Effect of pretreatment on the fouling of membranes: application in biologically treated sewage effluent

Abstract

Reuse of wastewater can help in maintaining environmental quality and relieving the unrelenting pressure on conventional and natural freshwater sources. Membrane processes find an important place in the wastewater treatment for reuse. Nonetheless, reverse osmosis (RO) and nanofiltration (NF), i.e. non-porous membranes require higher operational costs and energy. Thus, in this research NTR 7410 ultrafiltration (UF) membrane which is porous was used without and with pretreatment to treat biologically treated sewage effluent (BTSE). Four different pretreatment methods, namely, ferric chloride (FeCl 3) flocculation, powdered activated carbon (PAC) adsorption, flocculation followed by adsorption, and granular activated carbon (GAC) biofilter were used in this study to compare their relative merits. Experimental results indicate that the most suitable pretreatment was flocculation followed by adsorption leading to a total organic carbon (TOC) removal of 90%. To assess the suitability of the membranes, it is important to conduct a detailed membrane characterization. The fouled NTR 7410 membrane surface was analyzed in terms of contact angle, zeta potential, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), flux decline, and TOC removal. The contact angle of the fouled membrane surface was lower than that of the clean membrane surface. This suggests that the majority of the foulants may have been the hydrophilic organic compounds such as polysaccharides, urea, etc. which are the extracellular enzyme of microorganisms in BTSE. But, the fouled membrane surface after the pretreatment of flocculation followed by adsorption had nearly the same contact angle as that of the clean membrane, suggesting that the hydrophobicity of the membrane is preserved by this pretreatment. According to attenuated total reflection-Fourier transform infrared spectroscopy results, the peaks observed on the fouled membrane were ether (COC) and urea (RNHCONHR). On the other hand, the peaks obtained after the pretreatment of flocculation followed by adsorption were similar to those of clean membranes. The highest effluent organic matter (EfOM) concentration on the fouled membranes without any pretreatment was measured up to 0.011 mg EfOM/cm 2 membrane surface. The pretreatment of flocculation followed by adsorption reduced the EfOM concentration on the membrane to 0.005 mg EfOM/cm 2. The SEM images on the membrane cross-section revealed that there was practically no foulant layer on the membrane when a pretreatment of flocculation followed by adsorption was used.