Phosphoric acid is sold as a descaler for application in cooler sanitisation procedures. The manufacturers make no claim for disinfection and the technical data sheets refer only to use as descaler. Disinfectants are highly effective at reducing microbial counts, even at very low concentrations, for example, hydrogen peroxide and peracetic acid. Very strong acids will kill microorganisms, although their use in this manner is wasteful and can be dangerous. However, a common problem with coolers is a tendency to build up both scale and biofilm and acids can be useful in helping to remove both.
In its simplest description, biofilm is a colony of microorganisms attached to a surface and protected by a self-generated protective film of polysaccharide. This can occur in coolers and Pseudomonas aeruginosa is particularly adept at colonising surfaces and generating protective films. Surfaces covered with scale are particularly susceptible to attachment by microorganisms. They enjoy the ease with which a foothold can be made; smooth surfaces are more difficult to colonise.
Acids have the ability to dislodge and dissolve scale, provided the strength is high enough. In dislodging the scale, they will also dislodge the attached biofilm and therefore help in its removal. In this respect, acids will aid in removing microorganisms. However, biofilm attached to plastic or stainless steel surfaces can resist removal by acids, in fact older biofilms are very resistant to conventional chemicals such a phosphoric acid and chlorinated materials. In this case the biofilm will remain intact and continue to protect the bacteria, including Ps. aeruginosa, gradually releasing the bacteria into their immediate environment over a period of time. Oxidising acids, such as peracetic or nitric acids will attack biofilm successfully in this case. Phosphoric acid is not an oxidising acid.
Phosphoric acid is a useful descaler in the cooler sanitisation procedure. It will help in reducing microbiological count by removing scale which becomes coated with biofilm. However it cannot be considered as an effective disinfectant and is not “heavyweight” enough to fulfil this role. Descaling with phosphoric acid should always be followed by treatment with a disinfectant, such as hydrogen peroxide.