| The microorganisms, after being immobilized, can be kept in the reactor more effectively for an unlimited period time so that they can be used continuously and repeatedly. Cell immobilizing processes have been receiving the increasing attention in the field of water treatment in the recent decades. Although the immobilized microorganism technology was carried out over a wide range of research area, it was still in the laboratory tests and was difficult for large-scale industrial applications. The key point was that the current carrier materials were not stability. Therefore, a novel cell entrapment method that involves mild gelation and strong gel structure is desired.Based on the characteristics of polyurethane hydrogel and previous research, a novel microorganism immobilization method involving synthesis, gelation of polyurethane prepolymer has been developed. The feasibility of the proposed immobilization method was tested by nitrifying bacteria. Using macro monomer technology, IPDI/PMPO/HEMA prepolymer was synthesized by two-step reaction. The molecular weight and structure of polyurethane prepolymer were characterized by elemental analysis, IR and NMR spectra. Factors affecting prepolymer synthesis reaction were studied respectively to determine the order of group response, the appropriate reaction temperature, reaction time and the category of catalysts. Focus on the IPDI/PEG/HEMA macromonomer reaction kinetics; the reaction order and the order of groups were discussed. The reactions of PMPO and IPDI and IPDI polyurethane were confirmed to be the second-order reaction. The optimum reaction temperature was 65℃and reaction time was 4 h. And the reaction rate constants were 0.0165 mol/L·min and 0.0264 mol/L·min, and the corresponding activation energy were 55.02 kJ/mol and 101.52 kJ/mol, respectively. According to the laboratory test results and the theoretical analysis of reaction kinetics, through the scale-up response, a plant with the production scale of 200 tons/year was established.Radical polymerization was triggered by the KPS/TEMED as a two-component redox initiation system. The orthogonal test method was used to determine the best formula of preparation of polyurethane gel. The optimized polyurethane hydrogel contained:polyurethane monomer 10%, a promoter TEMED 1.0%, an initiator KPS 0.5%, an additive powdered activated carbon 3%. The structure of polyurethane hydrogels was characterized by IR spectra and scanning electron microscopy. According to Arrhenius equation, the lifespan of polyurethane gel pellets was up to 20 years at 25℃, pH 6～8 of the environment.According to effects of the immobilized reagents and operating conditions on the activity of nitrifying bacteria, we have established the following optimal preparation processes. Nitrifying bacteria concentrate (MLSS 2.0×104 mg/L) was mixed with 10% prepolymer emulsion, a promoter TEMED 1.0%, an initiator KPS 0.5% and an additive powdered activated carbon 3% in a special mold. The mixture was allowed to stand for about 10 min at a temperature of 25℃. The resulting polymerized gel carrier was cut into 3×3×3 mm cubes and then washed thoroughly with distilled water. Thus polyurethane immobilized nitrifying bacteria pellets were obtained. The intermittent and continuous domestication ways were adopted to restore the activity of immobilized nitrifying bacteria pellets, and the results confirmed that the method of continuously improving ammonia concentration was better for pellets domestication and ammonia oxidation rate could reach 350 mg-N/L-pellet·h rapidly. The SEM pictures showed the quantity of the ammonia oxidation bacteria increased substantially during the acclimation. Nitrification kinetics of immobilized nitrifying bacteria pellets in different initial ammonia concentration was studied. Nitrification reaction of immobilized pellets was followed zero-order reaction at the high concentration and first-order reaction at low concentration. Based on the sensitivity analysis, effects of different environmental factors for free and immobilized nitrifying bacteria were investigated under high and low ammonia concentration. The immobilized nitrifying bacteria showed advantage nitrification performance than the free nitrifying bacteria. The optimal operating point were:pH=9, DO=4 mg/L, temperature 30℃(low concentration); pH=8, DO=6 mg/L, temperature 30℃(High concentrations). The results of continuous operation to remove ammonia by immobilized nitrifying bacteria pellets confirmed that immobilized pellets had high ammonia removal efficiency in wastewater treatment system.The immobilized nitrifying bacteria pellets were used for nitrogen removal in various actual wastewaters, and the operation results showed ideal ammonia removal efficiency. The result verified that the polyurethane immobilized-cell method developed in this study has great potential for a variety of applications.
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