Researches On Granule Structure And Stability Based On Biological Nitrogen And Phosphorus Removal

Nitrogen and phosphorus removal process involves an abundant microbial community, including nitrifying bacteria (AOB/NOB), denitrifying bacteria (HOB), phosphorus-accumulating bacteria (PAO), glycogen-accumulating bacteria (GAO), etc. There exists a mutually competitive and cooperative relationship between these microorganisms, gradually developing a stable, mature granular sludge system. Over the past decades, we have developed a fairly sophisticated research framework on the community structure in this wastewater treatment technology, and achieved tangible achievements. Nevertheless, the microbial community changes with different cultivating conditions and particular disposal requirements. Therefore, more studies are still urgently needed to uncover the peculiar microstructure in bio-aggregates or evaluate its stability in wastewater treatment under different circumstances.This research related to the sludge system in nitrogen and phosphorus removal process, and particularly focused on several specific aspects, such as aerobic granular sludge cultivation, granule microstructure, its mechanical performance, entrapped mixed microbial cell technology, permeability evaluation of immobilization carriers, etc. To illustrate, this work referred to the cultivation and acclimation of robust and high bioactive AGS in simultaneously nitrogen, denitrogen and phosphorus removal process (SNDPR), the characterization of basic rheological properties and viscoelasticity of aerobic granules using the universal rheology methodology, the utilization of Wagner-type constitutive model to simulate the sludge simple flow and the employment of cellulose triacetate (TCA) to immobilize activated sludge in COD and ammonia nitrogen removal process. The main research contents and results are as follows:1) Based on enhanced biological phosphorus removal process (EBPR), AGS was successfully cultivated through controlling dissolved oxygen, reducing settle time and increasing shear rate. It showed that the mature self-immobilized aggregates had an average particle diameter at2mm, showing a regular, smooth and nearly round shape. Also the aerobic granules presented a dense and strong microbial architecture, with high biomass retention in an abundant extracellular polymeric substances matrix. On the other hand, this AGS could realize SNDPR process via nitrite pathway, and the periodic test of sludge cycle emphasized the story of denitrifying-PAOs. Finally, due to these results and discusses, we developed the following two research directions to promote the treatment efficiency in nitrogen and phosphorus removal. One was about a deeper observation of the microstructure within granules, and the other was about a more robust immobilized sludge system to replace AGS.2) Despite intensive researches on granular architecture and strategies to improve treatment efficiency, there are still some elusive material parameters needed to stimulate the granulation process or to elaborate on its microstructure development in AGS. Hence, the main aim of this study was to evaluate aerobic granular sludge innovatively using the universal rheology methodology, in terms of processability or quality and texture. Steady shear and oscillatory measurements were performed. Basic rheological characterization showed that AGS was a shear-thinning Herschel-Bulkley fluid with yield pseudoplasticity. Meanwhile, granular sludge presented characterized viscoelastic behaviors in dynamic sweeps highlighting its superiority to flocculent sludge and the significance of extracellular polymeric substances. Furthermore, a Wagner-type constitutive model incorporating a relaxation and damping function was introduced and able to describe the time-dependent and non-linear viscoelastic behaviors. This study could make a further step on predicting rheological properties, helping improve the actual sludge treatment process and the operation of sludge dewatering.3) For wastewater treatment with immobilized cells, the support material of choice needs to meet the following criteria:insoluble, not biodegradable, high mechanical stability, high diffusivity, simple immobilization procedure, high biomass retention, and preferably a low cost price. Through comparing different support materials, we successfully synthesized a mixed-culture cellulose triacetate entrapped microbial carrier (TCA), showing a robust stability and good settleability in high concentration phosphate solution (>10mg/L). The results of synthetic wastewater treatment experiments showed that TCA carriers had a high stability and good bio-activity, able to simultaneously realize bio-oxidation, nitrification and de-nitrification; the settling experiments demonstrated that TCA carriers had a dimensionless permeability factor ξ of10.9, and the fluid collection efficiency η of0.034, proving that the internal permeation of carriers was not sufficient to affect their settling behaviors, namely the bio-carriers could be considered as impermeable where their settling was concerned. Although we found the internal pore size was at nearly100μm, leading to the wash-out of regenerative microorganisms, this trial paved the way to realizing the sludge immobilization technology for simultaneously nitrogen and phosphorus removal.