From the description provided by you, looks like, you have a septic tank in South-West (SW) Direction, Please confirm.
As per Vastu, you should never have a septic tank in SW. West side of NW is the best location to have a septic tank. Some people also give second option as SE.
Best remedy for this is to close the existing one at SW and create a new septic tank.
No pipelines should have outlet from South, make sure to turn them towards North or East or NW or West.