Performances Of The Membrane-aerated Biofilm Reactor For Treatment Of Formaldehyde Wastewater

Formaldehyde（FA）is a volatile organic pollutant that is highly toxic and has carcinogenic properties.Traditional biological and physicochemical treatment methods of FA wastewater have many defects,such as long time,low microbial tolerance,complex processes,costly chemicals,and the generation of secondary pollution.In this study,a membrane-aerated biofilm reactor（MABR）was constructed to utilize its biofilm structure stratification and bubble-free oxygen supply to achieve efficient degradation of FA while avoiding secondary pollution.The experimental research contents include:（1）The oxygen transfer performance of the membrane modules in clean water under different operating conditions;（2）Start of the MABR before reactor modification;（3）Investigating the effects of hydraulic retention time（HRT）,aerated pressure and influent concentration on the degradation of organic pollutants during reactor continuous flow experiments;（4）The relationship between FA and cosubstrate methanol（Me OH）;（5）Studying the conversion of pollutants and the speculation of degradation pathways in a typical cycle experiment;（6）Analysis the performance of MABR after modification.Experimental results showed that:（1）In the experiment of clear water oxygenation,the oxygen transfer rate of the membrane module increased with the increase of aerated pressure and circulation flow.When the aerated pressure was increased from 0.010 MPa to 0.050 MPa,the oxygen transfer rate（OTR）increased from 13.73 mg O₂/h to 20.81 mg O₂/h.When the circulation ratio increased from 0 to 3,the oxygen transfer rate（OTR）increased from 17.15 mg O₂/h to 21.63 mg O₂/h.（2）After one month of acclimation,when the influent FA concentration was 119.09 mg/L,the aerated pressure was 0.010 MPa,the HRT was 24 h,the MABR had a stable removal effect on FA and COD,the reactor was successfully started,and the average removal rates of FA and COD were 99.95%and 90.74%,respectively.（3）When the influent FA loading was 2.99 kg HCHO/m³·d,the average FA removal rate reached 97.15%;There was no inhibition of FA-degrading bacteria by FA at an FA concentration of 500 mg/L;The degradation of FA in the MABR followed the pseudo first-order kinetic model,the kinetic constant was 1.23 h^-1 when the initial FA concentration was 500 mg/L and the aerated pressure was 0.030 MPa.However,in the presence of the cosubstrate Me OH,the increase of FA concentration had an obvious inhibitory effect on the degradation of Me OH.（4）Based on the biofilm layered structure,it is speculated that the degradation pathway of FA in MABR was as follows:FA was mainly transformed by a disproportionation reaction to form the intermediates formic acid and Me OH in the aerobic zone and anoxic zone.FA was polymerized in the reactor,the formaldehyde polymer was decomposed by acid-producing bacteria（APB）to form produces acetic acid and propionic acid in the anoxic zone,and finally these products were mineralized to form water and carbon dioxide.（5）After the membrane module was transformed into the open-end type to realize periodic exhaust,the biofilm grows rapidly and the start-up time was short.When the influent FA concentration was 512.61 mg/L,the HTR was 8 h,and the aerated pressure was 0.030 MPa,the influent FA load was 1.54 kg HCHO/m³·d,and the influent organic matter load was 2.37 kg COD/m³·d,the average removal rates of FA,Me OH and COD were 99.69%,96.15%and90.96%,respectively.This research indicated that the MABR constructed in this paper can effectively remove FA from FA wastewater while avoiding secondary pollution,and achieved higher removal efficiency under higher influent FA loading.The structure stratification of biofilm and the opposite diffusions of masses have a specific advantage for the degradation of FA.Compared wth the MABR using the dead-end membrane module,the MABR with an open-end membrane module is more promising in removing organic compounds.This study provides basic information and reference for engineering design and applications.