Soil Infiltration System (dispersal cell)
The drainfield (soil infiltration) is the final and most important step of the effluent treatment and dispersal. The size, elevation, location and shape fo the drainfield are all relative to the expected usage and soil characteristics.
The drainfield sizing is determined by the flow from the house (based on number of bedrooms) and the type of soil. Usually the more pervious the soil, teh smaller the drainfield, however, a certified soil tester makes this determination based on the many physical features within the soil, such as texture, structure, consistence and layering of the soil. This information is recorded on a "Soil and Site Evaluation Report" form.
The elevation and location of the drainfield is determined by the soil characteristics and depth of limiting conditions such as seasonal water table, saturation zones with the soil, or bedrock. For systems without pretreatment, it is necessary to maintain a safe vertical distance of 3 feet between the bottom of the drainfield and limiting conditions for proper treatment and renovation of effluent.
The shape of the drainfield is dependent upon the maximum length of suitable soil available, preferable along the contour. A drainfield design using trenches or narrow beds has been shown to improve aeration in the soil beneath the gravel thereby enhancing system treatement performance and longevity. When there are several trenches or beds, a distribution box may be incorporated to promote equal distribution of effluent.
Effluent inside the drainfield is dispersed in two ways - gravity or pressure distribution. Pressure distribution uses small diameter pipes and relies on a pump to force the effluent into the piping network. Research has shown that a pressure distribution network will effectively disperse the effluent throughout the drainfield evenly thereby delaying the over saturation of any one part of the drainfield. Saturation promotes clogging of the infiltrative surface. When a pressure distribution drainfield is lower in elevation than the septic tank, a siphon may be used to force the effluent through the system instead of a pump. A siphon must be checked periodically to ensure that it is properly discharging effluent in doses rather than "trickling" effluent into the drainfield.
Figures 1-4 depict typical drainfield layouts that are currently used in Wisconsin.
This type of system is used when the soil absorption bed is located higher in the landscape or where pressure distribution is desired.
3: At Grade
This system is similar to the At-Grade with washed sand placed on the plowed surface and the drainfield bed constructed in the sand. When finished, this will be approximately 3-4 ft above the grade.
Figures 5 & 6 depict advance pretreatment systems.
5: Sand Filter
Sand filters, utilize a sand media within a containment structure. Wastewater is treated as it passes over bacteria attached films on the sand media. As with the ATU, these units produce a high quality effluent to the soil.
6: Aerobic Treatment
This unit treats the effluent at the tank through aeration of the wastewater. This promotes the growth of bacteria that reduces wastewater constituents and produces a higher quality effluent to the soil absorption bed.
Note: Graphics supplied by the Department of Commerce