Below-grade systems are constructed in original soil with distribution of effluent occurring below the soil surface. In below-grade systems, the soil treatment area is designed and installed such that the infiltrative surface is below the original ground elevation and a final cover of topsoil stabilizes the completed installation, supports vegetative growth, and sheds runoff. The underlying soil treats the many harmful components in the effluent before it reaches the surface or groundwater.The two types of below-grade soil treatment systems commonly used are trenches and seepage beds. Generally these distribution systems have pretreatment prior in a septic tank but could also
