Septic tank Pump tank

Distribution pipeSand

Gravel

Geotextile fabric

On-site wastewater treatment systems

Mound systemBruce Lesikar and Vance Weynand

Associate Professor and Extension Agricultural Engineering Specialist, Extension AssistantThe Texas A&M University System

L-54144-02

Figure 1: A mound system for distributing treated wastewater to the soil.

A mound system for wastewater is a soil absorption system placedabove the natural surface of the ground. Mound systems are usedto distribute water on sites where there is minimal soil before

reaching groundwater, impermeable soils or bedrock.

For this type of system, anelevated “mound” of soil is builtabove the native soil to achieve therequired separation distance betweenthe wastewater application and thelimiting soil condition of the site.

A mound system includes apretreatment chamber, usually a septic

tank; a pump tank for dosing thewastewater; and a mounded drainfield containing a pressurized distri-bution system. The mound itself is alayered structure consisting of:

� A sand layer placed above thenative soil to obtain the required24 inches of separation between

the wastewater application andgroundwater, and 18 inches ofseparation from impermeablesoils or bedrock.

� An absorption area that storesthe pretreated wastewater until itpercolates down into the sandlayer. A variety of materials canbe used in the absorption area,including rock 3/4 to 2 inches indiameter, chipped tires 2 incheslong, leaching chambers and pipedrain fields.

� A low-pressure distributionsystem of pipes to distributeeffluent evenly throughout theabsorption area.

� Geotextile fabric placed over theabsorption area to prevent soilfrom entering the pores of themedia and clogging the absorp-tion area.

� A layer of sandy loam soil thatcovers the absorption area. Thissoil is used to retain some waterto be used by the plants in thevegetative cover.

� A topsoil cap of 6 inches of soilthat can support vegetation. Theplants growing on this capprevent erosion and give offmoisture through transpiration,which helps move the treatedwastewater into the air.

In mound systems, the effluentfrom a septic tank is pumped throughpipes to a soil absorption bed. Thewastewater is pumped at low pressurein controlled doses to ensure that it isdistributed uniformly throughout thebed. It flows through holes in thepipes, trickles downward through theabsorption area and percolates into thesand.

TreatmentWastewater must be pretreated

before it enters a mound distributionsystem. The pretreatment system isgenerally a septic tank, which re-moves the settleable and floatablesolids from the wastewater.

Advanced pretreatment systems,such as aerobic treatment units ormedia filters, can also be used toremove additional solids and organicmatter from the wastewater before it isapplied to the mound. These are used

when the sewage is stronger thannormal residential wastewater.

A mound application systempurifies wastewater through threemain mechanisms:

� Filtration, in which particles arephysically strained from thewastewater;

� Chemical sorption, in whichcontaminants stick to the surfaceof the soil and to the biologicalgrowth in the soil; and

� Assimilation, in which aerobicmicroorganisms (microbes) eatthe nutrients in the wastewater. Inthis process, the microbes trans-form the waste material intoanother harmless state.

Just as with conventional soilabsorption systems, the effluent inmound systems is treated as it movesthrough the fill material and into thenatural soil. The microbes andpathogens found naturally in the soilhelp remove nutrients (waste) fromthe water.

As the effluent is applied continu-ously over time, a biological matforms in the soil. This mat tends toslow the movement of water throughthe soil and help maintain unsaturatedconditions below the mat.

The water must move intounsaturated soil in order for themicrobes in the soil and in the mat tofeed on the waste and nutrients in thewater. The grass in a soil absorptionsystem also uses those nutrients for itsgrowth.

DesignThe pretreatment system for

residences is generally a septic tank.It provides the primary treatment of

the wastewater. Generally, wastewaterfrom a home does not require otheradvanced pretreatment before enteringa mound system.

After pretreatment, a pump tankcollects the wastewater and doses it tothe mound. A low-pressure distribu-tion system is generally used todistribute it into the mound’s absorp-tion area. The design for a pipenetwork and distribution piping for amound system should use the sameguidelines as for low-pressure dosingsystems.

Arrange the mound system alongthe contour of the slope to maximizethe down-slope movement of water.The mound bed should be as long andas narrow as possible. The total areaof the mound is based on the ability ofthe native soil to accept wastewater.

Also, make sure that the bottomof the absorption area is level toprevent any one part of the bed frombeing overloaded.

Before building a mound systemon a site, plow the soil to a depth of 6to 8 inches to help the water betterfilter into the native soil. The nativesoil should be tilled when it is dryenough that it crumbles and does notform a wire when rubbed betweenyour palms. Mixing some of the sandyfill material into the native soil canhelp the water move from the absorp-tion area into the native soil.

