Electrochemical oxidation has been proposed for the destruction of organic contaminants; however, this process is hampered by low oxidation efficiency and high energy cost. Accordingly, we developed a TiO2-based SnO2-Sb/polytetrafluroethylene resin (FR)-PbO2 electrode that was based on TiO2 nanotubes. We tested the performance of the electrode on an antibiotic, ofloxacin, and identified the major pathway of ofloxacin oxidation. We found growing TiO2 nanotubes on Ti material increased current efficiency, and the electrical efficiency per order (EE/O, kWh/m 3) for oxidation was decreased by 16.2%. Our electrode requires a large overpotential before electrons flow, which minimizes oxygen evolution, reduces hydrogen peroxide and ozone generation, and favors hydroxyl radicals (HO•) production. We found the electron efficiency (EE) during oxidation was as high as 88.45%. In other words, 88.45% of the electrons that flow out of the electrode cause oxidation. The effects of current density, initial concentration, pH value and electrolyte concentration were investigated. A differential column batch reactor (DCBR) was used to simulate the performance of continuous plug flow reactor and we found that the destruction of ofloxacin followed pseudo-first order model. We also evaluated the impact of mass transfer on electrochemical performance. The effects of fluid velocity and electrode spacing on oxidation rate were evaluated by determining the mass transfer coefficient and the effectiveness factor Ω (between 0-1). Our experiments and calculations indicated that the mass transfer reduced oxidation rate by more than 55% (Ω<0.45) for an electrode spacing of 1 cm at a fluid velocity of 0.033 m/s. Unlike studies carried out using completely mixed batch reactor, the DCBR can simulate the flow conditions in pilot or full scale reactors; consequently, observed pseudo-first order rate constants in the DCBR can be used for preliminary design.
A novel approach to rapidly initiate granulation of hydrogen-producing sludge was developed in an anaerobic continuous stirred tank reactor at 37 degrees C. To induce microbial granulation, the acclimated culture was subject to an acid incubation for 24 h by shifting the culture pH from 5.5 to 2.0. The culture was resumed to pH 5.5 after the incubation and the reactor was operated at hydraulic retention times (HRTs) of 12, 6, 2, 1, and 0.5 h in sequence. Microbial aggregation took place immediately with the initiation of acid incubation and granules were developed at 114 h. No granule was observed in the absence of acid incubation in the control test. Changing the culture pH resulted in improvement in surface physicochemical properties of the culture favoring microbial granulation. The zeta potential increased from -11.6 to -3.5 mV, hydrophobicity in terms of contact angle improved from 31 degrees to 43 degrees and extracellular proteins/polysaccharides ratio increased from 0.2 to 0.5-0.8. Formation of granular sludge facilitated biomass retention of up to 32.2 g-VSS/L and enhanced hydrogen production. The hydrogen production rate and hydrogen yield increased with the reduction in HRT at an influent glucose concentration of 10 g/L once steady granular sludge layer was formed, achieving the respective peaks of 3.20 L/L x h and 1.81 mol-H(2)/mol-glucose at 0.5 h HRT. The experimental results suggested that acid incubation was able to initiate the rapid formation of hydrogen-producing granules by regulating the surface characteristics of microbial aggregates in a well-mixed reactor, which enhanced the hydrogen production.
The cultivation of stable aerobic granules as well as granular structure and stability in sequencing batch reactors under different shear force were investigated in this study. Four column sequencing batch reactors (R1-R4) were operated under various shear force, in terms of superficial upflow air velocity of 0.8, 1.6, 2.4, and 3.2 cm s(-1), respectively. Aerobic granules were formed in all reactors in the experiment. It was found that the magnitude of shear force has an important impact on the granule stability. At shear force of 2.4 and 3.2 cm s(-1), granules can maintain a robust structure and have the potential of long-term operation. Granules developed in low shear force (R1, 0.8 cm s(-1) and R2, 1.6 cm s(-1)) deteriorated to large-sized filamentous granules with irregular shape, loose structure and resulted in poor performance and operation instability. Granules cultivated under high shear force (R3, 2.4 cm s(-1) and R4, 3.2 cm s(-1)) stabilized to clear outer morphology, dense and compact structure, and with good performance in 120 days operation. Fractal dimension (Df) represents the internal structure of granules and can be used as an important indicator to describe the structure and stability of granules. Due to the combined effects of shear force and growth force, the mature granules developed in R3 and R4 also displayed certain differences in granular structure and characteristics.
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