What type of barrier must be placed on top of the porous media before backfilling?

A permeable barrier is required on top of the porous media before backfilling because it allows for adequate drainage and airflow, crucial for the proper functioning of an on-site sewage facility (OSSF). This barrier aids in preventing the build-up of water or gases that could disrupt the treatment process. It ensures that the effluent can flow freely and promotes the necessary biological processes within the system.

By using a permeable material, the performance of the sewage system is enhanced, as it encourages the effective infiltration of treated wastewater into the surrounding soil while still protecting the media from contamination or excessive compaction during the backfilling process. This is vital for maintaining the system's efficiency and longevity.

In contrast, barriers such as plastic, reflective, or insulating types would create impediments to water flow or reduce ventilation, potentially leading to system failures or backups. Thus, utilizing a permeable barrier aligns with best practices in the installation and maintenance of OSSFs, ensuring they operate effectively and sustainably.