Treatment of formaldehyde wastewater by a membrane-aerated biofilm reactor (MABR): The degradation of formaldehyde in the presence of the cosubstrate methanol

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 achieve effective FA degradation, and the FA degradation kinetics and degradation pathway in the presence of cosubstrate methanol (MeOH) were analyzed. The experimental results showed that when the influent FA loading was 2.99 kg HCHO/m3·d, the average FA removal rate reached 97.15%. When the influent FA concentration was 116.31 mg/L, the hydraulic retention time (HRT) was 4 h and the aeration pressure was 0.010 MPa, the average removal rates of FA, chemical oxygen demand (COD) and MeOH were 99.90%, 81.50%, and 97.14%, respectively. The degradation of FA in the MABR followed the pseudo first-order kinetic model, and the kinetic constant of FA with an initial concentration of 500 mg/L was 1.23 h−1. There was no inhibition of FA degradation by bacteria at an FA concentration of 500 mg/L. However, an increase in FA concentration had an obvious inhibitory effect on the degradation of MeOH. MeOH and volatile fatty acids (VFAs) were produced during FA degradation. FA was mainly transformed by a disproportionation reaction to form the intermediates formic acid and MeOH because of the stratified biofilm structure. Long-chain polymers were formed due to FA polymerization in aqueous solution and were transformed into acetic acid and propionic acid by acid-producing bacteria (APB).