| OWDP Home > Online Resources > Treatment Systems > Wisconsin Mound | ||||
The mound system was originally developed by the North Dakota Agricultural College in the 1940’s and was known as the Nodak Disposal System. In 1976, the small scale Waste Management Project at the University of Wisconsin began a study of mound systems. In 1979, the project issued a design manual describing site conditions and design criteria for mounds. Ongoing research and monitoring of mounds has lead to further refinement of the location and design criteria. In 1990 an updated mound manual was released and a 2000 manual is scheduled for release later this year.
The mound system was originally developed by the North Dakota Agricultural College in the 1940’s and was known as the Nodak Disposal System. In 1976, the small scale Waste Management Project at the University of Wisconsin began a study of mound systems. In 1979, the project issued a design manual describing site conditions and design criteria for mounds. Ongoing research and monitoring of mounds has lead to further refinement of the location and design criteria. In 1990 an updated mound manual was released and a 2000 manual is scheduled for release later this year.
Mounds were developed to overcome three conditions: (1) slow or fast permeable soils, (2) shallow soil cover over creviced or porous bedrock, and (3) a high water table. A site that has any one, or a combination of, these conditions is not suited for a conventional system. Because acceptable soil conditions are not always found below the surface, the mound allows the conditions to be created above the surface.
There are three main components to the mound system: (1) a treatment unit such as a septic tank or secondary treatment device, (2) a pump chamber or dosing system, and (3) the mound disposal system.
Typical Mound System
The objective of the conventional septic tank or secondary treatment unit is to remove pollutants such as BOD and Suspended solids prior to discharge to the mound. BOD can cause in increase in the biomat that could ultimately lead to the sand media in mound to clog. The solids could fill void spaces in the media and also cause the mound to clog.
The pump chamber operates exactly as mentioned above in the Low Pressure Dosing System.
The mound is a leach field that is raised above the natural soil surface. The mound is composed of a sand fill that has a gravel-filled bed and a network of small diameter pipes called the distribution system.
From the pump chamber, effluent is pumped through the pipes in controlled pressure doses so that uniform distribution is achieved throughout the bed. The effluent comes out of the pipes through small orifices and trickles downward through the gravel bed and into the sand. Treatment occurs as the effluent moves through the sand and returns to the natural soil.
Two factors that determine the size and configuration of a mound are how the effluent moves away and the rate at which it moves away from the system. The prediction of the movement and rate of movement is done from the soil and site information obtained from test hole observations and percolation tests.
A suitable depth of soil is required to treat the effluent before it reaches the limiting condition, such as bedrock, a high water table, or a slowly permeable soil layer. The vertical separation distances between limiting layers varies, but typically ranges between one to four feet depending upon the amount of pretreatment.
Mound absorptions systems should be long and narrow and should follow the contour. The more restrictive the site, the narrower and longer the system should be.
A high water table is determined by direct observation and interpretation of soil mottling or other criteria. The bedrock should be classified as crevice, non-crevice semi-permeable, or non-crevice impermeable. This will help determine the depth of the sand media required.
Loading rates should be based on soil texture, structure and consistence, with the percolation test used to confirm the soil analyses. However, at this time percolation tests are used in Arizona to estimate the soil permeability by relating an empirical loading rate to the percolation rate.
Mounds can be constructed on sites with slopes up to 25%. The slope limitation is primarily for construction safety, because it is difficult to operate equipment on steep slopes. Furthermore, more fill material is needed for steeper slopes, which significantly affects the cost of the system.
Mounds should not be constructed in flood plains, drainage ways, or depressions unless flood protection is provided. Another location consideration is maintaining horizontal separation distances from wells, surface waters, springs, dry washes, road cuts, buildings and property lines.
The selection of the sand fill material is critical to the performance of the mound. Suitable sand is one that can be loaded at a reasonable rate and adequately treat the effluent. The sand should contain 20 percent or less material greater than 2.0 mm and 5 percent or less material finer than 0.053 mm. It should also have a size distribution that meets ASTMC-33 specifications.
For residential mounds, the daily wastewater volume is usually determined by the number of bedrooms in the house. Flows in Arizona for individual homes are up to 140 gallons per day (gpd) per bedroom. Mounds that are treating effluent with higher strength wastes have different criteria, possibly including pretreatment.
There are several things that can be done to help protect the mound and make it ascetically appealing at the same time. The right vegetation cover helps limit erosion from the mound and make it more attractive. However, the wrong cover can do irreparable damage to the mound. Do not plant trees, shrubs, or any plant with an extensive root system on the mound. The roots will interfere and possibly destroy the distribution system. The mound can be outlined with trees or shrubs, but they should be at least 20 feet away. Also, avoid planting vegetables or herbs on the mound.
The best things to plant on the mound are low-maintenance grasses, such as a mixture of creeping red, hard and sheep’s fescues, and perennial flowers such as daylilies and peonies. Because the mound will tend to dry on top, plant grasses and other ground cover that are resistant to water stress there. Cool season grasses and other plants may be planted on the sides of the mound
Use minimal tilling when planting and establish a cover as quickly as possible to limit erosion. The topsoil on the mound should be at least six inches, but no more than 30 inches deep. Keep traffic on the mound to a minimum, if you plant a lawn, plan to mow it only two or three times a year. If you have pets or children, you may need to erect a fence around the area. Do not irrigate or water the mound once you have vegetation on it.
The actual shape of the mound may be changed somewhat to suit individual landscaping and site needs. Constructing a contoured mound works well for hillsides. One built at a right angle can be used in the corner of a flat property. A rectangular mound is often used when there is plenty of room and a uniform slope. Landscaped areas around the mound can serve as a privacy barrier, a wind block or as a screen to block unsightly views.
With a few simple maintenance procedures, a properly designed and installed mound system will last for many 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 mounds is minimal. To reduce the possibility of failure to the mound all surface water and roof drainage should be diverted around the mound. As a minimum, the laterals should be flushed annually. Also, check the mound 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 mound system enables use of land that may otherwise be unsuitable for ground or at-grade systems.
· The natural soil utilized in a mound 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 mound area.
· Mounds can be utilized in most climates.
Disadvantages:
· Construction costs can be high do to expensive fill materials and the need for hand labor.
· Care must be taken to not damage the top permeable topsoil under the mound.
· The location of the mound 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).
· Mounds may not be aesthetically pleasing in some cases.