Lesson 6 - Brewing Product Water Final Treatment

Once we have treated the water, we can classify it as product water. We should still sterilise it to prevent any microbiological contamination. There are many sterilisation techniques available for treating water, including both chemical and physical methods.

Chemical sterilisation

Chlorination has served its purpose until now. Most municipal water supply companies still use this as their main form of sterilisation. In the summer months, chlorination can be higher than usual, as the municipality doses higher levels to pre-empt higher microbiological levels within these hotter periods.

However, once on the brewery site, chlorine is no longer a viable option as it will have been purposefully removed via an activated carbon filter. An alternative chemical is chlorine dioxide, which is a powerful oxidising agent (2.5 times more powerful than chlorine).

Chlorine dioxide is a neutral chlorine compound and it differs in structure and behaviour from elemental chlorine. It does not produce the associated flavour taints of chlorine, it is non-corrosive and is non-toxic (once in solution). 

Chlorine dioxide is an unstable gas that can be 50 times more toxic than carbon monoxide. Therefore, it needs to be produced at the point of usage. It is produced by reacting hydrochloric acid with sodium chlorite:

Typical levels of chlorine dioxide can be up to 1.5 mg/L.

Another chemical option is ozonation. Ozone is produced by passing a flow of air or oxygen through a high voltage field, causing the following reaction:

The ozone is injected into the water, to achieve a dissolved ozone concentration of 0.5 mg/L. These levels have bactericidal activity through oxidation of the bacteria cell membranes.

The advantages of ozone include the fact that it is unaffected by pH, does not form THMs and works very rapidly. Ozone also removes off-tastes, colouration, degrade pesticides and will even reduce phenols to harmless acids.

The biggest disadvantage of using ozone is the high running cost compared to other chemical sterilisation methods.

Instead of using chemicals, there are physical alternatives – UV light and sterile filtration.

Ultraviolet (UV) light in the range of 200-280 nm is shone through a small depth of water, which should be clear. The wavelength of light destroys the DNA of microbiological contaminates. Using UV light is non-corrosive, with no risk of residues or taints. However, its lack of residual killing potential means that sterilisation occurs at the point of irradiation only.

The final physical method is sterile filtration. We have already explored microfiltration. Sterile filtration involves using a porous membrane. The water passes through, but the membrane retains its microbiological contaminants. The pore size needed to achieve this is usually between 0.2-0.45 μm. The filter’s main advantage is that there is no flavour taint. However, they become easily blocked (especially if there is no pre-filtration to remove larger sized contaminants).

The following paper from the WHO covers the disinfection process of water in some detail – “Inactivation (disinfection) process”