Stability And Thermodynamics Of Aerobic Nitrifying Granules Under Different Substrate Limiting Conditions

In recent years, more and more attention and concern had been taken on the aerobic granular sludge technology. However, the poor stability of aerobic granular sludge was still an important factor limiting their practical application. Although it had been recognized that many factors could affect the formation and stability of the granular sludge, but there was still lack of quantitative description of the interaction between the microorganisms in the granular sludge and in-depth research and thorough understanding of the disintegration mechanism of the granules. In this study, the interaction between microorganisms and their influence on the stability of granules was studied through the detemination of zeta potential, contact angles, SOUR (specific oxygen uptake rate), EPS (extracelluler polymer substances), three-dimension fluorescence spectra, FT-IR (Fourier transform infrared), SEM (scanning electron microscopy) and the DLVO theory. This study could enrich the understanding of microbial interaction mechanisms and contrbute to predict the changes in granules stability. In this thesis, systematic research on the changes of the aerobic nitrifying granules in stability and thermodynamics under different substrate limiting conditions had been made. The main research contents and results were as follows:(1) Stability and thermodynamics of aerobic nitrifying granules under organic carbon source limiting condition were studied. The results showed that under the carbon source limiting conditions, the activity of heterotrophic microorganisms in the aerobic nitrifying granules gradually disappeared. Due to loss of competition of the heterotrophic microorganisms, the activity of the AOB (ammonia-oxidizing bacteria) and NOB (nitrite-oxidizing bacteria) ignificantly increased. The content of EPS increased slightly, but the value of the PN/PS reduced. The results of the three-dimensional fluorescence showed that the fluorescent substances in the EPS were protein-like things. As time grows, the structure of the protein in the EPS changed little. The surface functional groups of granular sludge had changed. The number of hydroxyl and amino groups increased, leading to the enhancement of the hydrophilicity of the granules. Surface thermodynamic properties of the granular sludge changed significantly. The repulsive barrier that microbes need to jump across for combination rised from the initial680KT for1950KT in the end. Thus the flocculation performance between the microorganisms was degraded, resulting in the deterioration of the stability of granular sludge. The calculation results of Gibbs free energy showed that the force between the micro-organisms evolved by the strong gravitation to the strong repulsion during the process. Finally micro-organisms at the surface of granular sludge fell off from the surface of the granules under the effect of shear stress and discharged with the effluent, resulting in the increasement of ESS (effluent suspended solids) concentration. The results of the particle size distribution showed that the particles in the system changed from mainly consisting of2.36-3.35mm large particles into the majority of small particles of1.00-1.18mm diameter. In summary, the system reached a new equilibrium. The aerobic nitrifying granules changed from the previous hybrid of heterotrophic and autotrophic kinds to the completely autotrophic type. Throughout the experiment, little granular sludge was ruptured, indicating that the aerobic nitrifying granules used were structural strong. The inhibition of the activity of heterotrophic strains in the granular sludge had no significant effect on their structural integrity.(2) Stability and thermodynamics of aerobic nitrifying granules under inorganic nitrogen source limiting condition were studied. The results show that under the matrix nitrogen limitation condition, the activity of the autotrophic nitrifying bacteria in the aerobic nitrification granular sludge decreased to a very low level quickly, and simultaneously the activity of the heterotrophic bacteria increased. The decreasement of the activity of the autotrophic bacteria caused the weakness of the structural strength of the granular sludge. Soon after the beginning of the research some of the larger granules disintegrated. As time grows, more and more granular sludge was ruptrued. At the fiftieth day, the concentration of suspended solids in the reactor has been reduced to a very low level. Finally, the granules in the reactor changed from mainly consisting of2.36-3.35mm large particles into the majority of0.43-0.85mm debris and flocs. The number of hydroxyl and amino groups on the surface of granular sludge declined. The hydrophobic groups like unsaturated carbonyl increased. All of those caused the granular sludge to be more hydrophobic. However, due to significantly reduce of content of the EPS, the bonding effect between the granular sludge microorganisms gradually disappeared. The DLVO theory can not reasonably explain the thermodynamic mechanism of the disintegration of the granular sludge. It is necessary to consider the impact of the EPS into the DLVO theory to fitted to the data.(3) In the end, after the removal of organic carbon from the water solution, changes in the DLVO theory could predict the stability of the granular sludge well. But when to remove the the inorganic nitrogen source, the DLVO theory could not explain the causes of the disintegration of the granular sludge. It was mainly because that under the nitrogen limiting condition, the extracellular polymer content of the granular sludge rapidly declined. The weakened bonding effect between the microbial caused structural deterioration of the granular sludge under the water shear stress in the end. However, the influence of EPS was not considerd into the DLVO theory. So the DLVO theory could not actually predict the changes in the stability of the aerobic nitrifying granules under nitrogen limiting condition.