Study On A Novel MBR-PBBR Combined System For Shortcut Biological Nitrogen Removal

Since eutrophication caused by nitrogen pollution,leading to "algae blooms" in lakes and "red tide" along coasts,is becoming a serious issue in China,an ever stricter nitrogen discharge standard has been established to protect ecosystems and human health. Conventional biological nitrogen removal technology has been widely applied,however,it has low TN removal rate and high energy consumption for the treatment of high-strength nitrogen wastewater with low COD/N.Therefore,current researches are ongoing to explore novel nitrogen removal technologies with high efficiency and low energy consumption.The partial nitrification and denitrification process being developed over recent years shows many economic advantages for the treatment of high-strength nitrogen wastewater with low COD/N, such as short reaction routes,low energy consumption,saving carbon resource,low sludge production and smaller reactor volume,etc.In this study,the performance of a novel two-stage MBR-PBBR(membrane bioreactorpacked bed biofilm reactor) combined system for shortcut biological nitrogen removal is investigated to improve its stability.The MBR and PBBR in the process are seeded with two kinds of functional microorganisms for partial nitrification and denitrification via nitrite.After fast start-up of the MBR-PBBR process,the effects of different ecological factors on the performance of MBR-PBBR combined system for shortcut biological nitrogen removal are studied to evaluate the feasibility of this novel process.The microbial mechanism is further explored to provide theoretical and technical guidelines for the application of this novel process.Firstly,the functional microorganisms for partial nitrification and denitrification via nitrite are obtained by the extinction dilution method.The results indicate that the functional microorganism for partial nitrification with spherical cell morphology is mainly composed of ammonia-oxidizing bacteria(AOB) belonging to Nitrosomonas,whose optimal conditions for ammonium oxidation are as follows:NH₄⁺-N 400 mg/L,pH 8.5,temperature 35℃,Alk/N 8.33.The functional microorganism for denitrification via nitrite with rod-shaped cell morphology belongs to facultative bacteria,whose optimal conditions for nitrite reduction are as follows:sodium citrate as carbon source,TOC/N=4,temperature 30℃,pH 9.0.Secondly,the novel two-stage MBR-PBBR combined system,seeded with two functional microorganisms,is constructed for shortcut biological nitrogen removal.The MBR-PBBR process is started up very fast and almost all nitrogen is removed via nitrite pathway during this period.Subsequently,stable shortcut biological nitrogen removal for a long period of 105 days is found in MBR-PBBR process with 0.4 L/min aerated rate and 30℃of operating temperature.However,as the decrease of HRT,ammonium oxidation rate in MBR decreases from 95%to 60%leading to the decrease of TN removal rate.The results from fluorescent in situ hybridization(FISH) and the most-probable-number(MPN) counts show that AOB(3×10⁸ MPN/mL) is dominated in MBR with less nitrite oxidizing bacteria (NOB,4.5×10³ MPN/mL) on day 105.Thirdly,the effects of three important ecological factors,i.e.the dissolved oxygen(DO), operating temperature and additional organic carbon,on the stability of shortcut nitrogen removal performance of the MBR-PBBR process are further studied The results show that low DO(0.8～1.2 mg/L) has negative effect on ammonium oxidation rate in MBR resulting in the decrease of TN removal rate from 95%to 80%.High DO(5～6 mg/L) contributes to the instability of partial nitrification,and only 70%of nitrogen is removed by nitrite pathway due to the growth of few NOB in MBR(from 4.5×10³ MPN/mL to 2×10⁵ MPN/mL) under sufficient DO conditions.The increase of operational temperature significantly improves the nitrogen removal rate and stability of MBR-PBBR process.As increasing the operating temperature from 20℃to 35℃,the ammonium oxidation rate and nitrite accumulation rate in MBR are improved from 45%to 90%and 55～80%to 85%,respectively.Small amount of additional organic carbon(TOC/N<0.2) can accelerate the ammonium oxidation rate from 60%to 75%in MBR.However,large amount of additional organic carbon concentration (TOC/N≥0.2) inhibits ammonium oxidation reaction,leading to sudden drawdown in ammonium oxidation rate to 25%.It is mainly due to the fact that the heterotrophs with rapid growth(11.5×10⁸ CFU/mL) could overeompete AOB for DO and NH₄⁺-N.96%of nitrogen is removed by nitrite pathway along with the addition of organic carbon.Because NOB is less competitive for enough DO for the nitrite oxidation,the number of NOB decreases to 1.0×10³ MPN/mL at TOC/N=0.3.Finally,the denaturing gradient gel eletrophoresis(DGGE) and FISH in molecular ecology technology are performed to analyze microbial community structure and their variations in MBR during different periods.The results indicate that the high efficiency and stable performance of shortcut nitrogen removal in MBR-PBBR process is attributed to the predominance of functional microbiology Nitrosomonas eutropha during the period of operation,which occupies over 54%of total bacteria.However,large amount of organic carbon can destroy the predominance of N.eutropha,whose proportion decreased to 31%at TOC/N=0.3.The diversity of microbial community in MBR increases along with operating time.The microbial community is mainly composed of three groups:β-subclass of Proteobacteria dominated by N.europha,CFB andα-,γ- subclass of Proteobacteria.The three groups form an orderly eco-chain for material metabolism and energy transfer by cooperating and coordinating with each other during the degradation of ammonium and organic matters.