Research On Effect Of F/M Ratio On Aerobic Granular Sludge Process And Its Control Strategy

Aerobic granular sludge has several properties,including compact structure,excellent settleability,high microbial diversity and biomass retention.In recent years,lots of researchers in the world have paid attention to aerobic granular sludge process due to the promising potential for engineering application.However,long startup period and granular instability under long-term operation are the main drawbacks limiting its application.Food to microorganisms(F/M)ratio is an essential parameter in biological wastewater treatment affecting microbial aggregation,sludge settleability and pollutant removal.The effect of F/M ratio on the startup and stability of the aerobic granular sludge process,and a new strategy for F/M ratio controlling were investigated in this thesis,using confocal laser scanning microscope(CLSM),three-dimensional fluorescence spectroscopy(3D-EEM)and high-throughput sequencing technologies.The major results are as follows:1.Two sequencing batch bioreactors(SBRs)were built to investigate the effect of F/M ratio on the startup of aerobic granular sludge process.Results showed that granules with mean size of 346±12 ?m were formed on Day 25 when F/M ratio was maintained at 0.7±0.03 gCOD/(gSS d),and the settleability of sludge got worse since flocs observed during further operation.However,granules with smaller size were achieved on Day 35 when F/M ratio was maintained at 0.5±0.02 gCOD/(gSS d),and the granules had better settleability with SVI30 of 53±2 mL/g and SVI30/SVI5 of 0.85±0.02.Meanwhile,the mixed liquor suspended solids(MLSS)increased to 7.3±0.3 g/L.Analysis of extracellular polymeric substances(EPS)contents showed that the mean protein/polysaccharide(PN/PS)ratio of EPS was 4.46 when F/M ratio was 0.5±0.02 gCOD/(gSS·d),which was higher than that of 2.37 when F/M ratio was 0.7±0.03 gCOD/(gSS·d).It indicating that PN secretion could be enhanced for formation of stable aerobic granules when F/M ratio was kept at 0.5±0.02 gCOD/(gSS·d).2.The effect of F/M ratio on the stability of aerobic granular sludge process was studied.Results showed that the MLSS increased at first and then decreased at a high F/M ratio(0.7-0.9 gCOD/(gSS d)).Meanwhile,the sludge retention time(SRT)decreased sharply to 2±0.5 d because of sludge bulking.In the reactor with lower F/M ratio of 0.2-0.3 gCOD/(gSS·d),the MLSS increased slowly to 5.4±0.2 g/L,and the mean granular size decreased gradually to 286±20 ?m,leading to the decrease of total nitrogen(TN)removal efficiency to 41.0±2.3%finally.In the reactor operated at 0.4-0.5 gCOD/(gSS d)of F/M ratio,the MLSS increased steadily and the SVI5 decreased to 45±2 mL/g.At the same time,its sludge yield was in the range of 0.25-0.32 gSS/gCOD,and the COD,NH4+-N and TN removal efficiencies achieved 99.7±0.1%,97.6±0.8%and 76.0±1.6%,respectively.The results of CLSM observation showed that aerobic granules formed at F/M ratio of 0.5 gCOD/(gSS·d)had dense structure with higher level of ?-polysaccharides,proteins and living cells.At the same time,the indices of ACE,Chao 1,Shannon and Simpson in granular sludge formed at F/M ratio of 0.5 gCOD/(gSS·d)were higher than those in other sludge samples.Several functional bacteria related with EPS secretion and denitrification were enriched in the stable granules,such as Paracoccus,Thauera,and Zoogloea.3.Based on previous results of F/M ratio,a new strategy for F/M ratio controlling via sludge discharge was established.Results showed that the MLSS,F/M ratio and SVI5 were stabilized at 4.0±0.3 g/L,0.4±0.03 gCOD/(gSS·d)and 45±3 mL/g with this strategy,respectively.Moreover,the mean granular size increased to 682±19?m and TN removal efficiency increased to 74±0.5%.3D-EEM analysis for the EPS content showed that the signal intensities of aromatic protein-like and tryptophan protein-like substances increased obviously during the operation,demonstrating that the secretion of hydrophobic protein-like substances could contribute to granular stability.In conclusion,F/M ratio could serve as a significant parameter during the design and operation of aerobic granular sludge process,and the stable aerobic granular sludge process with suitable sludge yield and microbial communities could be achieved via regulation of SRT.