I have a septic tank but when I asked our plumber he said the 25 gallons I put down the drain a couple of times of month is nothing compared to the volume of shower, washer, dishwasher and commode water that goes down it each month. He said it shouldn't bother the bacteria level in the tank but if I was worried to add some every few months. I also run the shower for a few minutes after dumping it to flush the pipes (PVC) out.