Molecular insight into variations of dissolved organic matters in leachates along China's largest A/O-MBR-NF process to improve the removal efficiency.

Dissolved organic matter (DOM) is a critical component of high-strength organic wastewater, and the study of them from molecular perspective could improve the removal efficiency. Leachate samples were collected from China's largest two stage anaerobic/aerobic membrane bioreactor and nanofiltration (A/O-MBR-NF) process, with the treatment capacity of 5000 t/d, and characterized by electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) from molecular perspective. High molecular weight (m/z > 500) compounds with 40-50 carbon atoms and 15-20 double bond equivalence (DBE) were biodegraded into medium molecular weight compounds with 10-20 carbon atoms and ∼10 DBE. Contribution of lipids and unsaturated hydrocarbons compounds in DOM turned into 42.1% and 2.5%, respectively, while contribution of condensed aromatics in DOM dramatically increased to 15.4% in leachates along the A/O-MBR process. Most of DOM was converted into higher polymerization degree and accumulated in concentrated leachate (CL). Sulfur-containing compounds, whose relative peak ratio accounted for 56.4%, were regarded as recalcitrant DOM in CL. Increase of retention time in anaerobic unit for raw leachate might be useful for decomposing the long-chain organic compounds, which could also reduce loadings for the following A/O-MBR process. Well-focus techniques such as sulfur-oxidation bacteria could be introduced into the MBR unit for better removing organo-sulfur compounds. Advanced oxidation processes for CL degradation would be efficiency for the removal of recalcitrant DOM. Thus, leachate could be disposed in a zero-discharge way based on the practical experience of such a typical working treatment process.