I too have a septic system and I use Iodophor. Not a good idea?
I personally like Star-san better because of it's many different uses (example) I wash my fruits and vegi's with it but:
If you treat your septic tank right otherwise a little bit or even 5 gallons of diluted iodophor won't cause any problems
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Can Some Conditions Kill Off Needed Septic Tank Bacteria?
If other conditions at a property have resulted in killing-off the (needed) septic tank bacteria (such as adding unusually large amounts of bleach, disinfectants, or antibiotics to a septic tank) some folks sell bacterial "starters" to "rejuvenate" the septic tank. To me this makes little sense for the following reasons:
Calculations of "septic tank die-off" which demonstrate that about 2 gallons of bleach is likely to harm septic tank bacteria have been based on a "static septic system", a fixed septic tank volume into which no new wastewater, sewage, and their diluting and re inoculating effect have been considered.
If you don't correct the conditions that have caused a bacterial die-off in the septic tank, no amount of starter or booster is going to make any difference.
Adding products such as enzymes which claim to break down grease risk destroying the floating scum layer in the septic tank, forcing unwanted oils and debris into the leach field.
As soon as you stop putting inappropriate bleach, disinfectant, or antibiotics into the septic system and after the first time someone uses a toilet, the septic tank has been re inoculated with what it needs.
Forcing hydrogen peroxide or other chemicals into drainfield or leach field soils can damage the soil and contaminate the environment.