Haiyin Xu

Effects of temperature and organic loading rate on the performance and microbial community of anaerobic co-digestion of waste activated sludge and food waste

Anaerobic co-digestion of waste activated sludge and food waste was investigated semi-continuously using continuously stirred tank reactors. Results showed that the performance of co-digestion system was distinctly influenced by temperature and organic loading rate (OLR) in terms of gas production rate (GPR), methane yield, volatile solids (VS) removal efficiency and the system stability. The highest GPR at 55 °C was 1.6 and 1.3 times higher than that at 35 and 45 °C with the OLR of 1 g VSL(-1)d(-1), and the corresponding average CH₄ yields were 0.40, 0.26 and 0.30 L CH₄ g(-1)VSadded, respectively. The thermophilic system exhibited the best load bearing capacity at extremely high OLR of 7 g VSL(-1)d(-1), while the mesophilic system showed the best process stability at low OLRs (< 5 g VSL(-1)d(-1)). Temperature had a more remarkable effect on the richness and diversity of microbial populations than the OLR.

The adsorption behavior and mechanism investigation of Pb(II) removal by flocculation using microbial flocculant GA1

In this work, microbial flocculant GA1 (MBFGA1) was used to remove Pb(II) ions from aqueous solution. A series of experimental parameters including initial pH, MBFGA1 dose, temperature and initial calcium ions concentration on Pb(II) uptake was evaluated. Meanwhile, the flocculation mechanism of MBFGA1 was investigated. The removal efficiency of Pb(II) reached up to 99.85% when MBFGA1 was added in two stages, separately. The results indicated that Pb(II) adsorption could be described by the Langmuir adsorption model, and being the monolayer capacity negatively affected with an increase in temperature. The adsorption process could be described by pseudo-second-order kinetic model. Fourier transform-infrared spectra and environmental scanning electron microscope analysis indicated that MBFGA1 had a large number of functional groups, which had strong capacity for removing Pb(II). The main mechanisms of Pb(II) removal by MBFGA1 could be charge neutralization and adsorption bridging.

Combination of cathodic reduction with adsorption for accelerated removal of Cr(VI) through reticulated vitreous carbon electrodes modified with sulfuric acid-glycine co-doped polyaniline.

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.

Anaerobic co-digestion of municipal wastewater sludge with food waste with different fat, oil, and grease contents: study of reactor performance and extracellular polymeric substances

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.

Assessing the effect of flow fields on flocculation of kaolin suspension using microbial flocculant GA1

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.

Electrochemical treatment of mature landfill leachate using Ti/RuO2–IrO2 and Al electrode: optimization and mechanism

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.

Drying behavior and thermo-gravimetrical kinetic analysis of foam-pretreated sewage sludge

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.