Above the native soil is a sandlayer, which consists of coarse sandwith a minimum amount of fineparticles (less than 5 percent).

The next layer is the absorptionarea, which can be built in either abed or trench configuration. Trenchesshould be used on clay soils or onslopes greater than 6 percent. Do notuse mound systems on sites with a

Figure 2: A mound system is placed above the natural surface of the ground.

Geotextile fabric

Distribution pipes

Sand

Gravel

Restrictive layer

Original topsoil

slope greater than 10 percent. Theconstruction materials and methodsfor the trenches and beds for moundsystems are similar to those used forconventional soil absorption systems.

Distribution pipes are placed inthe trenches. The holes on thedistribution pipes should face down-ward. If leaching chambers are usedin the absorption area, connect thedistribution pipes to the top of thechamber and face the holes up, withone or two holes down to allow thepipes to drain. Then fill the trencheswith rock or another approvedmaterial.

Cover the trenches with ageotextile fabric. On top of that, placea 6-inch layer of sandy loam soil sothat oxygen can move more easilyinto the absorption area.

The mound is then covered withtopsoil. After settling, there should bea minimum of 6 inches of topsoilcovering the edges of the absorptionarea and 12 inches covering the centerof the mound. The topsoil should beable to support vegetation.

Grade and landscape the area upslope of the mound to divert rainfallrunoff around the mound and preventponding of rainwater behind it.

After completing the final grade,seed and mulch the entire mound. Toprevent roots from intruding into thedistribution system, avoid plantingdeep-rooted woody vegetation on topof the mound.

Some considerations whenbuilding a mound system:

� Do not use mounds on slopesgreater than 10 percent. On claysoils, the slope should be 6percent or less.

� Mounds require 24 inches of fillbetween the absorption bed andgroundwater, and 18 inches of fillbetween restrictive horizons,which are impermeable soils orbedrock.

� To prevent soil erosion, the sideslope of the mound should bebetween 2:1 and 3:1. If themound system will be mowed,make the side slope 3:1 formowing safety.

� Install observation tubes tomonitor water infiltration into themound. The tubes will enable youto see if wastewater is ponding inthe absorption area.

Make the height of the mound asshort as possible while maintainingthe required 24 inches of separationbetween groundwater and restrictivehorizons. This will promote evapora-tion, water use by vegetation andoxygen diffusion to the absorptionsurface.

How to keep it workingThe most important maintenance

concern for a mound system is toensure that the pretreatment system(usually a septic tank) is operatingproperly. If solids, grease or scum aresent to the absorption area of amound, they will significantly reducethe life of the mound as a filtrationdevice. Follow the operation andmaintenance recommendations foryour pretreatment system.

Standard mound systems areintended only for residential-qualitysewage. Mound systems for strongereffluents must be larger than standardsystems or have additional pretreat-ment before wastewater is applied tothe absorption area.

One measure of wastewaterquality is biological oxygen demand(BOD

5), which is the amount of

oxygen used by microorganisms tobreak down waste material. Themaximum BOD

5 of pretreate waste-

The On-Site Wastewater Treatment Systems series of publications is a result of collaborative efforts of various agencies,organizations and funding sources. We would like to acknowledge the following collaborators:

Texas State Soil and Water Conservation Board USEPA 319(h) ProgramTexas On-Site Wastewater Treatment Research Council Texas Agricultural Extension ServiceTexas Natural Resource Conservation Commission Texas Agricultural Experiment StationUSDA Water Quality Demonstration Projects Texas On-Site Wastewater AssociationConsortium of Institutes for Decentralized Wastewater Treatment USDA Natural Resources Conservation Service

water entering a standard moundshould be about 140 mg/l.

The pressure distribution systemsneed periodic attention. You may needto flush the distribution lines toremove scum deposits from inside thepipe. If the discharge openingbecomes clogged with debris, removethe material to allow the wastewaterto flow through.

Inspect the mound surfaceperiodically for areas with little or toomuch water, which indicates that the

water is not being distributed uni-formly throughout the mound.

Here’s how to maintain yourmound system properly:

� Plant and maintain grasses andother ornamental ground cover onthe mound. These plants will usethe water and help minimize soilerosion from the cover. Becausethe top of the mound can dry outduring the summer months, besure that the plants can resistwater stress.

� Remove trees from near themound because their roots candamage the system.

� Divert rainfall around the system.Prevent runoff from accumulatingon the system’s up slope side.

� Develop good water conservationhabits at home. Excessive wateruse overloads the system andcauses it to fail.

This material is based upon work supported by the Rio Grande Basin Initiative administered by the Texas Water Resources Instituteof The Texas A&M University System Agriculture Program with funds provided through a grant from Cooperative State Research,Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. 2001-45049-01149.

TWRI 1002