The typical low-pressure pipe drainfield consists of a pressure manifold and trenches (although a bed-type configuration may be used). The laterals, usually 1¼- to 2-inch diameter PVC pipes, are surrounded by washed stone, gravel, plastic chambers, slotted irrigation pipe, or synthetic media placed in the trench. Trenches must be able to store the effluent until it is accepted by the soil. Because effluent exits the orifices at a relatively fast rate, temporary ponding occurs in the bottom of the trenches. Ponding can be monitored with observation ports.
Because straight septic tank effluent is being dosed, the development of a biomat at the trench infiltrative surface is to be expected. However, a fully developed biomat is not required for distribution of the effluent as is needed in gravity distribution trenches. The pipe network with orifices distributes the effluent. The dose volume and resting period between doses controls the quantity of ponding and distribution of the effluent. LPP systems can be either time-dosed (promoting flow equalization) or demand-dosed.