| Dyeing wastewater and chemical industry wastewater are two kinds of typical industry wastewater. Anthraquinone dyes constitute the second largest class of textile dyes after azo dyes, which are extensively used in the textile and dyeing industry because of their wide variety of color shades, high wet fastness profiles, ease of application, brilliant colors, and minimal energy consumption. Under typical industrial conditions, about 50 % of the dyes remain in the spent dye in unfixed or hydrolyzed form resulting in colored effluent, which brings major aesthetic problems for the industry. Thus, environmental concerns and the need of meeting the stringent international standards for rejecting wastewater have made the development of efficient and low cost processes for dealing with textile aqueous effluents an issue of major technological importance. Chlorophenols, such as 2,4-dichlorophenol (2,4-DCP), are used extensively in the manufacture of pesticides, herbicides, glue, paint, leather and pulp and wood preservatives. Due to its manufacture in large quantities and toxicity, DCP is an Environmental Protection Agency (EPA) priority pollutant identified in hazardous waste sites identified on the National Priorities List.The objectives of the present work were to investigate the feasibility of a batch operated biofilm and activated sludge hydrolytic-aerobic recycling process (HARP) in degrading anthraquinone dye Reactive Blue KN-R (KN-R) and 2,4-DCP in wastewater. This study may be valuable for experimental design and engineering application of high concentration organic wastewater treatment by the batch hydrolytic-aerobic recycling process.The results showed that recycling flux, hydrolytic/aerobic reactor volume ratio and aeration flux were the key process parameters for the batch operated biofilm and activated sludge hydrolytic-aerobic recycling process in treating KN-R and 2,4-DCP wastewater, which had great effects on removal efficiency and rate, stability of process, activity of microorganisms and characteristics of activated sludge and so on.When the synthetic wastewater of KN-R 600 mg/L and glucose 2000 mg/L was treated by the batch operated biofilm hydrolytic-aerobic recycling process with recycling flux 10 mL/min, aeration flux 3 L/min, hydrolytic/aerobic reactor volume ratio 2:2, chemical oxygen demand (COD) and color removal efficiency could be up to 94.2 % and 94.4 %, respectively. Increase in recycling flux would alleviate acidification degree in the hydrolytic reactor and decrease volatile fatty acids (VFA) accumulation. Moreover, COD and color removal efficiency and average removal rate could be greatly enhanced. However, when recycling flux was much higher, the dissolved oxygen (DO) in the hydrolytic reactor was gradually increasing, which resulted in the fact that the activity of hydrolytic microorganisms was depressed and removal efficiency of COD and color was decreased.When the synthetic wastewater of KN-R 200 mg/L and glucose 2000 mg/L was treated by the batch operated activated sludge hydrolytic-aerobic recycling process with recycling flux 10 mL/min, hydrolytic/aerobic reactor volume ratio 2:2, aeration flux 3 L/min, COD and color removal efficiencies reached 90 % and 85 %, respectively. With recycling flux increasing from 5 mL/min to 15 mL/min, COD and color removal efficiency increased and the maximum of VFA decreased. Meanwhile, the increase of the content of polysaccharide (PS) and protein (PN) in extracellular polymeric substances (EPS) of hydrolytic and aerobic activated sludge resulted in the increasing of Zeta potential and accelerated the conglutination and the settling of activated sludge.When the synthetic wastewater of 2,4-DCP 20 mg/L and glucose 2000 mg/L was treated by the batch operated biofilm hydrolytic-aerobic recycling process with recycling flux 10 mL/min, hydrolytic/aerobic reactor volume ratio 2:2, aeration flux 3 L/min, COD and 2,4-DCP removal efficiencies could be up to 95 % and 99 %, respectively. COD and 2,4-DCP removal efficiency rose with recycling flux increasing from 5 mL/min to 10 mL/min. However, when recycling flux was above 10 mL/min, the trace amounts of oxygen transported to the hydrolytic reactor by recycling solution from the aerobic reactor could reduce the activity of hydrolytic microorganisms. COD and 2,4-DCP removal efficiency could be greatly enhanced by increasing aeration flux.When the synthetic wastewater of 2,4-DCP 20 mg/L and glucose 2000 mg/L was treated by the batch operated activated sludge hydrolytic-aerobic recycling process with recycling flux 15 mL/min, hydrolytic/aerobic reactor volume ratio 1:3, aeration flux 3 L/min, COD and 2,4-DCP removal efficiencies reached 95 % and 96 %, respectively. With recycling flux increasing from 5 mL/min to 15 mL/min, COD and 2,4-DCP removal efficiency increased and the maximum of VFA decreased.Meanwhile, the increase of the content of PS and PN in EPS of hydrolytic and aerobic activated sludge resulted in the decreasing of Zeta potential. As a result, the conglutination and the settling performance of activated sludge were enhanced. The linear increase in PN/PS values with increasing of recycling flux was observed. Hydrolytic/aerobic reactor volume ratio had great effects on 2,4-DCP and COD removal. When hydrolytic/aerobic reactor volume ratio was 1:3, VFA concentrations were much lower, which indicated that the activity of hydrolytic microorganisms had not been inhibited and the whole process maintained a stable condition. The content of PS and PN in EPS of hydrolytic and aerobic activated sludge were highest and the Zeta potential was lowest at volume ratio 1:3. This implied that the stability of hydrolytic and aerobic activated sludge during the recycling process was enhanced.The above results suggested that the KN-R and 2,4-DCP in wastewater could be highly removed in the batch operated hydrolytic-aerobic recycling process. The exchange of metabolites between hydrolytic reactor and aerobic reactor could be compared to the metabolite exchange at interfaces between the anaerobic and aerobic zones of natural eco-process (sediments, bacterial colonies, stratified lakes and seas, microbial mats, biofilm, etc.). But the resistance to mass transfer across the hydrolytic-aerobic"interface"in reactors of the recycling process was much lower than it was in the natural process. The hydrolytic-aerobic recycling process benefited from the combination of"reductive and oxidative degradation mechanisms"and"cooperative metabolism"caused by the exchange of metabolites between hydrolytic and aerobic reactor. The metabolic and kinetic limitations to hydrolytic and aerobic microorganisms could be overcome in the recycling process. Meanwhile, the hydrolytic-aerobic recycling process could successfully solve the problem of over-acidification and effectively enhance the removal efficiency and rate of KN-R, 2,4-DCP and COD.
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