Articles & Case Studies

Controlling Chlorine Costs

Posted: Monday 2nd February 2009

The most widespread disinfectant used in the treatment of drinking water is chlorine, which can be applied in a variety of ways. History has taught us that, in bacteriological terms, chlorinating water is a proven way of disinfecting drinking water. After all, more than 75 years have passed since chlorine was used to disinfect drinking water for the first time.

Many 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:

1. Chlorine gas dosing
2. Dosing of sodium or calcium hypochlorite solution
3. 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, according to the equation NaClO + H20- NaOH + HClO and 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. Electrochlorination 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 reducing 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 substantially 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 the brine solution passes through the electrodes a DC power is applied to the electrodes 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 a fundamentally integral part of any electrochlorination system. As Hydrogen gas is explosive above a concentration of 4% (this threshold is termed the Lower Explosion Limit or LEL) in many electrochlorination systems the hydrogen is diluted to measureable levels below this threshold to ensure that the concentration of hydrogen never reaches an explosive concentration. This is commonly achieved through the use of centrifugal fans to force air ventilate areas where hydrogen can potentially accumulate within the system, such as the sodium hypochlorite storage tank.

As electrochlorination systems generate Hydrogen 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 and storage tanks which may contain hydrogen. Within this zone any electrical equipment must be suitably rated for use within a potentially explosive atmosphere.

The Selcoperm electrochlorination system manufactured by Grundfos Alldos has been engineered to remove the requirement to have an external zone around the electrochlorination system and the storage tanks. This innovative feature of the Selcoperm system 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 safely removes the majority of the hydrogen out of the sodium hypochlorite solution before it leaves the system. Tests have show that 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 to the external of the building.

Additionally both the electrolytic cell and degassing column are contained within a sealed enclosure which itself 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 that an explosive concentration can never be achieved. 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, air flow sensors and level sensors within the enclosure ensure that the integrity of the system is maintained at all times. The system fails safe and cuts power to the cells in the event of any one of these sensors detecting an unsafe condition.

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