Study on the Influence of Sludge Retention Time on Operation and Membrane Fouling in A/O Membrane Bioreactor
|School||Qingdao Technological University|
|Keywords||Sewage treatment and reuse A/O-SMBR Sludge retention time (SRT) Membrane fouling Soluble microbial product (SMP)|
The thesis was supported by State Key Lab Foundation of Western Architecture & Technology (Cultivating Base)–Study on Sewage Treatment and Recycling Technology in Scattered Neighborhoods (No. 08KF08). The thesis studied on operating characteristics of MBR used in sewage treatment and reuse.By consulting the relevant literatures, it was found that the experimental results were very different even opposite among different researchers on the MBR operating affected by sludge age (SRT). The research scope was concentrated for SRT in previous studies, which was at low/middle or middle/high level. Therefore, in this thesis the MBR operating conditions were observed under various SRT from low to high level, in order to reveal the essential reason of MBR affected by SRT, especially membrane fouling, and provide references for extending the stable operation cycle of MBR.The MBR technology was combined with traditional anoxic/aerobic (A/O) process to establish submerged A/O-MBR wastewater treatment system (A/O-SMBR). In the experiment, the campus sewage was treated by A/O-SMBR. The contents of the experimental research included:①The treatment efficiency and operation stability were observed under 10d, 30d and 90d SRT orderly, and optimal SRT level was obtained by contrast;②The impacts of SRT on the properties (MLSS, PSD, metabolites, SVI, sludge specific resistance and viscosity) of activated sludge mixed liquor were analyzed. And according to the stable operation period under corresponding SRT, the relationship was illustrated between each property and membrane fouling, in order to reveal the most direct impact factor, namely the essential reason of MBR affected by SRT;③The joint models of filtering mechanism were utilized to carry on curve fitting for the slow phase of membrane fouling at different SRT level. The fouling formation mechanism was studed in depth respectively.At various SRT level, there were differences in the treatment efficiency, but the pollutant concentrations in effluent all met reuse water quality standards. The retention rate of suspended solids was almost 100% in the microfiltration. The SS was not detected and the turbidness was less than 0.5NTU in effluent under each SRT. The system of A/O-SMBR possessed strong resistance capacity to shock loading. The CODCr concentration in effluent was always 40mg/L or less when organic loading fluctuated in 1.15~1.92. The removal rate of membrane to CODCr was high relatively under low and high SRT, which showed that a large number of macromolecular organic matter retained and deposited on the membrane. The removal rate of NH3-N and TN was 94.31% and 70.59% separately at 30d SRT, which was better than that at other SRT. The nitrification and denitrification abilities were week relatively under low and high SRT, for the constraint of DO and microbial metabolites et al. As the lack of strict anaerobic environment and the limit of sludge discharge cycle, the TP removal effect was not good for A/O-SMBR. The TP concentration was higher in effluent.It lasted about 12d, 33d and 90d repectively from the beginning of membrane filtration to TMP reaching limit value under 10d, 30d and 90d SRT. By contrast, the stable operation cycle was longer at middle SRT. The structure of fouling layers on membrane was adverse to membrane filtration and impacted on the reactor operating steadily at low and high SRT by SEM, whose filtering ratio resistance was high.The impacts of SRT on six properties of activated sludge mixed liquor were analyzed. MLSS, PSD and viscosity rised with SRT extending. Etracellular polymeric substance (EPS) production rate and sludge specific resistance decreased with SRT extending. Soluble microbial product (SMP) concentration and SVI were minimum at middle SRT. These properties impacted on membrane fouling directly or indirectly. The relationship between MLSS and fouling was complex. It was difficult that the fouling degree was determined only by MLSS, but whose content impacted on microbial metabolome production. The sludge particle size was smaller, the negative impact was greater on fouling. The mixture viscosity was higher, the fouling degree was more serious. EPS and sludge specific resistance had two-sided impact on membrane fouling, and they were too low or too high to make the reactor operating stably. When SMP concentration and SVI were least, the fouling degree was lightest.Membrane fouling was the result of all properties interaction, but the contribution degree of each property to fouling was different. SMP was the most direct factor after analysis and essential reason of MBR affected by SRT. SMP concentration was higher in the reactor, the membrane fouling was more severe and the stable operation cycle was shorter. According to the fitting results of joint models, standard plugging model was the main mechanism in the slow fouling phase. And the rate and extent of standard plugging was determined by SMP. Therefore, the production of SMP should be controled to achieve long-term stable operation in the MBR process.