•Impacts of high-strength food waste addition was comprehensively evaluated.•Multivariate analysis and visualization toolkits presented microbial cooperation.•Microbial dynamics showed possibility to indicate digestion deterioration.•A wide spread of rare species related to crucial steps in digestion process was observed.This study evaluated the impacts of FW addition on co-digestion in terms of microbial community. Anaerobic co-digestion (AcoD) reactors were conducted at gradually increased addition of food waste (FW) from 0 to 4 kg-VS m−3 d−1 for 220 days. Although no markable acidification was found at an OLR of 4 kg-VS m−3 d−1, the unhealthy operation was observed in aspect of an inhibited methane yield (185 mL g−1 VSadded), which was restricted by 40% when compared with its peak value. Deterioration of digestion process was timely indicated by the dramatic decrease of archaeal population and microbial biodiversity. Furthermore, the cooperation network showed a considerable number of rare species (<1%) were strongly correlated with methane production, which were frequently overlooked due to the limits of detecting resolution or analysis methods before. Advances in the analysis of sensitive microbial community enable us to detect the early disturbances in AcoD reactors.Download high-res image (75KB)Download full-size image

Generally, most biomaterials present high biosorption capacity for heavy-metal ions. In this study, alkaline reagents and microbial flocculant GA1 (MBFGA1) were combined to remove Ni(II) from aqueous solution. Response surface methodology was employed to optimize the flocculation and biosorption conditions with the Ni(II) removal rate as the response, as well as to analyze the biosorption capacity. At initial Ni(II) concentration of 100 mg L−1, the optimal conditions were predicted to be 1.3 × 10−2% (w/w) CaO, 6.59 × 10−3% (w/w) MBFGA1, and stirring time of 61.97 min, at which the Ni(II) removal rate and biosorption capacity of MBFGA1 could reach 99.35% and 225.16 mg g−1, respectively. The biosorption behavior, Fourier-transform infrared spectra, and environmental scanning electron microscopy analysis demonstrated that adsorption bridging with precipitation enmeshment was the most likely mechanism. Analysis of the mechanism and procedure indicated that synergistic flocculation and biosorption by MBFGA1 resulted in the significant Ni(II) removal.

•Electrocoagulation process for arsenic removal was evaluated.•Transformation and characterization of EC products were investigated.•Mechanism of As(III) oxidation and arsenic adsorption behavior were elucidated.•Future research needs for arsenic removal by electrocoagulation were suggested.Arsenic, classified as a carcinogen, is being subject to high concern due to its high toxicity especially in drinking water. Electrocoagulation (EC) has displayed a great potential as an effective and environmentally friendly method to remove arsenic from wastewaters. This review summarizes the recent development of arsenic removal in EC process including the effects of primary operating parameters, optimization of the EC performance, as well as the evaluation of EC reactor configurations. Production and characterization of EC products with respect to different electrodes are systematically discussed. Besides, this review sheds light on the debate about the mechanism involved in As(III) oxidation and further explores the arsenic adsorption behavior in EC process. Moreover, the performance of EC and other technologies are compared, and future research needs for arsenic removal in EC process are suggested accordingly. Overall, this review will contribute to deepening the understanding of EC process for arsenic removal and offer useful information to researchers in this field.

Herein, we report the creative combination of foaming and back mixing. Back mixing was simulated by the addition of dried foamed sludge (DFS) to raw sludge. Different ratios of (DFS + CaO), DFS shape and dosing sequence were investigated on the influence of sludge foamability and drying efficiency. Experimental results indicate that back mixing has positive effects on sludge foaming and sludge foam stability. Moreover, CaO is still dominant in the sludge foam. The best addition ratio is 10 g DFS + 10 g CaO for 1 kg of fresh sludge with an optimal dosing sequence of first CaO followed by DFS after 5 min. In addition, foam-mat drying for dewatered sludge is not greatly subjected to the DFS shape. During foam-mat drying, a high drying rate appears at a high foam density (>0.70 g cm−3). The foamed sludge of 0.80 g cm−3 has the fastest drying speed at 30 °C, whereas the best drying density is 0.90 g cm−3 at 50 °C. Moreover, the drying rates of foamed sludge were higher when the temperature was increased from 30 °C to 50 °C. In addition, mathematical modelling results demonstrate that the Logistic model is the most adequate model to describe the entire convective drying of thin layer sludge under the best drying density both at 30 °C and at 50 °C.

