Sidestream Injection Systems Ozone Water Mixing

Ozone water treatment began in the 1940’s and its effectiveness has been well documented. Over time, ozone technology has greatly improved and sidestream injection systems have increased in popularity over the years. Sidestream injection involves splitting off a portion of the main flow into a sidestream. Ozone is then injected into this sidestream and the sidestream is mixed back into the main flow.

At minimum, sidestream injection systems are composed of a booster pump, a venturi and nozzles for introducing the sidestream back into the main flow. A sidestream injection system can also include degassing equipment and in line mixers. Degassing is required since the feed gas is composed of typically 2-10% ozone. Therefore, most of the gas is either air or oxygen. This gas typically needs to be removed from the liquid and this can be accomplished in the sidestream or in the contactor after the sidestream has been mixed back into the main flow.

The higher the concentration of ozone in the gas, the smaller the volume of gas required to delivery the necessary dose of ozone. This means the sidestream can be smaller for a given ozone transfer efficiency. Additionally, this means the sidestream injection system can be smaller including the booster pump. As a result, this reduces both energy and capital costs. For large flows, this can be a critical consideration. In small systems it may not be important.

Schematics and photos of a sidestream ozone injection system for drinking water treatment can be viewed or downloaded here (see page 42):

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Sidestream Injection Design Approaches

There are two typical approaches to the design of sidestream injection systems. In the first, the degassing of the liquid stream is carried out in the sidestream using a degassing vessel. One design approach is sidestream injection in a pipeline.

Ozone Sidestream Injection in a Pipeline:

In the second design approach, the degassing takes place in the contact/reaction vessel. This design is an injection with a contact vessel.

Ozone Sidestream Injection with a Contact Vessel:

The injector with degas vessel approach is used when there is an interest in reducing: dissolved oxygen (DO) content of the water or the presence of gas bubbles downstream of the injector. Because the gas will spend less time in contact with the liquid, the liquid to gas ration must be higher, on the order of 10:1 to achieve good ozone transfer efficiency.

The use of a sidestream injection system without degas vessel is used when DO or gas bubbles down stream of the injector are not an issue. In addition, capital and operating costs are lower since the liquid to gas ratio can be 1.5-3.5:1. This means lower liquid flow and as a result smaller pumps and venturi. Additionally, the degas vessel is not needed.

In either approach, the industrial ozone generator has to be protected from back flow from the sidestream. In many cases with bubble diffusers the ozone generator is above the water level. Thus, the potential for back flow is reduced. This is not the case for sidestream injection systems. The ozone generator could be exposed to back flow at the line pressure. Multiple back flow preventors such as check valves and liquid traps are employed.

As with bubble diffuser systems, off gas should be passed through a vent gas treatment system, ozone destroyer. The difference is that with a degas vessel arrangement the off gas is under pressure and does not require a blower on the destroyer to pull the gas through the system.