Saturated salt water spa?

Tepelus · Jun 11, 2014

Hello everybody!
I am new to the forum and I do not have a spa yet, but I plan to get one. I am reading a lot about the pool chemistry. Few years ago during a trip to Grand Canaria I came across a spa called Dead Sea Spa with high concentration of salt. I think that it might be a good idea to try it because I do not see the need for any additional sanitizing method. The reverse of the medal is: difficult to dispose of the saturated salt water. Is this a stupid idea? Please comment.

chem geek · Jun 11, 2014

You would have a hard time finding a spa that would not have its metal components corrode very quickly in such concentrated salt water. Also, you are under the misunderstanding that there are no pathogens that survive in the Dead Sea (see this link). However, it's true that fecal bacteria from your body are unlikely to survive in such extraordinary salt levels. You would also have to be very, very careful not to get any water in your eyes. It doesn't sound like a very good idea.

Tepelus · Jun 12, 2014

Thank you for your comment. You are right. Some algae and some bacteria might survive, but in the natural environment they had many years to adapt. I plan to keep the spa covered when not in use, therefore the algae will have little chance to develop. Without algae will be less chance for bacteria to develop as well. I realize that I will have to oxidize the spa on a regular basis in order to remove the organic contaminants and that will reduce the risk of bacterial contamination. With respect of corrosion, if this spa is inert to chlorine or bromine I think it will be OK with saturated salt brine as well. Salt water in the eyes = painful experience, yes I know.

chem geek · Jun 12, 2014

How is your spa "inert to chlorine"? If you have any metal exposed to the water, it will corrode much faster with the higher salt levels. This is due both to the increase in conductivity which leads to electrolytic corrosion (even from dissolved oxygen if you didn't have any chlorine in the water) and from the higher chloride level that interferes with the reformation of the passivity layer in stainless steel. The only way this high salt spa will work for you is if you don't use any metal in the spa. So the pump impeller (actually the entire wet end exposed to water) would have to be entirely from plastic and not metal, for example. You couldn't use a gas heater unless you used a titanium heat exchanger (even cupro-nickel might not be good enough; I'm not sure if super-austentic stainless steel such as AL-6XN or super-ferritic stainless steel such as Sea-Cure would be good enough since they work for ocean-level salinity, not brine-level that you are talking about).

See the article on Marine Corrosion and consider that your saturated brine solution will have about 10 times as much salt in it (359,000 ppm) compared to the ocean (35,000 ppm) which itself is over 10 times the amount found in saltwater chlorine generator pools (3000 ppm) or spas (2000 ppm).

Tepelus · Jun 16, 2014

Thank you for your comments. Yes you are right, this saturated salt water is very corrosive. Only titanium, plastics and few other materials can tolerate it. For sure it will void the warranty for this spa, therefore I will not do it the first year. Still the fee floating feeling and the health benefits outweigh in my opinion, the trouble associate with this brine.

chem geek · Jun 16, 2014

One other factor to consider is that the very high chloride level will increase the chlorine gas concentration by 300 times or more compared to a normal spa with 1000 ppm or so salt (this would be towards the end of the water change interval). The concentration is still far lower than HOCl, but it's volatility is higher so whereas at 1000 ppm salt the HOCl outgassing is nearly 5000 times greater than Cl₂, at 359,000 ppm salt the outgassing rates are roughly equal. Chlorine gas is a much stronger oxidizer and more irritating and will also be more corrosive since dissolving in water produces very low pH due to the hydrochloric acid that is created.

So this means that you probably shouldn't try and sanitize the spa using chlorine. I agree with you that with the brine levels of salt that pathogens from your body likely won't survive, but the question becomes how to handle the bather waste from your sweat and urine. You will be soaking in that, so mostly urea as well as creatinine, ammonia and amino acids as well as small amounts of fecal matter. The filter should remove larger particles, but it will do nothing for the dissolved chemicals. So you can consider your brine spa soaks similar to reusing bath water over and over again. If you want to handle this bather waste, then you'd need an oxidizer but I don't know what you could use that would work with the brine water. Otherwise, you'll need to change the spa water and that means dumping brine solution into the environment which isn't a good thing.

Tepelus · Jun 17, 2014

Good points. Still, I know from my childhood experience that the saturated salt brine is perfect for a warm water spa. This was confirmed few years ago at a hotel in Grand Canarias. They had one. If they could do it, it means it can be done! Still it will be difficult to safely get rid of the brine when is time to do so. I can think of only three modalities to discharge this brine. First I think it will be safe to discard it in the sea. Second, I think of dilution and then discharge in the rain water drainage during a heavy rain. Third is to transfer it into an open container where it is left to dry during the hot summer days. Still, in case of a leakage it will make a mess in the garden. Some additional containment measures must be in place before I attempt any think. It will not be easy. I think that environmental considerations will stop me from doing this even if from a technical point of view this is doable and I will be left only with the option of going to the real thing.

With respect of bather contaminants, I think that a regular small dose of a strong oxidizer (chlorine or potassium monopersulfate) will do the job. In this brine this contaminants are not a health risk, but they should be removed anyway.

chem geek · Jun 17, 2014

If you use chlorine as the oxidizer, be sure to leave off the cover and stay away from the spa for a few hours. As I mentioned the addition of chlorine to the brine water will produce chlorine gas and you don't want to be around breathing that. As for potassium monopersulfate, it will oxidize some bather waste, but not all. It doesn't oxidize ammonia (to any great extent), for example.