Question

What is the difference between membrane fouling and membrane degradation? Classify membrane fouling (i.e. biofouling, organic, and inorganic) and suggest cleaning methods.

Answer 1

Membrane fouling is initiated by the accumulation of inorganic, organic, colloidal, and biological species on the membrane surface and/or within its pores. It results in a reduced flux, a rapidly increasing transmembrane pressure, and possibly deterioration of mechanical properties.

Membrane degradation occurs over time due to fouling.

There are four types of membrane fouling described as follows:

1. Inorganic fouling is caused by accumulation of inorganic precipitates such as metal hydroxides on membrane surface or in its pores.

2. Particulate/Colloidal fouling is caused due to biological inert particles or colloids (algae, bacteria,etc.) and inorganic colloids originated from weathering of rocks (silt, clay,etc.).

3. Microbial/Biological fouling is result of formation of biofilms on the surface of membrane.

4. Organic fouling is common in membrane filtration with source water containing relatively high natural organic matters.

Membrane cleaning methods can be divided into physical, chemical and physio-chemical process. In practice, physical cleaning methods followed by chemical cleaning methods are widely used in membrane applications. However, only the chemical cleaning methods are widely applied for RO desalination.

(a) Physical cleaning methods:

1. Sponge ball cleaning: In this method, sponge balls made of polyurethane or other materials are inserted into the
membrane modules for a few seconds for scrubbing the foulant from membrane surface.

2. Forward and reverse flushing: Forward flushing consists in pumping permeate water at high cross-flow velocity through the feed side in order to remove foulants from the membrane surface. Because of the more rapid flow and the resulting turbulence, particles absorbed to the membrane are released and discharged. In the reverse flushing method the direction of the permeate flush is alternated for a few seconds in the forward and for a few seconds in the reverse direction.

3. Backwashing: This is a reversed filtration process in which permeate is flushed through the membrane to
the concentrate side. In porous membranes, when backward flush is applied, the pores are flushed inside out. The pressure on the permeate side of the membrane is higher than the pressure within the membranes, causing the pores to be cleaned.

4. Air flushing: The air flushing or air sparging method generates a two phase flow to remove external fouling
and thus reduces the cake layer deposited on the membrane surface.

5. CO2 back permeation: This is a method traditionally used for hollow fiber configuration in which CO2 gas is forced from the permeate side through the internal fiber and out through them.

(b) Chemical cleaning methods: Chemical cleaning is the most common membrane cleaning method, especially in reverse osmosis membranes. In this type of cleaning, the choice of the cleaning agent is critical. The optimal selection of the cleaning agent depends mainly on membrane material and type of foulant. In general acid (nitric, phosphoric, hydrochloric, sulphuric and citric) are often used to remove precipitated salts or scalants, while alkaline cleaning is suitable for organic fouling removal.

(c) Physico-chemical cleaning methods use physical cleaning methods with the addition of chemical agents to enhance cleaning effectiveness.