How salt generators work

It has been said that, "There is nothing new under the sun." While salt generators have surged in popularity in the last decade, the technology goes back to 1800. Scientists back then discovered the technology of splitting molecules in a solution using what is called electrolysis. Salts like sodium chloride or sodium bromide are used in a solution that is subject to low voltage electrical current between a pair of cells with opposing charges. One cell, the Anode, contains positive charges and the other, the Cathode, contains negative charges. Electrical ions flow back and forth between the two cells. The result is that molecules are split and chlorine gas is produced at the anode. Hydrogen gas is produced at the cathode. Salt generators produce hydrogen gas, chlorine gas and a solution of sodium hydroxide. When salt generators are working properly they are continuously producing free available chlorine at set levels.

What happens when salt generators fail?

When a unit begins to fail it will not produce sufficient free chlorine to keep up with demand. There are numerous reasons for failure including dirty or calcified cells that need to be cleaned, no power to cells and insufficient levels of salt in the water. There is one main factor to consider first when a salt chlorine generator fails, and that is the presence of phosphates in the pool water.

When levels of phosphate exceed 500 ppb the unit can cease to produce enough free available chlorine to keep up with demand. Most manufacturers of salt chlorine generators will confirm that when there is a problem with production of free available chlorine, a phosphate test is recommended. If the phosphate levels are near or over 500 ppb, a phosphate removal treatment is advised to help the salt chlorine generator function properly.

A typical example of salt generator failure is what happened recently to Steve White, owner of Underwater Pool Masters in Massachusetts. He installed a new salt chlorine generator at an outdoor commercial swimming pool but could not get a decent chlorine reading for over two weeks after the installation. “The situation was such that we ended up having to continue to shock the water to sanitize the pool until we could make sure the new salt generator was producing the optimum chlorine levels,” he said. 

The phosphate connection

So, what is going on when phosphate levels climb in a salt pool? We need to look to the industrial uses of orthophosphate in water treatment. First of all it is important to understand that orthophosphate is the detrimental form of phosphate that exists in water. Orthophosphate will not only interfere with salt generators, it can also cause excessive algae blooms to occur in both traditional and salt pools as well.

A product known as zinc orthophosphate is used in drinking water systems because it adheres to metal pipes and acts as an anti-corrosion agent. So, from this we know that orthophosphate likes to cling to metals. The real interference of phosphates in chlorine generators is still somewhat theoretical. It appears that since orthophosphates attach to metals they attach to the anode and cause an interference with the flow of electrons between the anode and the cathode of the salt chlorine generator. We do know that higher levels of orthophosphate seem to cause a definite interference with the normal operation of the salt chlorine generators. 

Where are the phosphates coming from?

Phosphates can be introduced into swimming water from a multitude of sources including: fertilizers, organic debris and soil, detergent cleaners, tile cleaners and metal sequestering chemicals. Also, phosphate can come from human perspiration and urea. 

There are several different forms of phosphate depending on the source; all however eventually end up in the form of orthophosphate. Metal products can be one of the main culprits which cause failure in salt chlorine generator pools. This is because staining from metals is more prevalent in salt chlorine pools due to the potential of galvanic corrosion from high TDS of the salt and dissimilar metals present in the water. Most manufacturers of salt chlorine generators will recommend a metal sequestering product be used.  Most metal sequestering products available for pools are phosphonic or phosphoric acid based formulas. When these are added to a salt pool, the result will be a breakdown of the phosphate to free orthophosphate which is what will cause the problems with cells in the chlorine generator. For this reason it is best to use a phosphate free metal sequestering product.

White's experience at the pool in Massachusetts turned out to be the result of high phosphates introduced by the city’s water supply. The swimming pool in question used approximately 15,000 gallons of water which was replaced at the time of installation of the salt generator. About two-thirds of the new water added was delivered by truck while the remaining one-third came from the local town water supply. This community is known to have high levels of metals in its water — especially iron. As a result, the community regularly adds metal sequestering agents which are high in phosphates to avoid having the metals from pipes cause unsightly metal stains in sinks and toilets.

In doing his detective work to figure out why he wasn’t getting a good chlorine read, White checked the water in the bathroom of this pool facility and found that the phosphate levels were 1,500 ppb. He immediately started treating the pool with phosphate remover, and brought the phosphate levels back down to 100 ppb. This solved the problem and quickly restored chlorine production from the unit.

