Performance of gravity-driven membrane systems for algal water treatment: Effects of temperature and membrane properties.

Abstract

Gravity-driven membrane (GDM) systems are promising for algal water treatment. However, the algae-bacteria interaction in the biofilm on the membrane, which is highly dependent on temperature and membrane properties, is still unclear. Therefore, this study investigated the effect of temperature on the performance of GDM systems during the filtration of algae-rich water for 50 days using two types of membranes. The results suggested that the combined effect of the microbial growth (controlled by temperature) and organic rejection (related to membrane properties) determined the membrane biofilm structure and its hydraulic resistance. Increasing the temperature from 10 to 35 °C gradually improved the foulant removal by both polyvinylidene fluoride (PVDF200) and polyvinyl chloride (PVC0.01) membranes, corresponding to different microbial activities. The lowest removal observed at 10 °C was attributed to the algal cell rupture and limited bacteria growth. At 25 °C, the stimulated algae population was mainly responsible for nutrient removal, meanwhile the oxygenic environment encouraged the proliferation of heterotrophic bacteria for the organic removal. At a higher temperature of 35 °C, both the nutrient and organic removal were dominated by denitrification, accompanied by a strong increase in biological activity. Although PVDF200 membranes had 10 times higher initial fluxes than PVC0.01 membranes, they obtained comparable final fluxes. Unlike PVDF200 membranes exhibited the highest final flux at 10 °C (3.64 L/m2/h), the PVC0.01 membrane permeability increased in the order: 10 °C (1.58 L/m2/h) < 25 °C (2.20 L/m2/h) < 35 °C (4.00 L/m2/h). This is mainly because the PVDF200 membrane fouling was dominated by microbial biomass, while PVC0.01 membranes with smaller pores and higher hydrophilicity were more sensitive to changes in microbial metabolites. This study links temperature, membrane properties and biofilm physiology, with practical relevance for the hydraulic performance of GDM systems, hopefully leading to their wider application in algal water treatment.