A Preliminary Diagnostic Method for Membrane Fouling Using Extracellular Proteins Secreted in Pilot-Scale Membrane Bioreactors

Membrane bioreactors (MBRs) are advanced tools for biological wastewater treatment to obtain high-quality treated water using ultrafiltration or microfiltration membranes for solid–liquid separation2). Despite their advantages over the conventional activated sludge method, a major challenge associated with the use of MBRs is membrane fouling, which involves the clogging of membrane pores due to the attachment and accumulation of microorganisms, microbial products (e.g., extracellular proteins, polysaccharides, and lipids), and other organic/inorganic components on membrane surfaces. Membrane fouling results in an increase in transmembrane pressure (TMP), a severe decline in flux, and a reduction in treatment performance, causing high energy consumption. Although a number of studies have focused on controlling membrane fouling, some of which achieved progress in this regard, the complete prevention of fouling has not been possible. Therefore, membrane cleaning by physical and chemical treatment at periodic intervals is essential for the stable operation of MBRs4). Membrane fouling can be divided into different types based on the strength of attachment of the fouling materials or foulants to the membranes12). Reversible membrane fouling likely stemming from the loose attachment of foulants is resolved by physical cleaning methods, but it can transform into irreversible fouling by the formation of layered strong fouling matrices during continuous long-term filtration. Removal of these recalcitrant foulants cannot be achieved by physical cleaning, but can be done by chemical cleaning. In chemical treatment, a variety of washing agents, such as acids, bases, chelating agents, polymeric coagulants, surfactants, and oxidants, including sodium hypochlorite, have been employed3,8). However, over time, conventional chemical cleaning becomes only partially effective against irreversible fouling and the completely fouled membrane has to be replaced; this is frequently required12). Despite intensive efforts, the fundamental mechanism underlying membrane fouling remains to be clarified1). Membrane fouling of MBRs is usually detected by an increase in the TMP; however, the sensitivity of detection of increased TMP is insufficient to evaluate and prevent its occurrence. In practice, early prediction of the precise timing of membrane fouling and when chemical cleaning of a fouled membrane should be performed are important. Thus, alternative methods for effectively predicting membrane fouling prior to the increase of TMP need to be developed. In addition to microbiologically produced polysaccharides, proteins have been identified as one of the major foulants of MBRs5,6), and some proteins that cause membrane fouling were identified in pilot-scale MBRs7). In this study, we focused on extracellular proteins secreted in pilot-scale MBRs as an effective fouling indicator, and investigated the shifts in their concentrations throughout the operational period. Based on the relationships among the extracellular protein concentration, TMP, flux, and chemical oxygen demand (COD), we propose a preliminary diagnostic method for membrane fouling prior to the increase in TMP.