Over the years manufacturers have become increasingly aware of the importance of water purity and its effect on the quality of the final product. One of the largest problems that confronts pure water system operators is bacterial recontamination shortly after the water purification equipment.
Irrespective of how well a plant has been designed and constructed, when no special measures have been taken, it is virtually impossible to avoid microbial contamination in a pure water network. This is especially the case when a system is not continually replenished with fresh makeup water such as over night or over the weekend when production is shut down and the water stands in a tank or circulates in a closed loop. There are several recommended methods of either preventing or removing such contamination but most have inherent disadvantages.
The most effective method that has been adopted by leading food & beverage, pharmaceutical and semiconductor manufacturers, involves ozonation of the water in the system and reducing this prior to the first point of use with ultraviolet irradiation.
Worldwide, the Ozonia Membrel system (Datasheets) has a 55% market share in ultrapure water loop disinfection. Here we will look at some of the other disinfection methods that are employed and compare them to find out why.
Disinfection processes used in ultrapure water loops
| Process | Advantages | Disadvantages |
|---|---|---|
| Shock disinfection with chemicals | Low investment | Process must be stopped Residual chemicals in loop High maintenance cost Variation in water quality in between dose |
| Shock disinfection with vapour/steam | No chemical management No chemical residual in loop |
Process must be stopped Vapour must be sterile High maintenance cost Variation in water quality in between doses |
| UV only | No chemical management No chemical residual in loop |
Local effect only Requires shock disinfection |
| Sterile Filters | No chemical management No chemical residual in loop |
Bacterial development Danger of rupture Periodic replacement required Variation in water quality in between doses |
| Ozone Disinfection | No chemical management No chemical residual in loop No external agents introduced to ultrapure water loop |
Initial equipment investment cost |
Of the above mentioned standard methods shock disinfection with chemicals or shock sterilisation with steam seem to produce the best results providing that the service interruptions and fluctuating water quality can be tolerated. See below:
Fluctuating water quality with shock disinfection
An innovative and alternative method of disinfection of pure water loops, without any of the mentioned drawbacks, is by introducing ozone to the circulation flow. The advantages of using ozone and specifically the Membrel system are two fold:
Firstly, there are no objectionable by-products or residues when water is disinfected with ozone. In the absence of contamination ozone decomposes to form oxygen and where the ozone oxidizes substances only traces of carbon dioxide will form. These substances do not pose a problem with regards to the water quality.
Secondly and specifically for the Membrel system, the ozone is actually produced from the water being treated, this means that no external substances are being introduced to the water loop completely removing the risk of contamination that steam and chemical shock disinfection system bring.
Experience gained in the pharmaceutical industry has shown that very low ozone concentrations in the magnitude of 0. 1 to 0.2 mg/l are sufficient to keep germ counts below 1 c.f.u. per 100 ml.
Disinfection with ozone is a continual process and, in most cases, can be regulated by simple means. However, if required, the ozone production rate can be controlled by the process parameters in order to avoid incorrect dosing and to ensure optimised efficiency.
Owing to ozone’s limited half life, it has to be produced in-situ when and where it is required. Although this means that extra investment must be made for a minor additional amount of infrastructure in the form of an ozone generator it avoids the transportation of potentially dangerous chemicals or the high running costs of a steam sterilization unit.
System Integration
Most modem pure water networks, are constructed as closed loop systems in which the water is pumped through one or more circulation loops to different consumer points. Depending on the application of the process water, there will be differing amounts and types of equipment installed in the system.
For the vast majority of applications it is best to install the ozone generation system in the loop and return just before it re-enters the storage tank. The by-pass flow for the electrolytic system is tapped off before the loop’s pressure retaining valve and reintroduced on the low pressure side – with the aim of achieving 0.02 to 0.050 ppm residual ozone in the tank for optimal operation.
Typical Ultra Pure Water Loop
Ozone Destruction
Depending on the process or product in question, sometimes it is necessary to remove the ozone from the water prior to use. There are several methods of removing or reducing ozone (activated carbon filters, catalytic conversion units, thermal destructors, ultraviolet irradiation, etc.) but, because of the nature of the pure water and the associated systems, only ultraviolet irradiation comes into question.
Normal practice is to install a UV unit in the piping system shortly before the first point of use which is switched on shortly before starting the main process or commencing with production and then switched off when there is no demand for a longer period of time such as over night or over the weekend.
Standard germicidal UV units, with low pressure mercury lamps with a high UV-C output at the wavelength of 254 nanometer, are ideal for reducing ozone in pure water loops to below the measurable limits.
When dimensioning such lamps, special attention must be given to the fact that the UV dose delivered at the end of the lamp’s service fife, and at maximum water flow, is sufficient to achieve the required level of ozone reduction with 120mJ/cm2 being the recommended minimum dose.
Conclusion
Experience gained from numerous installations in various branches of industry and institutions indicate that the sanitisation of pure water systems with ozone is an easy and effective method that doesn’t require any specialised operator skills.
Because ozone disinfection is a continual process allowing very little chance of recontamination, many operators now rely solely on the ozone treatment and have dispensed with regular shock disinfection with either steam or other chemicals.
Contact Details
For further information on the products, or to obtain copies of the data sheets please follow this link: Ozonia Membrel MkIV
If you would like any advice regarding your specific application please get in touch at sales@daly-me.com or call us on +971 4 887 8185 and we will be happy to help.