Preparation And Performance Of Porous Microcapsule Membrane Carriers For The Immobilization Of Microbial Cells In Fluidized Bed Reactors

Biological treatment technology is the one of most economical and efficient methods for removing organic pollutants in wastewater treatments. Immobilized microbial techniques by using carriers are important ways to enhance the performance of the biological treatment of wastewater. Unfortunately, currently existing carriers for the microbial immobilization have some disadvantages. In this study, a novel type of carriers, which is a kind of porous capsules, is proposed to enhance the performance of microbial immobilization. First, interfacial precipitation polymerization and interfacial polymerization methods were introduced to prepare polyester and polyamide porous microcapsule carriers respectively. Then, polyethersulfone (PES) porous microcapsules with straight pores throughout the membranes were prepared by using gel-sol phase inversion method for the first time, and the fabrication principles and technological parameters for preparing porous microcapsule membrane carriers were investigated systemically. The performance of microbial immobilization using the fabricated porous microcapsule carriers and the metabolism process of microbial cells inside and outside the microcapsule carriers were also experimentally studied. The mass transfer process in the microbial cell aggregates in porous microcapsule carriers, which were called bio-granules, was described quantitatively. A mathematic model was established to predict theefficiency of the biotreatment using fabricated porous microcapsules as carriers for microbial immobilization.Microstructures of polyester porous microcapsule carriers prepared by interfacial precipitation polymerization were observed by SEM. The hollow structure was clearly observed. The inner surface and outer surface were both dense, but macro-void was observed in the membrane cross-section. Asymmetrical structure of prepared microcapsules usually appeared due to the limit of equipment and operation. The prepared polyester porous microcapsule membrane carriers were used for treating wastewater in anaerobic static equipment, but the performance was not satisfactory because the dense surface layer of the microcapsules prevent the mass transfer in the microbial immobilization and the bio-treatment. Therefore, the prepared polyester microcapsules were not proper to be used as carriers for microbial immobilization. SEM images of polyamide porous microcapsule membrane carriers prepared by interfacial polymerization showed that, the inner surface and outer surface were both porous and rough, and the total pore volume and total pore area were large, which were beneficial to the microbial immobilization. The prepared polyamide porous microcapsule membrane carriers were also used for treating wastewater in anaerobic static equipment, and the results showed that the cumulative biogas production using porous capsule carriers was higher than that both using porous matrix polymer carriers and no carriers. SEM observations showed that microbial cells were immobilized on both outer and inner surfaces of the capsule carriers. Bacillus and cocci existed on the outer surface, but only cocci appeared on the inner surface, because the spongy structure of the membrane cross-section of microcapsules slightly prevented the mass transfer across the membrane. Furthermore, the polyamide porous microcapsule membrane carriers could not endure high hydrodynamic load. In a word, the prepared polyester and polyamide microcapsules were proven to be not proper for microbial immobilizations.A novel type of PES porous microcapsule membranes with straight poresacross the whole thickness was successfully prepared with a gel-sol phase inversion method. The inner surface and outer surface were both porous and rough, and the pore volume and total pore area were as large as 5.65 cm3/g and 5.17 m2/g respectively. The porosity was as high as 88.8 %, and the pore diameters on the outer surface were in the range of 1.4-7.5 um, which was beneficial to the microbial immobilization. The PES carriers were heat-resistant and chemical-resistant. The microstructure of the PES porous microcapsules was affected by the polymer and additive concentrations in the membrane-forming solution, the temperature of coagulation bath and the dissolution time. The formation, transfixion and integrity of straight finger-like channels across the microcapsule membrane were significantly affected by the PES concentration in the polymer solution. With increasing the PES concentration, the membrane thickness increased, while the pore size and the porosity decreased. With increasing the additive lithium chloride in polymer solution, the pore size reduced and the number of small straight channels increased, and the membrane thickness increased while the porosity and pore volume decreased. Additives polyethylene glycol (PEG) and polyvinyl pyrrolidone (PVP) changed the phase