Ozone oxidising iron in Borehole Water

Iron and to a lesser extent, manganese are common contaminants in borehole water in South Africa. Various methods to eliminate iron are recommended but often the client calls for a chemical free solution and this is where ozone can play a role.

The following table shows the contaminants and the amount of ozone that is required to oxidise the contaminant leaving a residue which can easily be filtered or settled out after oxidation with ozone.

CONTAMINANT

MILLIGRAMS OZONE PER MG OF CONTAMINANT

Iron ( Fe)
0.43
Manganese (Mn)0.87

Hydrogen Sulfide (H2S)
3.0

Tannins
0.1

For Disinfection
0.5

So for example a water analysis of the raw water from the borehole may show 3 mgs/litre of iron and 2 mgs/litre of manganese.

The ozone demand for every litre of water will be:

For the Iron ( FE)  3 x 0.43 = 1.29 mgs of ozone required.

For the Manganese ( Mn) 2 X 0.87 = 1.74  mgs of ozone required.

Total ozone mgs per litre required to oxidise 3 mgs/litre iron and 2 mgs/litre of manganese = 3 mgs/litre of ozone.

If the flow rate is 1000 litres per hour then 1000 X 3 = 3000 mgs ozone = 3 grams of ozone required.

This is only an example for calculation of ozone demand purposes but should there be other contaminants in the water then bear in mind that ozone will react first with H2S then with Fe followed by Mn and lastly with tannins.

There are other things such as temperature of the water, organically bound compounds or seasonal variations that bear on the exact ozone treatment design so it would be sensible to add another 20% for example and increase contact time for as long as possible to factor in the unknowns.

We  recommend  that  when  treating  surface  waters with bacteria virus or cysts that ozonation becomes a 2 step process.

In Step 1 ozonation and filtration will purify the water (remove  iron,manganese, hydrogen sulphide etc.

In Step 2 ozone will again be injected and a ozone residual will be maintained to insure disinfection.

A possible alternative to the second point of injection would include the use of Giardia approved filter systems or ultra violet radiation post filtration.