Using Ozone in Potable and Drinking Water Systems

There is so much to know about ozone and how not to use it that I have decided to list some do’s and don’t’s, especially for those who have heard about ozone’s amazing properties and are interesting in applying to their projects but are not sure where to start.

In order to design and size an ozone system, several factors have to be taken into account.

The ozone gas is very unstable and will revert back to oxygen rapidly but is used for its remarkable oxidation properties in water treatment systems principally to eliminate microorganisms and also heavy metals such as iron and manganese.

Factors affecting the efficient use of ozone.

  1. Air preparation.
  2. Dosage level or ozone demand
  3. Mixing the gas with the water
  4. Contact time and off gassing
  5. Monitoring and controlling
  6. Safety and ozone destruction

Air preparation

The ozone generator modules change the molecular structure of oxygen by using high voltages to split the O2 atom and then adding these single O to another O2 molecule thus making an O3 molecule, this being Ozone.
It stands to reason therefore that the incoming gas to the ozone tube or dielectric must be dry and clean.
In atmospheric air with something like 15% oxygen in 100 mgs of air, there will only be 15 mgs of oxygen to convert. Therefore the percentage of ozone coming out of the tube will be very little and now the system design needs to incorporate a large injection system to accommodate a large volume of air with ozone in it to mix with the water.
However if straight oxygen is used and if the incoming gas entering the ozone module has 95% oxygen in it, then the ozone gas coming out of the tube or dielectric will be that much stronger and concentrated.
Oxygen can be produced on site using an oxygen generator ( Pressure Swing Absorption) system or Liquid Oxygen ( LOX) can be used from a gas supplier.
All systems require an oil free compressor and air drier to prevent fouling of the tubes and in the air drying case, to prevent the build up of nitrous oxides.

Dosage level or ozone demand.

Ozone is far stronger and quicker than chlorine in its oxidation capability and it disinfects 600 to 3000 times faster than chlorine.

For example it will take 0.1 mg/l of chlorine four hours to eliminate 6104 cells of E. Coli in water, whilst 0.1 mg/l of ozone will do the same in five seconds.
Also it will take 1 mg/l of chlorine two hours to disinfect water containing a poliomylitic virus whereas only two minutes will be required with only .05 mg/l of ozone.

In the case of iron oxidation, for every 1 mg of iron present in the water 0.43 mgs of ozone is required. For manganese it will be 0.87 mgs of ozone for every mg of manganese.

A sizing standard which could have a bearing on capital costs would be that one will have the same effect treating water with ozone at 1000 mgs/litre for 30 seconds as you will have treating with 500 mgs of ozone for 1 minute.

As a general rule of thumb when replacing chlorine for disinfection divide the mgs/litre of chlorine by 2.5 to obtain the ozone mgs/litre requirement using the same contact time.
Most ozone generators designed for residential water treatment are sized by the grams of ozone produced per hour of operation. To choose the right sized ozone generator for each job you must determine the potential work ozone can do. This work can also be thought of as the ozone demand, of the water.
Determining the ozone demand of the water requires an accurate water analysis and a little math using the demand each contaminant places on ozone, or the amount of ozone required to oxidize the contaminant. The amount of ozone needed for oxidation is known as the demand on ozone or the required dosage. There are as many different demand or dosage figures as there are people calculating ozone treatment.

Mixing the gas with the water

The two most common methods of injecting ozone into the water, are by either using a pump to force ozone through a diffuser or by using a venturi injector to suck the ozone in. Venturi injection is normally preferred because it is very efficient and requires no moving parts. Water pressure at the inlet of the venturi injector must be higher than the outlet pressure during the entire pump cycle.
One very vital factor to take in to account is protecting the ozone module from water backflow. The basic non return valves may work for a limited period of time but because of the low air pressure will not be reliable. A simple solution is to ensure the ozone generating tube is higher than the water level. On more sophisicated plants a solenoid system could be utilised to physically shut down the gas delivery pipe thus ensuring no back flow of water.

Contact time and off gassing

After injection the ozone must have time to do its work. This time is known as contact time. Contact time is the time ozone has to oxidise and disinfect or the time the water is allowed to hold the disinfectant. For ozone, contact time is measured from the point of injection to the filter. Depending upon the circumstances of each installation, there may still be ozone in the water after filtration but for purposes of calculating contact time, the filter marks the end. Suppose you inject ozone into a 100 litre contact vessel followed by a filter and the flow rate is 20 litres per minute, the contact time in this situation would be 100 litres divided by 20 litres per minute resulting in 5 minutes contact time.
The contact time needed varies with the matter to be oxidised. Time required for oxidation ranges from almost instantly to 10 minutes or more. A general rule of thumb is no less than 4 minutes contact time with a measurable residual of 0.1-mg/l ozone and more contact time is better.

Monitoring and controlling

In order to determine the efficacy of the ozone gas if one assumes that if there is no ozone in the water after the contact tank then more ozone is required to oxidise and do its job.
Similarly if there is residual ozone in the water then one knows that there is nothing left in the water for the ozone to oxidise.
On basic water treatment or bottling plants the residual ozone can be used using a handheld ozone analyser at certain periods of the day and either the ozone output decreased or increased or the timer that activates the ozone can be adjusted to lower or increase to ozone levels in the water.

On an ongoing basis an ORP ( oxygen reduction potential) or Redox metre can be used with a probe in line which will turn the ozone generator on or through 4 to 20 milliamp signal and maintain set high and low ozone levels. However as the Redox measurement is not actual ozone mgs per litre the actual ozone should be tested against the Redox signal using an ozone analyser on a regular basis.

On more sophisticated systems a proper ozone analyser is required to determine the actual ozone in the water in mgs/litre or ppm. The probes will be positioned where the operator needs to know the ozone levels and can be pre contacting or post contacting or both with the capability of adjusting the ozone levels again remotely through a Scada or PLC.

Safety and ozone destruction

Safety precautions must be taken to prevent prolonged exposure to ozone gas by humans or other animals.
In less sophisticated sites the unmixed ozone gas can be captured and discharged in to the atmosphere where it is safe to do so.
On other sites HAZAP regulations may determine that the leftover ozone gas must be destroyed prior to release to atmosphere.
Destruction could be by using heat or carbon filtration and there are Ultraviolet and catalytic ozone destruction systems.

Two commonly asked questions;

Will ozone take out hardness?
Ozone will not oxidize hardness minerals such as calcium. Incorporating a softener into your ozone water treatment system to remove hardness requires only two special considerations, whether all components are ozone compatible and where to place it. In a treatment system where ozone is employed, a softener is installed after oxidation and filtration processes.
Using ozone before your softener will likely decrease the work load of the softener as contaminates such as iron are removed prior to the softener and taken out of the softener sizing equation. This leaves only the hardness minerals for the softener to contend with and may reduce the size of softener needed and its operating cost while extending its service life.

Will ozone fix my low pH?
Ozone will not directly affect Low pH. Though ozone is relatively unaffected by normally encountered pH levels, if possible, adjust pH prior to ozonation as some contaminants are more readily removed with ozone where the pH is between 6.0 and 9.0. pH can be adjusted in a treatment system based on ozone just as it would without ozone.