Chlorine Demand Problem Every Year When We Open the Pool

[regatta333]

For the past several years, we've had a chlorine demand problem when we open the pool. No free chlorine registers and it's called for anywhere from 70 to 120 pounds of shock to "break" the
chlorine lock. At the pool store, they cannot tell me exactly what's causing the problem. I have tried using phosphate remover over the course of the last two summers, but it has not helped. They are saying that the chlorine is binding to nitrates in the pool and making it unavailable to sanitize the water.

One suggestion they offered was to drain half of the water in the pool and replace it. This is really not doable as it would create a swamp in our yard and our neighbor's. Has anyone else experienced this problem? How did you resolve it? With chlorine prices more than double what they were a few years ago, it's getting very expensive to dump so much shock in the pool on an annual basis.

 

[T_R_Oglodyte]

For the past several years, we've had a chlorine demand problem when we open the pool. No free chlorine registers and it's called for anywhere from 70 to 120 pounds of shock to "break" the
chlorine lock. At the pool store, they cannot tell me exactly what's causing the problem. I have tried using phosphate remover over the course of the last two summers, but it has not helped. They are saying that the chlorine is binding to nitrates in the pool and making it unavailable to sanitize the water.

One suggestion they offered was to drain half of the water in the pool and replace it. This is really not doable as it would create a swamp in our yard and our neighbor's. Has anyone else experienced this problem? How did you resolve it? With chlorine prices more than double what they were a few years ago, it's getting very expensive to dump so much shock in the pool on an annual basis.

Nothing unusual there. During the time when the pool is shutdown, oxidizable organic and nitrogen compounds accumulate in the water.

Chlorine is an oxidizer; it's a much stronger oxidizer than oxygen. When you open the pool and begin adding the chlorine, the chlorine reacts with those compounds. Until that chlorine demand is satisfied, no free chlorine residual will develop. Chlorine does react with nitrogen compounds (but not nitrate) to form chloramines, which are part of total chlorine residual, but are not free chlorine. As more chlorine is added, the chloramines are destroyed and free chlorine residual develops. Note the smell people associate with chlorinated water is actually chloramine. When water is chlorinated beyond breakpoint, most of the chlorine smell disappears.

Nitrate is nitrogen in it's most oxidized state - so chlorine doesn't react with nitrate. Phosphates also have no interaction with chlorine; they would come into play only if phosphate availability is the limiting factor in algae formation. But once you start maintaining a free chlorine residual, chlorine is the limiting factor in algae formation.

If you want to avoid the shock situation when you open the pool, the only way to do that without draining the pool is to maintain a chlorine residual during the time when the pool is closed.

The link below has a good explanation of the chlorination process.

Understanding Breakpoint Chlorination

Breakpoint chlorination is when chlorine has overcome the oxidant demand, and begins disinfection. Only after breakpoint can a free chlorine residual grow.

blog.orendatech.com

 

[regatta333]

Very interesting article. Thank you. I would love to get to the root cause of the problem. We have a solid cover on our pool and have not used lawn chemicals in the past two years. We only use water and a power washer to clean the pool deck. When we take the cover off the water is clear. There is always some quantity of arborvitae debris and worms, which have made their way beneath the cover, and that it is the first thing I vacuum out before I start trying to treat the water.

I am not sure what I can or should be doing to break this cycle of chlorine demand.

 

[MrockStar]

Salt water pool?

 

[regatta333]

No, chlorine.

 

[PigsDad]

I have thought about changing to a salt water pool? We have one in our FL home and it is great. It might be less expensive in the long run.

Kurt

 

[BJRSanDiego]

We have a local pool store called Leslie's. They have a computerized analyzer that identifies the things that are out of wack and the recommended remediations.

I noticed that you did not mention the total dissolved solids (TDS), the Cyanuric acid level (stabilizer), pH, or a few other things that Leslie's measures. Also, I'm wondering what your free chlorine level is before and after treatment and how quickly it drops.

On my first pool, they gave me a bunch of liquid chlorine (IIRC, sodium hypochlorite). I couldn't keep the chlorine level to the right level. I'd dump in a gallon and the next day it would read zero. I found out that I needed chlorine stabilizer (Cyanuric acid). I added it and it fixed the problem immediately.

What are you using for chlorine? Is it stabilized? Or liquid chlorine? or ?

Hopefully you have a Leslie's nearby or an equivalent store.

 

[regatta333]

Salt water pool would not solve the chlorine demand problem. I asked.

