Oxidising iron in Borehole Water using an Ozone generator

Ozone generators can oxidise iron, manganese, tannis, sulphides  and other contaminants common in South African water. 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.

Ozone is an extremely strong oxidant which in water treatment is capable of removing all bacteria, virus’s and cysts.

In addition to this it will, at the correct dosage remove iron, manganese, tannin and hydrogen sulphide from water.

Ozone treatment has no effect on salts, chlorides, calcium or the pH of the water and if these problems occur, then the end user will have to call on other service providers specialising in Reverse Osmosis or other products.

After oxidation by ozone there will be oxidised matter suspended in the water which should be filtered or settled out.

2 methods are used to introduce ozone in to the water.

  1. Using a venturi in the delivery pipeline, which depending upon the flow and back pressure, will suck the ozone gas into the mainstream.
  2. By blowing ozone gas through a diffuser mixing stone which hangs in the tank.
  1. The venturi system, although it is more efficient  is used commonly in industrial applications followed by a filter but relies on a constant water flow to work. This could be a problem in remote or domestic installations where the flow and suction cannot be monitored continuously. Also there is always the danger that, despite non return valves being placed in line, water could return through the venturi and damage the ozone generator.
  2. The Bubble diffuser system, although not as efficient, can be left on a timer to run for dedicated hours during the day blowing very fine ozone gas bubbles in to the water. Although not as efficient this can be a Plug and Play DIY system which can be left to run either continuously or for set periods of time.

The ozone system to run efficiently requires clean and dry air. Moisture in the air such as high humidity areas by the coast will result in less ozone being produced but more importantly, will affect the taste of the water, as nitrous oxides will be formed. Also the moisture could damage the ozone generator tubes. For this reason various types of air drier can be used the most efficient being the heat generated driers which have a high initial cost and are expensive to run. For small domestic or factory type of applications a silica gel drier system can be utilised where once the silica changes colour, this should be removed and dries in the son or oven.

Ozone Solutions offer the Aquazone Basic range of generators which have no air drier or flow metre and silica driers can be an optional extra.

The Aquazone Advanced range however, has the silica drier system and a flow metre so that the venturi gas flow can be monitored.

 

 

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.