Once the D-box lid is removed, inspect the interior to identify the blockage. When this critical component becomes blocked, wastewater may back up into the home, flood the drainfield, or contaminate surrounding soil and. A septic distribution box (D-box) is a concrete or plastic junction that evenly distributes wastewater from your septic tank to all drainfield lateral lines. When it fails, symptoms include uneven wet spots in the yard, slow indoor drains, and sewage odors. Its function is to receive pre-treated liquid waste, known as effluent, and distribute it evenly across the drain field pipes. When these distribution boxes malfunction, it can lead to a. Diagnose problems at the septic system drop box: procedures for troubleshooting leaks, smells, or backups & flooding in the septic system D-box.