Ozone Sizing: Know Your Goal
Tech Talk 72
In sizing an ozone system the most important design factor is getting the correct ozone dose for your specific application. Ozone is used mainly to achieve two different goals: sterilization/oxidation and microflocculation. Maximizing mass transfer (getting the ozone from the gas phase into the water) is of primary importance for both. The most efficient method of dissolving ozone (or any gas) is achieved by using a venturi educator, a device that passively pulls in ozone under a vacuum using the physical (motive) force of the water flowing in a pipe. The water enters the venturi where the velocity rapidly increases due to a cone-shaped restriction in the venturi throat. This increase in velocity causes a low pressure area to form at the point of maximum restriction (see diagram below), generating suction that pulls the ozone into the water stream. The venturi then rapidly expands in diameter, slowing the water down instantaneously, causing the water and gas to crash into each other at very high velocity and driving the gas into solution. The higher the pressure in the venturi and downstream piping, the more gas can be driven into solution.
Air diffusers and pressurized injectors are also sometimes used but have lower transfer efficiencies. The real advantage in using a venturi from a safety standpoint is that with positive pressure ozone delivery systems (where the ozone is pumped into the system under pressure) a leak in the delivery hoses or piping can let ozone leak into the environment. With a properly sized venturi, if a leak occurs under vacuum, surrounding (ambient) air will be drawn into the delivery tubing, so there's no chance of affecting nearby people with ozone.
Ozone has long been known to be a very efficient oxidant. In typical aquarium/aquaculture applications ozone can greatly reduce total organic carbon (TOC) levels by direct oxidation of the organics or indirect oxidation by other powerful oxidants that naturally occurs when ozone reacts with water (free radical oxidation). Applied ozone doses for oxidation and disinfection are similar and fall witin .1 to 1.0 mg/L. Another rule of thumb for ozone sizing for oxidation is based on food loading. An ozone dose of 15–20 grams of ozone per kg of food fed is recommended by Doctors Timmons and Ebling for aquaculture systems.
The other use of ozone not nearly as well known in aquatic systems is as a microflocculent. When dosed at rates roughly 1/10 of the oxidation dose (.01–.1 mg/L), ozone can act as a flocculent, causing very small particulates that normally pass through mechanical filters to clump into larger particles that mechanical filters can capture. The ozone does this by causing electrical charges on the surface of the particles so that they become attracted to each other like microscopic magnets. This type of ozone dose is typically used in foam fractionators (protein skimmers), so the flocculated particulates are carried out of the system water in the foam column.
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