Ozone is more effective than chlorine at water disinfection. Ozone also oxidizes several dissolved minerals, allowing them to be filtered. Due to its unstable nature (ozone has a half-life of approximately 22 minutes), ozone must be produced near the site of use and cannot be effectively stored or transported.
Municipal ozone water treatment
In municipal water treatment processes (surface water and ground water), ozone may be introduced in diffused (5 micron or less) bubbles mixed through the raw incoming water. The readily soluble ozone oxidized any organic material (bacteria, waterborne pathogens, etc) with which it comes in contact.
Diffused ozone also oxidized dissolved iron, sulfur and other materials, pulling them out of solution and allowing the new compounds to be filtered out. As the ozone (O3) breaks down, it releases its "extra" oxygen molecule to form oxidized compounds and passes the rest through as O2 for safe entry into the atmosphere.
As with chlorine disinfection, the method of determining water disinfection by ozone is based on Contact Time (CT), the amount of time the disinfectant is in contact with the raw water. The contact time required by a particular disinfectant is determined by experimental data showing how quickly a given disinfectant will reduce waterborne pathogens. Ozone is a favored disinfectant due to its effectiveness in destroying Giardia and Cryptosporidium which are two key waterborne pathogens.
Ozone production
Ozone is an unstable compound made up of three oxygen atoms. It readily breaks down into a stable oxygen (O2) molecule and a free ion. This is useful for its oxidation properties, but it means ozone cannot be stored or transported for industrial purposes.
Ozone production for water treatment (municipal or home processes) must be done on site by an ozone generator. Most ozone generation is done by passing oxygen through an electric corona (a controlled arcing) where a percentage of free O2 molecules are recombined as O3.
For home generators, the ~21% of oxygen in air will give safe quantities of ozone for small-scale use. In municipal operations, a 92 - 97% oxygen source is passed through large generators to produce high concentrations of ozone. Special safety precautions are required due to the toxic and highly flammable properties of high oxygen and ozone concentrations.
Dissolved Ozone and pass-through
In municipal operations, the high-concentration ozone dissolved into incoming raw water is monitored regularly by measuring the levels of dissolved ozone in the water. This aids in determining the effectiveness of disinfection (CT measurements) and as a troubleshooting method to ensure the ozone introduced is being diffused into the water effectively.
Since the oxygen-ozone mix diffused through the raw water will bubble to the surface, the atmosphere above the diffusion chambers will be monitored for the amounts of ozone passing through the water. Ozone passing through the water is ozone which did not oxidize waterborne pathogens. Also, EPA regulations are strict on the amount of ozone which may be released into the outside atmosphere. Unexpectedly high levels of undissolved ozone require process corrections or maintenance work by the water treatment facility.
Home ozone water treatment systems
There are a number of commercially available, safe home water systems. These systems are usually for purification of small quantities of water for drinking purposes. Since these systems use atmospheric oxygen and the ozone generation system usually converts (depending on temperature, humidity, and pressure) around 2% of oxygen into ozone, the resulting quantities of produced ozone are small enough to not represent a health hazard. These home systems will diffuse low ozone concentrations through fixed quantities of water to give the water a "polish" by reducing trace waterborne pathogens and dissolved metals.
Precautions
Ozone is classified as a toxic gas. Small quantities of ozone will cause headaches which last for a few hours. Contact with higher concentrations can lead to more serious effects or death. Commercially available home units will not produce dangerous levels of ozone unless badly malfunctioning and in enclosed areas where the ozone cannot dissipate.
Tags: waterborne pathogens, water treatment, ozone water, through water, breaks down, disinfection Ozone, incoming water