Function and Benefits of Saline Chlorination

There is an estimated 2-4 billion gallons of water used for the enjoyment of swimmers throughout various aquatic commercial facilities. These pools are located at hotels, apartments, health clubs, and gyms to schools, universities, neighborhoods and municipalities as well as water parks. All of these pools are treated with some form of sanitizer to kill bacteria and microorganisms to prevent the transmission of disease. Traditionally, chlorine has been the primary sanitizing agent used in the industry, especially on outdoor pools. There are several chemical compositions that can create the killing form of chlorine, hypochlorous acid (HOCl). Saline chlorination utilizes electrical energy to convert an aqueous saline solution to hypochlorous acid. Saline chlorinators are simple to install and operate while producing constant free available chlorine residuals using saline solution. Additional benefits include increased swimmer comfort and reduced amount of chemicals handled and stored on-sight. There are many manufacturers of saline chlorinators, but all devices work on the same principle.

 

History of Saline Chlorination

Sodium Chloride is a natural element found in the earth, in our bodies and in our homes. It is used to season and preserve food, rinse out contact lenses, soften drinking water, and to sanitize pool water through the process of saline chlorination. A saline chlorination system uses a small amount of high grade sodium chloride. Rock salt or any sodium chloride that contains additives such as “anti-caking agents”, “yellow prussiate of soda”, or “sodium ferrocyanide” compounds should not be used since it may cause staining of the pool surface. The sodium chloride applied to the pool water goes through a process called electrolysis, which converts it into free available chlorine.

 

Saline chlorination has been around internationally for nearly 100 years in the swimming pool industry.1 There are two types of systems that convert sodium chloride into hypochlorous acid; brine tank and in-line.

Brine tank chlorination is the less common saline chlorination method that uses a two- chamber tank to produce chlorine gas. The tank is divided into two chambers with positive electrode in one chamber and a negative electrode in the other. As electricity is applied to the electrodes, chlorine gas is formed at the anode and sodium hydroxide (NaOH) at the cathode. The chlorine generated is fed through a compressor or venturi jet system into the pool return line. A typical brine cell is illustrated in Diagram 1. This system requires proper ventilation to release hydrogen gas, since it can be explosive at high concentrations. The anode chamber needs to be refilled with high concentration of saline water periodically and the cathode chamber needs to be drained. Draining the cathode chamber should be done with great care since it contains caustic soda (sodium hydroxide) as a result of the chemical process.

1 Elefritz Jr, Robert A. Comparison of Alternate Methods of Disinfection for Reclaim Effluents, Florida Water Resources Journal. July 2000, p. 19.

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Function and Benefits of Saline Chlorination

Diagram 1

 

The basic components of the popular in-line systems include an electrolytic cell, an electronic control box, sensors and probes to monitor flow, temperature, and salinity level. Many of these systems can be integrated with controllers, remote monitoring, as well as ORP, ppm concentration levels, and pH probes.

 

Diagram 2

2 Basic Pool & Spa Technology. Second edition revised. Alexandria VA:, National Spa & Pool Institute. 1992. p. 223.

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Function and Benefits of Saline Chlorination

Chemistry of Saline Chlorination

Sodium chloride is added to balanced pool water to establish a saline solution. Typically, saline chlorinators contain electrolytic cells that are composed of titanium plates with ruthenium dioxide coating. The cells differ in size and configuration depending on the manufacturer. However, the principles of their operation remain the same. Sodium chloride dissolves in the water and flows through the electrolytic cell. A low voltage electrical charge is passed through the saline solution and the current breaks the sodium and chloride molecules and results in the formation of chlorine gas, hydrogen gas and sodium hydroxide (Scheme 1). The hydrogen gas is dissolved in the water and eventually vents to the atmosphere. The chlorine gas then dissociates into the active form of chlorine known as hypochlorous acid (HOCl), which provides a residual. Hydrochloric acid (HCl) is also formed as a result of the chlorine gas dissociating in the water (Scheme 2). The sodium hydroxide (NaOH) stays in the water to neutralize the acidic impact of the chlorine gas. Since twice as many basic sodium hydroxide molecules are formed compared to the one acidic hydrochloric acid molecule, there is a tendency for the pH to increase with saline chlorination systems. Therefore, an automatic chemical feeding device dispensing acid is often added into the return line to ensure pH consistency.

