Could be the second tank half pipe. Tank is divided, "solids" go on one side to be broken down by the buggies, the second tank is only for liquid and goes to the field. You want a pipe in that second tank cause if any solids slip over the top they could clog up the field line.
Could also be a simple clean-out and/or inspection pipe for your leech field. Our septic system has 11 pipes on it.
Could also be a simple clean-out and/or inspection pipe for your leech field. Our septic system has 11 pipes on it.