Performance And Mechanism Of Airlift Micro-pressure Dual-circulation Bioreactor For Treatment Of Domestic Wastewater In Cold Region

The traditional processes for biological nitrogen and phosphorus removal are mostly performed in two or more independent reactors or in the same reactor where aerobic and anoxic environments can alternate with time.As a result,it is more demanding in construction investment,operation cost and land occupation.When applied in urban sewage treatment,such processes are also subject to problems such as instability at low temperatures in winter,the need for organic carbon sources and poor resistance to shock load.This paper summarises the theory and technology of biological nitrogen and phosphorus removal from sewage in research and application.Based on this,a novel airlift micro-pressure dual-circulation bioreactor,named as AL-MPDR,was developed to create a circulating flow pattern of mixed liquid in the unit to strengthen antive sludge and improve the efficiency of material transfer.In the reactor,different oxygen environments coexist for the in situ removal of contaminants.To explore the effect and mechanism of the developed airlift reactor for sewage treatment and synchronous removal of contaminants,the characteristics of reactor flow field and the performance of different scale municipal wastewater treatment were studied in this paper,thus,laying a theoretical and technical foundation for promoting the application of the reactor.First,the flow field characteristics of the proposed AL-MPDR were studied via numerical simulation and actual measurement.The results showed that:with the increase of the aeration intensity,the liquid phase circulating flow pattern in the main reaction zone of the numerical simulation reactor gradually showed a trend of low flow velocity in the middle area and high flow velocity in the surrounding area.When the aeration rate was 0.6m³/h,the liquid phase circulating flow state was the most stable,the flow velocity in the central area was the lowest,and it was uniformly and symmetrically distributed with the main reaction zone of the reactor as the geometric center.It is further proved that the circulating flow state can be formed in the reactor by the test of flowing clear water,and the circulating time decreases with the increase of aeration intensity.Under the influence of liquid phase flow state in the reactor,the difference of Klas in different areas is also great.When the aeration rate is 0.6 m³/h,the liquid phase flow state in the reactor is the most stable,and the variation of K_Las is the biggest,the peripheral area is 0.4529,and the central area is only 0.1822.Also because of the special circulating flow state in the reactor that the reactor has flow field characteristics,such as the oxygen gradient distribution law with low dissolved oxygen value in the central region and high dissolved oxygen value in the peripheral region,and the sludge concentration distribution law with high dissolved oxygen value in the central region,low dissolved oxygen value in the peripheral region and lower sludge concentration at the reactor outlet.Based on the flow field characteristics of the reactor,the performance and mechanism of simultaneous removal of pollutants in the reactor were studied.The results show that:AL-MPDR showed better simultaneous removal effect of carbon,nitrogen and phosphorus under the aeration intensity of 0.104 L/(min·L),0.156 L/(min·L)and 0.208 L/(min·L),and HRT of 8h,10h,12h and14h,respectively.The nitrogen was removed by simultaneous nitrification and denitrification.The oxygen gradient environment in the reactor is the main factor that affects the diversity of microbial community in different areas.Because of the special flow field characteristics,Haliangium and Nitrospira with nitrification function,Acinetobacter and Zoogloea with denitrification function,Rhodoferax and Aeromonas with denitrification and phosphorus removal function were enriched in the reactor at the same time.And the system has a complete metabolic pathway of organic matter,nitrogen and phosphorus.In view of the characteristics of low temperature and low C/N in urban wastewater in China,combined with the flow field characteristics and the denitrification and phosphorus removal mechanism of AL-MPDR,the simultaneous removal performance and mechanism of AL-MPDR pollutants under low temperature and low C/N were studied respectively.The results show that:In view of the characteristics of large seasonal temperature variation of urban sewage in northern China,the operation mode of low sludge concentration at normal temperature and high sludge concentration at low temperature was adopted.After the stable operation,the effluent COD,NH₄⁺-N,TN and TP remain below 40mg/L,5mg/L,15mg/L and 0.5mg/L,respectively,which still maintains a strong synchronous removal performance of pollutants.At low temperature,the activity of TTC dehydrogenase decreased and the content of extracellular polymer increased.However,with the decrease of temperature and the change of operating conditions,the relative abundance of Bacteroidetes,Gemmatimonadetes,Nitrospirae and Firmicutes in the reactor increased,some of which had cold-tolerant and cold-loving bacteria genus,such as the relative abundance of Flavobacterium,Zoogloea and Rhodobacter also increased significantly.In addition,the relative abundance of functional bacteria such as Haliangium,Nitrospira and Aeromonas for denitrification and phosphorus removal also increased slightly.These functional bacteria were enriched in the reactor and formed the dominant bacteria group,which ensured the low temperature operation effect of the reactor.When the influent C/N ratio is between 3.2 and 9.4,the reactor maintains high simultaneous removal capacity of organic matter,nitrogen and phosphorus pollutants.With the decrease of C/N ratio,the settling performance of activated sludge in the reactor was not significantly affected,but the proportion of small particle size sludge was increasing.However,the effect of simultaneous nitrification and denitrification in the reactor was not affected by anoxic microenvironment,and the average SND rate at this time was still 88.67%.The abundance and diversity of microbial community in the reactor increased slightly with the decrease of C/N ratio.The special functional bacteria such as Denitratisoma,Thauera and Aeromonas were enriched in the reactor,and the relative abundance was increased.Therefore,there may be biological nitrogen and phosphorus removal mechanisms such as short-cut nitrification and denitrification,autotrophic denitrification and denitrification dephosphorization in the system,which greatly reduces the demand for carbon source in the reactor biological system and ensures the operation effect of the reactor at a low C/N ratio.The treatment performance of Al-MPDR in pilot scale was studied for a period of 368 days.The results show that:under an inlet water temperature of 6.9?16?and an environment with greatly fluctuating water quality and environment,with COD,NH₄⁺-N,TN and TP being111.30?2040.00mg/L,5.33?15.15mg/L,14.31?40.97mg/L and 1.89?13.12mg/L,respectively,the effluent indicators of the pilot-scale device under operation were better than the Class A discharge standard(GB18918-2002).It shows a higher effect of simultaneous removal of pollutants and a stronger ability to resist impact load.The flow pattern of the mixed liquid in the pilot-scale Al-MPDR unit tends to be more stable,and the microbial community in the reactor has higher abundance and diversity,and the diversity of the microbial community in different regions is great.Compared with the traditional biological nitrogen and phosphorus removal process,AL-MPDR has similar dominant flora structure,but the difference is that the number of dominant phyla with high relative abundance is more.Bacteria with the functions of nitrogen and phosphorus removal,such as Thermomonas,chlamydomonas,deschloomonas,Thaurea and Deschloomonas,were also enriched in the pilot plant.