With Singapore's Public Utility Board opting for sodium hypochlorite over chlorine gas for both its new desalination facility in Tuas and Waterworks in Lower Seletar, the use of this technology is gaining acceptance worldwide. Nadia Abboud looks at what this means for chlorine gas, currently widely used in the West.
Chlorination, one of the most common methods used to disinfect water and wastewater, has played a vital role in the reduction of diseases worldwide for more than 100 years. Highly effective against most pathogens, it is a popular choice as it provides residual protection required for drinking water, is operationally reliable and is typically the most cost-effective disinfection option.
On-site sodium hypochlorite generation process diagram
There are a number of methods to achieve chlorination, including chlorine gas, commercial sodium hypochlorite, on-site generated sodium hypochlorite and calcium hypochlorite (tablets or powder). Of these, chlorine gas is the most commonly used form.
According to the American Chemistry Council, 84% of large drinking water systems (>10,000 persons) and 61% of small drinking water systems (<10,000 persons) in the United States use chlorine gas as at least one of their disinfectants. Gaseous chlorine's use is even more dominant outside the United States. Various third party market research reports have shown the annual growth rate of chlorine gas to be between 5% and 7%, while the growth rate in the United Sates is at best flat, approximately 1%.
Despite being proven as an effective disinfection technology, it is the safety of chlorine gas that has come under increased scrutiny worldwide in recent years. At low levels, it can cause eye, skin and respiratory irritation, while exposure in high enough doses can be fatal. In addition to the side effects related to chlorine gas exposure, the nature of the gas allows migration to distances well beyond the point of release.
Large gaseous chlorination systems can pose a health risk to facility operators and a potential risk to the public. As a result, authorities have more stringently regulated the transportation, storage and use of pressurised chlorine gas.
Facilities using chlorine gas have increased operational requirements as a result of having to assess the risks associated with its use and develop a risk management program. In the United States for example, The Clean Air Act requires that a risk management plan is submitted to the U.S. Environmental Protection Agency and disclosed to the public if a site stores quantities of chlorine that exceed the regulated threshold amount.
This requirement raised awareness of the health risks posed by gaseous chlorine, the increasingly stringent requirements for safe operation of systems using gaseous chlorine and concern about the vulnerability of these systems to sabotage. Implementing a risk management plan also increased the costs associated with using chlorine gas as a disinfection technology.
Yet despite the potential hazards of gaseous chlorine, water and wastewater utilities throughout the world have achieved a strong record of safety in its use. The development of effective safety systems such as emergency gas scrubbers, container shut down systems and vent exhaust gas arrestors has contributed to the safe operation of chlorine gas systems.
Nevertheless, utilities have increasingly begun to evaluate alternative disinfection methods to mitigate the perceived risk associated with using chlorine gas and to reduce the costs associated with implementing it into their facility treatment processes. Utilities have also explored the use of alternative disinfection technologies to reduce the formation of disinfection by-products (DBPs).
Since the early 1970s, scientists have known that chlorine gas can form compounds such as trihalomethanes (THMs) and haloacetic acids (HAAs) when interacting with natural organic compounds in drinking water. When the Singapore Public Utility Board (PUB) began planning the construction of a new desalination plant in Tuas, all these factors were considered. In the end, the utility decided not to use chlorine gas.
PUB's drinking water plants
Singapore is an island city-state located at the end of the Malaysian peninsula in Southeast Asia. The island is small, 693 square km, with a population of 4.3 million people. PUB operates five main waterworks on Singapore Island: Chestnut Street, Choa Chu Kang, Woodleigh, Bedok and Bukit Timah. Chlorination or chloramination of finished water has been the typical disinfection practice. For years PUB plants have used chlorination and ammoniation equipment, including vacuum operated floor cabinet-mounted chlorine gas feeders, gas ammoniators and chlorine residual analyzers and controls.
Installed large-scale on-site sodium hypochlorite generation system
It was in 2005, Severn Trent Services provided ammoniation equipment for the Woodleigh, Bedok and Bukit Timah waterworks, which were all using existing Capital Controls® chlorination equipment. The waterworks at Chestnut Street and Choa Chu Kang were using existing chlorination and ammoniation equipment.
PUB has four sources (taps) of water: water from Johor, Malaysia, to the north; water from local reservoirs; water from a desalination plant, one of which is currently operating and another that is being built; and "NEWater" reclaimed water that has been treated with microfiltration, reverse osmosis and UV disinfection technologies. Water from Johor is ozonated and chloraminated in Malaysia before being pumped to Singapore.
The local reservoir water is treated in the local waterworks, with primary disinfection by gas chlorination and secondary disinfection by chloramination. At the Bedok plant, ozone is used for primary disinfection. The distribution system is monitored using online analyzers to confirm the presence of a secondary chloramine residual in the system.
Using these different disinfection technologies, chlorine residuals and chloramine residuals have mixed in the distribution system, causing taste and odor problems. So PUB has used chloramination after primary chlorine disinfection throughout its entire distribution system to address the problem.
Seeking an alternative disinfection method
The construction of PUB's new 60 MGD Waterworks in Lower Seletar will include on-site hypochlorite generation for primary disinfection, in lieu of chlorine gas. And when PUB began planning its new 70 MGD (265,000 m3/day approx.) desalination plant at Tuas, it also decided to use sodium hypochlorite instead of chlorine gas.
The use of on-site sodium hypochlorite generation has gained acceptance worldwide. The disinfection process allows for a utility to develop a flexible alternative disinfection strategy throughout its plant, enabling the facility to further ensure operator and community safety, while reducing hazardous training and meeting regulations concerning DBPs (disinfection by-products).
The ClorTec on-site hypochlorite generation system uses three common consumables in the sodium hypochlorite generating process: 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. The salt solution is then passed through electrolytic cells comprising numerous titanium plates divided into arrays of cell packs, each consisting of an equal number of anodic and cathodic plates.
The cell packs are configured electrically in parallel, while the overall cell is configured electrically in series. The cell is fed DC power from a rectifier and electrolyzes the diluted brine into a sodium hypochlorite solution. In simple terms, chlorine is evolved at the anode surface, while hydrogen and hydroxide is evolved at the cathode surface. The secondary reaction of chlorine, sodium and the hydroxyl ion produces sodium hypochlorite at a 0.8% solution. The sodium hypochlorite is stored in a tank until used. Liquid dosing pumps deliver the sodium hypochlorite to the process as needed.
Interest in the technology has grown in the Asia Pacific region, too, including recent on-site installations in Malaysia, China, Korea, Vietnam and Australia.
In addition to significant improvements in water quality, the use of on-site sodium hypochlorite generation does 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.
PUB's new Lower Seletar Waterworks is scheduled to be in operation by April 2013 and the new desalination plant in Tuas is scheduled to be in operation by May 2013. Because of the board's reputation for innovation and progressive operations, many other water utilities in the region will be monitoring the Lower Seletar Waterworks and the Tuas desalination plant and the sodium hypochlorite generating systems. With the promise of cost savings, improved safety and increased reliability, it is likely that use of on-site hypochlorite generation will continue to grow in the Asia Pacific.
Author's note: Nadia Abboud is on the board of directors of the Water and Wastewater Equipment Manufacturers Association (WWEMA) and is marketing manager for Severn Trent Services.