Superchlorinating
I'll use an example of 30 ppm CYA to give you an idea of what goes on with different FC levels below and above that and the corresponding 99.9% kill times assuming a 15,300 CT value for Crypto. I assume the pH is 7.5 in all cases so when shocking you need to significantly lower the pH prior to adding a hypochlorite source of chlorine or need to adjust with acid afterwards which can be tricky since the pH test becomes invalid at high FC levels (especially as high as needed for faster Crypto inactivation).
FC .
HOCl .
FCequiv .
Time for 99.9% kill
2 ... 0.027 . 0.055 ... 193 days ... this is FC/CYA of 6.7% so a little below the regular minimum
3 ... 0.042 . 0.085 ... 125 days ... this is FC/CYA of 10%
12 . 0.294 . 0.595 ... 17.9 days ... this is regular shock level
18 . 0.687 . 1.390 ... 7.6 days ... this is yellow/mustard algae shock level
25 . 1.597 . 3.230 ... 3.3 days
30 . 2.661 . 5.383 ... 47 hours (2 days)
35 . 4.097 . 8.288 ... 30.8 hours
40 . 5.851 . 11.837 ... 21.5 hours
45 . 7.832 . 15.843 ... 16.1 hours
50 . 9.962 . 20.152 ... 12.7 hours
So if your CYA test was off by 10 ppm then 40 ppm FC with 40 ppm CYA would be 41 hours instead of 22 hours so figure roughly double for that sort of error. So if you didn't go twice as long, then you would get a 96.8% reduction instead of a 99.9% reduction. Remember that the 15,300 is conservative for the worst-case strain.
Chlorine Dioxide
From
this paper and using the most conservative 1000 mg•min/liter for the CT for 2-log reduction which corresponds with 1500 for a 3-log reduction, 2 ppm chlorine dioxide would take 1500/2 = 750 minutes (12.5 hours) for inactivation of Crypto. Chlorine dioxide breaks down in sunlight so would be used overnight for outdoor pools. You can create chlorine dioxide by adding sodium chlorite to a pool that already has chlorine in it and the CYA in the pool is a good thing since the lower active chlorine level reduces the formation of chlorate. The product
Katadyn Micropur MP1 Purification Tablets has 6.4% sodium chlorite (the image of the package says chloride, but that's a misprint) and 1.0% Dichlor (dihydrate) to make chlorine dioxide for drinking water and is EPA approved for that purpose. On a molar basis, it's (6.4*90.442)/(1.0*237.95) = 2.4 times more chlorite than dichlor, but dichlor produces two HOCl in water so it's really 1.2 times more chlorite than chlorine for just a small excess. You could roughly use equal amounts so if you have 3 ppm FC in your pool then add roughly 3 ppm equivalent of sodium chlorite. 1 ppm sodium chlorite anhydrous in 10,000 gallons is (1 mg/L ClO
2)*(37854.1 liters)*(90.442 g/mole NaClO
2)/(67.45 g/mole ClO
2)/(1000 mg/g) = 50.758 grams or 1.79 ounces weight while for the trihydrate form of sodium chlorite it's (1 mg/L ClO
2)*(37854.1 liters)*(144.487 g/mole NaClO
2•3H
2O)/(67.45 g/mole ClO
2)/(1000 mg/g) = 81.089 grams or 2.86 ounces weight. So for 50,000 gallons and for 2 ppm chlorine dioxide (where the pool has roughly 2 ppm FC or slightly less), you need 10x this amount so 1.1 pounds of sodium chlorite anhydrous or 1.8 pounds of sodium chlorite trihydrate. It would be slowly added to the pool water as you would with any other concentrated chemical -- fortunately adding chlorite to chlorinated water minimizes by-products while the other way around would be more of a problem.
The chlorine dioxide manufacturers did not pay the money for efficacy and especially the expensive safety tests for use in swimming pools so it is not approved by the EPA for that purpose. However, for remediation purposes there is probably nothing against that in your state regs. The proposed CDC MAHC
Code and
Annex do allow for chlorine dioxide to be used for remediation when the aquatic venue is closed and no bathers are present and is allowed by the EPA under Section 18 (emergency exemptions) of FIFRA regulations.
The issue with an indoor pool is how to get rid of the chlorine dioxide. Since it breaks down from the UV in sunlight, you obviously want to turn off any UV system that you may be using, but if you have one then you can use it to get rid of the chlorine dioxide by turning the system on and circulating.
You can measure the chlorine dioxide level in a DPD (or FAS-DPD) test, but it looks very much like chlorine so you can't well distinguish between them. You can remove the chlorine dioxide using the same reducing agents you would use to dechlorinate.
Filtration
Also note in the Annex that it says that the use of a sand filter by itself may remove only 25% of oocysts, but when a coagulant is used it may remove 99% of oocysts. If this is the case per pass, then it would not take too long to reduce Crypto to safe levels. The MAHC specifies the following formula for determining the flow rate (Q, in gallons per minute) to achieve this is the following:
Q = V x (14.8 - ln(V)) / (60 x T)
or equivalently
turnovers = 14.8 - ln(V)
where V is volume in gallons, T is time in hours. So for a 50,000 gallon pool that would be 4 turnovers. This is because the large pool volume dilutes the oocysts from a diarrheal incident already. The standard isn't a 99.9% reduction, but rather an absolute reduction from an assumed 100 million oocysts from a single incident down to a concentration of 1 oocyst/100ml which is considered to be low enough to prevent infection.
