Chlorine on demand

A new portable system has been developed which generates sodium hypochlorite on demand. Fran House of Severn Trent Services discusses the benefits the unit can bring to a variety of applications - not least health and safety.

For 30-plus years, on-site generation of sodium hypochlorite has been an accepted technology within the water treatment industry for disinfecting drinking water.

This simple process uses three raw materials – salt, water, and electricity – to generate a consistent, safe, low-strength sodium hypochlorite solution, typically from 0.6 to 0.8% concentration.

Salt is dissolved in a tank using softened water to produce a saturated brine solution. Brine, typically a 30% saturated solution, is diluted 10:1 to achieve a 3% solution, which offers optimum conversion of the brine to sodium hypochlorite without wasting salt.

As the brine solution passes through each cell, DC power is applied to the electrodes, electrolysing the brine solution and causing a number of reactions to occur at the electrodes; chlorine and hydrogen are produced at the anode and, at the cathode surfaces, an additional reaction between the chlorine, sodium, and hydroxyl ion results in a solution of sodium hypochlorite:

Salt + Water + Energy = Hypochlorite + Hydrogen

This can be written as:

NaCl + H2O + 2E = NaOCl + H2

Household bleach is available in some parts of the world in concentrations as high as 5% to 6% solution. And commercial sodium hypochlorite is typically available as a 14% solution. The 0.8% sodium hypochlorite produced by electrolysis offers significant health and safety benefits for operators maintaining such systems.

A new system, the ClorTec SCT, has been developed by Severn Trent Services to service the low flow-rate market, operating in batch mode to produce up to 2kg/day of sodium hypochlorite solution.

The production rate of this new unit is ideally suited to applications such as swimming pools, food and drink production and cooling tower disinfection where low levels of disinfectant are required. But it is particularly suited for the water and wastewater sector, where electrochlorination has traditionally been used in the UK.

Traditional cell design for sodium hypochlorite production encourages the immediate separation of the hydrogen from the sodium hypochlorite solution. The presence of hydrogen has to be carefully managed and requires compliance with a number of European directives including ATEX and DSEAR.

ATEX regulations

ATEX is the name commonly given to the framework for controlling explosive atmospheres and the standards of equipment and protective systems used in them. ATEX Regulations apply to all equipment intended for use in explosive atmospheres, whether electrical or mechanical, and also to protective systems.

Manufacturers must ensure that their products meet essential health and safety requirements and undergo appropriate conformity procedures. Certification ensures that the equipment or protective system is fit for its intended purpose and that adequate information is supplied with it to ensure that it can be used safely.

The design of the new ClorTec SCT on-site hypochlorite generator obviates the need for European ATEX compliance.

Air is forced through the system at the point of production, which effectively dilutes the hydrogen to a level significantly below the safe limit. Tests carried out by Severn Trent Services (STS) have indicated this value to be more than ten times lower than this limit.

As a result, the SCT installation does not require any safety zoning around such systems. The system can be turned on to generate hypochlorite as soon as connections to the water supply, electricity and drainage have been established.

The SCT generation process is entirely a batch make-up system. The system cell fills with diluted brine solution until a level sensor indicates when the predetermined solution maximum has been reached and the filling process is stopped. This process is quick to complete, taking only two minutes.

The cell is then energised for about two hours. During this time, air is blown into the cell to dilute the hydrogen at the point of production. The brine solution is converted to hypochlorite at about 0.8% solution strength.

After two hours, the hypochlorite drains to the product tank, which takes about eight minutes. From the product tank, the solution can be dosed directly into the water supply and the process is started again.

The SCT batch production process ensures that the temperature of the solution is controlled since the system is not in constant production. Consequently the production of by-products like chlorate is minimised. This is an important consideration for customers in many industries where chlorate levels have to be minimised and future regulations may well set maximum levels of chlorate with water distribution systems.

The ClorTec SCT is a small, modular unit that incorporates all items required to generate sodium hypochlorite on site. The product tank, salt saturator, generating cell, water softener, control panel and hydrogen dilution fan are all provided on a skid measuring 1m wide by 0.7m deep by 1.7m tall.

Limited space

The small size of the system allows it to fit through standard doors and is easily manoeuvred on site to allow permanent or temporary location of the unit. And the SCT can be used to disinfect swimming pools, where space may be limited.

By minimising the number of components with moving parts, STS has simplified the installation process; reduced the level of maintenance that is required on other systems; and also ensured superior reliability over competing products.

The ClorTec SCT offers a reliable and cost-effective method of generating sodium hypochlorite on demand, the company says. Sized to fit through doorways, the system is easy to move and can be located permanently or temporarily. Generating low-level disinfectant on site reduces health and safety concerns associated with transportation and handling of chlorine gas or bulk sodium hypochlorite and eliminates the zoning requirements associated with hydrogen production.

Fran House is marketing manager at Severn Trent Services. T: 0121 313 2300

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