Science Portal

The growth of micro organisms in water can pose many problems for using or recycling water. The most important application is treating public drinking water to remove pathogens that could make people sick.

There are numerous choices for treating water to control or remove the growth of micro organisms including chlorine, UV and ozone. Each technique has its advantages and disadvantages. UV, for example, is an excellent way to remove micro organism from water that can transmit UV light readily, referred to as UV transmittance. However, UV only treats the water that passes by the UV lamps. If a contaminant enters the water after the lamp, say from the air, the water will not longer be protected. Chlorine and ozone, on the other hand remain in the water, maintain residuals, that move through the entire water system controlling the growth of micro organisms.

The advantages of ozone in particular are it is an extremely effective biocide that works against a wide range of micro organisms, it kills the organisms quickly, it is generated on site which eliminates the need to purchase or store the ozone, it forms limited disinfection byproducts and after use it breaks down to oxygen. This article will describe the how ozone systems are designed to treat water for the purpose of controlling or removing micro organisms from water.

As with many biocides, the effectiveness of ozone is determined by the average amount of ozone present over a specific period of time. This is expressed as a product of the average concentration of ozone (residual) multiplied by the time over which an organism is exposed to the residual. This product is referred to as CT and typically has the units mg-min/liter.

For a given organism and temperature, the CT will correlate to a certain log redcution of that organism. A four log reduction would mean that 99.99% of the organism has been removed. Each organism will have a different response for a given biocide. Below are some CT values for 3 log reduction (99.9%) of giardia lamblia:

Biocide - CT
Chlorine - 122
Chloramines - 2200
Chlorine Dioxide - 26
Ozone - 2

Comparing ozone to chlorine for this organism, for the same exposure time, it would require 1/60 (2/122) as much ozone to obtain a 99.9% reduction as chlorine. Stated another way, for the same residual concentration, it would take 1/60 the exposure to ozone than to chlorine to affect the 99.9% reduction. Therefore in teh case of giardia, ozone is 60 times more effective than ozone.

Ozone is effective against a range of important organisms besides giardia as shown below:

Organism - CT - Log Reduction
Virus - 1.2 - 99.99
E. coli - .02-.03 - 99.00
Stretococcus F. - .01-.03 - 99.00
Legionella - .30-1.1 - 99.00
Total coliforms - .19 - 99.9999

Ozone is therefore a powerful disinfecting agent for a wide variety of organisms found in water that can creates health or nuisance problems.

The challenge to designing an ozone disinfection system is knowing the C and the T. The concentration of ozone, as well as other disinfectant, drops with time as the ozone decomposes back to oxygen. So the C in CT is actually an average value for systems where the water passes only once through the treatment process, such as at a public drinking water treatment plant. In recycling system, it may be possible to continually add ozone to maintain a fixed concentration.

T is the time the organism is in contact with the ozone. One would think that if the flow of water is 10 gallons/minute and you have a 100 gallon tank, the time of exposure is simply 100 gallons divided by 10 gallons/minute to arrive at 10 minutes. Unfortunately this is usually not the case. Some of the water entering the tank can take a short cut through the tank to the exit, short circuiting the tank and leaving before the expected 10 minutes. To prevent this baffles are placed in these tanks to force the water to take a certain path through the tank to maximize the amount of time spent in the presence of the ozone. Even with baffles, the actual time in the tank will never be the tank volume divided by the flow rate.

Determining the C and T requires careful pilot studies, detailed engineering studies or both.

Different applications for disinfecting water with ozone apply the principles above with more or less rigor. In drinking water for public consumption the US EPA has strict rules for calculating the CT and specific tables for different organisms showing CT values and the associated log reductions. The EPA calculations are extremely conservative to insure that micro organism do not pass through the treatment plant and harm public health.

In some industrial applications the presence of micro organisms are tolerated up to the point where they cause operational problems such as odor or create biological films that clog machinery. The designs that are used in these cases are not as rigorous as with the drinking water applications.

Finally, it is worth noting that ozone is often used in conjunction with other biocides. For example in drinking water treatment ozone might be the primary disinfectant, but chlorine will often be used as the secondary disinfectant. This is done to minimize problems with the use of chlorine while retaining the advantage of its persistent residual in water distribution system.