Study On The Start-up And Wastewater Treatment Process Of Aeration-free Fixed-bed Biofilm Reactor Based On Zeolite Mixed Activated Sludg

PASND(Passive aeration nitrification and denitrification)is a novel low energy consumption biofilm wastewater treatment process.In this system,glycogen accumulation organisms(GAOs)and zeolite are used to remove the pollutants containing carbon and nitrogen in the alternating anaerobic and aerobic environment,and oxygen is supplied to the biofilm through natural ventilation through the drainage reactor in the aerobic stage,thus avoiding the large energy consumption caused by head aeration.It can effectively remove COD,TN in sewage under low energy consumption,and is considered a competitive alternative to activated sludge process.Until now,studies on PASND use highly enriched GAOs biofilms as seeds,and utilize the characteristics of GAOs to rapidly transform VFA(Volatile Fatty Acid)into PHAs(polyhydroxyalkanoates)stored in cells under anaerobic environment.The treatment effect of synthetic wastewater with sodium acetate as carbon source was studied.However,the application of PASND in practical engineering is still challenged: 1)The long time(60 days)required for the enrichment of GAOs may not meet the requirements of the start-up of the reactor in practical engineering;2)Fluctuating water quality and low organic carbon concentration in actual wastewater may not meet the growing needs of GAOs;3)Whether the biofilm dominated by GAOs can effectively treat the actual wastewater with complex carbon source components.Therefore,it is of great significance to study the rapid start-up of PASND process with activated sludge modified with zeolite as the sole seed,evaluate its treatment efficiency of wastewater under different conditions,and provide a theoretical basis for its application in practical engineering.In view of this,this study first fast starts PASND process with activated sludge modified with zeolite as the sole seed and explores its carbon,nitrogen pollutant removal mechanism,as well as the evolution characteristics of the microbial community in the reactor,and evaluates different conditions(carbon and nitrogen ratio,temperature,zeolite powder content,hydraulic retention time)on the reactor start-up and the influence of depurative effect for contamination removal,finally start reactor using the actual wastewater to explore the treatment effect of PASND process on actual wastewater.The main conclusions in this study are as follows:(1)PASND process was successfully started with activated sludge modified with zeolite as the sole seed.After running for 18 days,the bioreactor’s COD and TN removal efficiency reached 88.26% and 66.48%,respectively.GAOs(6.86%)and Thauera(18.29%)were successfully enriched in the biofilm.The synthesis of PHAs plays an important role in COD removal for the PHAsproduced/VFAconsumed at the anaerobic phase was 0.65 C mmol/C mmol,while glycogen metabolism is not the main pathway of COD removal as Glyproduced/ PHAsconsumed at the aerobic phase is 0.21 C mmol/C mmol,lower than that of relevant literatures with high abundance of GAOs.Compared with conventional wastewater treatment process,PASND process can save about 60% energy consumption and reduce sludge yield by 50%-85%.(2)The effects of C/N,temperature,zeolite powder dosage and HRT on the startup and operation of PASND process were studied in complex environment.It is found that high C/N is beneficial to GAOs enrichment,and the process can be successfully started when C/N is 12 and 5.When the C/N decreased to 2.5,GAOs could not be enriched,and NO3-accumulated in aerobic phase affected TN removal in the next cycle.In addition,low temperature can prolong the start-up time of PASND process,but does not affect the treatment effect on COD and TN in stable operation.The influence of different zeolite powder content on NH4+ removal was determined by adsorption experiment,and the optimal zeolite powder content was 0.4 g zeolite/g VSS.When zeolite powder content continues to increase,the rate of increase of NH4+ removal slows down.In addition,the reduction of HRT is beneficial to increase the COD treatment load.When HRT decreases from 12 h to 6 h,the COD treatment load increases to 1.86 times of that when HRT=12 h(0.57 kg COD/m3/d→1.06 kg COD/m3/d).(3)PASND reactor was successfully started with actual wastewater and high abundance of GAOs was enriched in biofilm.The enhanced ventilation in the aerobic phase was beneficial for shortening the reaction time in the aerobic phase,improving the reactor regeneration rate,and increasing the treatment loading.When HRT=4 h(3 h in anaerobic phase and 1 h in aerobic phase)and ventilation intensity is 0.006 m3/h,the COD treatment loading increases from 1.08 kg COD/m3/d to 1.86 kg COD/m3/d.The fitting results of PASND wastewater treatment capacity by using response surface methodology show that effective wastewater treatment with low energy consumption can be realized by adjusting influent COD and TN concentration,HRT,and ventilation intensity.The COD removal efficiency was 89.47%,TN removal efficiency was 83.12%,and COD treatment loading was 0.96 kg COD/m3/d.TN treatment loading was 0.045 kgN/m3/d,COD treatment energy consumption was 0.19 kWh/kg COD,TN treatment energy consumption was 19.11 kWh/kgN.