At Grade Mounds

The at-grade system is an on-site sewage disposal system that utilizes a raised bed of gravel or stone over the natural soil surface with a pressure distribution system constructed to equally distribute the pretreated effluent along the length of the gravel bed.

The operation of the at-grade system is a two-prong process involving both wastewater treatment and dispersal into the underlying soil. Final treatment is accomplished by physical and biochemical processes within the soil. The physical characteristics of the effluent, the loading rate, and the receiving soil can affect the physical and biochemical processes.

The at-grade system contains a distribution network placed on the raised gravel bed, which is constructed on top of the plowed or properly scarified soil dispersal area. The gravel bed and distribution network are covered with non woven geotextile fabric and quality topsoil. A soil setback of at least five (5) feet surrounds the gravel bed. Effluent is equally distributed into the bed where it moves into the soil and undergoes final treatment and dispersal processes. Figures 1, 2 and 3 depict the basic layout of an at-grade system.

A pressure distribution network allows effluent to be uniformly applied to the topsoil over the entire length of the gravel bed. Unsaturated flow conditions are created, enhancing treatment, and allowing the effluent to move laterally until absorbed into less permeable subsoils. Gravity distribution is not permitted.

The at-grade’s soil cover material provides frost protection and allows moisture retention sufficient to maintain a vegetative cover. The original soil below the gravel bed serves as the final treatment and dispersal medium for the effluent.

When sited, designed, installed and maintained in accordance with this manual, regulatory requirements and basic maintenance guidelines, the at-grade system provides treatment and dispersal of domestic wastewater in conformance with Code of Maryland Regulations 26.04.02.