Removing the phosphates

Phosphates can be removed from the pool water by a simple use of a phosphate remover. It is important to understand that orthophosphate in pool water exist in a soluble form. The most effective phosphate removers work by making the soluble orthophosphate precipitate out as a solid. This will cause some cloudiness to the water which can be filtered out readily with the use of a clarifier. However, keep in mind that the more phosphate, the more cloudiness there will be. Also, in extreme cases it can take up to 2 to 3 days for the cloudiness to clear completely. It is important during this cloudy period to have the salt chlorine generator turned off until the water clears. During this period liquid chlorine can be used to keep the residual up. The other option for extreme levels of phosphate — when levels are near or over 5,000 ppb — would be to drain and dilute some or all of the water.  The drain and dilution method may be better used for commercial pools where shut-down time is limited and cloudiness of the water is regulated. In these cases it is advisable to dilute out as much phosphate as possible and then use maintenance doses of phosphate remover to keep levels managed.

Keeping them out

It is important in a salt chlorine pool to be diligent about keeping phosphate levels down. The following are some guidelines that can help: 

1) Test water for phosphates on a weekly basis. 

2) Treat with a phosphate remover weekly to keep levels down. 

3) Use only non-phosphate metal products and cleaners. 

4) Clean and remove grass, leaves and any organic debris from pool as quickly as possible.

5) Test tap water to see what levels of phosphate are in the source. 

6) Test for and treat for phosphates after any periods of extreme weather or heavy use. 

7) For commercial pools, avoid using any phosphate-based cleaners on the decking or tile.

8) Operators should discourage patrons from visiting the pool after swimming in nearby lakes.

9) Care should be taken when fertilizing lawns and plants near pool area.

10) Source water should be tested for phosphates as well. Many water municipalities treat with straight orthophosphates at different times. If this is the case then phosphate treatment is recommended at filling and whenever topping off.

Winterizing A Salt Generator Pool

The method for winterizing a salt generator pool is not that different from a regular pool. In areas where winter is extreme and a hard shut down is used, the pool should be drained down and winterizing chemicals added as usual. The salt generator should be deactivated and in some cases disassembled and stored for the winter. Check with the generator manufacturer for specific winterizing instructions. In some areas a complete shutdown may not be necessary. In these areas you would simply turn down the hours of filtration operation and reset the generator to run in accordance with that schedule — typically this is 2 hours. For winter water temperatures, there should be sufficient chlorine produced from the unit in two hours a day.

Knowing he underlying water chemistry is pools is a key component to making sure that salt generators work properly.  In a salt chorine pool, keeping the phosphates low is the secret to letting the free chlorine flow!

