Shallow narrow drainfields are used to distribute wastewater that meets a minimum treatment level of 30 mg/L of both BOD5 and TSS. Shallow narrow drainfields are placed in the upper soil layers (6 to 12 inches from the ground surface) for maximum wastewater treatment by natural soil processes. Shallow placement maximizes vertical separation distance from the drainfield base to the groundwater.
A shallow narrow drainfield works by pressure-dosing treated effluent into a small PVC drainfield lateral which is typically ¾ to 2 inches in diameter. The drainfield lateral pipe usually has 1/8 to 3/16 inch orifices drilled every 18 to 24 inches. The pressurized effluent squirts up against a cover made of a 12-inch PVC pipe cut lengthwise (or it may be sleeved in a 4-inch corrugated pipe with perforations).
This half-pipe protects the trench and helps distribute the effluent evenly over the trench bottom just below the ground surface where biological activity is greatest. Effluent infiltrates the native soil surface and percolates down through underlying soil where additional nutrient and pathogen removal occurs.