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The Wisconsin At-Grade Soil Absorption system is one of several soil absorption systems that can be used to treat and dispose of on-site wastewater through the soil. It is a relatively new system with the first system installed in 1982. Since that time a number of systems have been installed and appear to work will on sites that don't meet the criteria for in-ground soil absorption systems but exceed the criteria for the Wisconsin Mound.
Why use a Wisconsin At-Grade System
The Wisconsin At-Grade system is designed for sites with soils too deep to justify a mound but too shallow to install a below-grade soil absorption system.
The Wisconsin at-grade soil absorption system accepts septic tank effluent, treats, and disposes of it in an environmentally acceptable manner. It serves the same function as in-ground soil absorption trenches or a mound system. Just like the Wisconsin Mound there are three main components to the at-grade system: (1) a treatment unit such as a septic tank or secondary treatment device, (2) a pump chamber or dosing system, and (3) the at-grade disposal system. The hydraulics and treatment concepts are very similar to the in-ground trench or the mound system.
The system configuration must meet the soil and site criteria. Prior to the design, the designer must use the soil profile to estimate the effluent acceptance rate of the soil and to determine the flow path. It is very difficult to determine the exact effective width that the system should be. A system that is too wide may leak at the down slope toe, other factors such as oxygen exchange beneath the absorption area are affected by the width. As with the mound it should be designed long and narrow whenever possible.
The system is installed as follows:
· The existing soil is tilled,
· Observation tubes and aggregate are placed,
· The distribution network is installed,
· The fabric covering is laid on the aggregate
· A soil cover is placed over the fabric and on the side slopes.
From the pump chamber, effluent is pumped through the pipes in controlled pressure doses so that uniform distribution is achieved throughout the absorption area. The effluent comes out of the pipes through small orifices and trickles downward through the gravel bed and into the tilled soil.
With a few simple maintenance procedures, a properly designed and installed at-grade system will last for several years. This system does have some mechanical equipment that will need care and replacement.
The treatment unit and pump chamber must be operated and maintained as mentioned in other sections of this document. For example, the septic tank should be checked for sludge and scum buildup and pumped as needed. The effluent filter should be removed and cleaned as needed.
The maintenance required for at-grade systems is minimal. To reduce the possibility of failure to the system all surface water and roof drainage should be diverted around it. As a minimum, the laterals should be flushed annually. Also, check the at-grade system for the following conditions:
· Ponding in the absorption area
· Seepage out of the side or toe
· Spongy areas developing on the side, top, or toe
· Clogging of the distribution system.
Some Advantages and Disadvantages are listed below.
Advantages:
· The Wisconsin at-grade system enables use of land that may otherwise be unsuitable for conventional trenches but exceed the criteria for the Wisconsin mound.
· Less costly than a mound because there is no need for the costly C33 sand.
· The natural soil utilized in at-grade system is usually the top layer, which is typically the most permeable.
· Construction damage can be minimized since there is little excavation required in the absorption area.
· At-grade systems can be utilized in most climates
Disadvantages:
· Care must be taken to not damage the top permeable topsoil under the absorption area.
· The location of the system may affect drainage patterns and limit land use options.
· Need for electricity and mechanical equipment (in some circumstances siphons can be used in place of pumps).