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Thread: Shocking - Time Or Concentration?

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    Shocking - Time Or Concentration?

    I was thinking about this earlier, as I bought another 4 gallons of bleack and realized my recycling bins are totally full.

    Say a pool is a semi batch reactor, and you have some reactions
    C(t) = Chlorine impulse
    ra = - kb*A^n*C^m - Algae destruction by chlorine
    ra = +ka*A^p - Algae growth
    rc = -kc*C^o - Chlorine Degredation

    So, If I'm trying to eliminate algae, I could say that
    dA/dt = + ka*A^p -kb*A^n*C^m
    dC/dt = C(t)-kc*C^o

    Does anyone know if these are 0, 1st, or 2nd order reactions?

    Either way, it would be interesting to see if your total chlorine usage (Integral of C(t) untill the pool is clear) is lower when you add all 25 gallons of bleach at one time, or if you add say 4 gallons per day during your shock treatment.


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    Mod Squad jblizzle's Avatar
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    Re: Shocking - Time Or Concentration?

    Well, pool companies tend to go with your first suggestion ... raise your FC up to 1000ppm and hope to kill everything at once so they do not have to come back.

    Our suggestion is to do the second suggestion so that you do not elevate the FC so high and maintain it.

    At some point, the high FC will start damaging equipment ... especially vinyl liners. Our shock process is designed to avoid the possibility of that damage.

    Of course this does not address your actual question of which uses more FC

    EDIT: although honestly, I would think a massive single dose would require less FC as nothing would continue growing at a low level ... but you put your equipment in danger.
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    Richard320's Avatar
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    Re: Shocking - Time Or Concentration?

    You need to factor in that chlorine degrades in sunlight, and the higher the concentration, the faster the drop. Just to confuse things more.

    Realistically, there is an upper limit to how high you can push FC without bleaching out vinyl liners.

    But now I'm wondering.... if you have an infection, and the doctor prescribes 2 pills/day for 10 days, do you take all 20 at once?
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    Mod Squad jblizzle's Avatar
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    Re: Shocking - Time Or Concentration?

    Good points Richard.
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    Re: Shocking - Time Or Concentration?

    Quote Originally Posted by jblizzle
    Well, pool companies tend to go with your first suggestion ... raise your FC up to 1000ppm and hope to kill everything at once so they do not have to come back.

    Our suggestion is to do the second suggestion so that you do not elevate the FC so high and maintain it.

    At some point, the high FC will start damaging equipment ... especially vinyl liners. Our shock process is designed to avoid the possibility of that damage.

    Of course this does not address your actual question of which uses more FC

    EDIT: although honestly, I would think a massive single dose would require less FC as nothing would continue growing at a low level ... but you put your equipment in danger.

    We bought the house with the pool, and last year paid someone to open it, since it was closed for 6 years. I heard grumbling that he was ill from the smell, then put something on the order of 24 gallons of 12% chlorine into it. This year, were trying it ourselves

    The pool calculator tells me I have added 100 ppm in total, over the course of 5 days. The water is near clear. Would 100 ppm at once be concerning?
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    Mod Squad jblizzle's Avatar
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    Re: Shocking - Time Or Concentration?

    Depending on your CYA (and probably even if it was 100+), that seems like a pretty high FC level ... at least triple the shock level. Although in reality, it would probably drop pretty quickly if there were a lot or organics.

    ChemGeek, would be able to tell if it was damaging.
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    Re: Shocking - Time Or Concentration?

    Quote Originally Posted by Richard320
    .... if you have an infection, and the doctor prescribes 2 pills/day for 10 days, do you take all 20 at once?
    I would argue no, because you run the risk of damage to the liner and the equipment (to keep the metaphors going).
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    Re: Shocking - Time Or Concentration?

