Identification Of Sludge Properties And Membrane Fouling In Membrane Bioreactors With The Addition Of Calcium Ion

Membrane bioreactor (MBR), as a new high-efficient wastewater treatment technology combining biological treatment with membrane separation, has been actively employed for municipal and industrial wastewater treatments. However, membrane fouling and its consequences in terms of plant maintenance and operating costs limit the wide-spread applications of MBR. Therefore, it is necessary to search for some methods to mitigate membrane fouling, which could increase membrane life and reduce the cost of MBR operation. A laboratory-scale submerged flat-sheet MBR treating synthetic wastewater was operated in this study in order to verify the impacts of calcium ion and the ratio of Ca to Mg on membrane fouling and sludge properties of MBR. The main research contents and the results were summarized as follows:(1) Impacts of Ca²⁺ on membrane fouling were investigated in a submerged flat-sheet MBR for synthetic wastewater treatment by changing influent Ca²⁺ concentration to 30 mg/L, 200 mg/L and 500 mg/L. The results showed that the membrane fouling in terms of growth tendency of TMP was effectively controlled with the increase of Ca²⁺ concentration, and the best permeability was achieved at Ca²⁺ concentration of 200 mg/L. It was not benefit to reduce membrane fouling when the Ca²⁺ concentration was much more than 200 mg/L, such as 500 mg/L. The increase of Ca²⁺ could decrease the concentrations of soluble microbial products (SMP) and loosely bound extracellular polymeric substances (LB-EPS) by enhancing the bioflocculation, thus helping reduce the external resistance due to the decrease of SMP and LB-EPS. Ca²⁺ could also increase the particle size of floc, and the larger floc size reduced the cake layer fouling. However, when the Ca²⁺ concentration was much higher, the average particle size would decrease due to the increase of inorganic particle with less size, and then the membrane fouling was deteriorated because the inorganic particle would increase the internal resistance by blocking the membrane pore.(2) A laboratory-scale submerged flat-sheet MBR for the treatment of synthetic wastewater containing 200 mg/L Ca²⁺ was operated under sludge retention time (SRT) of 10 d, 20 d and 30 d in order to verify the impacts of SRT on the performance of MBR with the addition of Ca²⁺. The results showed that the effluent quality was slightly influenced by SRT, while membrane fouling was effectively mitigated by extending SRT with the addition of Ca²⁺ due to the decline of fouling layer resistance. The concentration of SMP significantly decreased as SRT prolonged from 10 d to 30 d, which correlated well with the variation of membrane fouling. SMP and particle size played a more important role in membrane fouling in this study compared to other sludge properties. The Ca²⁺ accumulated in MBR as the SRT extended, and the accumulation of Ca²⁺ enhanced bio(?)occulation and resulted in the decrease of MLVSS/MLSS ratio and the increase of the average particle size. The accumulated Ca²⁺ in mixed liquor was mainly located in bound extracellular polymeric substances (BEPS).It might be indicated that the negatively functional groups such as polysaccharide in SMP were absorbed to BEPS by bridging effects of Ca²⁺, which led to the reduction of SMP and increase of BEPS at longer SRT.(3) Impacts of Ca: Mg ratios on membrane fouling and sludge properties were investigated in a submerged flat-sheet MBR for synthetic wastewater treatment by changing Ca: Mg ratios to 3:1, 1:1 and 1:3 with the total divalent cation of 200 mg/L in the influent. The results showed that the effluent quality was slightly influenced by Ca: Mg ratios, while membrane fouling was effectively controlled with the increase of Ca: Mg ratio due to the decline of fouling layer resistance. The concentration of SMP significantly decreased with the increase of Ca: Mg ratio, which correlated well with the variation of membrane fouling. Calcium ion played a more important role in bio?occulation in this study compared to magnesium ion, and the higher concentration of calcium ion increased the average sludge particle size, and the larger floc size reduced the cake layer fouling.