Use of Electrolytically Generated Ozone for the Disinfection of Pure Water
By Christian Stark
Pre-treatment and treatment of Pure Water
Increasingly high requirements are being specified today to achieve the necessary quality of the Pure Water used in production and cleaning processes in the pharmaceutical and microelectronics industries and in companies which process foodstuffs. Chemical and physical quality requirements can, in general, be easily met with water treatment methods such as single-stage and multi-stage reverse osmosis (RO), electrodeionization (EDI) and ion exchange (IX).
On the other hand, compliance with requirements for low microorganism counts and low pyrogen content is more difficult. The storage and distribution of purified and highly purified water, for example, requires technical measures which actively prevent the growth of germs—with the lowest possible operating costs and without the use of conventional chemical disinfectants. Specific limit values must be achieved (see Table 1).
Ozone as an effective disinfectant
Ozone is an allotropic form of oxygen with three atoms per molecule and is, after fluorine, the strongest oxidant and disinfectant. It has an oxidation potential of 2.07 eV and is up to 20 times more powerful than chlorine. It has been used for almost 100 years for the treatment of drinking water and water for general use. As an environmentally friendly substance, ozone can be produced in the necessary quantities wherever it is needed. In contrast to other oxidants and disinfectants, the use of ozone results in far fewer toxic disinfection by-products (DPBs). This is important since the disinfection process may not change the chemical or physical properties of the water, particularly in the treatment and disinfection of water for pharmaceutical purposes.
The effects of ozone result from its strong oxidation capacity with respect to chemicals and microorganisms in the water. The dwell time needed for reduction of microorganisms depends on the type of organism, other reactants in the water and the ozone concentration. Ozone directly attacks the outer surfaces of the microorganisms and destroys their cell walls and membranes. The cells burst and lose their cytoplasm when exposed to ozone, which means that they cannot regenerate themselves.
For the disinfection of demineralized water, experience has shown that a low ozone concentration of ≤ 0.020 ppm is sufficient. Ozone is metastable, which means it can maintain a non-equilibrium state for an extended period of time; it has a half-life in Pure Water of about 20 minutes. Since reinfection is always possible in Pure Water distribution systems, the disinfectant must be circulated through the entire system and protect the complete distribution network.
Table 2 shows various sterilization and disinfection methods and the related problems in the treatment of Pure Water. Compared with these disinfectants, ozone offers a significant advantage because, in addition to eliminating microorganisms, it also provides reliable and continuous protection for the complete network of storage tanks and distribution systems.
Methods of generating ozone
The most commonly used method for generating ozone is corona discharge. Here, a feed gas containing oxygen passes through a high-voltage field between a pair of electrodes and a dielectric; the oxygen is converted into ozone. For use in the area of Pure Water disinfection, however, this method has various drawbacks which include:
- High consumption of oxygen as a feed material.
- Contamination of the ozone gas with moisture.
- Reduction in the pH-value of the Pure Water caused by nitric acid formed from the residual nitrogen in the feed gas.
- Complex technology required (only when using air).
- Removal of superfluous gases (only when using air).
- Corrosivity problems resulting from the formation of nitric acid, which is caused by the reaction of water vapor with nitrogen in the feed gas.
Electrolytic generation of ozone
The electrolytic dissociation of water is particularly suitable for the disinfection of Pure Water. An electrolytic cell through which water passes contains a polymer anion exchange membrane which acts as the electrolyte. This solid electrolyte is compacted between the anode and the cathode, thus separating the two halves of the cell electrochemically.
DC voltage applied to the cell dissociates the flowing water into ozone, oxygen and hydrogen ions on the anode side and reduces it on the cathode side to hydrogen gas. The ozone generation takes place in a bypass connected in parallel to the main water circulation loop. The ozone which dissolves in the flowing water is carried out of the bypass line and into the main loop, including the storage tanks. This provides a reliable protection for the entire system which hinders the growth of microorganisms.
The efficient microorganism reduction and external contamination protection of this method make ozone an ideal disinfectant for Pure Water systems in the pharmaceutical industry, because it both kills the microorganisms and also decomposes the resulting endotoxins. Ozone can also be used for either systematic disinfection of distribution lines or for shock sanitization (such as after work has been performed on the system). While an ozone concentration of 0.015 to 0.020 ppm has proved sufficient for continuous disinfection, full sanitization should be carried out with a concentration of 0.050-0.060 ppm. Depending on the capacity requirements, ozone generators with outputs of 0.3-12 g/h are now available.
The use of such generators offers the system operator various benefits. It prevents contamination with ions, since the ozone is generated directly from the Pure Water and is dissolved in this water. It also effectively reduces the growth of microorganisms and reduces the TOC as well as the number of endotoxins at low ozone concentrations. The hydrogen generated is removed either by catalytic burning or brought as exhaust air ‘over the roof’ (depending on national regulations). The system can be operated continuously without periodic cleaning via chemicals or steam. Installation and commissioning are simple; the generator requires little maintenance and the operating costs are low.
