| Membrane bioreactor(MBR) employed membrane filtration to substitute conventional clarifier in activated sludge process to ensure effluent quality and stability. Researchers had found that activated sludge in MBR suspension and its characteristics are important factors to membrane fouling, which constrained MBR widespread application for wastewater treatment and reclamation. In contrast, attached growth process, i.e. biofilm system possessed many potential advantages for MBR combination, including a low biomass in suspension, a long sludge age and food chain, as well as a low liquid organic matter concentration, all of which would be beneficial for membrane fouling minimization. In the present work, membrane filtration was combined with a biofilm process to form a BF/MBR to evaluate its treatment performance, membrane fouling behavior and its control. Special attention was paid to the influence of biofilm content on pollutant removal and membrane fouling behavior, and the relationship between biofilm properties, liquid organic matters and membrane fouling was analyzed systematically.Four BF/MBR systems with different carriers area(R1 2.5:1, R2 5:1, R3 10:1, R4 18:1) were operated side by side, and compared with a conventional MBR under the same organic volumetric loading rate. The influence of carrier surface area on pollutant removal performance, membrane fouling evolution and supernatant specific resistance was investigated. Experimental results showed that there was insignificant differences between these four BF/MBR systems caused by carrier area variation. The average COD and ammonia removal efficiency was 92% and 95%, respectively. However, a larger carrier area would result in a relative high biomass, and thus to cause a high substrate utilization rate, which evidenced that BF/MBR could improve microbial activity to some extent. Carrier surface area related closely with the properties of suspended sludge, and a large area led to a large average particle size, which attributed to a great shear force around membrane surface that would reduce the particles deposition rate onto membrane surface. Moreover, the structure of floc on membrane surface was quite loose, and the porosity of cake layer was high, both of which may due to the dominant growth of biofilm in BF/MBR systems. These phenomena were significant to decelerate cake layer formation with continuous membrane filtration.The membrane fouling rate in BF/MBR systems was obviously lower compared with those of conventional MBR. Moreover, the supernatant specific resistance showed a positive relationship with supernatant soluble microbial products(SMP) and biopolymer cluster(BPC). TMP growth rate in R4 was slower than those in conventional MBR, and its possible reason was that supernatant SMP and specific resistance were kept stably lower than 7.17 mg/g MLSS and 1.75×1012 m/kg, respectively. The supernatant specific resistance related closely with the TMP growth rate. It was observed that SMP and BPC in R1 and R4 had a remarkably impact on supernatant specific resistance(R2 are 0.4246, 0.2898 respectively), and this relation was significant in R0. SMP proteins component was an important factor on membrane fouling in R1 and R2, while in R4 with a large number biofilm, BPC become an important, although its fouling rate decreased eventually.Based on the fact that a limited biofilm content would resulted in a high concentration of SMP and BPC that deduced a severe fouling, a side-stream ozonation method was used to reduce SMP and BPC concentration in the MBR mixed liquor to control membrane fouling. After a long-term lab-scale study, it was revealed that with an ozone dosage ratio of 0.10 mg O3/mg SMP, an improvement on biodegradability and a reduction of liquid specific resistance were observed, while there was insignificant effect to microbial activities and floc structures, and to pollutants removal efficiencies. |
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