Change And Mechanism Of EPS In Sludge Granulation Under Low Organic Load

In view of the general low COD of actual domestic sewage and the fact that there are few studies on the cultivation of aerobic granular sludge with domestic sewage,this study completed the cultivation of aerobic granular sludge with simulated domestic sewage with low organic load(0.63 kg COD m^-3·d^-1)in SBR reactor.In the process of aerobic granular sludge,the physical and chemical properties of sludge was regularly monitored and the sludge Extracellular polymers（EPS）was extracted,the secretion of organic load under the condition of low sludge EPS features and its change rule were studied,and its influence on aerobic granular sludge system was also analyzed.The main research results were as follows:（1）Under the condition of low organic load,the granulation process of sludge was slow,aerobic granular sludge began to form on the 38th day,and complete granulation was not realized until the 93rd day.The average particle size of the acclimated sludge was 72.37μm;Yellow and white granular sludge appeared in the forming stage,and the granules gradually increased;In the mature stage,granular sludge had compact structure and average particle size of 1254.7μm;During the acclimation period,the sludge sedimentation performance was poor,the removal rate of COD of organic pollutants decreased from the initial 80%⁹0%to 79.2%,and the removal rate of ammonia nitrogen decreased from the initial 99.48%to 45.21%;During the formation period,the sludge sedimentation performance gradually improved,and the removal rate of COD and ammonia nitrogen gradually increased.In the mature stage,the SVI value of granular sludge was 75.00±3 mL·g^-1,the removal rate of COD was about 95%,the removal rate of ammonia nitrogen was stable at more than 97.45%,with good sedimentation performance and pollutant removal efficiency.（2）In the process of sludge granulation,the content of EPS increased first and then decreased.The initial sludge EPS content was 76.13 mg·g^-1 MLSS.During the particle formation period,EPS content increased rapidly to 223.03 mg·g^-1 MLSS,which promoted the formation of granular sludge.The content of EPS in the mature stage decreased gradually,but the content was still higher than that in the domestication stage,so as to maintain the tight structure of granular sludge without losing stability.Proteins（Proteins,PN）in EPS content was the highest,and the content change rules consistented with EPS total change rule.The PN content in the mature stage was about twice of that in the initial stage,which played an important role in the granulation of sludge and the maintenance of particle stability.The content of Polysaccharide（PS）in EPS did not change significantly.（3）Fluorescence analysis showed that the main fluorescence components of EPS were aromatic proteins and soluble microbial metabolites.In the granulation process,the type of each component did not change,but the content changed significantly.The fluorescence intensity of aromatic protein was always higher than that of other peaks,which was an important material to promote sludge granulation;The infrared spectra showed that the main functional groups of EPS did not change significantly during the granulation process;According to XPS energy spectrum analysis,the elements with the highest content in mature granular sludge EPS were O,C,N in turn,among which C-O,C=O,-NH₂ and so on were the main chemical groups of EPS;The molecular weight of EPS in each period of sludge granulation ranged from 3.33 to 113.20 kD.During the granulation process,the molecular weight of EPS was gradually decreased,with 42.79kD at the domestication stage,34.97 kD at the formation stage,and 29.20 kD at the maturity stage;According to resin grading analysis,the sum of hydrophobic acid HOA and hydrophobic neutral material HON in each grading component of EPS accounted for 60%⁸0%of the total amount of EPS.The content of HOA and HON increased gradually during the granulation stage,and the relationship between HOB<HI<HON<HOA during the maturation stage indicated that HOA and HON were beneficial to the formation of granular sludge.（4）According to the statistical analysis,the contents of EPS components were significantly correlated with sludge particle size and SVI,but not with MLSS.Among them,PN and PS were significantly positively correlated with the mean particle size of sludge,and Pearson correlation coefficients were 0.531（P=0.028）and 0.638（P=0.006）,respectively.PN and PS were significantly negatively correlated with sludge SVI,and sludge settling performance tended to be better with the increase of PN and PS content in EPS.There was a significant positive correlation between EPS components and SV₃₀/SV₅,indicating that EPS components played an obvious promoting role in the process of sludge granulation.（5）The effect of granular carbon source on the granulation of aerobic sludge was simulated by adding starch into the intake matrix under low organic load.It was found that granular carbon source with appropriate proportion（25%）could stimulate the faster secretion of EPS and accelerate the granulation of aerobic sludge.On the 56th day,the sludge was completely granulated with an average particle size of 619.6μm and the maximum EPS increased to 170.23 mg·g^-1 MLSS.Compared with dissolved carbon source condition,the granulation period of sludge was shorter,but the particle size of mature granular sludge was smaller.The ratio of starch in the matrix was increased,the stability of granular sludge was decreased,the content of EPS was decreased,the sedimentation performance of granular sludge and the removal efficiency of pollutants were also decreased.It indicated that the proportion of granular carbon source in the matrix was too high（>50%）,which was not conducive to the stable operation of granular sludge.