Urban wastewater reuse: fouling of low pressure membrane by effluent organic matter

High-pressure membrane filtration using nanofiltration (NF) or reverse osmosis (RO) is a leading process in response to stringent regulations for wastewater reuse applications. However, despite of its successful implementation in wastewater reclamation plants, biofouling is still a major issue affecting the overall process performance, whereas it incurs large operational costs due the need for chemical cleaning. An efficient pretreatment step is therefore essential to ensure a long-term stable performance of the high-pressure membranes and minimize operational costs. Low pressure membrane filtration (i.e. UF or MF) is commonly used as pretreatment step and have been successfully applied to produce water of a high quality from secondary effluent. The aim of this work was to optimize the performance of the low-pressure membrane filtration step in order to enhance the efficiency of the RO process for municipal wastewater reuse. Laboratory-scale and long-term pilot scale experiments were conducted to determine the fractions of Effluent Organic Matter (EfOM) responsible for low-pressure membrane fouling in different secondary effluents and to determine the impact of membrane properties on the permeate water quality and organic fouling. The impact of physical and chemical treatment on EfOM was also studied using different adsorption and oxidation techniques. The fouling potential has been shown to be affected by the structural modification of the EfOM. Pre-oxidation processes prior the filtration resulted in a reduction in fouling due to their impact on the colloidal fraction of EfOM. The use of advanced analytical tools (Fluorescence EEM data and Size exclusion chromatography analysis) in combination with simple filtration tests proved to be a suitable approach to characterize the fouling potential of water/membrane combination. Protein-like material was particularly correlated with a high fouling potential of feed water. A comparative study of the behaviour of polymeric hollow fibre MF and UF membranes during filtration of secondary effluents highlighted the importance of the molecular weight cut-off and composite membrane materials. The so called “tight’ microfiltration membranes in PVDF showed to be less prone to fouling compared to PES UF and MF membranes. However, no significant differences were observed in terms of water quality produced at laboratory scale. All membranes tested showed a high retention of the colloidal fraction. This result was confirmed at pilot scale, by the comparison of two MBR systems (hollow fibre PVDF UF vs. flat sheet PVDF MF). Hence, the membrane type does not seem to have an impact of the wastewater quality when same biological treatment is used.