In addition, these systems are characterized by the following features:
- They require only a low voltage of four volts DC.
- There are no undesired external impurities in the Pure Water.
- They permit fine adjustment of the amount of ozone added to the water (according to Faraday’s Law, the ozone production rate is proportional to the current flowing through the cell).
- External contamination is avoided.
- They can be operated without additional safety measures (such as monitoring of the air in the room).
This method permits the generation of ozone at high concentrations from demineralized water with simultaneous solution of the ozone in the flowing water. The demineralized water thus acts simultaneously as a source of oxygen, as a solvent for the generated ozone and as a coolant for the electrolytic cell.
The special advantages of these systems are their longer operating lifetime (compared with conventional systems which results from the improved surface coating of the anode), their optimized arrangement of the anode, cathode and membrane, which allows the use of lower voltages and their simple construction permitting replacement of the membrane without special tools. Their electromagnetic compatibility (EMC) and the CE symbol indicate that the units comply with international regulations, such as those of the European Union and U.S. and have no negative effects on other systems. The on-line ozone measurement feature makes external control possible. Furthermore, such systems generally have a comprehensive system for detecting and signalling faults, which monitors the operating states and are also available with sterile connection facilities.
Application example in the pharmaceutical industry
In a system with a two-pass RO unit for the generation of Pure Water for pharmaceutical applications, a Pure Water tank acts as a reservoir to handle the peak demands. In order to keep this tank and the distribution system free of microorganisms, electrolytically generated ozone is added to the water in the return line of the loop. The water and ozone enter the storage tank and ensure active disinfection of the demineralized water.
Due to the low ozone concentration (generally approximately 0.02 mg/L) and dynamic operation, the ozone content of the pure water remains stable. The treated water circulates to the consumers and some of it is returned to the storage tank. This ensures that the distribution system is also disinfected. In applications where water needs to be free of ozone, a UV system breaks down the ozone, reducing the concentration to below the detectable limit of five ppb.
If necessary, the UV unit can be controlled by a timer which switches it off outside normal working hours, so that ozone remains in the water and disinfects the distribution system. During normal working hours, the UV unit is switched on and breaks down the ozone. The UV units use UV radiation with a wavelength of 254 nm to break down the ozone in a fraction of a second. If the UV unit is correctly selected to match the overall system, the ozone concentration is reduced to below the detectable level.
The results of validation tests demonstrate the efficient disinfection (Table 3). This system has been operating for several years and no disinfection with chemicals or steam has been needed to supplement the effects of disinfection with electrolytically generated ozone.
The operating and maintenance costs for disinfection with electrolytically generated ozone are extremely low, on the order of $0.01 (U.S.) per cubic meter of Pure Water. Maintenance is restricted to regular inspections and membrane replacement.
Measurement of residual ozone
The residual ozone content of the water is measured on-line, down to about five ppb, with electrochemical sensor systems which can be calibrated. These measuring devices can also be used for the regulation and control of the ozone generators.
These systems can be used wherever distribution systems and storage tanks need to be actively protected against the growth of microorganisms. Some typical application areas are:
- Pharmaceutical industry: for the production of USP Purified Water and Water for Injection.
- Cosmetics industry: for product water.
- Biotechnology: for rinsing and production water.
- Disinfection of existing distribution systems: dead-end lines and distribution systems which do not comply with the requirements for Pure Water.
The use of electrolytically generated ozone in systems for production of Pure Water guarantees low microorganism counts and environmentally friendly operation. Electrolytically generated ozone is a basic component of modern Pure Water production systems and is an economically superior solution to other methods (as shown in Table 2) when combined with methods such as RO and EDI.
About the author
Christian Stark studied sales and marketing, obtaining state diplomas in both areas. Today, he is the Director of Marketing & Communication at the Christ Water Technology Group in Aesch/Switzerland, one of the world’s leading manufacturers of Pure Water systems. Stark, who has held various positions within the company since 1990, has more than 15 years of experience in the area of Pure Water generation and specializes in all areas of pharmaceutical water treatment. He is a member of the ISPE and has previously published various articles on the use of ozone in Pure Water systems and various other water treatment subjects in the pharmaceutical and life science industries. Reach him at Christ Water Technology Group, Hauptstr. 192, 4147 Aesch/BL, Switzerland ; telephone: +41/61/7558-238 or via email: email@example.com
About the company
Christ is a global leader in technologies, components and services in the field of Pure Water as well as Ultrapure Water and drinking water production and the treatment of wastewater and sewage. The firm employs about 800 qualified staff members at more than 30 locations worldwide. A total solutions provider of turnkey water systems, Christ offers know-how, innovative drive, customer orientation and one-stop shopping. The company’s expertise ranges from project identification, planning, production and installation through commissioning and training backed by first rate customer service and support—all from a single source and corresponding to customers’ local needs.