic impacts. DIET primarily occurred on the surface sludge of the carbon cloth and replaced IHT to proceed the syntrophic metabolism and maintained stable anaerobic methanogenesis.Download high-res image (107KB)Download full-size image

•The Fe(OH)3 addition increased the COD removal and decoloration efficiency.•Autocatalysis of azo dye via intermediates was enhanced by Fe(III)/Fe(II) cycle.•Dissimilatory iron reducing bacterium enhanced the electron transfer efficiency.In this study, Fe(OH)3 was dosed into anaerobic reactor (R1) to stimulate the electron transfer process during the decoloration treatment. As results indicated, the chemical oxygen demand (COD) removal and decoloration efficiency were increased by 61.7% and 32.0% than that of the control reactor (R2), respectively. The cyclic-voltammetric analysis of the effluent demonstrated that the cycles of Fe(III)/Fe(II) and 1-imino-2-napthoquinone/1-amino-2-napthol (oxidative/reductive state of the decoloration intermediate product) had been established in R1. The concentration of cytochrome c and the conductivity of suspended sludge in R1 were also 3.2 and 2.1 times higher than that in R2. All experiments above indicated that the electron transfer process between substrates and azo bonds was accelerated efficiently. Furthermore, the abundance of iron reducing bacteria was higher than that of R2, indicating that dissimilatory iron reduction as a reason for the Fe(III)/Fe(II) cycle played an important role in the anaerobic decoloration treatment.Download high-res image (191KB)Download full-size image

Anaerobic reactors with ferric iron addition have been experimentally demonstrated to be able to simultaneously improve sulfate reduction and organic matter degradation during sulfate-containing wastewater treatment. In this work, a mathematical model is developed to evaluate the impact of ferric iron addition on sulfate reduction and organic carbon removal as well as the volatile fatty acids (VFA) composition in anaerobic reactor. The model is successfully calibrated and validated using independent long-term experimental data sets from the anaerobic reactor with Fe (III) addition under different operational conditions. The model satisfactorily describes the sulfate reduction, organic carbon removal and VFA production. Results show Fe (III) addition induces the microbial reduction of Fe (III) by iron reducing bacteria (IRB), which significantly enhances sulfate reduction by sulfate reducing bacteria (SRB) and subsequently changes the VFA composition to acetate-dominating effluent. Simultaneously, the produced Fe (II) from IRB can alleviate the inhibition of undissociated H2S on microorganisms through iron sulfide precipitation, resulting in further improvement of the performance. In addition, the enhancement on reactor performance by Fe (III) is found to be more significantly favored at relatively low organic carbon/SO42– ratio (e.g., 1.0) than at high organic carbon/SO42– ratio (e.g., 4.5). The Fe (III)-based process of this work can be easily integrated with a commonly used strategy for phosphorus recovery, with the produced sulfide being recovered and then deposited into conventional chemical phosphorus removal sludge (FePO4) to achieve FeS precipitation for phosphorus recovery while the required Fe (III) being acquired from the waste ferric sludge of drinking water treatment process, to enable maximum resource recovery/reuse while achieving high-rate sulfate removal.

Low hydrogen production from anaerobic digestion of sludge has greatly limited the application of biological hydrogen-producing technology. An Fe/graphite electrode was installed into an anaerobic digester to enhance the hydrogen production from waste sludge in this study. The electrode accelerated the decomposition of the sludge, and the production of short-chain fatty acids was 3.5 folds of that in a control anaerobic reactor with no electrode. The hydrogen production was 90.6 mL gVSS−1, while it was almost undetectable in the control. The results suggested that the hydrogen was produced from the cathodic reduction of H+ coupling with the anodic oxidation of sludge. The excessive consumption of the cathodic H+ drove the pH up to 9.2–9.5 in the electric-anaerobic system which inevitably inhibited the occurrence of methanogenesis. This led to quite low methane production in this electric-anaerobic system. The microbial analysis showed that exoelectrogens were enriched in the presence of an Fe electrode. It catalyzed the anodic oxidation of the sludge to produce more electrons for the cathodic hydrogen production. Raising the voltage supply boosted the H2 production, but the net energy output was obtained at 0.3 and 0.6 V.

