Comparison of treatment options for removal of recalcitrant dissolved organic matter from paper mill effluent

Recycling paper mill effluent by conventional water treatment is difficult due to the persistence of salt and recalcitrant organics. Elimination of dissolved organic matter (DOM) from paper mill effluent was studied using three treatment options, ion exchange resin (IER), granular activated carbon (GAC) and nanofiltration (NF). The removal efficiency was analysed based on hydrophobicity, molecular weight and fluorogenic origin of the DOM fractions. For IER, GAC and NF treatments, overall removal of dissolved organic carbon was 72%, 76% and 91%, respectively. Based on the hydrophobicity, all the three treatment methods majorly removed hydrophobic acid fractions (HPhoA). Further, IER acted on all fractions, 57% of HPhoA, 44% of transphilic acid and 18% of hydrophilics, substantiating that the removal is by both ion exchange and adsorption. Based on the molecular weight, IER and GAC treatments acted majorly on the high molecular weight fractions, whereas NF eliminated all molecular weight fractions. After GAC adsorption, some amount of humic hydrolysates and low molecular weight neutrals persisted in the effluent. After IER treatment, amount of low molecular weight compounds increased due to resin leaching. Qualitative analysis of fluorescence excitation emission matrices showed that the fulvic acid-like fluorophores were more recalcitrant among the various DOM fractions, considerable amount persisted after all the three treatment methods. Three treatment methods considerably differed in terms of removing different DOM fractions; however, a broad-spectrum process like NF would be needed to achieve the maximum elimination.