Membrane Biofouling

My thanks to ‘Greg’ for really rasing the bar on the level of discussion here. Greg’s comment is a little too long to reproduce, but you’ll find it below my previous post. It’s the comment starting “The Achilles heel of reverse osmosis (RO) membranes is biofouling…”.

“Biofouling” refers to the growth of biological organisms (bacteria, algae, slime, etc) on a moist surface. Sounds pretty gross, but the same types of “biofilms” currently line the insides of all our drinking water pipes. If we were to allow biofilms to grow on membranes unchecked, they would soon clog up the membrane. For some types of membranes, biofouling could also potentially damage the membrane as suggested by Greg. However, we have three very effective ways of preventing biofouling from becoming a problem:

1. Pretreat the feedwater prior to reaching the RO membranes.
This is the most important means of biofouling control. In almost all water treatment plants, RO treatment would be preceded by another membrane process with larger pores (usually either microfiltration or ultrafiltration). This removes the suspended solids and colloidal material before they reach the RO membrane, significantly reducing the biological organisms that can reach the RO membrane. If further removal of biological organisms is required, this can be achieved by “disinfecting” the feedwater to the RO. Common methods for disinfection include the use of chlorine, ozone or ultraviolet radiation (all the same disinfecting processes that are currently used for drinking water). Chlorine disinfection is the most common and the two most common forms of chlorine disinfection are chlorine gas and chlorine dioxide (some membranes are sensitive to chlorine and thus monochloramine is also commonly used). Yes, both are quite toxic substances (which, actually is the whole point!). However, they are both used very safely in all reticulated drinking water supplies in Australia. In most cases, this chlorine will be largely removed from recycled water, either by “dechlorination” immediately prior to RO or by the RO process itself.

These "pretreatment" methods of preventing biofilms from growing in the first place are a much more reliable approach than the one described by Greg (letting the biofilms grow and then hitting them with a strong dose of chlorine dioxide).

2. Select membranes made of materials that bacteria cannot degrade.
Membranes are available made from a variety of materials to be suited for a variety of applications (not just drinking water treatment). The most common materials include polypropylene, cellulose acetate, aromatic polyamides and thin-film composite membranes. Most of these are quite resistant to bacterial degradation, but cellulose acetate membranes may be expected to be more susceptible than the others. Aromatic polyamides are very resistant.

3. Regularly replace membranes.
Just like anything else, membranes have a life-span. This is normally caused by the pores eventually clogging up with the various components that they remove from the water. As this occurs, the pressure needed to force the water through the membrane gradually increases. The treatment plant operators would monitor this pressure and when it reaches a certain level, the membrane module would be taken off-line and a new one installed. Well managed modern RO membranes typically need to be replaced every couple of years.

In summary, I strongly agree with Greg’s comments that it is very important to control biofouling for reverse osmosis membranes. However, this can be quite easily and very effectively achieved. Greg’s further comment that “it relies on us trusting those operating the technology” is irrefutable. Just as we currently rely on those responsible for delivering our safe drinking water supplies (and so many other things in life), we will need to have confidence that people responsible for operating the system are well trained and following well established protocols.

Comments, corrections or criticism encouraged,

Stuart