2H2O+2NaCl(aq)␣Cl2(g) +H2(g) + 2NaOH Scheme 1

Cl2 (g) + H2O␣HOCl (aq) + HCl (aq) Scheme 2

During electrolysis, hydrogen and hydroxide ions are generated at the cathode and chlorine gas is generated at the anode (Illustration 1). Many electrolytic cells will reverse polarity to self-clean and prevent excess scale build-up on the plates. As with traditional chlorinated pools, the requirements for proper water balance must be maintained. This is especially important in electrolytic chlorination to aid in reducing the amount of build-up and deterioration of the cell plates. After killing the bacteria and algae, the chlorine reverts back to chloride ions, which are again converted to active hypochlorous acid as they pass through the electrolytic cell. The sodium chloride is only removed from the water through splash out, leaks, backwash, draining, and rain. It is not lost through evaporation.

Cathode ( - )

Anode ( + )

Cl- OH -

H2

OH-

O2 ,Cl2

Current of a solution of H2O and Salt (NaCl)

Illustration 1

4. Function and Benefits of Saline Chlorination

The amount of chlorine produced varies based upon the number and size of plates in the cell, the strength of the current passing through the cell, the saline concentration of the water, and pump run times. The majority of the manufacturers recommend maintaining salinity levels that range from 2,500 ppm to 7,000 ppm. These concentrations are significantly less than the saline content of ocean water at approximately 35,000 ppm.3 Tears in human eyes have a salinity level of about 8,000 ppm.4 The systems currently available produce from 1 lb to 25 lbs. of chlorine a day, allowing these systems to be effective for a wide range of pool sizes and bather loads.5 Other systems can handle larger pools by linking in series several in-line units together to accommodate the necessary sanitizer output needed to meet the pool’s demands. The saline concentration can be measured poolside using a conductivity meter or test strips. The proper free available chlorine residual can be measured using a DPD test kit, test strips, or any other approved method as determined by the state or county codes.

Saline Chlorination Components

The critical functionality of the in-line saline chlorination system is regulated through the controller box. There are many functions of saline chlorinator controller boxes. They monitor amperage to determine salinity level, contain diagnostic and troubleshooting features, which are equipped with audible/visual alarms to alert of an issue as well as boost modes for superchlorination, and contain a power supply. Most saline chlorinators are equipped with a control to regulate the amount of chlorine produced. Some systems can integrate ORP (Oxidation Reduction Potential) and pH monitoring capabilities. This allows for a fully automatic sanitation and pH controlling system.

The majority of the saline chlorinators used in the commercial segment use a hard wire installation for safety. Some units have standard 110V wall outlet plugs, but whether hard-wire or wall plug, the saline chlorinator unit power supply must be tied into the circulation system to ensure the unit does not operate if there is no circulation. All local, state and federal regulations should be followed to ensure proper installations are performed. Most manufacturers highly recommend a licensed professional install the units.

NSF ensures the design features of the approved electrolytic generators are in compliance with the NSF/ANSI Standard 50, Circulation System Components and Related Materials for Swimming Pools, Spas/Hot Tubs protocol. This testing includes accuracy of chemical output rates specified by the manufacturer, physical evaluation of the components for

3 Van der Leeden, F., F.L.Troise, and D.K. Todd. The Water Encyclopedia, Second edition. Chelsea MI: Lewis Publishers 1990. p.237. 4 Michail, D. Rev. Sud-americana Endocrinol. Inmunol. Quimioterap. vol. 21, pp. 130 – 1, 1938. (CA 32:304919) 5 NSF/ANSI Standard 50/ Circulations System Components and Related Materials for Swimming Pools, Spas/Hot Tubsfrom the Hwww.nsf.orgH

5. Function and Benefits of Saline Chlorination

suitability in swimming pools as well as longevity of the system components, automatic shut off in case of power failure and visual and/or audible alarm to warn of cell issues to help ensure safe operation. Alarms will identify if cell voltage is out of range or if the saline concentration falls below the recommended minimum level.6

Saline Chlorination Benefits

Saline chlorination systems have several benefits over using traditional chlorinating chemicals.

␣ From a safety standpoint, the NFPA (National Fire Protection Association) does not set restrictions for maximum storage capacity of sodium chloride and the DOT (Department of Transportation) does not regulate the transportation of sodium chloride. Since saline generators reduce the need to store chlorine based chemicals, more valuable storage space is available, and with a small area needed for the electrolytic cell, there is less space needed for the sanitizing system as compared to large traditional chemical feed systems found in many pump rooms.

