Ultraviolet/persulfate pre-treatment for organic fouling mitigation of forward osmosis membrane: Possible application in nutrient mining from dairy wastewater

The forward osmosis (FO) membrane process has recently established its potential as an alternative option to traditional membrane processes for producing clean water and recovering nutrients from anaerobically treated wastewater streams. However, organic fouling of membrane leads to reduced flux, and, thus, reduced amount of the extractable resources. In this study, the impact of ultraviolet/persulfate (UV/PS) oxidation pre-treatment for the mitigation of organic fouling in the FO process during processing of anaerobically treated dairy effluent (ATDE) was determined using a multi-cycle filtration method. The UV/PS performance was compared with control pre-treatments such as stand-alone ultraviolet (UV) irradiation and potassium persulfate (PS) oxidation. Size exclusion chromatography confirmed that flux reduction over successive filtration cycles was due mainly to the humic substances and building blocks i.e., sub-units of humic substances in the feedwater. Although all investigated pre-treatment options mitigated membrane fouling, UV/PS achieved a greater enhancement in flux and decrease in both reversible and irreversible foulant deposition than stand-alone UV and PS pre-treatments. This was because UV/PS could generate sulfate and hydroxyl radicals, which were effective for decreasing the bulk organic content and fluorescent organic content, and particularly for breaking down the large molecular weight (MW) hydrophobic compounds to small MW hydrophilic components, resulting in less organics adhesion to the membrane. This research shows the applicability of UV/PS pre-treatment for the organic fouling mitigation of FO membrane during processing of ATDE for applications such as nutrient mining from ATDE.