Start-up Of Aerobic Granular Sludge Reactor And The Effect Of Extracellular Polymeric Substances(EPS) On Sludge Aggregation

Aerobic granular sludge(AGS)has a compact structure and excellent sedimentation performance.It does not require the secondary setting tank and addition of flocculant to reduce the turbidity of the effluent.Therefore,it has broad application prospects because of saving area requirement and operating cost.However,due to the slow granule formation process and unclear granulation mechanism,the full-scale application of this technology has a challenge.Extracellular polymeric substances(EPS)plays an important role in the formation and stabilization of microbial aggregates.Meanwhile,the functions of microbial aggregates through different sub-fractions of EPS are distinct.Nowadays,many researshers work on the change of content and structure of protein(PN),polysaccharide(PS)in EPS during granulation.But the effect of different sub-fractions of EPS on sludge aggregation during granulation is unclear.In order to explore the granulation mechanism,the change of sludge morphology and characteristics during the start-up of AGS reactor was studied in this work.Furthermore,the components of different sub-fractions of EPS and the effect on the surface characteristics and aggregation of sludge were measured.Finally,based on the extended DLVO theory for qualifying and quantifying the surface energy of sludge and contribution of EPS and their sub-fractions in sludge aggregation was calculated.This provided a further understand of the granulation mechanism and the effect of different sub-fractions of EPS in the granulation process.Meanwhile,it made for a theoretical basis for start-up and stable operation of AGS reactor,as well as,the EPS regulation.The main conclusions obtained in this study are as follows:①Through decreasing setting time,the AGS reactor could be start-up in 40 days.The MLSS and MLVSS in the reactor were 8.73±0.08,7.39±0.08 g/L,respectively.In addition,the SV30/SV5 and the degree of granulation in the reactor were 0.93±0.02,90.22%,respectively,indicating that granulation was achieved.The hydrophobicity and zeta potential of the sludge increased from 45.67±0.88%,-12.27±0.59 mV to 76.67 ±2.17%,-8.09±0.71 mV,respectively.The confocal laser scanning microscopy(CLSM)analysis indicated the structure of mature granule sludge was compact and the bioactivity was highly.In this granule sludge,living cells decreased from the out layer to the inner layer.Moreover,a large number of filamentous dead bacteria in the sludge was skeleton of the granule sludge.The protein formed the core,and the β-PS was distributed in the form of filaments in the granule sludge,which served as the skeleton.Through analysis of the morphology,surface characteristics and structure of sludge during granulation,the proposed granulation mechanism was:filamentous bacteria serves as skeleton,meanwhile,the increasing hydrophobicity promotes the aggregation of cells.Then those cells adheres to filamentous bacteria and rapidly grow and reproduce.Finally,under the hydraulic shear,the sludge becomes mature granular sludge with smooth surface and compact structure.②During the granulation process,the content and relative hydrophobicity of PN increased by 2.72 and 1.83 times respectively,but the content of PS did not change significantly.In situ visualization of EPS components of inoculated sludge and mature granular sludge using CLSM revealed that the PN formed a core in granular sludge,in addition,the PS structure changed from loose structure to filamentous skeleton structure.It indicated that PN and PS in EPS have important effects on the sludge aggregation and structural stability,respectively.The composition and variation of sub-fractions of EPS were different in the granulation process.The dissolved EPS(S-EPS)content was lower.For loosely bound EPS(LB-EPS),the content of PS decreased from 11.63±0.11 mg/gVSS to 3.33±0.26 mg/gVSS,however,the content of PN did not change significantly.The TB-EPS was the main component of EPS,accounting for 72.8193.11%.During the granulation process,the content and relative hydrophobicity of PN in TB-EPS increased from 30.38±0.12 mg/gVSS,38.90%to 83.99±2.05 mg/gVSS,71.25%respectively,which made for cell aggregation.Through analysis the surface characteristics and aggregation performance of the sludge before and after EPS extraction,it was revealed that the zeta potential,hydrophobicity and aggregation capability decreased after TB-EPS extraction.It indicated that the TB-EPS was conducive to cell aggregation and granulation.Furthermore,during the granulation process,the contribution of TB-EPS to the surface characteristics of sludge kept increasing.In contrast,the effect of S-EPS and LB-EPS on the surface characteristics of sludge were weaker.Pearson correlation analysis revealed that the content of TB-EPS increasing was beneficial to reduce the negative charge and enhance the hydrophobicity of sludge.Therefore,the aggregation ability of microorganisms was improved,thus accelerating the granulation process.However,the content of LB-EPS increasing would worsen the setting ability of sludge and hinder the granulation process.In addition,S-EPS had no significant correlation with sludge aggregation and setting sbility.TB-EPS played a major role in EPS,and the increasing content of PN could promote sludge granulation and improve setting ability.③Surface thermodynamic analysis indicated that the Lewis acid-base interaction energy(△GadhAB)acted as a significant contributor to the interface adsorption free energy(AGadh).During the granulation process.△adhAB decreased from 13.57 mJ/m2 to-58.92 mJ/m2.Thus,the hydrophobicity of the sludge surface increased promoting microorganisms aggregation.After the TB-EPS extraction,the variation of △Gadh increased significantly during the granulation process.It indicated that the TB-EPS played a key role in the sludge aggregation.The effect of LB-EPS on the △Gadh changed from negative to positive,when activated sludge transformed into granule.Based on the result of XDLVO theory analysis,the acid-base interaction(WAB)played a significantly decisive role in the total energy of interaction(WTOT).During the granulation process,the effect of WAB on microorganisms aggregation changed from repulsion to attraction.Therefore,the energy potential barrier of sludge decreased from 3146.59 KT to-3.69 KT,in addition,the second minimum value decreased from-12.94 KT to-22.55 KT.It revealed that the aggregation performance and stability of sludge had been continuously improved.Moreover,the LB-EPS and TB-EPS had different contributions on the sludge aggregation.TB-EPS played a key role in the contribution of EPS to WTOT.The interaction energy of TB-EPS is always negative,suggesting that the TB-EPS always displayed a positive effect on the sludge aggregation during the granulation process.However,the role of LB-EPS in sludge aggregation is depended on the sludge characteristics.That is the LB-EPS displayed a repulsive and attractive contribution to the WTOT in the actived sludge and granule sludge respectively.