Leslie's is not conveniently located to us, but our local pool store does computerized water analysis. Total alkalinity, PH, TDS. and CYA are all in the correct range.
I use Costco shock and chlorine tablets and baking soda to increase alkalinity. Other chemicals (algaecide, PH balancing, scale inhibitor, etc.) I get from the pool store.

 

[Patri]

Now I see why pools are so much work!

 

[joestein]

Salt water pool would not solve the chlorine demand problem. I asked.

Leslie's is not conveniently located to us, but our local pool store does computerized water analysis. Total alkalinity, PH, TDS. and CYA are all in the correct range.
I use Costco shock and chlorine tablets and baking soda to increase alkalinity. Other chemicals (algaecide, PH balancing, scale inhibitor, etc.) I get from the pool store.

The Costco pool products, especially the shock, are substandard.

I would check the shock bags to see how much chlorine is actually in the bag. I use Drytec Shock because it was 69% chlorine - I think the costco shock was in the 30s or 40s % chlorine. I was paying $100 for a 24 bag case.

I used to think they were a great deal, but when you take the effectiveness into account, they are quite expensive.

Joe

 

[davidvel]

The Costco pool products, especially the shock, are substandard.

I would check the shock bags to see how much chlorine is actually in the bag. I use Drytec Shock because it was 69% chlorine - I think the costco shock was in the 30s or 40s % chlorine. I was paying $100 for a 24 bag case.

I used to think they were a great deal, but when you take the effectiveness into account, they are quite expensive.

Joe

The devil's always in the details!

 

[Passepartout]

Definitely First World Problems.

 

[regatta333]

I would check the shock bags to see how much chlorine is actually in the bag. I use Drytec Shock because it was 69% chlorine - I think the costco shock was in the 30s or 40s % chlorine. I was paying $100 for a 24 bag case.

The bag says 56.44% calcium hypochlorite. Looks like Costco price is comparable to the Drytec Shock, which is available at Walmart, so if Drytec is 69%, definitely a better value. I will give it a try.

I put 45 pounds of shock in last night (40% dichloride shock and 60% Costco shock). Water was crystal clear even before the shock. This morning, I vacuumed up small amounts of dead algae and put my alkalinity out of whack. I added baking soda to get it back up and will repeat the shock tonight with the dichloride shock and Drytec shock.

 

[T_R_Oglodyte]

I put 45 pounds of shock in last night (40% dichloride shock and 60% Costco shock). Water was crystal clear even before the shock. This morning, I vacuumed up small amounts of dead algae and put my alkalinity out of whack. I added baking soda to get it back up and will repeat the shock tonight with the dichloride shock and Drytec shock.

Hypochlorite is a base - the opposite of an acid - and will raise the pH of the water. For chlorine to work effectively, the pH of the water should be no higher than 7. That's why acids are usually added in conjunction when hypochlorite is used as a disinfectant.

You can also use liquid bleach; bleach is just a sodium hypochlorite solution. Just make sure the bleach contains only sodium hypochlorite; some bleach products targeted for laundry use contain other added ingredients.

 

[BJRSanDiego]

I put 45 pounds of shock in last night (40% dichloride shock and 60% Costco shock). Water was crystal clear even before the shock.

Is this a typo ? Or do you have an absolutely HUGE pool ?

If the water was crystal clear before the shock, then why would you put in so much? I'm guessing that your free chlorine level was on the order of 100 ppm. That would make it unhealthy to use for swimming. I think that 1-2 ppm is considered good and safe. Do you know how to measure free chlorine and also total chlorine?

I avoid calcium hypochlorite because the calcium causes calcium scale on my tiles. I typically use a stabilized shock (tri-chloral triazinine IIRC).

When my pool has gotten out of wack (only in the hottest months and never in the winter in southern Ca.), I've used an algicide and a flocculant plus stabilized shock (one the advice of my local Leslies). BTW, my pool holds about 18,000 gallons and I probably put in 1 - 3 pounds of shock when it becomes cloudy or I get algae on the walls.

Perhaps you should hire a pool service for a couple of months. They can do the necessary detective work and get your pool in shape. Then it might be easier to carry on by yourself.

Edit: an afterthought - - when you open your pool, is there a lot of debris in the pool, like leaves, insects, etc. ? If so, you should vacuum it out first. If your pool water is clear, pH okay, phosphates okay, CYA okay, TDS (or hardness) okay when you open the pool, then why would you have to super-duper-duper hyper chlorinate your pool. I guess that I'm very puzzled.

