Experimental work on a laboratory recirculating apparatus, for the simulation of a cooling water system has yielded information on the effectiveness of ozone as a biocide for biofouling control. Biofilms were developed within glass tubes of the simulated cooling water system using filtered mainswater and Pseudomonas fluorescens as the test bacteria. A summary of the results obtained is presented and an interpretation of the these results obtained is presented and an interpretation of these results relating to the full-scale application of ozone is provided.
Ozonated water was produced using a contact system specifically designed for these tests by Ozotech Ltd. Ozone residual concentrations in the order of 0.1 mg/L were found to be capable of removing 80-99% of the biofilm in single applicatins. However, effectivemenss of ozone was dependent on morphology, thickness nad age of the biofilms. Intermittent application of ozone, using residual concentraions < 0.1mg/L, was found to be capable of weakening the biofilm, but a minimum period of 3-h was required for effective control.
The fuild velocity was found to affect the rate amount of biofilm removed. The effects of fluid velocities in the range 0.5-2.5 m/sec were examined. In general, the higher the velocity the greater initial rate of removal and the percentage of the biofilm removed; this suggested that ozone action was mass transfer dependent.
The effect of ozone on bacterial cell structure was investigated using scanning electron microscopy techniques. Changes in cell structure were revealed after contact with ozone and residuals above 0.4 mg/L were required for 100% kill of bacterial suspensions.
In conclusion, the effectiveness of ozone for full-scale application will vary depending on the morphology of the biofilm, the velocity of the fluid and the ozone dose applied. An ozone dosing regime for commercial application is suggested.
Water is the most common and effective means of cooling in industries, however most water contains microorganisms causing biological contamination of the systems the water comes into contact with, resulting in restricted flow, corrosion and reduced efficiency. Chlorine is the most commonly used biocide to reduce the microorganisms causing this problem. More tests are showing the adverse effects of clorine due to its toxicity causing more industries to consider alternative solutions. Ozone is considered an effective alternative to chlorine due to its natural characterics and disinfectant capabilty.
A ozone generator dispersed into a 200 L capacity tank through a venturi injector at a flow rate of 500 L/h. The ozonated water was recirculated in the tank continuously to obtain the desired residual. Biofilm removal tests were conducted in three areas; single application, effect of velocity and intermitent dosing. Single application effectiveness was determined by the age of the biofilm, initial size, and biofilms adapted to chlorine. Increasing the fluid velocity transfers more ozone to the biofilm. Intermitent doses at different quantities of ozone (.1 mg/L to .6 mg/L) was tested because it is more economical for large scale operations.
Ozone was found to be an effective biocide in biofilm removal in water cooling systems. The age and growth of the biofilm must be evaluated in determining the level and method of injecting ozone in the water system. Each commercial cooling water system will require investigation into the levels of biofilm and most economical and effective means of ozination to eliminate biofilms.
OZONE SCIENCE & ENGINEERING
Vol. 14, pp. 517 - 530
International Ozone Association