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The Wisconsin Mound System

What is it and how does it work?

Wisconsin mounds were developed in the United States in the 1970’s as a system designed to treat and dispose of human wastewater in areas identified as generally unsuitable for conventional septic tank and soil absorption systems.  Over the years the design of the mound has been modified and improved to the present design that if sized and installed correctly is capable of appropriately disposing of domestic wastewater in a number of locations within the Port Stephens Local Government Area that have specific site limitations making them unsuitable for conventional systems.

Mounds are pressure-dosed sand filters constructed above the natural ground surface and discharge indirectly to the natural soil.  Their main purpose is to provide additional treatment to domestic effluent that has already undergone either primary treatment in a septic tank or secondary treatment in a secondary treatment system.  Mound systems are designed to overcome site restrictions such as:

• Highly permeable soils (sands);
• Slowly permeable soils (clays);
• High groundwater tables;
• Shallow soil cover over creviced or porous bedrock.

There are three components to a mound system, these being a pre-treatment tank, dosing or pump chamber and the elevated mound (refer figure 1).

Figure 1. Schematic of a Septic-Wisconsin Mound Wastewater System (Source Converse and Tyler (1987a), copyright © by the American Society of Agricultural Engineers)

The pre-treatment tank can be a septic tank (primary treatment), a wet composting treatment system (enhanced primary treatment) or a secondary treatment system such as an aerated wastewater treatment system (AWTS).  Whichever system is used, the main purpose is to remove suspended and organic materials from the wastewater.  Pre-treated effluent flows from the pre-treatment tank into a dosing or pump chamber that contains a pump. 

The pump distributes the effluent into pipework within the mound where it is evenly distributed over the infiltration surface.

The mound is made up of layers of sand, non-crushed gravel aggregate through which a network of small diameter perforated pipes is placed, a permeable fabric and a soil cover that has an established grass cover.  The network of perforated pipes distributes the effluent evenly over the gravel aggregate from where it moves vertically through the sand layer and then into the natural soil.  The effluent receives treatment through physical, biological and chemical means as it filters down through the aggregate and sand layers.  Once it reaches the natural soil the effluent has been treated to a level considerably superior to that obtained within conventional absorption trench systems.  If a mound is used in conjunction with a pre-treatment system providing a higher level of treatment than a septic tank then the quality of effluent reaching the natural soil is of an even higher standard.

When a mound system is installed properly, it should continue to operate for a considerable period of time.  It should be noted that the greater the level of pre-treatment the better the mound will operate and the longer it will last.  Another factor that determines the performance and lifespan of the mound is the type of sand that is used in the construction.  Coarse sand with a specific grain size and uniformity coefficient must be used to ensure the correct flow rate of effluent occurs vertically through the mound.  A mound should always be installed by a licenced tradesperson with suitable experience in their construction.

Maintenance requirements for a mound system are generally low.  Maintenance issues to consider include:

• A desludge of the pre-treatment tank and pump well should be performed on average every five (5) years;
• The pre-treatment tank and pumpwell must remain watertight.  It is critical that surface water and groundwater be prevented from entering the tanks;
• The vegetation cover over the mound should be maintained on a regular basis with the clippings always removed from the surface of the mound;
• Possible cleaning out of the distribution pipework to prevent clogging.

Years of monitoring the performance of mound systems have shown that they can consistently and efficiently treat and dispose of domestic wastewater providing the mound has been sized, installed, operated and maintained correctly.

Should you require further information regarding mound systems contact Councils Wastewater Management Officers on 49800106 or 49800394.

Contact details
Waste Water Management Officer
02 49800394 (ph)
andrew.weekes@portstephens.nsw.gov.au

© 2008 Port Stephens Council
This page: http://www.portstephens.local-e.nsw.gov.au/environment/44034/44037.html