Can you backwash your pool to your septic tank?

No, the chlorine concentration in your pool water is designed to kill all of the bacteria that end up in the water in order to keep your pool clean. The bacteria in your septic tank are good bacteria that break down what you flush down the toilet so that it doesnt get backed-up. Dumping your pool water into the septic tank will kill these good bacteria and stop your septic tank from working. THIS WILL BE VERY BAD.

So the bottom line is definitely NO. If you need somewhere to empty your pool, get a long enough hose to dump it into a storm drain on the street. (thats even what the pros do)