Today, improving the elimination of refractory pollutants in landfill leachate through electrochemical oxidation technology has attracted considerable attention. In this study, a combination of anodic oxidation and cathodic coagulation process using Ti/RuO2–IrO2 and Al electrodes, was adopted to treat the mature landfill leachate with a very low biodegradability ratio (BOD5/COD) of 0.12. The effects of current density, pH, and the chloride ion concentration on the removal of chemical oxygen demand (COD) and ammonia nitrogen (NH3–N) were investigated by response surface methodology (RSM). The optimum condition of 83.7% COD and 100% NH3–N removal was achieved with a current density of 0.1 A cm−2 and a pH of 6.37, the chloride ion concentration 6.5 g L−1, and an electrolytic time of 150 min. In addition, heavy metals were partly removed. A main degradation mechanism of the pollutants, including oxidation, coagulation and precipitation, was elucidated by gas chromatography-mass spectrometry (GC-MS), environmental scanning electron microscopy coupled with energy dispersive spectrometer (ESEM/EDS) and Fourier transform infrared spectroscopy (FT-IR) analysis of organic components in landfill leachate and sludge generated at the cathode. These results indicated that the electrochemical processes could be a convenient and efficient method for the treatment of landfill leachate.

•Initial pH, Cr(VI) and APC could affect the behavior of dissolution/passivation in Fe-EC.•A dissolution/passivation region was constructed with different initial pH-Cr(VI).•The film was rich in Fe and Cr at high Cr(VI), whereas with lots of Fe but negligible of Cr at low Cr(VI).•The film was non-protective at long TAPC, but became more stable and protective at short TAPC.•Behavior of dissolution/passivation and passive film transformation in Fe-EC was elucidated.The passivation behavior of an iron anode for electrocoagulation (EC) was first investigated using response surface methodology (RSM). Tested initial pH range, Cr(VI) concentration and alternating pulsed current (APC) were 4.0 to 8.0, 52 to 520 mg L−1 and 10 to 590 s, respectively. The distance between electrodes was 25 mm, and K2SO4 (1 g L−1) was used as the supporting electrolyte in a 2.5 L EC reactor. Results confirmed that initial pH, Cr(VI) concentration, and APC significantly influence the extent of passivation. Then, based on the interaction effect on passivation behavior between initial pH and Cr(VI) in RSM, a pH-Cr(VI)-dissolution/passivation diagram was constructed with galvanostatic measurements. The diagram showed an optimal dissolution region for EC operation. This optimum was characterized by a reasonable final pH for extended precipitation and little passivation. Results of the cyclic voltammetry and X-ray photoelectron spectroscopy revealed a significant difference in the composition and stability of oxide films in the region with more pronounced passivation. Interestingly, the APC had both positive and negative effect on the passivation behavior. Long period of APC (TAPC = 590 s) produced a non-protective film, which favored the Fe0 dissolution. However, a more stable and protective passive film with a uniform structure of Fe and Cr oxides was formed by short TAPC (10 s). Based on the above results, this study elucidated the behavior of dissolution/passivation and the transformation of passive films during the Fe-EC process for Cr(VI) treatment.

An innovative pretreatment technology, in which CaO was jointly added with NaOH followed by appropriate mechanical whipping, was investigated for the foaming and drying of sewage sludge (SS). Foaming efficiency, appropriate density of the best drying performance and drying temperature were examined. Considering high foaming efficiency and low energy consumption, the optimal mixture was found to be 1.0 wt% NaOH with 1.0 wt% CaO, which takes only 50% of the foaming time to reach the appropriate density (0.6 g mL−1) compared to that with 2.0 wt% CaO. When compared with SS, the optimal mixture also saved 20%–26% drying time to reach 40% moisture content. Non-linear regression techniques (Nonlinear Curve Fit (Dose Resp)) were applied to simulate the drying process of S2:2. The kinetics of sludge was studied by thermogravimetry analysis using the Starink method with three heating rates (10, 20 and 50 K min−1). Different trends in apparent activation energy support the conclusion that differences appear in the chemical compositions of sludge samples that have different ratios of additives. Moreover, the TG analysis results provide referable combustion data for the sludge samples in this research.

