U.K. County Turns to Sodium Hypochlorite Generation
for Disinfection Needs

The county of Derbyshire, situated in the north midlands of England, is known for its “firsts.” Its Peak District National Park was designated the country's first national park in 1951. The country’s first modern factory – a silk mill in the town of Derby – was built in 1717; and its first water-powered cotton mill was constructed in 1771 in Cromford. Rolls Royce Ltd. established the company’s first factory – also in Derby – in 1908.

Today, agriculture, mining, manufacturing and engineering are the major industries in the county – each dependent upon water, which is provided by the River Trent; its major tributary, the River Derwent; and the River Dove. In fact, water itself is a major industry in Derbyshire, as Severn Trent Water operates the Ogston Water Treatment Works in north Derbyshire. The company is the world's fourth largest privately-owned water company serving more than 8 million customers across the United Kingdom. The company also provides a broad range of water and environmental services to businesses.

Historically, the Ogston facility employed two electrochlorination systems as part of its overall potable water disinfection system. When one of the systems recently required replacement, the decision was made to invest in a new, larger electrochlorination system suitably sized to accommodate the entire chlorination requirements of the treatment works.

Severn Trent Water called upon its sister company, Severn Trent Services, to provide two ClorTec™ CT-900 on-site sodium hypochlorite generation systems. The systems provide a cost effective, reliable and safe alternative to increasingly costly chlorine gas and bulk sodium hypochlorite disinfection methods.

The ClorTec system is approved for potable water within the United Kingdom and is compliant with ATEX requirements. Each unit produces chlorine equivalent using salt, water and electricity as its raw materials. The system operates by feeding softened water into a brine dissolver. The salt dissolves to form a brine solution, which is further diluted to the desired salt solution. The salt solution is then passed through electrolytic cells, which apply a low voltage DC current to the brine to produce the sodium hypochlorite. The sodium hypochlorite is then safely stored in a day tank and, when it reaches the low-level set point, the system automatically restarts to replenish its supply. The unit typically requires operator visits once a week in order to keep appropriate levels of salt in the brine tank.

Since the ClorTec unit produces sodium hypochlorite on demand, the technology alleviates the problem of chlorate by-product generation that typically results with the storage of bulk sodium hypochlorite. The ClorTec system also features clear acrylic cells which allow the reaction of salt, water and electricity to be visible and maintenance issues to be obvious. The split flow of brine to the cell ensures the economical, low chlorate production of 0.8% sodium hypochlorite solution.

Severn Trent Water is using the ClorTec units in a duty/standby arrangement. Over a seven-hour period, each unit is capable of generating a sufficient amount of sodium hypochlorite to satisfy the facility’s demand under average-to-maximum flow conditions. As a result, the electrochlorination system generates enough chlorine equivalent during off-peak hours to take advantage of off-peak electricity tariffs, further ensuring low operational costs.

The new ClorTec systems were integrated with the existing salt saturators, product storage tanks and plant room. During the generation of sodium hypochlorite, hydrogen is formed as a byproduct. In order to maintain hydrogen levels at or below 25% of the hydrogen lower exposure limit, the hydrogen dilution blowers at the works were upgraded.

With increased electrochlorination capacity, a new, larger water softener was installed to pre-treat water as it entered the system. A new hardness monitor further ensures that the water quality entering the system is within acceptable and optimal operational limits. The ClorTec system is comprised of clear acrylic cells where the reaction of salt, water and electricity to generate sodium hypochlorite can be monitored visually. This feature enables rapid assessment of system performance without having to shut the plant down.

In a county of firsts, the Ogston Water Treatment Works’ first foray into hypochlorite generation is providing benefits for both Severn Trent Water and to the residential and commercial customers of Derbyshire.

For more information, email info@severntrentservices.com