␣ The saline chlorinators only require infrequent maintenance-level sodium chloride applications into the pool to produce chlorine. This reduces the need to continually add chlorinating products to the water, thus making this system low maintenance. As the hypochlorous acid oxidizes undesirable compounds and kills bacteria, it leaves the chloride ion behind. This chloride ion is now available to enter the electrolytic cell and be converted back to hypochlorous acid. This efficient recycling of the chlorine is continuous; therefore, only occasional maintenance additions of sodium chloride are needed.

␣ Another low maintenance benefit is the self-cleaning function of the cell. By reversing polarity, in which the anodes and cathodes switch electrical charges back and forth periodically, there is a reduction in the accumulation and scale buildup on the plates. The majority of the units today use an automatic step down transition to lower the current then switch the charge on the plates. This means less frequent cleaning. The cell should be inspected according to the manufacturer’s recommendations. This usually occurs every few months to ensure it is working at optimum efficiency. If scale is noticeable on the electrolytic cell, the plates can easily cleaned by soaking the cell in a mild acidic solution then rinsing with water. When compared to continuous filling and cleaning a calcium hypochlorite feeder, due to the build up of calcium or scale at the influent and effluent injection points, the saline chlorination system is almost maintenance free.

␣ An additional benefit to saline chlorination versus dry calcium hypochlorite feeders includes high calcium levels, which can cause complicated water balance leading to scaling and cloudy water.

6 Pool and Spa Bulletin. Winter 2004. Ann Arbor, MI, NSF International.

6.Function and Benefits of Saline Chlorination

␣ Manufacturing chlorine on site eliminates the need to routinely handle liquid chlorine, granular chlorine or chlorine tablets. It also saves time from having to routinely apply chlorine chemicals or fill chemical feeders.

␣ These systems have a high efficiency production of chlorine with low operational cost. Cost is primarily associated with initial purchase and installation. Sodium chloride itself is very economical and the other costs associated with these systems include electrical supply and balancing chemicals. Savings are also realized by not having to ship, handle and transport traditional chlorine products. Automated systems save on down time, as well as time of adding products to the pool, thus saving money.

␣ Controller boxes and systems have additional audible/visual alarms features to make maintenance easy while protecting the electrical components of the system. Some of these features include automatic function for shut off in case of low temperature, no flow, or low salinity levels. Some systems have special alarms for high salinity levels as well as other special features.

␣ The saline concentration and temperature play an important function in resistance on the system. Low temperatures, usually below 45oF, may cause destruction to the ruthenium coating; therefore, an automatic shut off switch will protect the coating on the cell plates. Resistance is measured in ohms. The higher the saline concentration or the higher the temperature, the higher electrical resistance, thus putting stress on the system. The resistance can cause the control box to overheat and cause damage to the control board, although some units have been designed to function at higher saline concentrations.

Ohms Law: R = V/ I Resistance = Voltage / Amps

Power Formula: P = V*I Power = Voltage * Current

Resistance is measured in ohms (R) Difference in potential is voltage (V) Current though the resistance and measured in amps (I) Power is measured in watts (W)

␣ Automatic shut off switches, for no or low flow, are usually standard to prevent damage to the equipment. Inspection of the cell is recommended when low or no flow is detected and cleaning may be necessary.

␣ Maintaining a proper salinity level is recommended for maximizing the longevity of the cell plates. Some units are equipped with an automatic shut off for high salinity levels. High salinity levels cause

7. Function and Benefits of Saline Chlorination

the voltage to increase, which then increases the resistance. Depending on the manufacturer, when the current has reached its maximum amperage level the current limiter will shut off the power supply to the unit. Units that do not have current limiting capabilities will have electronics failures if the saline concentration is too high. There are other units that are configured in such a way to handle high salinity levels. Some units that allow for production at high salinity levels will have a current limiter and/or a transformer sized so that power outputs only reach certain levels. These measures protect system electronics from becoming damaged.

␣ When a low salinity is detected, it means there is more oxygen than hypochlorous acid present in the cell column, which can destroy the coating and result in low chlorine production. Most saline chlorination manufacturers require a minimum level of saline to provide protection of the cell plates. If a low saline level is detected, the unit will automatically shut off.

Saline chlorination’s future is on the rise for commercial swimming pools. This is due to benefits of reduced management and system maintenance time, enhanced swimmer comfort, and reduced handling, storage and transporting of traditional chlorine chemicals, reduction in pool chemistry adjustments and the ease of pool automation. An additional benefit of using a saline chlorination system is the monitoring and troubleshooting capabilities within the controller box. The primary benefit of using a saline system is the continuous chlorination from the regeneration of saline to hypochlorous acid, which kills algae, microorganisms and oxidizes contaminants.