| Azo dyes are the largest chemical class of dyes and are frequently used for textile dying and paper printing industries due to cheap costs, firmness, and a variety of colors compared to natural dyes. However, azo dyes are highly toxic and persistent to biodegradation. Besides, the intensive color of dye-containing wastewater leads to severe aesthetic problems and obstructs light penetration and oxygen transfer into water bodies, adversely affecting aquatic life. For these reasons, the color removal from dye-containing wastewater is one of major concerns in China where textile industry has grown exponentially in recent years. Azo dyes should be removed from wastewater before being discharged to water body. More recently, Bioelectrochemical system (BES) is emerging as a promising technology in which microorganisms function as catalysts to convert chemical energy into electrical energy. BES has been tested for many potential applications besides electricity generation, including biohydrogen prodution, metal reduction and recovery, desalination, organic products synthesis and treatment of various wastewaters.This study focused on the configuration improvement of BES. Aliarin Yellow R (AYR) was selected as a model azo dye and was removed from wastewater using BES. A novel up-flow biocatalyzed electrolysis reactor (UBER) was developed. Then, anaerobic process was coupled with UBER to establish a process named AD-UBER. Basing on this, a small pilot scale anaerobic baffled reactor coupled with UBER (ABR-UBER) was developed.Firstly, we developed an economical BES lacking membrane (an up-flow biocatalyzed electrolysis reactor (UBER) for azo dye removal. This design avoided the problems of traditional bioelectrochemical reactor, such as high cost and difficult scaling-up. The toxic inhibition of azo dye (AYR) to the bioelectrochemical microoganisms was tested in a dual-chamber biocatalyzed electrolysis reactor to identify that AYR was highly toxic to the anodic bacteria. Anodic bacteria lost activity when AYR concentration reached40mg/L in the culture and current generation stopped. Thus, the cathode of UBER was placed at the bottom of the reactor to well protect the bioanode that set above the cathode. The elimination of membrane decreased the cost greatly. With the supply of external power source0.5V in the UBER, AYR decolorization efficiency reached up to94.8±1.5%. Electron recovery efficiency based on AYR removal in cathode zone was nearly100%at HRT longer than6h. Relatively high concentration of AYR accumulated at higher AYR loading rates (>780g/m3·d) likely inhibited acetate oxidation of anode-respiring bacteria on the anode, which decreased current density in the UBER; optimal AYR loading rate for the UBER was680g/m3·d (HRT2.5h).To optimize the performance of UBER, the effect of cathde volume, which was a key parameter of UBER, was investigated at a constant andoe volume. Decolorization efficiency was improved with increasing cathode size in UBERs, but AYR removal rate and current density did not increase in proportion to cathode size indicating that bigger cathode was more efficient for azo dye removal while most of the cathode could not be utilized effectively. However, smaller cathode volume was disbenefit to the stable operation of UBER. The best performance of UBER was obtaind when the volume ratio of cathode to anode was2:1where the charge transfer resistance Rct (39.5Ω) was minimal. AYR and its reductive products were further mineralized in the subsequent aerobic bio-contact oxidation reactor (ABOR). Decolorization efficiency and COD removal efficiency was93.8±0.7%and93.0±0.5%in the combined process of UBER and ABOR in overall HRT6h (HRT2.5h in UBER+HRT3.5h in ABOR). The Chroma in ABOR effluent was80times, which was satisfied with the texile wastewater discharge standard II. A possible AYR tranfermaion pathway was aproposed.Based on the design of UBER, we developed an anaerobic digestion-up-flow biocatalyzed electrolysis reactor (AD-UBER). A sludge bed was set below the UBER. Bioelectrochemical reaction was cooperated with anaerobic biological reaction for azo dye removal and the decolorization efficiency was higher than90%. The mechanism of AD-UBER was analyzed by comparing with the process of UBER and AD. The best performance of AD-UBER was obtained when the eletrodes were placed in the aqueous phase.Finally, we intended to combined several AD-UBERs together and enlarge the reacor. Therefore, a small pilot scale system that integrated UBER with ABR by installing UBER module into each compartment of ABR (called, ABR-UBER) was established for azo dye wastewater treatment. The ABR-UBER was operated without and with external power supply to examine AYR reduction process and reductive intermediates with different external voltages (0.3,0.5and0.7V) and hydraulic retention times (HRT:8,6and4h). The decolorization efficiency in the ABR-UBER (8h HRT,0.5V) was higher than that in ABR-UBER without electrolysis, i.e.95.1±1.5%versus86.9±6.3%. Higher power supply (0.7V) enhanced AYR decolorization efficiency (96.4±1.8%), VFAs removal, and current density (24.1A/m3·TCV). Shorter HRT increased volumetric AYR decolorization rates, but decreased AYR decolorization efficiency. This indicated that each compartment of ABR-UBER could endure a high AYR loading rate, while the capalities of former compartements were the limit fact to the overall performance. The latter compartments still worked well at shorter HRT of4h. The novel ABR-UBER with membrane-free provided a new concept for BES scaling-up to energy-efficient treatment of azo dye wastewater. |
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