Intensive routine cleaning for mitigation of fouling in flat-sheet ceramic membranes used for drinking water production: Unique characteristics of resulting foulants

Abstract

Ceramic membranes are physically and chemically robust, and intensive membrane cleaning can therefore be used. The use of ceramic membranes may alleviate the problem of membrane fouling. In this study, fouling in flat-sheet ceramic membranes operated with intensive mechanical cleaning (scouring with granular materials) was investigated in the context of drinking water production. Samples collected from multiple drinking water treatment plants were used as the feed. Bench-scale experiments were carried out in a realistic style: with a constant flow rate and periodical hydraulic backwashing. Granular materials for the purpose of scraping off the fouling layer from the membrane surface were placed in the membrane tank (10 % v/v) and they moved freely by the aid of periodical aeration provided during the routine backwashing. Operation under the condition of a high membrane flux of 125 LMH was possible with little fouling when granular scouring was carried out with pretreatment using PACl coagulation. As for irreversible fouling, in-line chemical membrane cleaning at a low frequency (1-hour cleaning/120 h of filtration) using oxalic acid followed by NaClO exhibited a very high cleaning efficiency (permeability recovery of > 99 %). Analysis of the foulants extracted from the fouled ceramic membrane revealed that the properties and compositions of the foulants causing the irreversible fouling in ceramic membranes were different from those in polymeric membranes. Analysis using liquid chromatography with organic carbon detection (LC-OCD) suggested that humics were dominant (35 % of the total organic carbon) in the foulant extracted from the fouled membranes, instead of biopolymers (11 % of the total organic carbon) that have been shown in recent studies to be the major foulants for polymeric membranes. However, spectral analysis of the foulant using infra-red and fluorescence and filtration data obtained with pretreatments suggested the feature of biopolymers, particularly polysaccharides. These discrepancies indicate the necessity for new analytical methods to distinguish overlaps between humics and biopolymers.