Three-dimensional fluorescence characteristics analysis of DOMs in water treated by the ultrafiltration-reverse osmosis process

Abstract

Ultrafiltration (UF) is often used as a pretreatment for reverse osmosis (RO) units due to its high rejection of microorganisms and particulate matter. UF has different removal effects on different kinds of dissolved organic matter (DOM) in water and affects the pollution of RO membrane. This study aimed to investigate the changes of DOM during the ultrafiltration-reverse osmosis (UF-RO) process. The ultraviolet–visible absorption spectrum and three-dimensional excitation-emission matrix spectrum (3DEEMs) were used to examine the DOM components of the effluent from membrane bioreactor (MBR), tap water and surface water treated by the UF-RO process. The results revealed that UF removed 3.19 %–36.85 % of aromatic DOM components and 5.71 %–32.25 % of hydrophobic DOM components. Based on parallel factor analysis (PARAFAC), all the water samples predominantly contained protein-like substances (e.g., high-excitation and low-excitation of tryptophan and tyrosine), and fulvic acid-like substances. Due to the different quality of feed water, the removal rate of the same type of DOMs varied significantly. While the highest removal rates for both protein-like substances and fulvic acid-like substances in tap water were higher than 80 %, the highest removal rates for these substances in MBR effluent were only about 40 %. For surface water, the highest removal rates were 28.07 % for fulvic acid-like substances and 70 %–80 % for protein-like substances. Pretreatment by UF membranes of different pore sizes had no impact on the content of DOMs in RO effluent. However, after treatment with the UF-RO process, the removal rates of all types of DOMs exceeded 80 %. It was found that the membrane fouling was reduced by pretreatment of tap water and surface water, while the fouling deteriorated faster after pretreatment of the MBR effluent. This study suggested that based on types of water to be treated, UF membranes of the appropriate pore sizes can be selected for use in the UF-RO process to better achieve the desired effluent quality, water flux, and economy.