Saltwater systems have become extremely popular over the past five to 10 years. The system’s popularity, however, began with pools, to improve the swimming experience while simultaneously reducing the number of traditional chemicals needed to sanitize the water. Add to this the safety benefit of eliminating the need to store large quantities of chlorine on-site.
Now that salt chlorine generators are universally accepted and proven to work wonders on pools, the technology has expanded to include systems for hot tubs. This water treatment system makes a lot of sense for smaller bodies of water, such as a hot tub as users prefer to soak in the water for health and relaxation and are not interested in immersing themselves in a tub of chemicals.
As retailers and service professionals encounter salt chlorine generators on hot tubs more frequently, it is important to understand how these systems work and how they differ from those systems used on pools. With this knowledge, the industry will be better prepared to service these hot tubs and pro-actively deal with issues that arise from the use of salt chlorine generators.
What is a salt system?
First, it is important to understand how salt chlorine generators work and how this method of water treatment is applied to pools and hot tubs.
Salt systems use a method known as electrolysis, which splits atoms of molecules apart in solution to create active chemicals. The chemicals produced are based on what the molecule is in the solution. In 1800, two scientists discovered the water molecule H2O could be split into active oxygen (O) and hydrogen (H) through electrolysis. Electrolysis has been used in treating drinking water and wastewater since the early 1900s. Recently, the process of splitting salt molecules to create active sanitizer in hot tubs has taken off. The two most common molecules for hot tub water treatment are sodium chloride (NaCl) and sodium bromide (NaBr). These two halogen salts are split by electrolysis to create active chlorine (Cl) or bromine (Br).
How does it work in a hot tub?
Salt generators work to produce active sanitizer in water. The term ‘salt system’ is a misnomer as it leads to the conclusion this is a non-chemical system. In fact, salt water is part of a process to produce active sanitizer. Sanitizers are needed (required) in hot tubs to disinfect against pathogenic bacteria and viruses present in water as a result of bather use. Effective sanitizers for hot tubs will act to sanitize, oxidize, and leave a small residual behind in the water. The two most common and effective sanitizers for this purpose are chlorine and bromine, which are both categorized as halogens. This means they are created from inert salts and, after they have reacted in water, they return to salts again.The word halogen comes from the Greek words meaning ‘salt-former.’ Most salt generator systems work by taking a simple solution of either sodium chloride or sodium bromide that is circulated through a device with two cells made of a noble metal—platinum or titanium are two of the most commonly used. A low-voltage electrical charge is introduced through the cells. One cell is defined as an anode with positive charges, while the other is known as the cathode with negative charges. Chloride (Cl−) is a negative-charged atom that bonds to the positive-charged sodium (Na) atom (Cl−+ NA+= NaCl).
In solution, the negative-charged Cl atom is pulled toward the positive-charged anode and the positive-charged Na atom is pulled to the negative-charged cathode. After which the molecule is split. At the anode, the chloride is oxidized to chlorine. Sodium is pulled towards the negative-charged cathode where hydrogen is being created. The atoms of Cl and Na rejoin after this process to once again form NaCl sodium chloride.
As long as the NaCl sodium chloride molecules are present in the water, the reaction of creating active chlorine is continuous. Bromine is created the same way. NaBr sodium bromide in solution passes by the anode and cathode. At the positive-charged anode, bromide is oxidized to active bromine, while at the negative-charged cathode, sodium hydroxide (NaOH) is created. The positively charged anode of a salt generator always produces oxygen, which acts to oxidize either chloride or bromide. As these units also produce oxygen, there is increased oxidation, which breaks up combined chlorine and helps to reduce waste. Increased oxygen leads to better water quality and less draining. At the cathode, hydrogen is created. Sodium mixes with water and hydrogen to create sodium hydroxide along with some hydrogen gas.
How do salt systems for hot tubs differ from standard units?
Traditional hot tub water needs to be properly balanced to keep the hot tub free from damage (e.g. scale and corrosion). The elements of balanced water are calcium hardness, total alkalinity (TA), pH, and total dissolved solids (TDS).
In traditional hot tubs, calcium hardness levels should be maintained between 150 and 400 parts per million (ppm). In salt water hot tubs, the calcium level can be maintained more towards the lower end. Some manufacturers recommend calcium hardness levels be no higher than 50 ppm. In these cases, they provide a device for lowering calcium hardness with the salt generating system. Higher levels of calcium in salt systems can cause scale to form on the metal cells and reduce sanitizer output. This is a specific problem in hot water environments because calcium precipitates into a solid much quicker.
Total dissolved solids (TDS)
Because of the high TDS and the production of sodium hydroxide in the system, the total alkalinity will also increase. Therefore, the total alkalinity in a salt system can be maintained at a lower level as well. Many manufacturers of salt systems recommend no more than 80 ppm of total alkalinity. Normal total alkalinity levels usually range between 80 and 120 ppm.
pH in salt pools tends to stay fairly balanced due to the fact an acid and base are produced and mixed from the reaction of the cells. The ideal pH for salt systems is 7.4. High TDS in a standard hot tub is undesirable. However, in salt systems, there will be an increased TDS level from the sodium. This is beneficial for the creation of the sanitizer. TDS of the fill water should be tested and written down before adding salt or sodium bromide to the water. TDS should be retested after adding salt or sodium bromide. Afterward, TDS should be tested monthly. Whatever the TDS reading is, the difference should be subtracted between the fill water and the test after salt or sodium bromide was added. If the TDS reading is 1500 ppm over the fill water, then it is time to drain and refill. The recommended TDS salt level is 1000 ppm.
