Four types of Membrane Fouling

There are four categories of membrane fouling. They are (a) inorganic fouling/scaling, (b) particle/colloids fouling, (c) microbial fouling, and (d) organic fouling. A brief description on the nature of fouling, relevant water quality as indicators, and control measures are summarized below for each type of membrane fouling.

Inorganic Fouling

Inorganic fouling or scaling is caused by the accumulation of inorganic precipitates such as metal hydroxides, and “scales” on membrane surface or within pore structure. Precipitates are formed when the concentration of chemical species exceeding their saturation concentrations. Scaling is a major concern for reverse osmosis (RO) and nanofiltration (NF). RO and NF membranes reject inorganic species. Those species form a concentrated layer in the vicinity of membrane-liquid interface - a phenomenon referred to “concentration polarization”. For microfiltration (MF) and ultrafiltration (UF), inorganic fouling due to concentration polarization is much less profound, but can exist most likely due to interactions between ions and other fouling materials (i.e., organic polymers) via chemical bonding.

Particulate/colloid Fouling

Algae, bacteria, and certain natural organic matters fall into the size range of particle and colloids. However, they are different from inert particles and colloids such as silts and clays. To distinguish the different fouling phenomena, particles and colloids here are referred to biologically inert particles and colloids that are inorganic in nature and are originated from weathering of rocks.

Microbial/Biological Fouling

Microbial fouling is a result of formation of biofilms on membrane surfaces. Once bacteria attach to the membrane, they start to multiple and produce extracellular polymetric substances (EPS) to form a viscous, slimy, hydrated gel. EPS typically consists of heteropolysaccharides and have high negative charge density. This gel structure protects bacterial cells from hydraulic shearing and from chemical attacks of biocides such as chlorine.

Organic Fouling

Organic fouling is profound in membrane filtration with source water containing relatively high natural organic matters (NOM). Surface water (lake, river) typically contains higher NOM than ground water, with exceptions. For source water high in NOM, organic fouling is believed to be the most significant factor contributed to flux decline (Mallevialle et al., 1989; Lahoussine-Turcaud et al, 1990). Microfilters usually remove insignificant amount of organic matter, as measured by dissolve organic carbon (DOC). DOC as an indicator for organic fouling is probably neither proper nor adequate. Efforts to identify the effects of subgroups of NOM on membrane fouling have yet been able to draw definitive conclusions.

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