Analysis On The Oxygen Transfer Efficiency Of Fine-pore Aeration System

Aerobic biological treatment is an efficient and practical wastewater treatment technology.The aeration system is one of the core technical units of aerobic biological treatment,and is also the main energy-consuming unit,occupying 45 to 75% of the total energy consumption of the entire wastewater treatment plant.In 2018,the total power consumption of China’s municipal wastewater treatment plants accounted for 0.28% of the total power consumption of the whole country,and it showed an upward trend year by year.The fine-pore aeration system is widely used in municipal wastewater treatment plants due to its high oxygen transfer efficiency.During the operation,fine-pre diffusers inevitably foul with pollutants accumulating on the surface and within the pores of fine-pore diffusers,resulting in a decrease in aeration performance and an increase in pressure loss.Therefore,it is necessary to increase the air flux provided by the blower and the output pressure of the blower to maintain the same process performance,which aggravates the energy waste of the aeration system.Therefore,reducing elevating energy consumption of the aeration system has become a key to the energy-conservation and waste-reduction the wastewater treatment industry.This dissertation studied the composition of the foulants accumulated on the surface and in the pores of the fine-pore diffuser during long-term operation,and related the foulants to the decrease in oxygen transfer efficiency.By comparing different cleaning methods,optimal strategy for oxygen transfer performance recovery was provided.Through field testing of the oxygen transfer efficiency of the fine-pore aeration system along the aeration tank,the influence of wastewater quality characteristics on the oxygen transfer performance of the fine-pore aeration is verified.These results are of great significance for the precise control and energy-saving operation of the fine-pore aeration system.First,the foulant morphology and elemental composition of the fine-pore diffuser used in Zhuyuan No.2 Plant for 10 years were analyzed.According to the scanning electron microscope and energy spectrum analysis results of the surface of used and chemical cleaned fine-pore diffusers,the foulants accumulated on the fine-pore diffuser can be divided into microbial,organic and inorganic foulants.Mechanical scrubbing removed half of the organic foulants,but had no effect on the removal of inorganic foulants.The total amount of extracellular polymer substances（EPS）in the external foulants scraped off the surface of the fine-pore diffuser was 448 mg TOC / g VS,which was 10 times that of bulk sludge.NaClO removed 60% of the organic foulants remaining in the fine-pore diffuser after mechanical scrubbing by oxidizing organic functional groups.Chemical cleaning through HCl can remove all the inorganic foulants.Quantitative analysis revealed that the accumulated metal cations on the fine-pore diffuser were 3.45 g Ca²⁺,1.20 g Fe³⁺ and 1.92 g Mg²⁺.The inorganic foulants of each fine-pore diffuser were: CaCO₃ of 9.62 g/m,MgCO₃ of 6.75 g/m and FePO₄ of 3.20 g/m.To further investigate the microbial foulants components,the difference analysis of the microbial population structure between the diffuser surface and the bulk sludge was carried out by Illumina-Mi Seq sequencing.The dominant microbial population on the diffuser surface was identified.During long-term operation,microorganisms are enriched on the diffuser surface,thereby increasing the abundance and diversity of microorganisms.Microbial diversity and population analysis indicated that the microbial contaminants on the diffuser surface are mainly derived from activated sludge.Because of the different environment of the diffuser surface,microorganisms enriched on the diffuser surface were not the same as those in the bulk sludge.Microorganisms enriched on the diffuser surface were mainly nitrite oxidizing bacteria with oxygen affinity,filamentous bacteria and EPS secreting bacteria.Oxygen transfer efficiency tests were carried out to link the decrease in oxygen transfer efficiency to diffuser fouling.The oxygen transfer efficiency of cleaned fine-pore diffusers were also performed on.The performance decline of diffusers was observed with dynamic wet pressure increased by 3.2 times and standard oxygen transfer efficiency dropped to 73%,which contributed to 26.81% increase in air flow and 15.00% increase in total energy consumption of wastewater treatment plant.The cleaning treatment has a certain degree of recovery for oxygen transfer efficiency.After NaClO cleaning,the oxygen transfer efficiency of the fine-pore diffuser was restored to 83.00% of a new diffuser,and the pressure loss was reduced to 45.45% of a used diffuser,which showed effective foulant removal.However,the conventional cleaning agent HCl in the wastewater treatment plant failed to restore oxygen transfer efficiency of the diffuser.From the confocal laser scanning microscope and scanning electron microscope observation,it can be seen that HCl eliminated the structural support formed by inorganic pollutants,making the organic pollutants broken into smaller piece and distributed more dispersed,therefore re-blocking the pores.The combined cleaning treatment results showed that using NaClO and then HCl reached 86.00% of the oxygen transfer efficiency of a new aerator,and achieved the best recovery.The quantitative analysis result of fouling showed that the external foulant of the diffuer（i.e.,the biofilm growth）accounted for 34.00% of the total fouling,and the internal fouling（including organic and inorganic fouling）accounted for 45.42% of the total fouling.The irrecoverable fouling accounted for 20.58% of the total fouling.Field tests on the pollutant removal effect along the aeration tank showed that chemical oxygen demand（COD）and ammonia nitrogen（NH₄-N）at the outlet of the aeration tank were not affected by temperature.The removal rates of COD and NH₄-N were 84.38% and 99.97%,respectively.Both COD and NH₄-N in the effluent reached GB18918-2002 Class A discharge standard.The migration and transformation of nitrogen species in the aeration tank showed that NH₄-N was oxidized to nitrate,and no simultaneous nitrification and denitrification occured.The results of soluble COD（SCOD）fractionation showed that the proportion of SS in summer was 57.75%,which was 7.03% lower than that in winter.The degradation rate of SS in Zone1 area of the aeration tank was 92.74 ± 3.08%,showing high load in the front of the aeration tank and low load in the middle and at the end.Oxygen transfer efficiency comparison in summer and winter indicated that in summer,αSOTE showed little change along the tank,with average value of 25.32 ± 0.77%;but in winter αSOTE fluctuated alone the tank,with 24.55% in Zone1 and 10.85% in Zone3,indicating that diffuser fouling may occure.According to the comparison of oxygen transfer rate and oxygen demand in the aeration tank,the oxygen supply in summer was 35.94% higher than that oxygen demand,and the oxygen supply in winter was 25.94% higher than oxygen demand.Excessive air flux was provided,causing energy consumption waste.