Chlorine, the safer way

The most widespread disinfectant used in the treatment of drinking water is chlorine, which can be applied in a variety of ways. In bacteriological terms, chlorination is well proven, writes Tosh Singh of Grundfos Alldos. It is more than 75 years since chlorine was used to disinfect drinking water for the first time. And the years of experience have shown that acute toxicity can be excluded, when chlorination is executed correctly.

Generally, three methods are used for chlorinating drinking and process water:
  • Chlorine gas dosing
  • Dosing of sodium or calcium hypochlorite solution
  • Electrolytic chlorine generation
The disinfecting properties of sodium hypochlorite, a common form of disinfection, is based on the fact that hypochlorous acid (HClO) is produced when it is dissolved in water. And it depends strongly on the pH value; the optimum pH value being less than 7.5.

Electrochlorination, a common method of generating sodium hypochlorite on site, has a number of advantages over other means of disinfection. It is a simple and effective process that uses only widely available raw materials - salt, water and electricity, to generate a high-quality, low-strength sodium hypochlorite solution (up to 0.8% concentration).

In fact, the quality of electrolytically generated sodium hypochlorite is controlled by the quality of the raw materials used in the process. This process is considered safer for operators to use rather than chlorine gas or commercial sodium hypochlorite as the COSHH regulations do not apply to the product of electrochlorination.

Other principle advantages of electrochlorination include handling and storage of salt, which does not degrade and can be stored on site indefinitely. The sodium hypochlorite generated is made on demand, which reduces the need for bulk storage of chemicals on-site. Additionally, the sodium hypochlorite produced is not subject to the same rate of degradation as commercial hypochlorite. This improves the reliability of the dosing system and reduces the calcification of injection points.

The electrochlorination process is based on passing a brine solution through a series of electrodes contained within an electrolytic cell. As brine passes through the electrodes, a DC power is applied, which results in the production of sodium hypochlorite. The by-product, hydrogen gas, is diluted and safely vented to atmosphere. Sodium hypochlorite can then be stored in a product tank and dosed into the application as required.

Safe hydrogen management is an integral part of any electrochlorination system. The gas can be explosive above a concentration of 4% - known as the lower explosion limit (LEL). It is commonly diluted using centrifugal fans to force air to ventilate areas where hydrogen can potentially accumulate within the system, such as the sodium hypochlorite storage tank. The ATEX Directive covering the use of equipment in a potentially explosive environment applies to the design of these systems.

ATEX requires that a zone is applied to any equipment or storage tanks that may contain hydrogen. Within this zone, any electrical equipment must be suitably rated for use within a potentially explosive atmosphere.

However, an electrochlorination system has been engineered to remove the requirement to have an external zone around the electrochlorination system and the storage tanks. This feature of the Selcoperm system, manufactured by Grundfos Alldos, permits the unit to be safely installed within any existing plant room without having to apply a zone in the room.

The Selcoperm system includes a hydrogen degassing system, which removes the majority of the hydrogen from the sodium hypochlorite solution before it leaves the system. According to Grundfos Alldos, tests have shown any residual entrained hydrogen that passes through the degassing system is at such a low level that the concentration of hydrogen does not exceed the LEL once within the product storage tank. The hydrogen evolved from the degassing system can then be safely vented out of the building.

Additionally, both the electrolytic cell and degassing column are contained within a sealed enclosure. This is force air ventilated to ensure that any hydrogen leaking into the enclosure, as a result of poor maintenance or mechanical failure, is always sufficiently diluted. Cells are arranged vertically to ensure that hydrogen freely levels the system even when the system is not generating.

Safety interlocks such as bund sensor, airflow sensors and level sensors within the enclosure maintain the integrity of the system. The system cuts power to the cells
in the event of any one of these sensors detecting an unsafe condition.

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