Sludge Granulation Promoted By Mycelial Pellets In A Square Sequencing Batch Airlift Reactor And Analysis Of Flow Regime And Efficiency

Aerobic granular sludge process has excellent sedimentation performance and higher biomass concentration,which can save land and attained sludge reduction.At present,cylindrical SBR or SBAR with large H/D are mostly used in laboratory research.Too high a structure will greatly increase the cost of construction and cylindrical structures are not easy to construct and arrange.Therefore,it is of far-reaching significance to research square AGS reactors with low effective water depth for AGS engineering application.In this study,different configurations of square SBAR and cylindrical SBR were simulated to explore the difference between the two AGS reactors and analyze the flow conditions suitable for sludge granulation.A square SBAR with low effective water depth was selected for actual operation to analyze the changes of water quality index and sludge characteristics,Combined with the change of microbial community structure,the sludge granulation in the reactor was analyzed,and the operation efficiency of the reactor was evaluated.The results show that the gas holdup in square SBAR is mainly affected by aeration intensity and the H/L ratio has little effect on it.The ascending velocity inside the draft tube is mainly determined by the aeration rate,while the descending velocity outside the draft tube is jointly affected by the height length ratio（H/L）and the aeration intensity.The flow pattern in the SBAR reactor is a large-scale regular circulation flow,and the shear stress is concentrated in the draft tube.With the increase of height diameter ratio（H/D）in the cylindrical SBR reactor,the number of vortices in the reactor increases and the shear rate increases.The square SBAR reactor with high aspect ratio（H/L=3.75）was selected for experimental verification.Under the condition of aeration intensity of 3L/min,the particle size increased in the first 14 days and the settling performance gradually improved.After 20 days of operation,microfilaments were found.The reason was that the local aeration was uneven due to insufficient aeration intensity.On the 20th day,the aeration rate was adjusted to 6L/min,and mycelial pellets（MPs）were added to the system to inhibit the growth of microfilaments.From the 26th day,the system gradually recovered,the sludge settling performance improved,the particle size began to increase,and the microfilament content in the system decreased.On the 50th day of operation,the sedimentation performance is good,with obvious boundary,and the particle size is376μm.In addition,the square SBAR reactor has the tendency to form mature granular sludge.In the whole operation stage,the effluent quality is good,COD removal rate is more than 90%,TN removal rate is about 60%-70%,NH₄⁺-N removal rate is more than95%,indicating that the reactor has a good effect on pollutant removal.Through high-throughput sequencing,it was found that the abundance of Proteobacteria increased and Actinobacteriota decreased with the decrease of sludge settling performance,which verified that the transformation of community structure from Actinobacteriota to Proteobacteria might indicate sludge bulking.The increase of microthrix abundance is the key factor for the decline of sludge settling performance.