    For the most part, the reactions are all first-order in chlorine except for the loss in sunlight (in terms of being proportional to FC) because the FC/CYA ratio is no longer valid as a measure of active chlorine level when that ratio gets high -- to shock level and above. The following is a table of active chlorine level at various FC assuming a 50 ppm CYA level and pH of 7.5:

    FC . FC/CYA . HOCl + OCl-
    -- . ------ . -----------
    3 ..... 0.06 ........ 0.050 ..... just above minimum FC/CYA for SWCG pools
    5 ..... 0.10 ........ 0.088 ..... typical target FC/CYA for non-SWCG pools
    10 ... 0.20 ........ 0.210
    20 ... 0.40 ........ 0.629 ..... shock level for green and black algae
    30 ... 0.60 ........ 1.53 ....... shock level for yellow/mustard algae
    40 ... 0.80 ........ 3.40
    50 ... 1.00 ........ 6.97
    60 ... 1.20 ...... 12.73
    70 ... 1.40 ...... 20.30
    100 . 2.00 ...... 47.51

    The loss from sunlight comes from fairly rapid loss of HOCl and OCl- as well as a slower loss of chlorine bound to CYA and both losses are proportional to their respective concentrations though the latter doesn't seem to break down CYA itself any further, possibly from the following:

    HClCY- + h? --> HCY-• + Cl•
    HCY-• + H2O --> H2CY- + OH•

    which is essentially equivalent to what happens when hypochlorous acid or hypochlorite ion break down:

    HOCl + h? --> OH• + Cl•
    OCl- + h? --> O-• + Cl•
    O-• + H2O --> OH• + OH-

    The radicals are powerful oxidizers and eventually terminate usually resulting in oxygen gas and chloride ion. Basically, there are diminishing returns once the chlorine is able to kill algae substantially faster than it can grow. The losses from sunlight become too substantial relative to the clearing rate.

    The loss rate of a 50/50 HOCl + OCl- is a half-life of 60 minutes in a typical pool at noon or a loss rate constant of 60*ln(2)/60 = 69% per hour. The loss rate of chlorine bound to CYA is not as well known, but I estimate it to be around 5% per hour at noon when the CYA is 50 ppm. So with the FC at 5 ppm from above, the hourly chlorine loss at noon is 0.088 * 0.69 + (5-0.088) * 0.05 = 0.31 ppm per hour (which sounds about right since over 8 equivalent noon hours that's a loss of about half the chlorine in a day). At normal shock level, we have 0.629 * 0.69 + (20-0.629) * 0.05 = 1.40 ppm per hour or about 56% per day (11 ppm per day). At yellow/mustard shock level, we have 1.53 * 0.69 + (30-1.53) * 0.05 = 2.48 ppm per hour or about 66% per day (20 ppm per day). With an FC equal to the CYA level, so 50 ppm in our example, we have 6.97 * 0.69 + (50-6.97) * 0.05 = 6.96 ppm per hour or 111% per day (assuming one is maintaining the 50 ppm level, so one loses around 56 ppm in a day to sunlight!).

    Now, IF the chlorine (not bound to CYA) alone was clearing the algae, then the higher level for a shorter time would use less cumulative chlorine because most of the loss is from chlorine bound to CYA which does next to nothing to clear the algae (except, perhaps, if the breakdown from sunlight creates hydroxyl radicals) so shifting to higher FC/CYA ratios that shift the amounts towards active chlorine in a non-linear way would make sense. Unfortunately, chlorine only goes so far so one might argue to blast the algae initially to kill it and then have lower chlorine levels during the filtering (cloudy) phase. It would make the most sense to shock to the highest levels at night when there is no sun on the pool.

    Technically, the "damage" to pool surfaces and equipment would be a product of concentration and time, again assuming first-order in chlorine reactions, so IF you were able to clear the pool proportionately more quickly, then the cumulative "damage" should be similar. In practice, however, clearing a pool of algae is more than trying to oxidize it all with chlorine. A substantial amount of algae chemistry is not nitrogenous compounds and those need to be filtered out of the water. That is the typical cloudy stage that takes much longer when circulation is poor, even if one maintains a higher chlorine level. Chlorine can only go so far.

    So the slower clearing that is not proportional to chlorine level conspires against trying to make the process go faster with higher chlorine levels, at least through the entire process. The only argument for the blasting with high chlorine would be initially during the "kill the algae" phase which pretty much is during the first day except in very fouled pools (i.e. it usually goes from green to cloudy in 24 hours).
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