This study sought to identify factors responsible for enhanced fouling of ultrafiltration membranes used in tertiary wastewater treatment under challenging conditions of high flow and low temperature. A detailed analysis of full-scale membrane operating data was conducted, and this was supported by data gathered through a field sampling campaign. Higher average fouling rates and average recoveries were observed during periods of highest flows and lowest temperatures. The results demonstrated that the negative impact of seasonal changes on short-term fouling are readily reversible, while hydraulically irreversible fouling, which is responsible for intermediate and long-term fouling rates, is not effectively recovered by maintenance cleans (MCs) but is recovered by recovery cleans (RCs). An examination of membrane feedwater quality revealed that high fouling rates correlated to an increase in dissolved organic carbon (DOC) concentrations, with the biopolymer fraction of the DOC being most important. Increased capillary suction time (CST) values, which indicate reduced sludge dewaterability, were also observed during high fouling events. It was concluded that seasonal variations result in the increased release of extracellular polymeric substances (EPS) by microorganisms, which leads to higher membrane fouling and worsened dewaterability of the activated sludge.