Characterization of dissolved organic matter in the secondary effluent of pulp and paper mill wastewater before and after coagulation treatment.

Meeting the regulatory discharge standards for pulp and paper mill wastewater has become ever more difficult because of its recalcitrant and colored dissolved organic matter (DOM). In this study, the variation of DOM from the secondary effluent of pulp and paper mill wastewater before and after coagulation was investigated based on hydrophobicity/hydrophilicity, apparent molecular weight (MW) and fluorescence. DOM fractions of the secondary effluent were all with the apparent MW <20 kDa. Particularly, the hydrophobic acids (HOA) fraction, mainly composed of humic-like materials, was the major component, and it also had the highest color and SUVA254 (UV254 to dissolved organic carbon ratio). Hydrophilic bases (HIB) and hydrophilic neutrals (HIN) fractions were the other important parts besides HOA. Coagulation can remove all DOM fractions to different extent, and it was more effective for organic compounds with MW > 5 kDa. The removal efficiencies of humic-like, fulvic-like and soluble microbial by-product-like constituents in HOA and HIB fractions were much higher than in other fractions. Even so, the coagulation effluent still contained large amounts of contaminants with complicated fluorophores and apparent MW <5 kDa, and HOA and HIN fractions were also the major parts in the coagulation effluent.