Terry Arko is AQUA Contributor of AQUA Magazine.
This is one of the best explanations of how salt generators work. Most pool sites dumb it down to the point of being useless. My salt generator electronics board died so I removed it and simply hard wired the cell thru a rectifier and DPDT polarity reversing switch. After that I noticed that the cell only generated chlorine in one polarity. So I gather one "polarity" of the places was affected by phosphates. I did clean the cell with CLR (used to remove rust and calcium in home) I will try with hydrochloric acid.(the science name for muriatic acid!) Does anyone have a link to more scientific information? Things such as how often to change polarity (I figure I do it manually every morning) and what voltages to use. This particular unit has a center tap 24 volt transformer but only applies 12 volts to the cell.. Is there a higher voltage cell rejuvenation cycle?
Correction for Kate: That should be 500 PPB of phosphate in SWG NOT 500 ppm.
In response to your question. Many SWG manufacturers recommend maintaining levels below 500 ppm of phosphate.
Terry, thank you for the article. Do you have any new data on recommended "real world" levels of PO4 from SWG manufacturers? I'm quite curious because I am on well water, and will be switching to SWG after a major removal of PO4 to the order of 25,000 ppb. Historically, I have been able to maintain clear, algae-free water by keeping my FC at 7.5 - 10% of my CYA range.
Terry, Thanks for your response. I made mistake with my decimal point when I said 300ppm I should have said 3000ppb. We find the D.C. municipal water (also used in Arlington, Falls Church and some parts of Fairfax, Va. ) to be pretty consistent at 2500ppb. We do a good job of keeping our service route pools with a 1.0-3.0 chlorine level and phosphate levels are not a factor unless an unusual load from soil runoff, or something jacks it up. As far as phosphates reducing the effectiveness of chlorine output from a chlorine generator goes - how do you know that's happening? If our service tech finds the chlorine level a little low from week to week he increases the output a little. At some point in time the output is might set at 100% and the chlorine level starts to fall unacceptably low anyway, so we test the water for everything. The usual culprit is too low cyanuric, but if that isn't the problem a chlorine demand test finds that there is a demand. I don't know if phosphates are detected in the chlorine demand test but the treatment of the chlorine demand condition almost always resolves the issue. I think we only had one service pool (out of 700) last year that required (as a last resort) phosphate removal treatment. You are right that pool owners who do not have good service routines start running into problems with algae blooms if there are phosphates in the water and these are difficult recover from. When we diagnose the problem we ask the customer about his maintenance procedure and almost always we find that he is not adding a maintenance or preventative dosage of algicide. I am still searching for a level of phosphates which can be labeled "too high", but it would appear that 3000ppb is in the "acceptable" range. Paul
One correction and some clarification on the attaching of orthophosphate on the unit cells, I should have said that the orthophosphate could cause interference at the anode NOT the cathode. My references on zinc orthophosphate in drinking water pipes was from the EPA and there is plenty of information on this on their website for any one who is interested in researching. Most chlorine generator manufacturers have references to phosphate removal and proper levels in their user manuals. Terry Arko SeaKlear
Thanks Paul! Your question and concerns are very valid. The phenomenon of phosphate interference with salt generators in pools is still relatively new. There really isn't to my knowledge any direct scientific data related to results and exact measurements in swimming pools. The reports and solutions at this point are mainly anecdotal from the field. As with many problems in the pool industry professionals in the field many times are ahead of those in the lab in the observance and solutions to various water quality challenges. You yourself have collected your own set of data just in the fact that you have seen hundreds of pools that you have tested levels under 300 (ppm or ppb?) with no observable problems. In all 15 + years that we have worked with phosphates and phosphate removal we have found that levels around the 200 ppb range are fine. Once above 500 is where in salt water generators problems may be reported. This is based again on field observations reported to us and to chlorine generator manufacturers. The one reason I recommend a weekly or at least some sort of regular phosphate removal system in salt water pools is because increasing levels of phosphate are a continual maintenance issue based on the fact that phosphates can be contributed to pool water from so many sources. From our experience in many areas phosphate levels need to be managed or they tend to spike up. Many times the source of the phosphate contribution is not always easy to find. So as to your question on the acceptable maximum level of phosphate there are many "experts" who will say that in regular chlorine pools the level of phosphate doesn't matter because algae can always be controlled by simply keeping proper chlorine levels and using algaecide. While they may be right, the question to pose is how many pools are kept at the perfect Cl levels 24/7. There are lots of demands on chlorine particularly in season and so if the Cl level goes down and isn't dealt with quickly you have more risk of a quicker outbreak of algae due to an abundant food supply present. We do know however that when the phosphate levels reach 500 ppb+ in salt/chlorine generator pools there is a reduction in output of free available chlorine. You are correct that the answer is not zero and pools will NEVER be able to maintain zero readings of phosphate for any feasible length of time. Phosphates are pretty much omnipresent. They can exist and be contributed to pool water from the source water, airborne fertilizers, cleaners, pool chemicals, animals, organic debris and swimmer waste (sweat and urine). If you do some searching in lake and pond industry you will discover that the problem of phosphate pollution is a huge one overall and one of the biggest challenges in water sustainability.
Terry, A very good explanation of the orthophosphate problem with salt chlorine generators. However, i do take exception with one part of your recommended treatment schedule: Item #1 - test for phosphates - Good. #2 - treat with phosphate remover weekly - Why? I am a big believer that professional pool operators should test and then apply the appropriate corrective chemical when needed. Simply applying phosphate remover every week is unnecessary if the phosphate level is low enough to be acceptable. My frustration is that no one seems to be able to get a concensus on what level is "acceptable". That opens the door to the idea that any level above zero is acceptable and the use of phosphate remover is required. I have tested hundreds of pools with low levels of (under 300ppm) which had no problem maintaining chlorine level, no algae bloom, no algae whatsoever. So, no phosphate remover was needed. So, the big the question for the experts is "What is the acceptable maximum level of phosphate in swimming pool water?" and the answer is not zero.