Performance And Mechanism Of Nitrogen And Phosphorus Removal In A Vertical Flow Aerobic Granular Sludge Reactor At Low Temperature

The quality and quantity of domestic wastewater in cold regions of northern China are greatly affected by seasons.Meanwhile,facilities of wastewater collection and treatment in villages are still insufficient,leading to low wastewater treatment rate,which need for improvement of the water environment quality at low temperature.For wastewater treatment plants in villages in cold regions,low temperature in winter increases various costs in the wastewater treatment process,and the effect of nitrogen and phosphorus removal in the effluent also shows a significant downward trend,which becomes a bottleneck problem in the operation.Aerobic Granular Sludge（AGS）has become a popular technology in the field of nitrogen and phosphorus removal due to its advantages such as high biological activity,dense granular structure,and strong ability to resist impact loads.However,the current domestic wastewater treatment technology with AGS has problems such as long startup time,strict operating conditions,and difficulty in stably forming in urban wastewater treatment plants.In addition,low temperature conditions can inhibit biological enzyme activity,limiting the application of AGS technology in the field of domestic wastewater treatment at low temperature in northern China.In response to the above issues,the unstable load of domestic wastewater in northern villages was taken as the raw water to construct vertical flow AGS reactors.Continuous experiments were conducted under the water temperature conditions of18～25℃and 6～11℃to research the rapid startup,anti-shock load capacity,and adaptability to low-temperature environment of Vertical Baffle Reactors（VBRs）.At the same time,the relationship between the structure of VBRs and the formation process of AGS as well as the characteristics of bacterial community was explored to reveal the mechanism of nitrogen and phosphorus removal by AGS in VBRs at low temperature.The main research contents are as follows:（1）VBRs with a daily sewage treatment capacity of 20 m³ has been designed,equipped with a composite reflux system and a structure with a high H/D ratio conducive to the formation of AGS.High concentration sludge load（MLSS=8000～10000 mg/L）was the key for VBRs to withstand continuous and unstable loads of domestic wastewater,while VBRs can achieve high-concentration sludge loads by providing favorable conditions for the formation of AGS.Based on the above study,combined with the characteristics of large seasonal variations in wastewater temperature in cold regions of northern China,the rapid startup,optimization of operating parameters,stable operation,and pollutant removal efficiency during low-temperature operation of VBRs were explored.Using domestic wastewater with unstable loads as the raw water（COD=120～556 mg/L,TN=19.97～81.54 mg/L）,the VBRs were successfully started up within 15 days under temperature of 18～25℃.Reducing both aeration and HRT can inhibit the nitrification process and affect the removal efficiency of ammonia nitrogen.By increasing the sludge concentration within the VBRs,it was to enhance pollutant removal efficiency under low-temperature conditions.When the water temperature dropped to 6～11°C,the average MLSS was controlled within the reactor to increase from 7901 mg/L to 9211 mg/L.After 80 days of continuous stable operation,the COD in the effluent was successfully controlled below 38 mg/L,NH₃–N concentration was maintained below 0.8 mg/L,and the concentrations of TN and TP were kept below 15 mg/L and 0.5 mg/L,respectively.（2）The interrelationship between AGS cultivation and microbial characteristics was elucidated,and the physicochemical properties of AGS during the granulation process of activated sludge were clarified.The mechanism of AGS denitrification and phosphorus removal in VBRs under low temperature conditions was revealed.Research has shown that the high sludge reflux ratio of VBRs and the turbulent effect of baffles provide conditions for AGS self cultivation.The increase in polysaccharide（PS）concentration in EPS promotes the bridging between particles to form AGS,which has a large particle size（average particle size of about 1.00 mm）,high extracellular polymer content（44.2 mg/g VSS）,and rich pore structure.The rich biodiversity within the VBRs demonstrates the formation of an aerobic-anoxic-anaerobic layered environment from the outside to the inside,providing space for the enrichment of various functional bacterial communities.The microbial composition and abundance of aerobic sludge under normal and low temperature conditions exhibit significant differences.At low temperatures,the functional diversity of the microbial community is relatively concentrated,with Flavobacterium,which has low-temperature denitrification and phosphorus removal capabilities,and Rhodobacter,which has denitrification capabilities,forming dominant bacterial groups.Their abundance increases at low temperatures,promoting efficient nitrogen and phosphorus removal under low temperature conditions.（3）On the basis of the above pilot studies,the practical application of a new VBRs in rural domestic wastewater was carried out.Taking a rural domestic wastewater treatment plant with a scale of 3000 m³/d in the northern cold region as an example,the impact of VBRs on nitrogen and phosphorus removal efficiency was investigated under low temperature and continuous fluctuations in water quality and quantity.The results showed that the VBRs can operate stably throughout the year,and the effluent quality is far superior to the Class A discharge standard in GB18918-2002“Pollutant Discharge Standards for Urban Sewage Treatment Plants”.Meanwhile,the combined recirculation system of the VBRs and the high aeration rate in the aerobic zone resulted in increased endogenous sludge consumption,which was the primary reason for the low residual sludge production.The vertical flow bioreactor constructed in this study can effectively treat domestic wastewater at low temperature in cold regions,avoiding the disadvantages of traditional biological treatment processes such as poor treatment efficiency at lowtemperature,low sludge activity,and large amounts of residual sludge.The research results have broad promotion and application prospects for the treatment plants（stations）of unstable domestic wastewater in the cold regions of northern China.