Chlorous acid has some chlorine dioxide in it, but is generally not used for water purification because of the high amounts of disinfection by products formed during purification.
However, to purify water that has been filtered to remove the large chunks you need a CT of 1000. In an emergency you can use chlorous acid made following a modification of the Miracle-Mineral-Supplement protocol. You would mix up 1.13 ml (about 19 drops) of 22.4% sodium chlorite and add 1.13 ml (about 23 drops) of 10% citric acid and let this activate for 10 minutes. This would then be added to 1 gallon of water and after 4 hours the water would be ready to drink. If you are using 50% citric acid you would only need 0.23 ml (about 5 drops) of the 50% citric acid .
The problem with using chlorous acid to purify water is that you end up with water that has an off taste and it tends to be acidic.
You can approximate using chlorine dioxide by changing acids used for activation. The problem with this is that MMS is an industrial strength chemical and requires an industrial strength acid to activate. At these strengths the reaction is violent and there is some danger involved in mixing.
If you dilute MMS down to a 5% sodium chlorite solution, you can use 6% hydrochloric acid to activate it. In this case you would use 0.5 ml (about 10 drops) of 5% sodium chlorite and add 0.5 ml (about 10 drops) of 6% HCl. This activation takes about 30 seconds, then you can add that to 1 gallon of water and it will be purified in 4 hours.
The CT of 1000 assumes that you have cysts in the water (giardia and cryptosporidium). If you remove those by filtering, the hold time for disinfection is reduced to 15 minutes.