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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.
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