balance of polymer solution system. When the PEG and PVP dosages were proper, large straight finger-like channels could form easily in the microcapsule membrane. With increasing the PEG and PVP dosages, the diameter of straight finger-like channels decreased and the channel numbers increased, whereas the membrane thickness decreased and the porosity and pore volume increased. With increasing the temperature of coagulation bath, the pore size in the membrane became smaller and the membrane thickness decreased, while the porosity and pore volume increased. With increasing the dissolution time to remove the dense skin layer, the membrane thickness decreased while the microcapsule porosity and pore volume increased slightly. With decreasing the microcapsule diameter, the porosity and pore volume increased.The prepared PES porous microcapsules were used as carriers for the microbial immobilization in the wastewater treatment in both static and dynamic equipmentwith both anaerobic and aerobic microbes. The results showed that the treatment efficiency using prepared PES porous capsule carriers was better than that both using matrix porous polymer carriers and no carriers. SEM images showed that many cocci microbes were immobilized inside both the straight finger-like channels and the inner space of the PES capsules. The PES porous microcapsule carriers were also used for treat wastewater in the aerobic area of an IAOR, and the results showed that the aerobic bio-granules were active and the CODcr removal efficiency was always about 70 %. Many aerobic microbial cells were found inside both the straight finger-like channels and inner space of the PES capsules. That is, the hollow structure and the straight pores across the membrane were both beneficial to the aggregation of microbial cells.The prepared PES porous microcapsules were also used as carriers for immobilizing microbial cells in wastewater treatments in an AFB reactor. The start-up time of the experimental AFB reactor system was 36 days. The microcapsule carrier samples during the start-up time were observed by SEM at different time. SEM Image showed that many microbial cells were aggregated inside both the straight finger-like channels and the inner spaces of the PES capsule carriers, and the amount of immobilized microbial cells increased with time. NaHCCb was used to adjust pH value of experimental wastewater, and the pH value was usually maintained in the range 7.5 to 8.5. When the pH value was less than 7 or larger than 9, CODcr removal efficiency slightly decreased gradually. The microcapsule membrane could prevent the microbial cells immobilized inside the microcapsule from suffering the exceptional pH fluctuation of the wastewater. The organic cubage load was varied by changing the hydraulic residence time and the feed CODcr concentration. When hydraulic residence time maintained constant, the CODcr removal efficiency was always larger than 40%, but reduced slightly when the organic cubage load increased. When the feed CODcr concentration maintained constant, the CODcr removal efficiency decreased when the organic cubage load increased with decreasing the hydraulic residence time. With decreasing theexpansion rate of the fluidized bed reactor, the CODcr removal efficiency reduced, and vice versa. However, when the expansion rate increased too much, the CODcr removal efficiency decreased again. When the expansion rate was in the range of 30 to 50 %, the CODcr removal efficiency was always larger than 45 %.The concentration distribution inside the bio-granules, mass transfer across the liquid layer, solute diffusion across the bio-granule surface, and the mass balance in the bioreactor were analyzed. Accordingly, a mathematical model describing the mass transfer inside the bio-granules was established, from which the outlet concentration could be predicted on the basis of the feed concentration and volume flowrate of the bioreactor.In summary, a novel family of porous microcapsules, which was featured with a hollow structure and with straight microchannels across the membrane, has been successfully developed with a simple and effective method for the immobilization of microbial cells. The membrane of the fabricated microcapsule was entirely composed of closely packed straight microchannels, which acted as unblocked passages not only for the transfer of nutrients/reactants and products but also for the movement and metastasis of the microbial cells. The anaerobic microbial cells have been found to be satisfactorily immobilized both in the microchannels and inside the microcapsule. Furthermore, the microcapsule membranes could act as a protective layer for the immobilized microbial cells inside the microcapsules, and consequently could enhance the performance of bio-treatment. The developed porous microcapsules provided a new mode of the carriers for the immobilization of microbial cells, which is highly attractive for the immobilized cell systems in wastewater biotreatments and biochemical processes.