I am on a septic that was emptied 3 mo ago. My kitchen sink started recently to back up filling both sides and making pipes explode underneath sink due to pressure. I snaked lines and found thick black sludge in line. After using snake, hot water and drain cleaner, it seems to no longer be thick. How do I know if line is clearede?

Official way, run a camera scope down to see if it is clear.

Check flow rate ( drain) on your sink to water going down clear pipe of same size.

Wait to see if it backs up again.

I am on a septic that was emptied 3 mo ago. My kitchen sink started recently to back up filling both sides and making pipes explode underneath sink due to pressure. I snaked lines and found thick black sludge in line. After using snake, hot water and drain cleaner, it seems to no longer be thick. How do i know if line is clearede?
The black gunk's a combination of grease, rotting food and anything that sticks to it. If you fill one tub with hot water and it drains out then you have cleared the line. Good job! Regards, Tom