Research On Formation And Characteristics Of Aerobic Granular Sludge In A Continuous Airlift Fluidized Bed

Most studies on aerobic granulation have been done in sequencing batch reactors（SBR） by now, while little successful aerobic granulation has been reported in thecontinuous equipment. This work is the first to cultivate aerobic granules in acontinuous flow airlift fluidized bed reactor （CAFB） with activated sludge as seeding.Study on hydrodynamic and mass transfer of CAFB, formation and mechanism ofaerobic granules, physical, chemical and biological characteristics of aerobic granulesand effects on formation with Ca²⁺-augmented aerobic granules in this process arediscussed in this thesis.Compared with SBR and AUSB, CAFB is a novel completely mixed continuousreator. Gas holdup, oxygen mass transfer coefficient KLa and hydrodynamic shearforce in these two areas increased as the increase of superficial air velocity whensuperficial gas velocity was less than34mm·s-1. Due to Quarter surge phenomena,each parameter was stable or decreasing when superficial gas velocity was more than34mm·s-1. It should control proper superficial gas velocity to ensure CAFB＇s normaloperation. After several researches about operation conditions of CAFB, theappropriate conditions parameters which could avoid the overgrowth of filamentousbacteria and keep stability of CAFB were summarized, such as COD/N/P/, DO,HRT and feed composition etc.Two column-type CAFBs under different organic loading rates （OLR） werestart-up at1.5³.5g COD·L^-1·d^-1（R1） and1.5⁴.8g COD·L^-1·d^-1（R2） where theOLRs were gradually raised. Results showed that aerobic granules with an averagediameter of250³00μm successfully formed at2days in two reactors when it wasneeded20to50days to form the same granules in SBR. COD removal loading rateof R2increased from0.7g COD·L^-1·d^-1to3.5g COD·L^-1·d^-1when R1was from0.2gCOD·L^-1·d^-1to2.5g COD·L^-1·d^-1. The granule formation rate of R1, R2also increased form5.9%to71.9%and58.6%（Day32） respectively. It was found thatthere was a significant increase in the cell surface hydrophobicity and EPS of theaerobic granules compared to SBR. The high cell surface hydrophobicity and highcell protein contents were believed to play an important role in the formation ofaerobic granules. Due to inner-circulated between the riser and the downcomer of theCAFB, the drag force exerted on the particles which favors flocs and granulesaggregation mainly comed from fluid in the CAFB was100³00times stronger thanthat in the SBR. The CAFB had the potential advantages of more effective phase mixand interphase mass transfer. Therefore, aerobic granules could formatted in theCAFB that much more quickly than other reported aerobic granule sludge process.According to the fluorescent staining results of EPS and cells of aerobic granulesin the CAFB, it indicated that the main components of EPS were protein and β-Dglucopyranose polysaccharide. They distributed throughout the interior of the granuleuniformly. The dead cells accompanied with the distribution of living cellsthroughout the granules. The KLa values of the CAFB were almost twice than thoseof the SBR under the same aeration strength, due to the special operational conditionand hydraulic condition of the CAFB.It was showed that adding Ca²⁺could accelerate the prosess of granulation.Ca-rich granules had more regular and rigid structure and more steady distribution ofparticle size in CAFB. Examination with a SEM-EDX showed that Ca and P wasmainly accumulated entire granules uniformly. Ca₄H（PO₄）₃·2.5H₂O was found to bethe main calcium precipitate in the granules with XRD.