Please help another pool store guy out!(Needs adv. chem)

[terbum]

Hello, TFP members. I'm another pool store worker, and although you guys like to talk down about us, I was wondering if I could ask for some help. However, my problem is a little different than what's been posted here already. Let me warn right now the problem might require advanced chemistry. I hope I'm posting this in the right place. I apologize if I haven't.

Here's the story: my boss is currently working on a vanishing edge pool on a service call. The pool itself, which has plaster walls, is ~18000 gallons with a 2000-3000 gal. basin at the bottom of the waterfall. The water gets recirculated, not through a filter, into a spill over spa of ~600 gal. The pool has an ozonator, a Delozone TrioPure to be exact. It recommends that at least 3500 ppm of salt is kept in the water.

Because of the whole waterfall system, it is estimated that around 100 gallons of water is lost a day. So, there is an auto refill system to add fresh spring water to the pool when needed. Let's assume that whatever lost is gained in a course of a day.

Now here's the problem I'm stuck with: because of the whole water loss situation, no matter how miniscule the loss, my boss wants me to model how each chemical level changes with time (keep in mind this is only a summer job for me and this is just a mom and pop pool store). This includes calcium hardness, salt concentrations, stabilizers, chlorine, total alkalinity, and (most unfortunate of all) pH. Everything. He wants this done so he can ensure the clients keep not only the chemicals where they need to be, but they have the right settings on the ozonator.

Here are the current levels from tests I did on the water today (done with a Taylor 2005 kit):

FC and TC: 0
TA: 80
pH: >8.0 (It was busy in the store today and because this was my last test, I didn't have time to do an acid demand test)
CH: 50
CYA: <30 (It's low)
Salt: 700 ppm

Some of these will be easy. Some of my assumptions include the spring water not including calcium hardness (probably a bad one) and cyanuric acid, so it's just simple solution chemistry to figure it out. If they are bad assumptions, these were only starting points.

The other chemical levels, not so much. Chlorine is being generated by the electrolysis of the salt in the water, which is being done at an unknown rate. However, the hypochlorous acid being formed can be degraded in the sunlight. Even worse, these rates change with the presence of CYA, some of which is being lost everyday. There's also the chlorine being lost through the waterfall. All of that has to be considered.

Salt wouldn't be so bad except it being electrolyzed and lost at the same time. I'm not familiar with the electrolysis rates of the ozonators, and no matter how hard I look I can't find any information on it. So, I have no idea how fast the salt is being degraded.

Alkalinity and pH are going to be the very, very hard ones. Already, there is major aeration, contributing to a possible climbing pH. Without knowing the appropriate reaction kinetics, I don't know how fast this climb takes place. I know baking soda and soda ash come together to make a buffer, but I'm not very familiar with the specific equilibrium chemistry you can use to determine how something specific is affected for that system. Also, there is the problem of the incoming water being of a different alkalinity and pH than the pool water. Plus, the pool's pH is going to need to come down, and how is the chemical ultimately used for that specifically going to affect the buffer, let alone the pH and alkalinity? This is probably going to be way too complicated to figure out by hand, but no one is taking "I'll try to figure it out" or "This is way too hard" as answers. So, I'm stuck.

Basically, where do I start to begin figuring this out? Where should the chemical levels be in a pool like this? I'm adept in chemistry, so if anyone can help me out, use all the jargon and mathematics you want. If this is too hard of a question on this board, is there anywhere else I can go? Unfortunately, in my situation, it's not good enough to simply say, "set up and check and adjust the chemical levels frequently", so I don't want to be pressuring or anything, but I need at least a ballpark model to go back with.

If anyone can help me, I greatly appreciate your help. Thank you.

 

[spishex]

You're going to need to know the chemistry of the fill water, and if possible a better idea of how much water is added over a given period of time.

 

[terbum]

Unfortunately, information about the fill water is an unknown (it wasn't given to me and it's not known at this time, but it can be figured out, but unfortunately not in a timely enough fashion.). 100 gallons is added everyday to replace the 100 gallons that is lost everyday.

Even if I don't know what exactly the fill water consists of, what is the general chemistry I'm going to be considering, so when I do, it's just simple calculations to figure it out?

 

[JasonLion]

This water is really seriously out of balance. That pool should not be left in that condition for long. You should start with Water Balance for an SWG Pool.

You are missing some basic facts. Hardly any chemical evaporate, really only water evaporates. If you replaced evaporation with distilled water most of the levels would stay essentially stable. The problem is that the fill water levels probably aren't zero, so anything that is present in the fill water will be constantly rising. It is essential that you get good readings on the fill water TA and CH levels.

You don't have to worry about the FC level very much at all once the pool is balanced. The SWG produces chlorine and chlorine is lost to sunlight. Because higher FC levels result in more chlorine being lost to sunlight, those two effects form a stable system. It is fairly easy to adjust the SWG to produce a stable FC level. Once you get the SWG adjusted correctly, it will only need small seasonal fine tuning.

Your big problem is going to be figuring out how to balance PH and TA, given the negative edge. Negative edges have huge amounts of aeration, which tends to result in rapid PH increases. If the negative edge is on every day, the aeration is so extreme that lowering the TA only helps a little bit. Most of your challenge is going to be here, doubly so if the pool owner has any control over when the negative edge runs.

 

[terbum]

Thank you for the quick responses. Yes, I do understand that chemicals do not evaporate with water. That's very basic stuff. The problem, according to the person who was filling me in, was that the water from the waterfall simply misses the basin and losses are coming from this, not evaporation (I don't know if that can happen, but that seems like that was implied). I apologize if I didn't make that clear. That's why I put into consideration water losses. If it were simply from evaporation it wouldn't be such a big deal.

Thank you again. The response was very helpful.

 

[Guest]

It is going to be very hard to determine the amount of change of the various levels without tracking this pool for a period of time with at least daily testing. The Triopure is a combo SWG/Ozonator and with only 700 ppm salt you are NOT generating any chlorine. With less than 30 ppm CYA any chlorine that is introduced is going to be lost to UV.
I understand your dilemma, I have worked in retail pool/spa supplies and also have done commercial maintenance but in this case the first thing needed is to balance the water, then monitor it for a few weeks to see the average rate of change. Also, you do need info on the fill water so you know what is also going into the pool.
With the negative edge I would run the TA at a very low number, around 50 or 60 ppm, and raise the CH higher to keep the CSI in balance. I would also keep close tabs on the pH and NEVER let it climb above 7.8!. Adding 50 ppm borates can be helpful in fighting the constant pH rise from aeration but no matter what you do it's going to be a losing battle.
This is just one of those pools that is never going to be easy and trouble free.