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Mound Septic System Information

Mound Septic System Information Page

A mound septic system is used for aberrant soil conditions.  In order for effluent (waste water) to be completely treated it must pass slowly through 3 feet of dry soil.  If you have a high water table, lets say at 1 foot below the surface, you need to bring in 2 feet of soil (sand) to get that 3 foot of separation.  Then you build the drainfield on top of that….thus a mound septic system. The pipes in a mound septic system are laid out in a double H pattern. The pipe is 1½” in diameter with ¼” holes. 

A mound septic system use 2 tanks…this first tank is used as a normal tank (for settling of the solids) and the second tank has a pump in it that when the water level reaches a certain point or with a timer, the pump kicks on and pushes the effluent to the  mound…because of the small pipe and holes, the effluent is distributed (under pressure) evenly throughout the mound.      

A mound septic system is also used when you have rocky soil because the effluent will run right through it without being treated.  Conversely, if the soil is too slow mounds can be used because a large portion of the effluent is returned to the atmosphere via evaporation so a mound will make up for slow soils.      

The mound septic system is really the grand daddy of alternative systems.  They were developed in 60’s by the University of Wisconsin and can pretty much be used in any situation.    

The drawbacks to a mound septic system are:   

·        They are more expensive.  To dig a trench and fill it with gravel is fairly easy and therefore relatively inexpensive.  To build a mound is a lot more involved (even requiring special tracked equipment) and obviously cost more to build.  

·      They are ugly.  Essentially they are a lump of dirt in your yard and very few people find this attractive.  However there are people that are using paver blocks and decorative plants to landscape them and many of them actually enhance the yard because they are so beautiful.  The University of Minnesota Extension Service has a publication on landscaping mounds.    

·        They fail more frequently than a conventional system.

The 3 main things that kill a mound septic system are:   

1. –Suspended solids in the effluent.  A mound septic system, like an aerobic, sand filters, drip and spray irrigation and wetland systems, use pumps to move the effluent through the system.  They also have smaller diameter pipes with smaller diameter holes.    

The primary cause of pump burnout and pipe plugging is solids in the effluent.  These solids damage the impellers in the pumps and plug the holes in the pipes.  These solids also plug the pores of the soil (which is the major source of any system failure…tanks don’t fail, the soil fails and the soil fails when it gets plugged with solids).  This is why it is critical to use effluent filters in the tank and washing machine filters in the house to keep these solids out of system.  The total cost of these filters is about $300.    

Often the contractor or homeowner plays it cheap and doesn’t want to spend the extra money on these filters, but when the system fails in 3, 5 or 7 years, and they find out it is going cost another $6,000 to replace it, that $300 looks pretty small and they wish they would have spent the extra dough in the beginning.  Even replacing a pump can cost $300 to $600 so these filters pay for themselves in short order.        

2. –Overloading the system with water. Systems are sized according to the worst case scenario. Most families will only use 100-250 gallons of water per day…but the system will be sized to 500 gals per day, plenty of cushion…but if you have leaking fixtures like toilets and faucets you can overload the system in a matter of days. You avoid this by making sure you don’t have plumbing leaks.   

3.-Putting too many harsh chemicals into the system killing off the bacteria in the tank and soil. Your system works on a natural bacterial process of bacteria in the soil and tank that “eat” the nasty things in the wastewater. If you put in chemicals that kill these bacteria the system will fail and you prevent these problems by paying attention to what you are putting down the drain (normal amounts of cleaning supplies will not hurt a system).   

If the system is overloaded with water you stop using so much and the system will dryout and start working again. If you kill off the bacteria you can stop using chemicals and they will grow back…but if you plug the system with solids it gets tough to recover it…you can jet the lines but you will still have solids in the soil…sometimes you can get them to breakdown, sometimes you can’t. The best way to prevent solids from reaching the mound is to install a washing machine filter to keep the very fine solids out of the system and an effluent filter (in the tank) to prevent the larger solids from getting out to the mound or any type of system for that matter.   

Not cheap to install and maintain.  Uses pumps that burn electricity and burn out.  Good for removal of pathogens and better at removing nitrogen…an issue near bodies of water.   

Sand filters-with a sand filter you have 2 tanks (just like a mound) but the effluent comes out of the tank and is sprayed over a box of sand that is buried underground.  The effluent filters through the sand then is pumped to a regular drainfield or mound.   

Not cheap to install and maintain.  Uses pumps that burn electricity and burn out.  Sand gets plugged and needs removal/replacement.  Good for removal of pathogens and better at removing nitrogen…an issue near bodies of water.   

Peat filters-same as a sand filter except peat takes the place of the sand.    

Not cheap to install and maintain.  Uses pumps that burn electricity and burn out.  Peat degrades and needs replacing every 6 to 17 years.  Good for removal of pathogens and lots better at removing nitrogen…an issue near bodies of water.  

Recirculating Sand filters-with a sand filter you have 2 tanks (just like a mound) but the effluent comes out of the tank and is sprayed over the buried sandbox.  The effluent filters through the sand then is pumped to a tank where it is pumped back through the sand filter 4-6 times (for better nitrogen removal) then discharged to a regular drainfield or mound.    

Not cheap to install and maintain.  Uses pumps that burn electricity and burn out.  Sand gets plugged and needs removal/replacement.  Good for removal of pathogens and better at removing nitrogen…an issue near bodies of water.