For both sodium chloride and sodium bromide systems, the salt levels can run anywhere between 1000 to 2000 ppm. Therefore, it is important to always check the manufacturers’ directions for details on proper salt levels.
Dealing with metals
In salt water pools there is a greater propensity for corrosion and staining from metals. These metals can be in solution in the source water used to fill the pool and or hot tub. Some examples of dissolved metals that can be present in water include iron (Fe), copper (Cu), and manganese (Mn). Also, depending on the type of system, there could be some copper present that could corrode. The reason for this in salt water hot tubs is due to galvanic corrosion. In a high TDS solution such as a salt system, the water is highly conductive. This means electrical charges travel more quickly and efficiently.
With the presence of a low-voltage form of electricity in solution with dissimilar metals there is a greater potential for staining. As pointed out previously, most salt systems use platinum or titanium for the anode and cathode cells. Both of these are high on the list of noble metals. This means they are the least likely to corrode in an environment where other metals may be present. Iron, copper, and manganese are all lower on the list of metals; therefore, if any of them are present in the water of a hot tub using a salt chlorine generator, they will act as a sacrificial anode and corrode more readily.
The stains from galvanic corrosion of these metals will appear brown, turquoise, black, and purple. The use of metal sequestering chemicals can help, along with the use of a zinc anode in the system. Zinc is one of the lowest grade metals on the list of noble metals so it will corrode more readily without staining surfaces.
What else is needed?
Saltwater hot tubs actually should allow for fewer chemicals needed to maintain proper water quality. Slightly more acid may be needed to keep total alkalinity levels from drifting upwards. Also, high levels of phosphates (PO43−) can cause interference in salt systems that reduce the output of sufficient sanitizer. For this reason, monthly phosphate testing should be performed.
Phosphate levels should be maintained below 500 parts per billion (ppb); 200 ppb is ideal. Phosphate removers are available to rid excess phosphate that may be present in fill water. Also, extra metal protection is needed in salt systems to ensure any metals in source water do not drop out and cause staining; therefore, be sure the metal product is a phosphate-free formula. Clarifiers can help to floc small suspended particles and keep water clear as well as help keep the cells cleaner.
Benefits of salt
There are great benefits to a saltwater hot tub system. The main benefit is the continuous production of an active sanitizer. Many users also claim the water feels softer, there are no irritating chemical smells, and less draining is required. Salt or sodium bromide hot tubs offer one striking benefit over standard units—they provide pure chlorine or bromine, unlike granular chlorine which contains fillers and stabilizers, or bromine tablets which comprise fillers and preservatives. The byproducts of packaged chlorine and/or bromine end up in the water and the buildup leads to increased TDS, poor water quality, and greater draining frequency.
It is important to realize hot tub owners want to reduce their overall chemical usage and are turning to alternative sanitizers such as salt chlorine generators and ultraviolet (UV) systems. In fact, many hot tub manufacturers are now including UV sanitizers within their original designs. Further, several manufacturers are also offering retrofit chlorine generators that simply ‘drape over’ into the hot tub water. These units generate the sanitizer needed—with a simple tap on the device—to keep the water clean, while also reducing the number of traditional chemicals needed.
What about draining?
Great care should be taken when draining salt system hot tubs. First, the chlorine or bromine levels should be allowed to drop to as close to zero as possible. The salt generator should be shut off and the hot tub should be left uncovered for a day or two to allow the active sanitizer levels to drop before the water is drained. The best place to drain the water is directly into the sanitary sewer through a clean out located beside the exterior of most homes. If a clean out cannot be located, the pump hose can be placed inside a bathtub or toilet. Be sure to use a low-flow sump pump to prevent backflow. The pump flow should be no more than 75.7 liters per minute (lpm) (20 gallons per minute [gmp]).
Water should never be drained onto lawns or into gardens as the sodium will burn the grass and kill plants. Any type of hot tub or pool water should never be drained into storm drains. There are strict regulations against draining chemically treated water into the storm drain. Storm drains lead directly to creeks and streams and eventually end up in larger rivers and lakes. Thus, it can have a detrimental effect on fish and wildlife in the surrounding eco-system. Hot tubs and pools should also never be drained directly into a creek or stream. Two years ago, more than 1,000 wild salmon were killed in a stream in Vancouver, B.C., when a pool contractor inadvertently drained chlorinated pool water into a storm drain that led directly to the stream. In many areas in Canada and the U.S., there are fines and penalties for draining pool and/or hot tub water directly into storm drains or streams. Fines can range up to $2,000, or in some cases even imprisonment.
Saltwater hot tubs may be emptied onto drain fields or rock pits, but this should be verified with local authorities first. There are some who claim the sodium bromide treated pools can be drained safely onto lawns without any risks of burning or bleaching the grass. In this case, however, it may be wise to test a small inconspicuous area of the lawn first before doing a complete drain.
Keeping salt system hot tubs clean and maintained is fairly simple, too. A bit of acid may be needed from time to time to keep alkalinity levels down. Phosphate levels should be checked and managed monthly. For metals, a phosphate-free metal treatment should be used to sequester the metal ions. Also, a natural clarifier should be used weekly to keep the water clear, while also effectively removing any organic solids to the filter.
This article was written by Terry Arko and originally appeared on Pool & Spa Marketing [link].