Characterization of Tertiary Membrane Fouling in Cold Climates

Abstract

Cold climates have limited the application of membrane technologies in tertiary treatment by affecting both secondary biological and tertiary membrane processes. There is a need to develop a fundamental understanding of fouling mechanisms and to identify solutions to fouling of tertiary membranes under cold climates. A combination of bioprocess modelling and advanced SMP characterization illustrated the effect of secondary temperature (8, 14 and 20 ℃) and HRT (10, 15 and 20 h) on biomass process rates and SMP production. Liquid chromatography-organic carbon detection (LC-OCD) results revealed a significant increase in total SMP concentration as secondary temperature decreased with the fractions of polysaccharide and protein being more sensitive to temperature. In contrast, reducing HRT did not significantly affect total SMP concentration in effluents whereas SMP composition varied. Both biomass decay (BAP) and substrate utilization (UAP) yields increased as secondary temperature decreased while only UAP yields decreased as HRT decreased. A strong correlation was observed between secondary temperature and BAP/UAP yields whereas the generation of BAP was more temperature sensitive than UAP. The development of resistances due to fouling as a function of secondary temperature and HRT was assessed. Reducing secondary temperature resulted in a substantial increase in total membrane resistances over multiple filtration cycles which was contributed by increased hydraulically reversible and irreversible resistances. A two-stage pattern in fouling rates development was observed indicating potential fouling mechanisms of cake and pore blocking. Shortening HRT led to increased hydraulically reversible and irreversible resistances whereas the degree and pattern of the change were affected by temperature. When filtration tests were conducted at the secondary operating temperature, the hydraulically irreversible permeability further decreased relative to filtration at 20 ℃. Water viscosity and intrinsic membrane resistance were observed to be responsible for 20–29% of the total decrease in hydraulically irreversible permeability. These declines established the upper limit to which fouling mitigation strategies could enhance hydraulically irreversible permeabilities. The hydraulically irreversible permeability decline beyond that associated with changes in viscosity and intrinsic membrane resistance was attributed to narrowed membrane pores that retained additional SMP fractions and modified membrane-foulant interactions. Correlation analysis revealed that the fouling of tertiary membranes was governed by high and low molecular weight (MW) organics which were generated to a greater extent at low secondary temperature and short HRT conditions. Multiple linear correlation analysis results indicate that filtration temperature presented the highest contribution to tertiary membrane fouling followed by secondary temperature and HRT. The effect of coagulation with alum (0‒1.0 mM) on fouling of tertiary membranes was studied at secondary temperatures of 8 and 20 ℃. The removal efficiency of high MW organics by coagulation was consistently higher than that of low MW organics. The coagulated effluent concentrations were higher for the SBR effluent generated at 8 ℃ than those generated at 20 ℃. The results of filtration tests revealed that pre-coagulation can effectively reduce total membrane resistances; however, at the preferred dosages, the values of hydraulically reversible and irreversible membrane resistances were higher for the SBR operated at 8 ℃ due to higher concentrations of SMP fractions remaining in the coagulated effluent. The present study provides basic knowledge and insights into factors that should be considered when developing strategies for mitigating fouling of tertiary membranes under low temperature and high flow conditions. The approach of process modelling and differentiating the effects of secondary HRT, temperature and filtration temperature developed in the present study provides insights into fouling development of tertiary membranes and the results can be employed to understand the limits of tertiary fouling control under challenging conditions. The potential of pre-coagulation in membrane fouling alleviation and effluent quality improvement can be limited at low temperatures and high flows. Further investigation should be conducted to explore the potential of other pretreatments or adjustments of secondary operating conditions (such as solids retention time (SRT)) to achieve more effective fouling control under cold climates.