The main objective when using low-pressure distribution is to evenly distribute wastewater over the entire surface of the drainfield trenches. To accomplish even distribution, pumps and small diameter pipes with small diameter orifices are utilized.
Two general categories of low-pressure drainfields (LPD) are recognized based upon the strength of wastewater being dosed: low-pressure pipe (LPP) and shallow narrow drainfields. LPP is typically installed in a trench and the pipe is supported by stone or other material. A shallow narrow drainfield is typically installed at the base of a shallow trench. It is critical for the service provider to have access to original LPD designs and as-built drawings, as valve/manifold arrangements can vary widely.
After the wastewater is pretreated by at least a septic tank, it flows to the pump tank. The pump tank houses a pump that discharges wastewater to the LPD system three or four times a day. The distribution system consists of a small pipe with holes drilled in it, laid in narrow 6 to 12 inch wide trenches.
Once in the trench, the wastewater seeps into the soil. The soil provides most of the wastewater treatment. Soil particles filter solids and organic matter from the wastewater. Microbes in the soil break down the solids and kill the bacteria and pathogens in the wastewater.