A Spanish utility adopts onsite sodium hypochlorite generation for disinfection
How does an ancient Spanish city located on the shores of the Mediterranean Sea, known for its ancient Roman ruins, monuments and 249-meter-long aqueduct, minimize health risks associated with sodium hypochlorite disinfection? In the case of Tarragona, an alternative disinfection solution was sought.
Home to the Aqueducte de les Ferreres, Tarragona’s historic aqueduct no longer provides drinking water for the city’s 140,000 residents. For more than 20 years, Consorci d’Aigües of Tarragona (CAT), one of the largest water utilities in the region, has provided potable water for residents, businesses and industry through reservoir 118, a distribution tank in the St. Pere i St. Pau section of the city. The plant has a treated flow rate of 1 cu meter per second. From the day of its initial start-up, disinfection at the plant was provided through the use of bulk sodium hypochlorite. But safety standards for the storage and transportation of chlorine have become increasingly stricter, and the generation of chlorate byproduct that results from the storage of bulk sodium hypochlorite has also become a concern for many water treatment facilities. Additionally, potable-grade bulk sodium hypochlorite solution ranges from 12 to 15% in concentration, which is considered to be hazardous and subject to containment requirements.
Seeking to Minimize Health Risks
In 2010, seeking a reliable disinfection system that would minimize health risks, CAT installed two ClorTec CT100  onsite hypochlorite generating systems from Severn Trent Services  in a duty/standby arrangement. Producing 100 lb per day of chlorine equivalent, the reservoir is utilizing the largest electrochlorination system in Spain.
The use of onsite generation is growing rapidly throughout the world because of its advantages over alternative means of chlorination. For example, unlike bulk sodium hypochlorite, which degrades with exposure to ultraviolet light, the 0.8% sodium hypochlorite solution is stable.
Onsite generation also has multiple advantages compared with gaseous chlorine. The onsite disinfectant is produced and stored in liquid form. Therefore, there is no danger of leaks from chlorine gas cylinders. It is also not necessary for facilities using onsite sodium hypochlorite generating systems to develop and maintain a risk management plan. HAZMAT or other safety training is not required for handling the disinfectant; nor is there any need for the use of self-contained breathing apparatuses. In addition, onsite sodium hypochlorite disinfection systems do not suppress finished water pH to the extent that gaseous chlorine disinfection does. Therefore, the amount of pH-adjustment chemical (i.e., lime or caustic) necessary before distribution of finished water is reduced.
The onsite generation process is simple, as three common consumables are used: salt, water and electricity. 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 and then passed through electrolytic cells. The cells apply a low-voltage DC current to the brine to produce the sodium hypochlorite. The solution is then safely stored in a storage tank with a capacity of 6 cu meters. When the solution reaches the low-level set point, the system automatically restarts to replenish its supply, generating a consistent solution strength on demand, so sodium hypochlorite is generated only when required. The 0.8% sodium hypochlorite solution is non-hazardous; the only byproduct is hydrogen gas, which is safely vented to the atmosphere.
Commitment to Improving Water Quality
“Consorci d’Aigües of Tarragona selected electrochlorination technology because of our commitment to improving the quality of water that we supply to our consortium members, which includes 68 city councils and 33 industrial clients,” said Salvador Plana, managing director of CAT. “In addition, we will be installing a series of plants across our extensive network in the years to come. The ClorTec technology will enable us to improve how we control chlorine dosing for the drinking water we supply to our members and eliminate the need to transport hazardous chemicals and store them on site. Ultimately, onsite generation improves the safety of our facility and minimizes risks to the public, our employees and the environment.”
Fran House is marketing manager for Severn Trent Services. House can be reached at firstname.lastname@example.org  or +44.1827.266.000.