Question 1

How much chlorine do I need to disinfect a 500,000-gallon storage tank?

Accepted Answer

At AWWA C652 Method 1 (50 mg/L starting dose), a 500,000-gal tank needs ≈209 lbs of 100% chlorine — about 200 gal of 12.5% sodium hypochlorite or 321 lbs of 65% HTH calcium hypochlorite. Hold 24 hr and verify a free chlorine residual ≥10 mg/L.

Question 2

What is the AWWA C652 chlorine dose for tank disinfection?

Accepted Answer

AWWA C652 Method 1 specifies filling the tank with potable water dosed to a starting free chlorine concentration of approximately 50 mg/L and holding 24 hours, with a minimum residual of 10 mg/L at the end of the contact period. Methods 2 and 3 use a 200 mg/L solution applied to interior surfaces.

Question 3

Can I use HTH (calcium hypochlorite) for tank disinfection?

Accepted Answer

Yes — HTH tablets or granules at 65% available chlorine are common for tank and main disinfection. Pounds of HTH = lbs of Cl₂ needed ÷ 0.65. Dissolve in a portable batch tank before introduction; never mix dry HTH with petroleum, ammonia, or organics.

Question 4

Do I need to dechlorinate before discharging the super-chlorinated tank?

Accepted Answer

Yes. AWWA C655 and most NPDES permits require dechlorination of any water with >0.1 mg/L free chlorine before discharge to surface water or storm sewer. Sodium bisulfite, sodium thiosulfate, or ascorbic acid are the standard reducing agents.

Tank Disinfection Calculator – AWWA C652 Chlorine Dose for Storage Tanks