Frequently raw water containing bacteriological impurities, e.g. surface water or bank-filtered water, is used in potable water recovery processes. Here, water disinfection is an absolute necessity. The disinfection of production water used in a variety of industries, e.g. the food and beverage or pharmaceutical industry, is also self-evident nowadays. The disinfection of circulating water, e.g. in cooling, ventilation, and air conditioning systems, has also proven advantageous and in some cases is essential. In recent years risk of infection by presence of Legionella in warm- and cooling-water circuits was focused by public interest.
For the disinfection of these different types of water, an array of disinfection methodologies and technologies is available, each having its specific advantages and disadvantages. Which technology is best suited for an individual application depends on many parameters and boundary conditions. Sounds complicated? Don't worry. ProMinent offers a wide portfolio of technologies for water disinfection. This means our specialists can identify and offer the optimum technology for your specific requirements. Often, a combination of different technologies is suited best to fulfill your requirements regarding water disinfection in the most effective and cost-efficient way.
Follows is a synopsis of water disinfection processes:
Chlorination
In chlorination, chlorine gas, sodium or calcium hypochlorite is added to water. The required quantity depends upon chlorine demand of the water and on the disinfection requirements. A minimum reaction period of 20 minutes is also required for effective water disinfection. The efficiency of the chlorination is extremely dependent upon the raw water’s pH value. Particularly in organically contaminated water, odour and taste of the water can be impaired by using chlorination for water disinfection, and undesirable by-products including haloforms may be produced. The prevention of bacterial aftergrowth by chlorine in the distribution system is generally overestimated, as there is a rapid decrease in the chlorine concentration due to the inevitable chlorine demand of water in the supply network. Chlorination is the disinfection method used most and is applied in very many different areas to disinfect water.
Chlorine Dioxide
Chlorine Dioxide is an instrument for water disinfection, which replaces chlorine in more and more applications due to its multiple advantages. It’s disinfection force is stronger and independent upon water’s pH value. Due to its specific chemistry, no chlorinated by-products can develop. The much longer half-life affords better depot action in treated water. In opposition to chlorine, Chlorine Dioxide is able to remove biofilms in pipe systems and tanks to abolish growth of Legionella. Chlorine Dioxide has already proven its advantages as instrument for water disinfection in a wide field of applications, such as potable and waste water treatment, Legionella prevention, treatment of product- and process water in the food and beverage industry, as well as in cooling water treatment.
Ozonisation
Ozone is the strongest disinfectant and oxidant suitable for water treatment. It’s major advantage is, that no undesired by-products are developed and ozone itself decomposes to oxygen. It’s disadvantage is a low half-life and a weak solubility in water. Ozone meets increasing popularity for disinfection of bottled water as well as disinfection of production- and rinsing-water in the food and beverage or cosmetics industry. Additional classic applications are the treatment of potable water, water treatment in swimming pools and zoos, and in cooling circulations.
Ultraviolet Disinfection
In ultraviolet disinfection, water is exposed to short wave ultraviolet light. This is an effective germicide and does not affect the water quality. UV-Disinfection is used to treat potable and waste water as well as for the treatment of product and production water in industrial applications.
Silver Ion Sterilisation
In silver ion sterilisation, silver ions are added to water in a concentration of 0.05 to 0.1 mg/l. The silver ions act as an (oligodynamic) germicide although the precise interrelationships are not known. Several hours are required for effective action to take place. Currently the process is used only for disinfection of potable water on ships or for potable water supplies in disaster areas.
Sterile Filtration
High costs confine sterile filtration to medical or pharmaceutical applications. This process uses ultra filtration membranes with pore sizes of < 0.5 µm. These have to be disinfected at regular intervals. Simple ultra filters are also used in small potable water filters, e.g. for domestic use. There is always a danger, however, that bacteria may pass through the filter. Even silvering of filters has been shown not to offer a reliable long term solution to this problem.
Thermal Water Disinfection
Thermal water disinfection is a reliable process irrespective of water quality. Due to its high energy use and the costs associated with heating the water to a minimum of 75 °C, thermal disinfection is mainly used to fight Legionella in hot water systems, to disinfect pipework in the pharmaceutical and food industries and to disinfect waste water from micro-biological laboratories and from critical areas in clinics.
Slow Sand Filtration
Slow sand filtration with filter speeds of approx. 0.1 m/h achieves a significant reduction of the micro-biological count. This technology is used primarily for waste water treatment and potable water treatment. However, due to the large filter surface required, and the time-consuming maintenance of the filter system slow sand filters are becoming increasingly rare in potable water treatment. In many cases additional water disinfection is required particularly with critical raw water.
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