Effects Of Magnetic Biocarriers On The Performance And Microbial Characteristic Of MBBR At Low Temperature

Moving-bed biofilm reactor（MBBR）have widely applied in enhancement of the biological wastewater treatment due to its higher diversity and richness of bacterial community.However,the activity of nitrifiers in MBBR were inhibited at low temperature（≤15℃）,which affected the nitrification process and limited the nitrogen removal capacity of the system.Magnetic field could improve the biodegradation rate of pollutants by the magnetic effect of microorganisms,but there are some problems such as easy loss of magnetic particles and small size of magnets in the application.In this study,the magnetic biocarriers were prepared by blending and extrusion technology,which were made of Polyethylene and Nd₂Fe₁₄B magnetic powder.And MBBR process was constructed by adding magnetic biocarriers.Firstly,the effect of magnetic biocarriers with different intensity on wastewater treatment capacity was analyzed.Then,the effects of magnetic biocarriers on nitrification performance and biofilm growth characteristics were investigated at low temperature,and the changes of microbial characteristics of biofilm were analyzed.Our results could explain the mechanism of magnetic biocarriers from microbiology.The main research results were shown as follows:（1）The magnetic biocarriers with different intensity were made of PE,polyquaternium-10（2 phr）and a series of Nd₂Fe₁₄B magnetic powders（2 phr and 4 phr）by blending and extrusion technology.The average magnetic induction intensities on the surface of the two magnetic biocarriers were 0.3 m T（2 phr）and 0.5 m T（4 phr）,respectively.Under the same conditions,three MBBRs with different biocarriers were operated.The results showed that the magnetic carriers could improve the wastewater treatment effect,biofilm quantity and aerobic microbial activity of the reactor,and the magnetic field intensity is positively correlated.The average removal rates of COD and NH₄⁺-N by adding 0.5 m T magnetic biocarriers were 7%and 20.8%higher than those of commercial biocarriers,respectively.In the stable operation stage of the reactor,the biofilm amount and specific oxygen uptake rate of the 0.5 m T magnetic biocarriers were 1.2 times and 2.4 times than that of commercial biocarriers,respectively.（2）We found that the magnetic biocarriers have better biological affinity through the characterization of the biocarriers,which could conducive to the attachment growth of microorganisms in the early stage of biofilm formation.Compared with the commercial biocarriers,the surface of the magnetic biocarriers displayed positive electricity（0.7 m V）,and hydrophilicity and roughness were improved.Two MBBRs were operated at low temperature.The results showed that the COD and NH₄⁺-N average concentrations of effluent of reactor with magnetic biocarriers were 37 mg/L and 7.60 mg/L at 9℃±1℃,respectively.And the average removal rates of COD,NH₄⁺-N and TN were 3.6%,16.2%and 12.1%higher than those of commercial biocarriers,respectively.The removal effect of NH₄⁺-N was improved markedly.The magnetic biocarriers changed the composition and content of EPS,and the content of Total EPS in the biofilm was 1.04 times higher than that in the commercial biocarriers.The secretion of more EPS could maintain and improve the morphology and structure of the biofilm,and ensure microbial metabolic activities at low temperature.（3）Based on high-throughput sequencing analysis,higher diversity and richness of bacterial community was established in the biofilm of magnetic biocarriers at low temperature.There are obvious differences in microbial community structure of biofilm between the two biocarriers duo to bacterial magnetic susceptibility.Relative abundances of nitrifiers（e.g.,Nitrosomonas,Nitrospira）and denitrifiers（e.g.,Sphaerotilus,Zoogloea）were increased in the magnetic biocarriers.Functional prediction analysis with PICRUSt2 showed that the microorganism of magnetic biocarriers has better total gene function expression level,which were significantly increased than commercial biocarriers in gene representing signal transduction mechanism and intracellular trafficking,secretion,and vesicular transport.Meanwhile,the magnetic biocarriers could improve most functional metabolic pathways of biofilms,which were significantly increased than commercial biocarriers in material transport and signal transduction.The increase of genes related to EPS synthesis and transport also showed that magnetic biocarriers could regulate population behavior and improve the potential of EPS secretion by enhancing intercellular signal transduction.Furthermore,most of the abundances of nitrogen metabolism genes were raised in the biofilm of magnetic biocarriers.For example,genes amo,hao that were responsible for nitrification,and genes nap,nor that were responsible for denitrification.Magnetic biocarriers increased biofilm potential for denitrification at low temperature.