The linkage between reactor performance and microbial extracellular polymeric substances (EPS) was investigated in three groups of semi-continuous mesophilic anaerobic co-digestion (ACoD) systems, treating municipal waste sludge (MWS) with food waste (FW) with different fat, oil and grease (FOG) contents. The addition of FOG to the test reactors enhanced the co-digestion process significantly in terms of reactor performance and microbial activity. During the process, no major variations in pH and VFA/Alk were observed. Moreover, the daily yield of biogas peaked at 810.3 mL per g VSadded when the FOG load reached 42% of volatile solids (VS), with an organic loading rate (OLR) of 5.2 g VS L−1 d−1 and a hydraulic retention time (HRT) of 20 days. However, an excessive FOG load (55% of VS) reduced biogas production by 40.3% when compared with the control unit (539.3 mL per g VSadded). At the end of digestion, 195 L, 381 L and 351 L cumulative biogas were obtained in the three systems, respectively. Further analysis of extracellular polymeric substances (EPS) showed that the accumulation peaked at 648.5, 772.3 and 640.9 mg L−1 with the optimal digestion parameters, respectively. The proportion of LB-EPS was always less than that of TB-EPS, which accounted for about 40% and 60%. The FOG-enhanced systems (R2 and R3) produced considerably higher levels of EPS than the control system (R1) for both humic acid substances (HS) and proteins (PN). Moreover, variations in EPS revealed that the three systems experienced an accommodation phase followed by a vigorous phase and an exhausted phase with elevated levels of added FOG. However, enhanced units may undergo exhaustion prematurely due to “doping” phenomena.

In this study, electrocoagulation (EC) with hybrid Fe–Al electrodes was used to remove antimony from contaminated surface water. Response surface methodology was applied to investigate the interactive effects of the operating parameters on antimony removal and optimize these variables. Results showed that the relationship between operating parameters and the response was well described by a second-order polynomial equation. Under the optimal conditions of current density 2.58 mA/cm2, pH 5.24, initial concentration 521.3 μg/L, and time 89.17 min, more than 99 % antimony were removed. Besides, the antimony adsorption behavior in EC process was also investigated. Adsorption kinetics and isotherms studies suggested that the adsorption process followed well the pseudo-second-order kinetic model and the Langmuir adsorption model, respectively. Adsorption thermodynamics study revealed that the reaction was spontaneous, endothermic, and thermodynamically favorable. These results further proved that the main mechanism involved in antimony removal in EC process could be chemisorption.

Extracellular electron transfer (EET) of microorganisms represents a communicative bridge between the interior and exterior of the cells. Most prior EET studies have focused on Gram-negative bacteria. However, fungi and Gram-positive bacteria, that contain dense cellular walls, have rarely been reported. Herein, two model dense cell wall microorganisms (Bacillus sp. WS-XY1 and the yeast Pichia stipitis) were identified to be electrochemically active. Further analysis indicated that the two microorganisms were able to secrete flavins to mediate their EET. The discovery, that dense cell wall containing microorganisms can undertake mediated EET, adds to the body of knowledge towards building a comprehensive understanding of biogeochemical and bioelectrical processes.

In this work, the effect of flow fields on flocculation efficiency in a kaolin suspension using microbial flocculant GA1 (MBFGA1) was studied. The flow fields were controlled via mechanical mixing of a Rushton turbine in a fully baffled flocculation reactor and simulated by a three-dimensional Computational Fluid Dynamic (CFD) model. Good agreement between experimental and simulated results confirmed the validity and applicability of the CFD model. By integrating flocculation tests with CFD simulations, it was shown that the impeller with different speeds generated different flow fields, and hence offered different efficiencies for flocculation. Flow fields of rapid mixing at 400 rpm then slow mixing at 80 rpm, which formed the largest flocs (538 ± 18 μm), achieved the optimum flocculation efficiency, i.e. the lowest residual turbidity (3.03 ± 0.10 NTU) and the maximum flocculation rate (98.2 ± 0.8%). The two-loop flow pattern associated with the distribution of velocity magnitude, local shear rate and turbulent kinetic energy provided an improved understanding of flow behaviors within the reactor. Additionally, charge neutralization and adsorption bridging were proposed as the main flocculation mechanisms of MBFGA1.

