Side Stream Injection Ozone Water Mixing

Side stream injection involves splitting off a portion of the main flow into a side stream. Ozone is injected into this side stream and then the side stream is mixed back into the main flow. The side stream injection system is composed at a minimum of a booster pump, a venturi and nozzles for introducing the side stream back into the main flow. A side stream 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. So, most of the gas is either air or oxygen. This gas typically needs to be removed from the liquid. This can be accomplished in the side stream or in the contactor after the side stream 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 side stream can be smaller for a given ozone transfer efficiency. This means that the side stream injection system can be smaller including the booster pump. 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 side stream injection system for drinking water treatment is shown in the presentation
Overview of Ozone Water Treatment of Drinking Water.

The advantages of a side stream injection system are:

  • Lower maintenance costs relative to bubble diffusers.
  • Greater flexibility in contactor layout since dissolution is separated from reaction
The disadvantages of this approach are:
  • Somewhat higher energy costs associated with the booster pump versus bubble diffusers
  • Less flexibility in turn down possibly requiring multiple injectors
There are two typical approaches to the design of a side stream injection system. In the first, the degassing of the liquid stream is carried out in the side stream using a degassing vessel. In the second approach, the degassing takes place in the contact/reaction vessel.

Side Stream Injection into Pipeline
Side Stream Injection with 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 side stream 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. In addition, the degas vessel is not needed.

In either approach, the ozone generator has to be protected from back flow from the side stream. 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 side stream 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 an 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.