Why Can't Water "live" Forever?

[Whoneedsavacation]

So we put sanitizer in the water to kill little beasties and the sanitizer gets used up at the same rate we put it in, so all we are adding, net, is dead beasties which presumably get filtered out. We put in shock, but the result is ditto, we aren't building up a residue of shock if we are doing things right. We put in soaps and oils and salts and more beasties from our bodies and clothes, but these either get killed by the sanitizer, broken down by the sanitizer and shock and/or enzyme products into smaller/inorganic compounds and get filtered out either with or without the help of a flocculent. The filter gets cleaned periodically. Do we need to change water every so often because none of the these processes, especially the filtration, are perfect? Or if we got close to perfection in our maintenance routines could we make our water last a very long time?

[tony]

So we put sanitizer in the water to kill little beasties and the sanitizer gets used up at the same rate we put it in, so all we are adding, net, is dead beasties which presumably get filtered out. We put in shock, but the result is ditto, we aren't building up a residue of shock if we are doing things right. We put in soaps and oils and salts and more beasties from our bodies and clothes, but these either get killed by the sanitizer, broken down by the sanitizer and shock and/or enzyme products into smaller/inorganic compounds and get filtered out either with or without the help of a flocculent. The filter gets cleaned periodically. Do we need to change water every so often because none of the these processes, especially the filtration, are perfect? Or if we got close to perfection in our maintenance routines could we make our water last a very long time?

With almost everything we add to the water, something is left behind. Chlorine, MPS, Spa Up, Spa Down, etc all leave something in the water what we collectively call TDS or Total Dissolved Solids. TDS and cyanuric acid are two main reasons water needs to be changed. It is interesting that you brought this subject up because I just changed my water last weekend after five months. It was my first full season using bleach in my spa and I honestly felt I didn't need the change. I've felt this in the past using dichlor but when I drained and filled, I could see how my water had deteriorated over time and lulled me into thinking it was still good. Of course, there is nothing like new water. This time, though, I found no difference. At the three month mark I was noticing how nice the water still was, but I needed to verify this with the water change. My TDS levels seemed extremely low for five month old water. I normally change my water every four months. Lack of TDS and of course CYA (which also adds to TDS) is what I summize accounts for the difference. Needless to say, I will continue with my "experiment" another season to see if the results continue.

[chem geek]

The Total Dissolved Solids (TDS) by itself is not a problem nor does it make water dull. It is WHAT the TDS is composed of that makes the difference. When you use bleach or chlorinating liquid as your main source of chlorine, most of the chlorine that gets used up gets converted to chloride ion so the net result is simply an increase in the salt (sodium chloride) level in the water. Salt is very soluble and doesn't cause "old" water issues nor problems unless it gets really high (saltwater chlorine generator pools are at 3000 ppm, for example, and most spas won't get above 1500 ppm even when using bleach). Just figure that with bleach, for every 10 ppm FC that gets added, it also adds 16 ppm salt when accounting for addition AND chlorine usage.

If you use Dichlor, then the Cyanuric Acid (CYA) builds up and that's an organic molecule that might not be quite as well dissolved as salt in terms of not refracting light to make the water look dull (though I do not know for sure). Most of the components of sweat and urine (mostly urea and ammonia plus some amino acids) get fully oxidized though it can take some time.

Generally, it is oils and algae that can make water dull -- mostly items that don't fully dissolve. Some of these items can get caught in the filter if they are consolidated by a clarifier or are otherwise clumped together, but generally by themselves they would simply pass through the filter. In a well-maintained spa, there is very little bacteria or algae growth so that is essentially negligible. The biggest problem would mostly be that of saturated hydrocarbons such as oils. If one is clean getting into the spa, then this is minimized. If one has suntan or other lotions, then such oils can build up and even chlorine may not break them down well enough. MPS can be useful in this case as can some enzymes.

It sounds like Tony is a pretty clean guy!

Richard

[Whoneedsavacation]

Generally, it is oils and algae that can make water dull -- mostly items that don't fully dissolve. [...] The biggest problem would mostly be that of saturated hydrocarbons such as oils. If one is clean getting into the spa, then this is minimized. [...] MPS can be useful in this case as can some enzymes.

We got 5 1/2 months out of our last water change, but the last month was stretching it. The main issue was sluggishness/foaming so I am guessing that our problem is probably residual oils. If we were more aggressive with MPS and enzymes, could we expect to get as much as 6 months, or is that just wishful thinking? Since we hear on the radio that we should only be watering our lawns every other day, dumping all those gallons into the street makes me a bit self-conscious.

[ieremia]

I guess a better way to determine the age of the water would be to figure out how often you're using the spa. It's like comparing a 5 year old car with 20,000 miles to a 2 year old car with 50,000 miles. Is how long the water should last determined on its age or its usage?

If I only use my spa 3 times a month and keep the water well maintained, should I be dumping it out as often as every 4 months?

[chem geek]

MPS won't touch the saturated hydrocarbons. The foaming is probably sudsing from soap and a higher Calcium Hardness (CH) helps prevent that, but one needs to be careful since you don't want to produce scale (so 150 ppm CH is probably the most one should have while 100 ppm would be safer to prevent scaling; if the CH is lower than that, then foaming is more likely). So to break down the oils, a good enzyme might work, but I don't know which products are most effective at that. The enzymes break down by chlorine so don't last long -- they need to accelerate the breakdown of the oils before they get broken down themselves.

As for how long the water should last, it isn't really based on age, but on what's been put into the water. This is why the general Water Replacement Interval (WRI) in days is (1/3)x(Spa Volume in U.S. Gallons)/(Number of Bathers per Day). The denominator should really be something more like bather-hours and the current WRI formula probably assumes something like 30 minute soaks (or less), but you get the idea. However, it isn't just your soaking that adds chemicals to the water, but also the addition of chlorine or bromine and related chemicals. If you are using Dichlor, then you are adding Cyanuric Acid (CYA) to the water. If you are using bromine tabs, then you are adding 5,5-dimethylhydantoin (DMH). If you are using any chlorine product, you are adding salt (sodium chloride) which is usually not a problem unless the level gets quite high, but with bleach you accumulate twice as much salt as with Dichlor. With bromine reactivated from a bromide bank (not from tabs), you accumulate bromide ion which is usually not a problem. With MPS, you accumulate potassium sulfate, usually not a problem but don't know for sure what high sulfate levels will do (it can be corrosive to stainless steel when also present with chloride but don't know what level of sulfates causes this problem -- chlorides need to be around 3000-5000 ppm before more rapid corrosion may occur).

Richard