 

[regatta333]

Our pool holds 33,000 gallons. Even after the 45 pounds of shock (half of what was called for on the overnight chlorine demand test that was done at the pool store),
free chlorine in the pool was 0.5 ppm. They are telling me that something is "binding" with the chlorine and making it unavailable to sanitize the pool. Apparently, a number of pools they are treating also have this problem.

I put the second 45 pounds in last night and will see what the reading is this morning. We had the same problem last summer and I opted to slowly chip away at the chlorine demand instead of trying to break it in one go, but it meant that I was having to shock the pool much more frequently and it was consuming an inordinate amount of chlorine just trying to maintain 1ppm of free chlorine.

Where do you buy your tri-chlor? We are also having the problem with the calcium scale.

 

[BJRSanDiego]

Our pool holds 33,000 gallons. Even after the 45 pounds of shock (half of what was called for on the overnight chlorine demand test that was done at the pool store),
free chlorine in the pool was 0.5 ppm. They are telling me that something is "binding" with the chlorine and making it unavailable to sanitize the pool. Apparently, a number of pools they are treating also have this problem.

I put the second 45 pounds in last night and will see what the reading is this morning. We had the same problem last summer and I opted to slowly chip away at the chlorine demand instead of trying to break it in one go, but it meant that I was having to shock the pool much more frequently and it was consuming an inordinate amount of chlorine just trying to maintain 1ppm of free chlorine.

Where do you buy your tri-chlor? We are also having the problem with the calcium scale.

I have been buying my Trichlor... (pool shock) at Home Despot. That is the same stuff that is in your stabilized pool tablets. I buy it in granular form in maybe 10 pound tubs. Your own pool store should be able to supply.

Frankly, I am amazed and dumbfounded with your problem. If everything else is within reasonable specs you shouldn't have to use nearly that much chlorine Considering that your pool is double the volume of mine, it still doesn't make sense. Also, considering that your local pool store says that others are having the same problem (which is highly unusual), kind of raises a red flag (at least to me). Calcium hypochlorate dissipates quickly. On my first pool, I couldn't keep the chlorine level in spec. But that was because there wasn't any stabilizer (Cyanuric acid). Once I added stabilizer and switched away from liquid chlorine (calcium hypochlorate) things got better. But since your pool company thinks that your CYA level is fine, that puzzles me.

I use stabilized chlorine tablets for routine maintenance. One of the long-term drawbacks is the it can bind up the chlorine. So, when I have to, I shock the pool with perhaps 2 # of shock. I do that - - maybe - - once a year or twice a year. For your pool size, that would be 4 #. An order of magnitude less than what you need to use currently.

I'd consider hiring a pool service for a few weeks and/or find another pool chemical company. Just my own opinion....

 

[youppi]

When a pool is balanced, you need 1 pound of shock per 10,000 gallons to shock a pool.
For 33,000 gallons, you need 3.3 pounds normally.
When a pool has a chlorine demand problem, a way to fix it, is to over-shock by using 3x more shock but you over-shocked by 13.6 (45 pounds / 3.3) twice and it didn't fix the problem.
The other way to fix it, is to partially drain your pool. To avoid flooding your yard, just drain your pool a little every day in place of draining half of your pool in one shot.

Is your pool water clear or murky ?

When you do a backwash, the exit water is clean or dirty ?

What do you use to filter ? Sand ? Cartridge ? Others ?
If you use sand, when did you change the sand ?

A poor filtering may force using more chemicals than usual.

Also, too much chlorine can bleach out the reagent of the test and you will think that there is no chlorine but in fact there is too much.

Water Test Fails: 12 Simple Pool Testing Mistakes

12 simple pool water testing pitfalls to avoid. You could be making these simple pool testing mistakes and not even realize! Dr. Pool pool water test tips!

blog.intheswim.com

 

[regatta333]

Well, yesterday, after the second 45 pound shock treatment, my free chlorine is once again 0.5 ppm, with total chlorine of 2 ppm. Even though I used the 60% regular shock/40% dichlor ratio they recommended to maintain PH and total alkalinity balance, both were out of whack the following morning. Morning after first treatment, total alkalinity was 89, PH 6.6. I had to add 20 pounds of baking soda and 2 pounds of PH increaser. Morning after second treatment, total alkalinity was 64 and PH 6.6 and I had to add 32 pounds of baking soda and 5.5 pounds of PH increaser. They are running another chlorine demand test overnight and will call me with the results today.

The article about the pool testing mistakes was very interesting. I do reach down into the deep end of the pool to get water samples, although maybe not quite to elbow level. I will start switching to mid-way point of pool and reach down farther. I don't think bleaching the reagents is the issue, because the pool ordinarily consumes around 6 chlorine tablets per week and I shock every other week with 3 pounds of chlorine. Now, it is consuming 6 tablets every day or two.