Catalytic ozonation of Reactive Red X-3B in aqueous solution had been carried out in an ozone oxidation reactor where Mn-Fe-ceramic honeycomb was used as the catalysts. The presence of Mn-Fe-ceramic honeycomb catalyst could obviously improve the decoloration efficiency of Reactive Red X-3B and the utilization efficiency of ozone compared to the results from non-catalytic ozonation. Adsorption of Reactive Red X-3B had no obviously influence on the degradation efficiency. Addition of tert-butanol significantly decreased the degradation efficiency, indicating that the degradation of Reactive Red X-3B followed the mechanism of hydroxyl radical (OH·) oxidation. The operating variables such as reaction pressure and ozone supply had a positive influence on the degradation efficiency, mainly attributing to facilitate the ozone decomposition and OH · formation.

It is important to develop efficient technologies on removal of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) from water due to their wide distribution and potential threat to human health. In this work, a durable and convenient electrosorption device was designed in continuous flow mode to investigate the adsorption of PFOX (X = A and S) on multiwalled carbon nanotubes (MWNTs) from water under electrochemical assistance. The electrosorption experiments were conducted under different influent and electrolyte concentrations, hydraulic retention time (HRT) and electrode distance to optimize the operation. The results showed that the highest removal efficiencies toward both PFOA and PFOS could come up to 90 % at 1 V. Compared with open circuit (OC) adsorption under the same conditions, the removal efficiencies were improved by 4.9 times (PFOA) and 4.2 times (PFOS) respectively. In addition, the MWNTs electrode was found to be reusable. These findings provide an efficient method to remove PFOX from water by electrosorption in continuous flow mode.

In order to investigate the contribution of various black carbon (BC) contents to nonlinearity of sorption and desorption isotherms for acetochlor on sediment, equilibrium sorption and desorption isotherms were determined to measure sorption and desorption of acetochlor in sediment amended with various amounts of BC. In this paper, two types of BC referred to as BC400 and BC500 were prepared at 400°C and 500°C, respectively. Higher preparation temperature facilitated the formation of micropores on BC to enhance its sorption capacity. Increase of the BC content obviously increased the sorption amount and reduced the desorption amount for acetochlor. When the BC500 contents in total organic carbon (TOC) increased from 0 to 60%, Freundlich sorption coefficient (Kf) increased from 4.07 to 35.74, and desorption hysteresis became gradually obvious.When the content of BC in TOC was lower than 23%, the sorption isotherm had a significant linear correlation (p = 50.05). In case of desorption, a significant nonlinear change could be observed when the content of BC was up to 13%. Increase of BC content in the sediment would result in shifting the sorption-desorption isotherms from linearity to nonlinearity, which indicated that contribution of BC to nonlinear adsorption fraction became gradually remarkable.

•Organic removal and methanogenesis were improved in sulfate-containing wastewater.•Adding stainless steel enhanced methanogenesis to resist the sulfate impact.•Potential direct interspecies electron transfer was enriched on the stainless steel.•Methanogenesis via DIET is faster transfer to electron than sulfate reduction via IHT.Direct interspecies electron transfer (DIET) is an alternative to syntrophic metabolism in natural carbon cycle as well as in anaerobic digesters, but its function in anaerobic treatment of sulfate-containing wastewater have not yet to be described. Here, conductive stainless steel was added into anaerobic digesters for treating sulfate-containing wastewater to investigate the potential role of DIET in the response to the sulfate impact. Results showed that adding the conductive stainless steel made the anaerobic digestion less affected by the sulfate reduction than adding insulative plastic material. With adding stainless steel, methane production of the digesters increased by 7.5%–24.6%. Microbial analysis showed that the dissimilatory Fe (III) reducers like Geobacter species were enriched on the surface of the stainless steel. These results implied that the potential DIET of methanogenesis was established associating with stainless steel to outcompete the sulfate reduction.Download high-res image (166KB)Download full-size image

