Killing Algae and maintaining pool balance

Jun 22, 2008
6
#1
Hello,

I really screwed up my pool last year and I am paying the price this season. I have quite a bit of algae that I am trying to kill while keeping my CSI negative to help with a scaling problem from last year.

Is it ok to add muriatic acid to the pool while in the process of shocking to kill algae?

25,000 Gallons
PH 7.5
FC 5+ (test kit limitation)
TA 120
CH 430 (Fill water is at 400 ppm)
CYA 35
Temp 70 Deg.
CSI 0.15

Equipment: 1.5 hp generic pump, 225 lb sand filter, Hayward 8lb inline chlorinator, 1.5" pvc schedule 40 throughout, Leslies copper therm 260,000 bt/h pool heater

Location: Western Suburbs of Chicago w/ exposure to lots of sunlight.

Thank You,
Bill
 
Jun 22, 2008
6
#3
Thank you.

I am apparently having an acid demand problem as well. The acid demand test took 30 drops to lower the PH from 7.5 to 7.2 and I don't believe that the volume of acid generated by the the pool calculator is high enough given the results of the acid demand test.

I am going to periodically add muriatic acid in small quantities until i get the desired results. Is this the right course of action?

According to the acid demand test I would need 5.37 gallons of acid to lower my PH is this the correct amount to add over time?
 

duraleigh

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#4
Bill,

As a general rule, we don't use the acid demand test for much.

I am not familiar with it (someone will be along soon who is) .

Something is very wrong in the calculation, however. Far less than that is needed to lower your pH from 7.5 to 7.2. I have not calced it but I would guess between a quart and a half gallon.
 

JasonLion

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#5
It would be nice if you could put the basic specs of your pool into your signature. That makes it easier for us to help you.

I calculate that you will need about 6 cups of acid, if your PH and TA readings are correct.

If your FC level is high enough the PH test might be reading incorrectly, which could dramatically throw off the acid demand test. That is the problem with adding acid while shocking, the PH test doesn't work at high FC levels.
 
G
#7
You might not have an acid demand problem. Are you testing the pH with the chlorine at shock level? High chlorine levels will interfere with the pH test and make it read higher than it really is.
(also algae will be killed more effectively at a higher pH than an lower one because the higher pH will favor the formation of monochloramine 7.8 to 8.0 is the most effective so I don't understand why you want to drop your pH to 7.2).

Wait until you chlorine levels drop back to normal and THEN test your pH.
 
Jun 22, 2008
6
#8
Is this statement not true then?

http://www.edstrom.com/DocLib/MI4148.pdf

Disinfectant Strength Comparison

The germicidal strength of different forms of chlorine in water are ranked as follows:

HOCl > OCl– > inorganic chloramines > organic chloramines

HOCl is 100 times more powerful an oxidant and disinfectant than is the hypochlorite ion.
Consequently, free chlorine is most effective at a pH of 5 to 7 where HOCl is the predominant
form. The effectiveness declines with increased pH.
 

JasonLion

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#9
Shark0311 said:
Is this statement not true then?
That article does not cover what happens when there is CYA in the water. They are assuming no CYA. CYA changes things significantly. You can look at this post for more information.

There are also some interesting, but more complex, chemical reactions that can happen with or without CYA when PH is higher that can kill algae, which are outside the scope of that article. For example, chlorine can react with algae to form monochloramine, which can then kill more algae.