Cells Proliferation And Decay Of Immobilized Nitrifiers And Nitrification Characteristics

Compared to suspended nitrifiers, immobilized nitrifiers have moresuperiority for the higher biomass concentration, easier to separate solidfrom liquid and better resistance to impact loading rate. In this study,immobilized nitrifier pellets made of PU （polyurethane） wereindependently developed by our research group. Cell proliferation anddecay in immobilized pellets and its nitrification characteristics wereinvestigated. The main conclusions were listed as follows:A immerse method was established to calculate the effectivediffusion coefficient （D_e） of immobilized nitrifiers pellets. D_eof ammoniain immobilized pellets before cultivation was0.826×10^-9m²/s,42.1%ofthat in pure water. Likewise, D_eof oxygen in immobilized pellets beforecultivation was0.468×10^-9m²/s,23.6%of that in pure water. Substratediffusivity of this study was a little higher than that of otherimmobilization methods in literatures. It showed a better permeability ofthe immobilization carrier in this study and less effect on nitrification rate.In addition, prepolymer concentration and MLVSS（Mixed liquor volatilesuspended solids） would influence D_e. The higher prepolymerconcentration and MLVSS, the less D_ewas.Characteristics of proliferation of immobilized nitrifiers and masstransfer in a cultivation process were obtained by means of qualitativeand quantitative methods for microbial biomass and activity. Efficiencyof cultivation in continuous feed was better than that in batch feed. Cultivation could be finished in a short time and maximum activity as DOwas controlled at3⁴mg/L.In70days of the cultivation, respiratory rate, nitrification rate andmicrobial biomass all increased fast at first, and gradually slowed down,and achieved stable state in50days or so, which indicated that thecultivation was finished.Activity of unit biomass of immobilized pellets decreased22%astime went by. As biomass increased, D_edecreased to0.492×10^-9m²/s,59.6%of that before cultivation. Owing to transfer limitation,microcolonies near the surface of the carrier grew faster, larger anddenser than the colonies near the center, resulting in a greater difficultyfor substrate to spread to the center. Specific growth rate （μi） ofimmobilized nitrifiers was obtained via biomass balance in the reactor.The results made it clear that in the cultivation process, μikept reducingclose to0, which meant that the biomass released to bulk solution wasequal to the net growth biomass.The results of electric microscope and PCR-DGGE analysis showedthat some of the nitrifiers adhered to the surface of the carrier, and theothers were still encapsulated in reticular structure of the carrier. Viablecells and dead cells coexisted in microcolonies. AOB and NOB inimmobilized pellets formed stable microbial community structure. AOBare almost Nitrosomonas europaea, and NOB are mostly Nitrobacter219and Nitrobacter Sp.263.When O2was insufficient, the decay rate of activity of immobilizednitrifiers was0.935×10-6s-1（5.88%of μmax）. The activity returned to thehighest of1207.8mgO2/L-pellet·h10d later after O2was added. Whenammonia was insufficient, the decay rate was4.20×10-6s-1（26.4%of μmax）.The activity did not return to its highest activity25d later after ammoniawas added. Therefore, when ammonia concentration was limited, DO concentration had to be cut down to reduce microorganism decay.Diffusion-reaction model of immobilized nitrifiers was studied. Theresults showed that effect of ammonia on internal diffusion wasneglibible, while oxygen internal diffusion would affect on nitrificationrate. When oxygen concentration was0.5,1,2and4mg/L, the averageDO penetrating thichness was0.511,0.599,0.692and0.808mm,respectively. The results indicated that the higher DO concentration inbulk solution, the less anoxic area in the center of pellets.Comparative studies of nitrification between immobilized nitrifiersand biofilm on elasticity plastic filler showed that immobilized nitrifierswere adaptable to the changes of environmental conditions, and had amore stable nitrification rate. The optimum environmental condition was:pH=8.0, DO＝4mg/L and T=30℃. Sensitivity analysis indicated that pHhad a more evident effect on nitrification activity than DO andtemperature. In the continuous experiment, the immobilized nitrifiersreactor kept higher nitrification rate all along. In addition, immobilizedpellets were used in treatment of rice wine wastewater containing highconcentrated organic matter and nitrogen. Due to too high concentrationof organic matters, growth of heterotrophic bacteria inhibited the activityof nitrifiers, so that the maximum loading rate and nitrification rate in thereactor were all lower than that in the laboratory experiment.The study of nitritation and nitratation process and inhibition of freeammonia combined with FISH（Fluorescence in situ hybridization）method obtained several factors to realized stable short-cut nitrification:（1） AOB was the dominant species in the immobilized pellets, and thespecies polulation determined that ammonia oxidation rate was far higherthan nitrite oxidation rate;（2） half saturation constant of AOB was greaterthan that of NOB, which made the former easier to obtain oxygen andgrow at a low DO contration;（3） AOB kept a higher oxidation rate even at high FA contration.