Infiltration of wastewater in buried soil infiltration systems has been promoted as a low cost, effective alternative for treatment and disposal of wastewater flows from commercial developments and small communities. Unlike many mechanical wastewater treatment systems, soil infiltration systems are subject to complex, far-reaching influences related to natural site conditions and the interaction of wastewater with a dynamic soil and ground water system. As a result, design and performance relationships are not always well defined and systems are often implemented based on local tradition and empiricism. Consequently there is a need for improvement of design criteria especially for large subsurface wastewater treatment systems.
Successful performance starts with thorough site investigations, where large scale infiltration tests or tracer studies might be needed. The hydraulic loading rate is a principal design parameter. An integrated approach for assessment of the hydraulic loading rate based on soil type and wastewater quality is suggested. In general, trench design should be preferred over beds, but rational criteria for selection of optimal geometry is lacking. Purification performance of subsurface wastewater infiltration systems is generally good. Estimates of purification on the basis of soil grain size, soil depth and loading rate can be given.