Study On The Mechanisms Of Ammonia-Nitrogen Removal From Micro-Polluted Water By Biofilter Under Low/Room Temperature Conditions

China’s drinking water sources have been threatened by the potential micro-pollution of ammonia nitrogen.Especially in the low temperature environment in winter,the self-purification ability of water bodies is weakened,which makes the ammonia nitrogen pollution in water bodies or source water more serious.The biological filter denitrification method is often used for pretreatment or advanced treatment of micro-polluted source water due to its advantages of low cost,small secondary pollution and simultaneous removal of multiple pollutants.However,the growth and metabolic activity of denitrifying bacteria decreased under low temperature conditions,resulting in a significant decrease in the denitrification performance of the system.Therefore,it is one of the important technical measures to ensure the safety of drinking water and people’s health in winter to construct a low temperature resistant biological denitrification mixed bacteria system and maintain stable denitrification efficiency.This paper attempts to use the sand filter attachment containing a variety of nitrifying bacteria as an inoculum to start the biofilter for the pretreatment of low-temperature ammonia nitrogen micro-polluted water,and to explore the ammonia nitrogen removal performance and microbial degradation mechanism of the filter under different low temperature and filter material types.The attached microorganisms containing nitrifying bacteria,mainly including ammonia oxidizing bacteria（AOB）,nitrite oxidizing bacteria（NOB）and complete ammonia oxidizing bacteria（Comammox Nitrospira）,were successfully extracted from the sand filter packing of drinking water plants.The results showed that:（1）Low temperature（2-15℃）would significantly reduce the rate of Ammoxidation of bacteria,and increase the accumulation time or peak concentration of NO₂^--N in the system,and the increase of ammonia nitrogen concentration would promote the accumulation of NO₂^--N under low temperature conditions;However,a medium to high temperature environment（≥37℃）will completely lose bacterial activity.（2）The increase of NO₃^--N concentration（>20-30mg-N/L）will significantly inhibit the Ammoxidation rate of the flora,but the flora can still maintain a high Ammoxidation activity.（3）When the temperature drops below 15℃,the growth of Nitrifying bacteria will be significantly inhibited,but the flora can improve the efficiency of energy metabolism by stimulating metabolic pathways such as Oxidative phosphorylation,which comes from self protection and resistance to the adverse effects of the external cold environment.（4）Nitrospira has a higher competitive advantage in growth compared to other nitrifying bacterial communities under low temperature（10-15℃）and low ammonia nitrogen load conditions,suggesting that Comamox Nitrospira may play a dominant role in ammonia nitrogen removal.The ammonia nitrogen removal characteristics and microbial degradation mechanism of quartz sand filter under low/room temperature conditions（2,5,10 and 25℃）during start-up and operation stages were studied.（1）After 40 days of start-up and operation of the sand filter,the removal efficiency of ammonia nitrogen in the reactor at 5-25℃was high（86%-98%）.The extremely low temperature（2℃）environment makes the filter almost lose ammonia oxidation activity.（2）After 120 days of long-term operation,the removal rate of ammonia nitrogen in the filter decreased significantly,and the lower the temperature,the stronger the inhibition.Prolonging the hydraulic retention time is helpful to the recovery of ammonia oxidation performance of sand filter under low temperature conditions.（3）After 372 days of stable operation of the filter,the filter had formed a stable low-temperature resistant nitrification community composition,and the autotrophic Nitrosospira bacteria played a major role in the ammonia nitrogen removal process of the sand filter.（4）The results of environmental Scanning Electron Microscope showed that the pores of the filter material were conducive to the growth of biofilm,and more flocculent secretions were observed on the surface of the filter material at low temperature（5-10℃）.At the same time,low temperature（5-10℃）stimulated the flora to secrete more proteins and polysaccharides to maintain cell growth.The physicochemical-microbial coupling mechanism of nitrogen transformation and ammonia nitrogen degradation in BAC filter under low/room temperature conditions（2,5,10and 25℃）was studied.（1）The results of dynamic adsorption test showed that the adsorption capacity of activated carbon to’three nitrogen’gradually decreased with the decrease of temperature.When the temperature was reduced to 5℃,the activated carbon had no significant adsorption removal efficiency for nitrite and nitrate nitrogen.When the temperature is reduced to2℃,the activated carbon has no significant adsorption removal efficiency for ammonia nitrogen.（2）Low temperature（2-10℃）made the start-up time of BAC filter longer,and the filter had significant nitrite accumulation at extreme low temperature of 2℃.（3）In the environment of2-5℃,microbial degradation plays a leading role in the process of ammonia nitrogen removal in BAC filter.At 10-25℃,the process of ammonia nitrogen adsorption-microbial degradation-readsorption may be the main mechanism of ammonia nitrogen removal in BAC filter.（4）Low temperature will promote the secretion of bacterial proteins to resist the effects of low temperature stress,but extreme low temperature（2℃）will reduce the potential of bacterial flora to stimulate protein secretion from self-protection and regulation.（5）After 372 days of operation,a stable low-temperature nitrogen removal system was successfully constructed.The main nitrifying bacteria were Nitrospira,Nitrosospira and Nitrosomonas,and with the decrease of temperature,Nitrospira gradually competed with Nitrosomonas.It is speculated that Comammox Nitrospira plays a key role in the ammonia nitrogen removal process of BAC.In this paper,eight groups of biological filter devices were started by using the attachment of sand filter in water plant.Except for the sand filter operated at extremely low temperature of2℃,the stable and efficient removal of ammonia nitrogen in micro-polluted source water was achieved（>80%）.At the same time,compared with sand filter,BAC filter also has higher total nitrogen removal efficiency（>60%）.This study will provide important theoretical basis and technical support for the bioaugmentation removal of ammonia nitrogen in source water under low temperature conditions,and is of great significance for ensuring the safety of drinking water in winter.