Chlorine Levels High For Long Time

[tan8856]

Hi

The problem im having is that it takes ages for my chlorine DPD levels to drop to level in which its safe to swim. For example, I shocked the pool on Tuesday which took its reading to 6 (mg/l) which is the maximum. Its now saturday and its still 6. So we cant swim as its too high.

Is this normal? On the shock instructions it said 'shock the night before a pool party so your water is sparkling clean the next morning!' How is it possible for the levels to reduce down to swimmable overnight?!

How can i rapidly reduce the chlorine? It could be days before its at a swimmable level again...

Thanks

[PaulR]

When that happens, the most likely causes are (1) your chlorine level was actually much higher than 6, (2) you are using stabilized chlorine and your stabilizer level is sky high.

In your other post, you mentioned having a 15' steel framed pool. Assuming this is a round above-ground pool with the usual 4' of water, that works out to around 5000 gallons. Is that what you have?

When you shocked the pool, what kind of chlorine did you use, and how much? The main ingredient is what I'm after, not the brand name. Common forms of powdered/granular chlorine are calcium hypochlorite, sodium dichloro-s-triazinetrione, and sodium trichloro-s-triazinetrione. Liquid chlorine would be sodium hypochlorite.

What do you use to chlorinate the pool when you are not shocking it?

Finally, a complete set of water testing results would be VERY helpful. If all you can test is chlorine and pH, then please take a sample (~1 quart) to a nearby pool store, they usually will do water testing for free. You get what you pay for but it's better than nothing.

--paulr

[tan8856]

When that happens, the most likely causes are (1) your chlorine level was actually much higher than 6, (2) you are using stabilized chlorine and your stabilizer level is sky high.

In your other post, you mentioned having a 15' steel framed pool. Assuming this is a round above-ground pool with the usual 4' of water, that works out to around 5000 gallons. Is that what you have?

When you shocked the pool, what kind of chlorine did you use, and how much? The main ingredient is what I'm after, not the brand name. Common forms of powdered/granular chlorine are calcium hypochlorite, sodium dichloro-s-triazinetrione, and sodium trichloro-s-triazinetrione. Liquid chlorine would be sodium hypochlorite.

What do you use to chlorinate the pool when you are not shocking it?

Finally, a complete set of water testing results would be VERY helpful. If all you can test is chlorine and pH, then please take a sample (~1 quart) to a nearby pool store, they usually will do water testing for free. You get what you pay for but it's better than nothing.

--paulr

Hi Paul and thanks for your reply.

Im new to the pool world and havent heard of stabiliser level, what is it and how does it impact?

The pool is 3' deep which equates to around 3300 gallons (or 15000 litres) i think.

I used Calcium Hypochlorite (75%) to shock the pool 150g as per instructions. The regular chlorine I use is Sodium Dichloroisocyanurate Dihydrate.

All was well with the pool for the first 6 weeks but then we developed an algae problem (i think due to me not looking after the pool enough - the weather is rubbish and we havent been using it much and i guess i got complacent!) About 80% of the algae has now gone due to me cleaning and shocking and algaeciding but i cant seem to shift the final 20% which sits on the bottom like a fog. Is it safe to swim in water with a bit of algae in?

The PH is around 7.4 sometimes up to 7.6.

What am i doing wrong?

[PaulR]

Okay, a lot of info coming at you here.

15000 litres would be around 3300 Imperial gallons or 4000 U.S. gallons, which matches the dimensions you gave. I should have guessed you think in metric as you quoted the chlorine reading in mg/l.

The sodium dichloroisocyanurate (or "dichlor" for short) is chlorine bound to an almost equal amount of cyanuric acid (CYA), also known as stabiliser or conditioner. Dichlor adds 9 mg/l of CYA for each 10 mg/l chlorine. However, the chlorine will get used up by algae and whatnot, and needs to be replaced; while the CYA essentially never goes away (more specific details on request).

Some stabiliser is a good thing, as it helps protect the active chlorine from sunlight. Lots of stabiliser is a bad thing, as it protects the chlorine so thoroughly that there is essentially no active chlorine left to kill the algae and whatnot. The ideal range for CYA is 30-50 mg/l, unless you have a salt-water chlorine generator (wants more) or the pool is indoors (wants less).

In your 15k litre pool, 1kg dichlor will put you right at the 30mg/l level for CYA. If you have added more than 2kg dichlor total since you filled it, then you have a seriously overstabilised pool, and your chlorine is unable to attack the algae properly. The "fog" you are seeing is almost certainly a low level of algae, which has reached some kind of balance where it is growing about as fast as your pitiful active chlorine level can kill it off.

The only practical option I can suggest is to completely drain and refill the pool, then use dichlor as your chlorine source until you've used up 1kg, then switch to the calcium hypochlorite as your regular chlorine source. You could also use sodium hypochlorite, which in the U.S. is sold as "liquid chlorine" and also at lower strength as regular laundry bleach. (I find the Pool Calculator invaluable for figuring out how much of which things to add to the pool.)

If this is the kind of pool where you drain it and put it away for the winter, then calcium hypochlorite will be fine; in the long run you could get an excessive amount of calcium in the water (cal-hypo adds 7mg/l calcium for each 10mg/l chlorine) but over the course of one summer, it wouldn't be enough to worry about.

I hope this is not too overwhelming, it's a lot to take in at once. Post back any questions you might have.

--paulr