Chlorine cost analysis

ti_chris

Member
Apr 30, 2017
12
San Jose, CA
Hi,

I'd be curious to hear about everyone's experience with chlorination strategies to figure out what would work best for me. For many years, I've been using trichlor pucks along with calhypo for shock with mixed success. It's been pretty painful to deal with the high levels of CYA and the calcium deposits that have been around the tiles (I'm in the process of painfully cleaning all of those ugly marks out). I've been looking at liquid chlorine as an alternative since it's been highly recommended here in the forums, but it all comes down to what's most cost effective in the end. For my area, The best price (when not on sale) I could find per concentration is as follows:
Chlorine typeCost / unitUnit costAmount to raise 10k pool by 1ppmCost to increase 10k pool by 1ppm
trichlor (90%)$145/50lbs$0.18125/oz (weight)1.5oz (weight)$0.27
calhypo (73%)$180/50lbs$0.225/oz (weight)1.8oz (weight)$0.41
liquid chlorine (10%)$13/3gallons$0.034/oz12oz$0.41

For every 1ppm of chlorine added, I understand that there's the following negative effect from each chemicals:

Chlorine typeTDS per ppm of chlorineCYA per ppm of chlorineCalcium Hardness per ppm of chlorinepH
trichlor (90%)+1.4ppm+0.6ppm3
calhypo (73%)+1.2ppm+0.7ppm10.8
liquid chlorine (10%)+1.6ppm13


Now turning my attention to the effect of pH first. When used together, trichlor and calhypo tend to have their pH cancel each other out so I'm ignoring the cost of balancing the pH for that combination. It's not quite true of course but to simplify the calculations, I'm making that assumption. As for liquid chlorine however, according to my math, it would require roughly 12oz of muriatic acid to compensate for the negative effect of the ph caused by liquid chlorine. So looking at my options here:
Acid typeCost / unitUnit costAmount to lower 10k from 7.6 to 7.5Cost to lower 10k from 7.6 to 7.5
Muriatic Acid (14.5%)$11/2gallons$0.043/oz8.3oz$0.36
Sodium Bisulfate (93%)$52/25lbs$0.13/oz (weight)5.1oz (weight)$0.66

It seems like the clear winer is muriatic acid here at a lower cost and lower TDS. If this is true; the true cost of using liquid chlorine to raise by 1ppm basically rises by roughly $0.688 (16x0.043) for a total of $1.10/ppm.

Finally, I turn my attention to the water refills required to remove the extra 'junk' added to the pool. That one's a little trickier to calculate due to the seasonality of the chlorine demand unfortunately. I turn my attention to the CYA only given that it's the most damanging and least tolerant of all of them. TDS has a generally high tolerance of 1.5k while CYA has a nasty relationship where the higher it is, the more you need and thus the more you end up adding of it. Water costs are quite high where I am. It's at $6.55/CCF or $0.0087627 per gallons.

I'm assuming that the CYA will start at 30 and that the pool will be halved when it reaches 60. I'm ignoring the effect of higher levels of CYA in terms of demand for the sake of keeping things simple for now. It will take a total of 50ppm increases to increase the CYA to 60 at which points it would cost $43.81 to refill a 10k pool or $0.8762/ppm. Bringing the total cost for trichlor to at least $1.1462/ppm. Now if we look at the effect the demand over time since the CYA is increasing and requiring us to put in my ppm of chlorine after every addition

Liquid chlorine will also require a refill due to TDS at some point. Playing the same game and allowing a range of 750-1500 TDS before a refill indicates that it will require ~469ppm to raise the level by 750. By the same prior logic, we have $0.093/ppm. Bringing the total to rougly $1.193/ppm.

On the surface it seems slightly more expensive to go down the liquid path, but if I undo the assumption that the CYA has no effect on ppm demand for effectiveness the story changes. And add that we want to keep the effectiness of the chlorine throughout the CYA addition, we would actually need to change the water after adding 50ppm in the equivalent of 36ppm of liquid chlorine (obtained by modeling chlorine intake assumign a start point of 2.25 @ CYA-30 and with a FC demand equal to 7.5% CYA). Meaning that the net cost of trichlor is 1.39times more expensive or averaged at $1.52/ppm which is clearly higher than the liquid alternative.

Of course the cost structure here is a function of my local economy and the cost of things in my area. I'd love to hear if anyone else did the analysis for their region and figured out if the math works out. Feel free to point out areas where I may have made a mistake or wrong assumption.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
23,770
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
As for liquid chlorine however, according to my math, it would require roughly 12oz of muriatic acid to compensate for the negative effect of the ph caused by liquid chlorine.

Liquid chlorine is pH neutral across its cycle. It will temporarily raise pH when added and lower pH as the chlorine is consumed.

With TFP methods we don’t care about TDS, it is not a factor in pool water chemistry, and never a reason for draining. CYA and CH accumulation can require draining to keep at manageable levels.

I would not use Trichlor to chlorinate my pool long term even if it was free.

You left out the SWG from your analysis. .
 
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mgtfp

Bronze Supporter
Mar 5, 2020
780
Melbourne, Australia
Pool Size
66000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Astral VX 7T
One comment to your pH considerations. Chlorination with Hypochlorites (Cal-Hypo or liquid chlorine) has no significant net effect. It is true that adding liquid chlorine or Cal-Hypo initially raises pH. But all processes that consume chlorine (UV-decay, sanitation) are acidic processes. Once the chlorine that had been added has been consumed, the pH is more or less back where it started.

What raises pH is outgassing of CO2, driven by high TA.

Edit: Looks like Allen and I were typing at the same time, but he hit Post a bit sooner...
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
23,770
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Prior threads on the topic...


 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
4,664
NW Ohio
Liquid chlorine will also require a refill due to TDS at some point. Playing the same game and allowing a range of 750-1500 TDS before a refill
Where did you get these numbers? SWG pools run just fine with 4000+ TDS, so how do you get that you can't run higher than 1500?
 

ti_chris

Member
Apr 30, 2017
12
San Jose, CA
Liquid chlorine is pH neutral across its cycle. It will temporarily raise pH when added and lower pH as the chlorine is consumed.

AH interesting. Thanks to everyone who replied on this topic. That's good to know. It makes the cost analysis even more black/white.

With TFP methods we don’t care about TDS, it is not a factor in pool water chemistry, and never a reason for draining. CYA and CH accumulation can require draining to keep at manageable levels.

My understanding was that high levels of TDS affects chlorine. Is that inaccurate?

I would not use Trichlor to chlorinate my pool long term even if it was free.

You left out the SWG from your analysis. .

Yes that's a good point. I didn't include SWG because they're simply not an option for me. My main drain is a copper pipe and I have a lot of pavers all around the pool both of which would get destroyed by a SWG solution.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
23,770
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
My understanding was that high levels of TDS affects chlorine. Is that inaccurate?

Yes. Please explain the chemistry of how you think TDS affects chlorine?


Yes that's a good point. I didn't include SWG because they're simply not an option for me. My main drain is a copper pipe

Salt does not effect copper. Many pool heater heat exchangers are copper. They are not self destructing in salt pools. It is low pH that is detrimental to copper pipes, not salt.

and I have a lot of pavers all around the pool both of which would get destroyed by a SWG solution.

What type of pavers? How does salt affect your pavers?

All pool water accumulates salt over time. Chances are you have a salt pool even if you do not use a SWG. Salt is an ingredient in liquid chlorine and other chemicals. Have you tested your pool water specifically for salt?

The science and chemistry do not support your understandings.
 
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