Lesson 7 - Brewing Product Water Final Treatment SAQ

**Chlorine is very effective at killing bacteria. Why is it removed during water treatment, when we know that potable and brewing water requires the water to be free of bacteria? How is the water dechlorinated? How could you alternatively sterilise brewing product water?

Response:

Chlorine can be toxic to yeast in high levels. It can cause an off-flavour in beer, known as chlorophenol. In addition to this, chlorine can damage membrane filtration (such as reverse osmosis filters) in the water treatment plant and can negatively affect WAC ion exchangers.

Chlorine can be removed by activated carbon filtration, addition of metabisulphite or the use of UV.

The most effective treatment is adsorption on porous material, which traps the chlorine molecules in its pores.

Activated carbon based on coal or coconut can be used to adsorb chlorine from water. It does this either through physical adsorption into its honeycomb structure (a reversible process), or by a chemical reaction that is irreversible. A material with high internal pore volumes favours both types of reaction.

Another option is the addition of metabisulphite to the water. Sodium metabisulphite or potassium metabisulphite (Camden tablets) reacts with water to form sulphur dioxide (SO₂):

The sulphur dioxide then reacts with chlorine to form chloride and sulphate:

A final option is UV. UV light at 180-200 nm breaks the molecular bonds of the chlorine, reducing it into its component ions, thus yielding chloride and water.

Once the water is dechlorinated, it is now susceptible to microbiological infection; the water thus needs to be sterilised in another way.

UV was mentioned as a dechlorination technique, but it also kills bacteria and viruses. Ultraviolet 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.

UV is non-corrosive and carries no risk of residues or taints. However, its lack of residual killing potential means that sterilisation is achieved at the point of irradiation only.

There are also chemical options, such as chlorine dioxide. It is a powerful oxidising agent (2.5 times more powerful than chlorine). It does not produce flavour taints and is both non-corrosive and non-toxic (once in solution).

It needs to be produced at the point of usage, due to its instability. It is produced when hydrochloric acid reacts with sodium chlorite.

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

Another chemical option is ozonation. Ozone is produced by passing a flow of air or oxygen (O₂)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 are that it is unaffected by pH, does not form THMs, and works very rapidly.