We have a sand filter that seems to be working effectively to filter out even small particles of dead algae. I will have to check with the pool store about when it was last replaced. Pool water is crystal clear and when I backwash, it is cloudy at first and shows clear after a minute or two. We have actually put it a good amount of new water because the pool was drained to a lower level than normal at closing last year. It was about 6-8 inches below the tile line compared to 3 inches normally. Since opening, I've added a couple of inches of water beyond that. Not even close to half the pool volume, but something.

I will report back after they call me today.

 

[regatta333]

Just got a call from the pool store. The test came back with a demand for 62 pounds of shock!

They are recommending I just "run with it", just trying to keep up a minimum level of free chlorine. They say that with everything that has been dumped into the pool, it's becoming harder and harder to balance. This is exactly what I did last year, consuming way more chlorine and shock than normal. I guess I can try draining and adding water bit by bit, but I am out of ideas.

 

[joestein]

Well, yesterday, after the second 45 pound shock treatment, my free chlorine is once again 0.5 ppm, with total chlorine of 2 ppm. Even though I used the 60% regular shock/40% dichlor ratio they recommended to maintain PH and total alkalinity balance, both were out of whack the following morning. Morning after first treatment, total alkalinity was 89, PH 6.6. I had to add 20 pounds of baking soda and 2 pounds of PH increaser. Morning after second treatment, total alkalinity was 64 and PH 6.6 and I had to add 32 pounds of baking soda and 5.5 pounds of PH increaser. They are running another chlorine demand test overnight and will call me with the results today.

The article about the pool testing mistakes was very interesting. I do reach down into the deep end of the pool to get water samples, although maybe not quite to elbow level. I will start switching to mid-way point of pool and reach down farther. I don't think bleaching the reagents is the issue, because the pool ordinarily consumes around 6 chlorine tablets per week and I shock every other week with 3 pounds of chlorine. Now, it is consuming 6 tablets every day or two.

We have a sand filter that seems to be working effectively to filter out even small particles of dead algae. I will have to check with the pool store about when it was last replaced. Pool water is crystal clear and when I backwash, it is cloudy at first and shows clear after a minute or two. We have actually put it a good amount of new water because the pool was drained to a lower level than normal at closing last year. It was about 6-8 inches below the tile line compared to 3 inches normally. Since opening, I've added a couple of inches of water beyond that. Not even close to half the pool volume, but something.

I will report back after they call me today.

45 lbs of shock at todays prices have to $300. That is crazy.

Also, you can add some fiberclear or alternative (I think Leslies has their own version which is cheaper) to your skimmer. It will enable your sand filter to catch smaller particles. I have been using it for years.

Pool And Spa Filtration Media

Fiber Clear is a safe, effective filtration media that can be used instead of diatomaceous earth, which is a known carcinogenic. Use Fiber Clear in any pool filtration system: Sand, Filter or Cartridge.

fiberclearpoolandspafiltration.com

 

[PigsDad]

Seems to me it would be cheaper and possible more efficient to hire a pool service for a few months. Mine charges $135/month, comes out weekly, and includes any chemicals needed. And I'm sure there are less expensive pool service providers.

Kurt

 

[BJRSanDiego]

If it was me, I think that I'd dump the pool water and start the water conditioning from scratch. The super-flexible blue discharge hoses are cheap and connecting a few in series would allow me to dump it down at the street if I wanted to (our soil is porous, so that probably wouldn't be necessary).

I did a Google search and found this: https://naturalpoolproducts.com/wea... breaks down chlorine,just two or three hours.

The only thing that jumped out at me was the best pH seems to be higher than the OP is able to keep it.

 

[klpca]

Seems to me it would be cheaper and possible more efficient to hire a pool service for a few months. Mine charges $135/month, comes out weekly, and includes any chemicals needed. And I'm sure there are less expensive pool service providers.

Kurt

Exactly what I was going to suggest. Having John, our "pool guy", pays for itself because not only does he supply the chemicals, the pool water stays in balance a lot better than when we DIY'd it. I am a penny pincher, but hiring someone to maintain the pool almost pays for itself.

 

[regatta333]

Seems to me it would be cheaper and possible more efficient to hire a pool service for a few months. Mine charges $135/month, comes out weekly, and includes any chemicals needed. And I'm sure there are less expensive pool service providers.

Kurt

I am looking into this. At this price, I agree, it would be well worth it. Our local pool store charges $149 per visit, so not really feasible, but I have reached out to another company that offers a weekly service.