Trying to find out information on how phosphates effect the operation of salt generators? We are having problems with a salt generator that is oversized for the size of pool that it is installed on, it is controlled by and ORP controller. The generator will not produce enough chlorine to maintain an appropriate residual amount of chlorine in the pool.
We have tested the phosphates in the pool and have a level of around 1850 ppb and we are adding a phosphate remover to lower the levels down.
Any information would be appreciated.
You can read more about Phosphates and Phosphate Removers. The phosphates are not directly doing anything to interfere with chlorine production, but they do provide algae with nutrients to grow so if your Free Chlorine (FC) level is too low relative to the Cyanuric Acid (CYA) level, then algae can grow faster than chlorine can kill it and that can appear to be mysterious chlorine demand if the algae has not grown enough to become visible. Higher phosphate levels will make such algae growth faster, though there is a limit to such growth rates. I had over 3000 ppb phosphates in my pool as have others and yet we were all able to maintain chlorine levels, some with manually dosed pools while others had saltwater chlorine generator (SWCG) pools.
There are several other factors. First is that the Cyanuric Acid (CYA) level may be too low for the outdoor pool. SWCG manufacturers often recommend higher CYA levels, though that may conflict with local/state regulations if this is a commercial/public pool. You say that the salt generator is oversized for the size of pool, but it really depends on the bather load, not just the size of pool. Also, don't forget that SWCG ratings are 24 hour ratings, but during the day the rate of chlorine depletion is higher due to both sunlight (UV) exposure and to bather load.
If your Free Chlorine (FC) level is not at least 5% of the Cyanuric Acid (CYA) level, then this will not be a high enough chlorine level to kill algae faster than it can grow, regardless of phosphate level. Also, if algae growth has already started because you got behind with too low an FC/CYA ratio, then it takes a higher shock level of FC to get ahead of such growth. There is no question that a pool high in phosphates is more "reactive" in this way and therefore can seem harder to manage, but it CAN be managed if you beat off the algae and then properly maintain a sufficient FC/CYA ratio. Some people find it worth spending money on phosphate removers or algaecides while others would rather just maintain the right chlorine amount.
Why don't you give us more specifics and we can figure out some details. What is your target Free Chlorine (FC) level? What is your Cyanuric Acid (CYA) level? What is the size (volume) of the pool? What is the chlorine production rating for your SWCG? What is the bather load for the pool in person-hours per day?
There are some theoretical reasons for interference with Cl production of salt generators from high phosphates. So far in the pool industry no one has done any hard data work on this situation. Some theories are that since ortho-phosphates are used as anti-corrosion in iron pipes they may have an affinity to cling to metals and very possibly to the metal cells of the generator. Another theory is that since phosphates act as a nutrient for algae growth the higher levels could cause algae spores present in the water to begin the blooming process thus creating a higher Cl demand from this process. Lastly phosphates are also a part of TDS total dissolved solids and would be detrimental as opposed to the TDS from the salt which is needed by the generator. What is known anecdotally is that when levels of phosphate reach above 500 ppb the efficiency of the generator goes down. Generator manufacturers, phosphate remover manufacturers and the pool organizations such as NSPF and APSP should come together and research this further to obtain more factual data!
Though I agree that there may very well be theoretical reasons for direct interference with salt generator materials, as there is with high sulfate levels (though those are orders of magnitude higher before a problem is seen), there has not been a single pool out of any of the saltwater chlorine generator pools with high phosphates (> 500 ppb) at Trouble Free Pool (nearly 30,000 pool owners on that site) that was not able to be controlled by using the proper FC and CYA amounts. In addition, we had people measure FC levels out of the returns and they were just as expected, which would not indicate a problem with chlorine generation.
Also, the corrosion inhibitor levels for orthophosphate are typically far higher than the few ppm levels seen in pools with high phosphates and it isn't orthophosphate itself that clings to pipes, but rather the calcium phosphate precipitate/solid that is formed. It's just another form of metal surface protection by formation of scale, though using calcium phosphate instead of calcium carbonate so as not to require as high a calcium level. Sometimes zinc is added to form zinc phosphate scale since that precipitates even more readily than calcium phosphate and this is typically done using 9-18 ppm orthophosphate with 1.5-3 ppm zinc. So a pool high in zinc and high in orthophosphate might produce more scaling. Organic phosphates (phosphonates) as well as some polyphosphates inhibit metal corrosion by cathodic inhibition rather than forming scale. Also note that with polarity-reversing cells, the scale would have to form quickly and thick enough to prevent it from being redissolved under the acidic conditions when cell polarity is reversed. That is, the problem is not that much different than traditional calcium carbonate scaling.
I agree that research should be done, but until then, the actual experience would indicate that the primary problem is with feeding algae growth and not having a high enough FC/CYA ratio to kill it or not having a high enough CYA level to keep up with sunlight. I can tell you with absolute certainty that the traditional industry recommendations from the SWCG manufacturers of 1-3 ppm FC with 60-80 ppm CYA is NOT sufficient to prevent algae growth when nutrients are present. The minimum FC level is 5% of the CYA level while the shock level if you've already got significant algae growth (even if not yet visible) is far higher, but once you get ahead and kill off the algae, the maintenance FC/CYA level works. Now, that said, a pool is much more sensitive to getting into trouble if not properly maintained when the phosphate level is higher so lowering phosphate levels can be seen, like algicides, to be insurance just in case something goes wrong.