Micropollutants removal and tecnological development of membrane bioreactors

Abstract

The increasing worldwide contamination of aquatic environment with pollutants introduced by anthropogenic sources has become of great concern. Although present at low concentration, many of these pollutants have considerable long-term impacts on the ecosystem, such that extremely challenging legislative limits on their concentration in effluents are being proposed. This has led to the examination of membrane bioreactor (MBR) technology for wastewater treatment, since it offers the best and most consistent treated water quality of all biotreatment processes. However, both a review of the literature and experimental study reveals that MBRs appear to offer insufficient benefit over conventional processes to make their implementation for this duty viable, given their significantly higher cost. Notwithstanding this, the fate of micropollutants in MBR processes represents the most rapidly growing research topic in the general MBR subject area. Despite the wide range of products commercially available, the majority are hollow fibre (HF) products based on polyvinylidene difluoride (PVDF) or polyethersulfone (PES) and almost all are in the pore size range between 0.04 and 0.4 μm. Whilst differences in module design across the whole range of products constrains their interchangeability, the increased acceptance of and confidence in this technology is reflected in the increased rate of implementation of large installations and the overall exponential growth in the market of 11-13% per year. However, there appears to be a dysfunction between the needs of the industry and the primary research area within the academic community, with practitioners identifying clogging as the main impediment to sustainable operation while 31% of all research papers published to end 2009 were based on fouling and less than 1% on clogging. Experimental study of operation of an MBR to treat municipal wastewater based on a new HF material has revealed that studies based on aeration step provide a more realistic indication of critical operation than classical flux step experiments, since the latter involve imposing a hydraulic shock on the membrane. Simple measured sludge fouling propensity parameters did not appear to relate to fouling rate, with a counter-intuitive relationship arising under some operating conditions.