If your home runs on a septic system, you have a tank that stores the solid waste after filtering through the waste water. This tank is often of good quality and lasts a long time, but it probably won't last forever. Here are some signs that it is time to get the septic tank replaced. There is a Strong Odour A common sign that your septic tank needs to be replaced is when you smell bad odours either in your home or outside close to the septic tank or drainfield.