A mound is a drainfield built on top of the ground instead of in it. You'll see a long, grassy hump in the yard. Under the grass sits a bed of gravel and pipe on a thick layer of specified sand, and a pump feeds it a few times a day.
A mound septic system is a drainfield built above natural grade in specified sand. A pump doses effluent into a gravel bed, and the sand and underlying soil provide treatment and dispersal. A mound may suit some shallow-soil sites, but Tennessee still requires suitable natural soil, acceptable slope, setbacks, and a replacement area.
Every mound has three parts: a septic tank, a pump chamber (also called a dosing tank), and the mound itself.
Wastewater leaves the house and settles in the tank. Sludge sinks, grease and scum float, and the clearer liquid in the middle (effluent) moves on. An outlet filter keeps stray solids out of the pump.
Effluent flows into a second tank with a submersible pump. When the level reaches the "pump on" float, the pump runs. A separate high-water float sounds an alarm if the level keeps rising.
The pump pushes effluent through a small pressure manifold into perforated pipes laid in a level gravel bed near the top of the mound. Every hole gets about the same amount, so no one spot gets soaked.
Effluent trickles down through at least a foot of specified sand. Microorganisms and filtration in the sand reduce organic matter and pathogens; the water is still wastewater, not safe drinking water. This is the part a thin natural soil can't do on its own.
Below the sand, the treated water soaks into the plowed native soil and spreads out sideways and downward. On a slope, it moves mostly downhill, which is why the ground below a mound has to stay undisturbed.
Dosing a few times a day, then resting, lets air back into the sand so the bacteria stay aerobic. Grass on top holds the cover in place and pulls out some moisture.
Tennessee spells out the mound's layers in Rule 0400-48-01-.15(4). Your designer uses the soil map to size the bed and determine whether the site needs more than these minimums.
| Layer (bottom to top) | What it does | Tennessee minimum or limit |
|---|---|---|
| Native soil | Final treatment and dispersal | At least 24 inches of soil over rock or another restrictive layer (20 inches for a modified mound with 4 extra inches of sand) |
| Plowed surface | Opens the soil so water can enter | Plowed on contour at least 8 inches deep, when dry, with no wheeled traffic afterward |
| Sand fill | Main treatment zone | At least 1 foot under the gravel bed; mostly medium sand, no more than 20% fine sand, free of silt and clay |
| Gravel distribution bed | Holds the pressure pipes and spreads flow | At least 9 inches thick, 6 inches of it below the pipes; no wider than 10 feet; built level |
| Pressure laterals | Deliver even doses | PVC at least 1 inch in diameter with 5/32 to 1/4 inch holes; drain after each dose |
| Separator | Keeps the cap out of the gravel | Either 6 inches of uncompacted straw, untreated building paper, or approved fabric |
| Clay cap | Sheds rain and holds heat and moisture | 12 inches of clayey subsoil at the center, tapering to 6 inches at the edges |
| Topsoil and grass | Erosion control | 6 inches of soil that will grow grass over the whole mound |
| Side slopes | Carry the mound down to grade | No steeper than 3 feet horizontal to 1 foot vertical |
Tennessee sizes the gravel bed at 1.2 gallons per square foot per day of sand capacity and designs alternative systems at 150 gallons per bedroom per day. The site must also hold a full-size duplicate area for a future repair.
Sources: Tennessee Rule 0400-48-01-.15(4), mound systems · EPA mound system fact sheet
Bigger than most people picture. Take a three-bedroom house in Tennessee: 3 bedrooms times 150 gallons is a 450-gallon design flow. At 1.2 gallons per square foot, the gravel bed alone needs about 375 square feet, so roughly 10 feet wide and 38 feet long.
Then stack the minimum layers: 9 inches of gravel, 12 inches of sand under it, a 12-inch cap, and 6 inches of topsoil. That's a little over 3 feet at the crest before any extra sand for slope. At a 3-to-1 side slope, every foot of height adds 3 feet of slope on each side. A small mound on flat ground ends up around 30 feet wide and 60 feet long. Slower soil, a sloping lot, or a bigger house makes it larger, and Ohio State Extension notes a three-bedroom mound can run as long as 200 feet.
Ask for a scaled plan and have the corners staked before you sign. Look at it from the road, the patio, and the kitchen window. It's far easier to shift or soften the shape on paper than after the sand is placed.
| Three-bedroom example (level lot) | Rough figure | How it's figured |
|---|---|---|
| Design flow | 450 gallons per day | 150 gallons per bedroom per day (Tennessee alternative-system rule) |
| Gravel bed area | About 375 sq ft | 450 divided by 1.2 gallons per sq ft per day |
| Height at the crest | A bit over 3 feet | 9 in gravel + 12 in sand + 12 in cap + 6 in topsoil, minimums only |
| Overall footprint | Roughly 30 by 60 feet or more | Bed plus 3-to-1 side slopes; basal-area and slope rules often make it bigger |
| Extra land held back | A second, equal area | Tennessee requires 100% duplication for a future repair |
This worked example uses the rule minimums. Your design must also meet basal-area and slope requirements, which can make the footprint much larger.
