Study On The Treatment Of Low Temperature And Low C/N Wastewater By Biochar Coupled With Heterotrophic Nitrification-aerobic Denitrification Strain ZO3

Soluble nitrogenous pollutant accumulation has been happening for more than 20 years in over 95%of China's water bodies.In order to control the discharge of nitrogenous pollutants in domestic sewage,the total nitrogen discharge standards of sewage treatment plants have been gradually improved.The screening of denitrification microorganisms and improvements of current denitrification technologies have also become one of the research hotspots.Heterotrophic nitrification-aerobic denitrification(HN-AD)microorganisms,as a new type of denitrification bacteria,can utilize organic matter in influent as carbon sources and carry out nitrification and denitrification processes simultaneously under aerobic conditions.Compared with conditional denitrification microorganisms,HN-AD microorganisms can achieve higher growth efficiency at higher ammonia and nitrogen concentration comparing to traditional denitrification microorganisms,thus,it can be employed in the treatment of high concentrations of nitrogenous wastewater at relatively low costs.However,this process is susceptible to a series of factors such as carbon-nitrogen ratio(C/N),dissolved oxygen(DO),temperature and p H conditions,and the relevant properties are not fully elucidated.With the implementation of source control and emission reduction nationwide in China,the C/N ratio in the water body has been greatly reduced.Furthermore,the microbial activity is inhibited under low-temperature conditions in winter.Therefore,improving the abilities of HN-AD microorganisms to treat low-temperature and low C/N wastewater is one of the current problems to be solved urgently.Biochar,as a carbon-rich material with complex size structures,can be used as a solid carbon source in the biological denitrification process.In order to improve the denitrification effect of the HN-AD system under low temperature,a psychrotrophic HN-AD strain Pseudomonas fluorescent sp.Z03 was isolated and screened from aerobic activated sludge of a pig farm wastewater treatment system in winter.This study carried out physiological and biochemical identification and denitrification characteristics researches of the strain Z03 and identified its denitrification functional genes.On this basis,in order to improve the denitrification effect of strain Z03 under low C/N conditions,a sequencing batch experiment of biochar(made of chestnut shells)coupled strain Z03 denitrification under low temperature and low C/N conditions was designed.The denitrification characteristics and denitrification mechanism of the SND system have been further studied.The main research contents are as follows.(1)A novel psychrotrophic HN-AD bacterium was isolated from a piggery sewage treatment system in winter and identified as Pseudomonas fluorescent sp.Z03.This strain can degrade a wide range of nitrogenous pollutants including ammonia,nitrate and nitrite nitrogen,and its removal efficiency can reach 1.75 mg NH₄⁺-N/(L·h),1.63 mg NO₂^--N/(L·h)and 1.67 mg NO₃^--N/(L·h),respectively.Above of that,the optimum growth and nitrification conditions of strain Z03were 5.0%dosage,sodium citrate as carbon source,15?incubation temperature,initial p H 6.5?9.0,C/N ratio 10 and 0.3%salinity.The existence of nitrogen removal genes nxr B,nap A,nir S,nor B and nos Z further confirmed the heterotrophic nitrification-aerobic denitrification capability of strain Z03.The denitrification process belongs to complete nitrification and denitrification,and the main product was nitrogenous gas,with almost no accumulation of NO₂^--N and NO₃^--N throughout the process.(2)In this study,using chestnut shells as biomass,biochar BC400 and BC700 were prepared through oxygen-limited and temperature-controlled carbonization methods at 400 and 700°C.The physical and chemical properties of biochar are characterized by SEM,BET,FITR,XRD,RS,DOC,zeta potential and elemental analysis.The characterization results showed that both BC400and BC700 were carbon-rich materials with complex pore structure and oxygen-containing functional groups.BC700 has a higher specific surface area and a more complex multilayered pore structure,with increased aromatic condensation,higher crystallinity and a more ordered structure.BC400 and BC700 both have good redox activity,while the further demonstrates stronger electron donor capacities and the later shows better electron transfer properties.In addition,both BC400and BC700 contain relatively high levels of DOC,reaching at 62.95mg/g and 51.617mg/g respectively.(3)The nitrogen removal system coupling single biochar with strain Z03 can significantly improve the treatment effect of low-temperature and low C/N wastewater compared with the control group(only biochar/only strain Z03).The nitrogen removal effects of the system raised with the increase of biochar dosing.At a dosage of 4.0 g/L,the removal rate of ammonia nitrogen raised from 41.03%(controlling group with Z03 only)to 95.16%(BC400+Z03)and 84.37%(BC700+Z03),respectively.The biochar-coupled-microbe nitrogen removal systems provide higher nitrogen removal efficiencies than single biochar-coupled-microbial nitrogen removal systems.When the dosage reaches 3.0g/L,it can achieve more than 90%ammonia nitrogen removal rate within 48h,while dosage ratio and the initial p H can slightly affect the denitrification effect.(4)From the experimental results and existing studies,it is assumed that there are three main reasons why biochar may enhance microbial denitrification.Primarily,BC400 and BC700contain a relatively high DOC content and provide an additional carbon source for microbial growth and metabolisms.Secondly,biochar as a p H retarder,can neutralize acidity caused by nitrification reactions and to some extent reducing the inhibitory effects of p H fluctuations on enzyme activities.Moreover,BC400 and BC700,as materials with good redox activity,may play a role in promoting electron transport system activity and enzyme activity in biological denitrification processes.It is speculated that the denitrification effect of a high dosage of BC400+Z03 is slightly better than that of BC700+Z03 because BC400 can promote the activity of nitrase and denitrase,while the significant increase in the reaction rate of BC400+BC700+Z03compared with the former two may be due to the electron transfer ability of BC700 which promotes the activity of BC400 as an electron donor.However,after 72h-reaction,the difference between the three systems was significantly smaller than that of 48h.Therefore,it was speculated that the mixed biochar system could improve the reaction rate by enhancing the electron transfer system activity,and the final denitrification effect was mainly related to the content of soluble organic carbon contained and p H sustained released in the biochar.