Exposure to antimony (Sb) and arsenic (As) through contaminated surface water poses a great threat to human health. In this work, Sb and As were removed simultaneously by electrocoagulation (EC) using Fe–Al electrodes. The effects of current density, pH, initial concentration, aeration intensity, and anions were investigated. A higher current density achieved better removal performance. The optimum pH range was 5.0–7.0. Sb and As removals were slower at higher initial concentrations. Preoxidation was beneficial to As removal, whereas anoxic conditions were more favorable for Sb removal. Nitrate and sulfate had little influence on the performance of the EC process, but significant inhibition was observed in phosphate-rich solutions. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) analysis demonstrated that adsorption onto iron and aluminum hydroxides/oxyhydroxides was the predominant mechanism involved in Sb and As removal. Finally, over 99% of Sb and As were removed from a practical wastewater sample, indicating that EC using Fe–Al electrodes provides an alternative method for Sb and As removal.

•RVC/PANI-SA-GLY electrode was applied as a novel electrode material for accelerated removal of Cr(VI).•Faster reduction kinetics of Cr(VI) was observed by RVC/PANI-SA-GLY electrode when compared with RVC/PANI-SA and RVC electrode.•Cr(VI) removal experienced an adsorption-reduction system built by RVC/PANI-SA-GLY electrode.•The stability of RVC/PANI-SA-GLY electrode was relatively satisfactory.Improving the reduction kinetics is crucial in the electroreduction process of Cr(VI). In this study, we developed a novel adsorption–electroreduction system for accelerated removal of Cr(VI) by employing reticulated vitreous carbon electrode modified with sulfuric acid–glycine co-doped polyaniline (RVC/PANI-SA-GLY). Firstly, response surface methodology confirmed the optimum polymerization condition of co-doped polyaniline for modifying electrodes (Aniline, sulfuric acid and glycine, respectively, of 0.2 mol/L, 0.85 mol/L, 0.93 mol/L) when untraditional dopant glycine was added. Subsequently, RVC/PANI-SA-GLY showed higher Cr(VI) removal percentages in electroreduction experiments over RVC electrode modified with sulfuric acid doped polyaniline (RVC/PANI-SA) and bare RVC electrode. In contrast to RVC/PANI-SA, the improvement by RVC/PANI-SA-GLY was more significant and especially obvious at more negative potential, lower initial Cr(VI) concentration, relatively less acidic solution and higher current densities, best achieving 7.84% higher removal efficiency with entire Cr(VI) eliminated after 900 s. Current efficiencies were likewise enhanced by RVC/PANI-SA-GLY under quite negative potentials. Fourier transform infrared (FTIR) and energy dispersive spectrometer (EDS) analysis revealed a possible adsorption–reduction mechanism of RVC/PANI-SA-GLY, which greatly contributed to the faster reduction kinetics and was probably relative to the absorption between protonated amine groups of glycine and HCrO4−. Eventually, the stability of RVC/PANI-SA-GLY was proven relatively satisfactory.

Phosphate (P) removal is significant for the prevention of eutrophication in natural waters. In this paper, a novel adsorbent for the removal of P from aqueous solution was synthesized by loading zirconium oxide and iron oxide onto activated carbon nanofiber (ACF-ZrFe) simultaneously. The adsorbent was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The results showed that P adsorption was highly pH dependent and the optimum pH was found to be 4.0. The isotherm of adsorption could be well described by the Langmuir model and the maximum P adsorption capacity was estimated to be 26.3 mg P/g at 25 °C. The kinetic data were well fitted to the pseudo-second-order equation, indicating that chemical sorption was the rate-limiting step. Moreover, co-existing ions including sulfate (SO42−), chloride (Cl−), nitrate (NO3−) and fluoride (F−) exhibited a distinct effect on P adsorption with the order of F− > NO3− > Cl− > SO42−. Further investigations by FT-IR spectroscopy and pH variations associated with the adsorption process revealed that ligands exchange and electrostatic interactions were the dominant mechanisms for P adsorption. The findings reported in this work highlight the potential of using ACF-ZrFe as an effective adsorbent for the removal of P in natural waters.