Which materials are inappropriate for septic tank backfill?

Using large rocks and non-porous materials for septic tank backfill is inappropriate due to several reasons. Septic systems rely on the ability of the surrounding soil to both filter and absorb effluent effectively. Large rocks can create voids that can impede the proper distribution of effluent, leading to potential wastewater pooling and system failure. Non-porous materials, on the other hand, do not allow for adequate drainage or aeration, which are crucial for the natural biological processes that occur in the septic system. This can prevent the system from performing its intended function of treating sewage effectively.

In contrast, materials such as sand, gravel, topsoil, and permeable soil are preferable for backfill because they promote proper drainage and prevent the accumulation of water. They support the soil's role in filtering harmful contaminants and allowing wastewater to be treated effectively before it reaches the underlying groundwater or surface water.