•Methanogenic activity was suppressed after heat/alkali pretreatment.•Zero-valent iron recovered the methanogenic activity from heat or alkaline.•Acetoclastic methanogenesis and H2-utilizing methanogenesis were enhanced.•Methane production increased by 91.5% with zero-valent iron.Heat or alkali pretreatment is the effective method to improve hydrolysis of waste sludge and then enhance anaerobic sludge digestion. However the pretreatment may inactivate the methanogens in the sludge. In the present work, zero-valent iron (ZVI) was used to enhance the methanogenic activity in anaerobic sludge digester under two methanogens-suppressing conditions, i.e. heat-pretreatment and alkali condition respectively. With the addition of ZVI, the lag time of methane production was shortened, and the methane yield increased by 91.5% compared to the control group. The consumption of VFA was accelerated by ZVI, especially for acetate, indicating that the acetoclastic methanogenesis was enhanced. In the alkali-condition experiment, the hydrogen produced decreased from 27.6 to 18.8 mL when increasing the ZVI dosage from 0 to 10 g/L. Correspondingly, the methane yield increased from 1.9 to 32.2 mL, which meant that the H2-utilizing methanogenes was enriched. These results suggested that the addition of ZVI into anaerobic digestion of sludge after pretreated by the heat or alkali process could efficiently recover the methanogenic activity and increase the methane production and sludge reduction.

•Anaerobic digestion with different types of conductive materials was investigated.•Magnetite significantly enhanced the decomposition of complex organics to simples.•Magnetite supplemented raised the hydrogen partial pressure of anaerobic system.•Granular activated carbon (GAC) accelerated the syntrophic metabolism of simples.•GAC had less effects on syntrophic metabolism following the failed acidogenesis.Conductive materials have been widely investigated to accelerate and stabilize the conversion of organic wastes to methane. However, the potential mechanisms involved with different types of conductive materials are still unclear. In this study, magnetite (Fe3O4) and granular activated carbon (GAC), as the two typical conductive materials, were respectively supplemented to acidogenesis and methanogenesis of a two-phase anaerobic digestion (AD) system in an attempt to explore their different mechanisms. The results showed that, magnetite supplemented to the acidogenic phase could enhance the decomposition of complex organics into simples, but significantly raise the hydrogen partial pressure as well as enrich the hydrogen-utilizing methanogens, which were not expected for aceticlastic methanogenesis known as a mainstream of methanogenesis in most of traditional digesters. GAC supplemented to the methanogenic phase had less influences on syntrophic metabolism of alcohols and fatty acids when acidogenesis was ineffective or out of work. Microbial community analysis suggested that direct interspecies electron transfer (DIET) had been established on the GAC, though the insignificant improvement of performances. Once magnetite was supplemented to the acidogenesis to improve the acidification efficiency, the syntrophic conversion of alcohols and fatty acids to methane in the GAC-supplemented methanogenic phase was significantly improved. These results suggested that, DIET was unlikely to participate in the direct decomposition of complex organics, even in the presence of GAC, but it could work effectively once acidogenesis functioned well.Download high-res image (393KB)Download full-size image

•H2 produced in anaerobic process acted as electron donor for denitrification.•Adding nitrate could accelerate the sludge granulation in acetogenesis.•Butyrate was optimum carbon source for sludge granulation and denitrification.•Microbial communities in butyrate digester presented the most active state.Sludge granulation is a key factor to sustain anaerobic systems operating efficiently and steadily. Nitrate as a H2 consumer was added into anaerobic digesters to investigate its effects on the sludge granulation. The results showed that adding nitrate increased the sludge granule size by 289%, 325% and 790% with acetate, propionate and butyrate as substrates, respectively. Butyrate was preferable to the denitrifying bacteria because it was capable of releasing more electrons available for denitrification during acetogenesis. The analyses of fluorescence in situ hybridization, scanning electron microscope, and denaturing gradient gel electrophoresis indicated that denitrifying bacteria and volatile fatty acid (VFA)-oxidizing bacteria in the butyrate digester were richer than those in the other digesters. Taken together, addition of nitrate accelerated the decomposition of VFA and simultaneously improved the granulation of anaerobic process.

The startup of a hybrid system consisting of an upflow anaerobic sludge blanket (UASB) and an anaerobic fixing filter (AFF) with internal hydraulic circulation and external sludge circulation was investigated. The reactor was rapidly cultivated using municipal sludge as a seed 38 d after a failed startup. During the operation, the average size of the granular sludge increased from 111 μm to 264 μm, and the sludge was uniformly distributed in the reacting region. Efficient performance was attributed to good hydraulic contact between the substrate and sludge and the low loss of sludge. However, excessive hydraulic circulation resulted in a sharp decline in the effectiveness. After a pause in the operation, a second startup was rapidly completed in 15 d, during which time the organic load reached 15.4 COD/(m3 d).