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AGROIWATECH Report Summary

Project ID: ICA2-CT-2002-10010
Funded under: FP5-INCO 2
Country: Germany

Pilot reactor for post-ozonation of wastewater for indirect potable reuse

Since water contents can react with dissolved ozone, one major task is to dissolve the ozone in the liquid. Commonly this is done with injector for the gas and a booster-pump for the liquid. The advantage of such a system is a fine bubble-size distribution and therefore a good mass-transfer for the ozone. The disadvantage is the high energy input needed.

The new system consists of a stirred bubble column reactor with installations to improve the back-mixing behaviour. The ozone intake is realised with a ceramic diffuser with small pore sizes to enable a small bubble-size distribution with a very low energy intake. The installations divide the reactor in 4 parts. The volume can be divided into volumes of 7.5 L, 11.0 L and 14.5 L and is operated as cascaded bubble column. The concentration of ozone in the gas was measured with a ozone HC-500 by PCI-Wedeco. The reactor was equipped with a bypass for the gas, thus making it possible to measure the ozone concentration in the inlet to the reactor as well as in the off-gas from the reactor. The ozone was produced with a generator SWO 100 by WEDECO from technical oxygen.

During the tests, the ozone production of the generator was adjusted via measurement in the bypass of the reactor prior to switching the gas for inlet into the reactor and measuring the off-gas. The amount of ozone transferred into the liquid was controlled by the time of ozonation.

The residence time distribution was measured with the HYDROMES-System made by the institute for thermal process-engineering of the Technical University of Clausthal. This device can measure the concentration of a tracer as differences of conductivity. The difference of the conductivity is directly proportional to the concentration of a salt-tracer. For the tests, 100mL of a salt solution was applied to the inlet of the reactor as a pulse mark during the continuous operation of the reactor.

The reactor was operated with several flow rates of demineralised water and air. The investigation of the residence time distribution has shown a strong influence of the stirrer and the volumetric flow rate of the fluid into the reactor. A strong influence of the gas flow rate could not be found. By varying the active volume of the reactor, it can be adapted to different needs. Compared with a CSTR, the residence time distribution showed an improved back-mixing behaviour with all tested reactor volumes.

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