Once the distribution box is free, you can remove it. With the old box removed, you need to provide a flat area for the new distribution box. A distribution box — also called a D-box, junction box, or header box — is a small concrete, fiberglass, or plastic chamber typically 12 to 24 inches in diameter. It is installed at the beginning of the drain field, receiving the outlet pipe from the septic tank and distributing flow into multiple. A distribution box splits septic effluent evenly across your drain field. Learn how d-boxes work, what failure looks like, and what replacement costs in 2026. I can't get the lid off, though.