Sources: Tennessee Rule 0400-48-01-.15(4), mound systems · Ohio State Extension mound fact sheet
Mounds cost more than a conventional drainfield because you're paying for imported sand, a pump system, engineering, and careful construction.
The biggest swing items are sand volume and haul distance, how much rock and brush has to be cleared, and whether the pump has to lift effluent a long way uphill. Weather matters too. Tennessee won't let the site be prepped while the soil is wet, so a rainy spring can stretch the schedule.
When you compare bids, make every contractor price the same accepted plan and sand specification. A low number that swaps in ordinary fill dirt, runs equipment over the plowed area, or leaves out the electrical work and final grading isn't a bargain. The engineered system cost page covers how mound pricing compares with LPP and drip.
| Cost item | Typical figure | Notes |
|---|---|---|
| Mound system, national cost guides | $10,000 to $20,000 materials only (Fixr); $25,000 to $50,000+ installed (HomeGuide) | Fixr covers materials; HomeGuide includes installation |
| Installed mound system, planning budget | $15,000 to $30,000+ | Rock, haul distance, sand volume, and slope affect the final price |
| TDEC permit and inspection | $500 + $200 | Published state fee for a new alternative system up to 1,000 gallons per day; verify before paying |
| Soil work | Get a site-specific mapping quote | Mounds need an extra-high-intensity soil map from a TDEC-approved soil consultant |
| Pump replacement | Many pumps and controls need replacement in 10 to 20 years | EPA's range for pumps and controls; price depends on the pump, panel, and access |
| Pumping and inspection | Every 1 to 5 years for tanks; yearly check of the pump system | Ohio State pumping range and EPA's yearly inspection advice for systems with pumps |
We use $15,000 to $30,000+ for early budgeting while local installed-price data are limited. Compare itemized bids for the same approved plan, including electrical work and restoration.
Sources: Fixr septic installation cost guide (updated Jan. 2025) · HomeGuide engineered septic system cost · Ohio State Extension mound fact sheet · EPA: how to care for your septic system
EPA identifies mounds as a possible approach to three site problems: shallow soil over creviced or porous bedrock, a high water table, and soil that drains too slowly or too quickly. Tennessee adds hard limits on top of that.
Thin soil over limestone is an important siting concern in Maury County. The Tennessee Geological Survey's Environmental Geology Atlas maps sinkholes and shallow rock across much of the county, and those are exactly the lots where a conventional trench field won't fit. Some of them pass for a mound. Others need LPP, drip, or aerobic treatment, and a few can't support any onsite system.
TDEC's Columbia Environmental Field Office reviews Maury County applications. Before it evaluates a mound, it wants an extra-high-intensity soil map from an approved soil consultant. Get that map before you close on land that depends on a mound, and don't read much into a neighbor's system. Soil can change within a few yards on karst ground.
| Site condition | Can a mound help? | Tennessee limit that still applies |
|---|---|---|
| Shallow soil over limestone | Often. The sand adds the treatment depth the native soil lacks | At least 24 inches of natural soil (20 for a modified mound) must remain over rock |
| Seasonal high water table | Often. Raising the bed keeps an unsaturated zone below it | Wetness that drainage can't correct rules out the extra percolation-test route |
| Slow-draining clay | Sometimes, with a wider basal area | Soil must test 10 to 75 minutes per inch on the soil map, or up to 120 if there are no water problems |
| Sloping lot | Up to a point | No mound on slopes over 12%, or over 6% when soil runs 61 to 120 minutes per inch |
| Small lot | Rarely | The site needs room for the mound, its slopes, setbacks, and an equal duplicate area |
| Floodplain, drainageway, or depression | Needs specific siting review; raising a bed does not establish approval | Tennessee groundwater and site restrictions still apply |
Sources: EPA mound system fact sheet · Tennessee Rule 0400-48-01-.15(4), mound systems
| Pros | Cons |
|---|---|
| Makes a buildable lot out of some shallow-soil or wet sites | Costs well above a conventional gravity system |
| Specified sand adds treatment depth where the natural soil alone is insufficient | Needs a large footprint plus a duplicate area |
| Little excavation in the mound area, so native soil stays intact | A visible hump you can't flatten, plant with trees, or build on |
| Pressure dosing loads the whole bed evenly and lets it rest | Runs on electricity; the pump, floats, and panel wear out |
| Has no direct discharge to a ditch or stream | A leak at the toe can mean partly rebuilding the mound |
Source: EPA mound system fact sheet
A mound's parts age separately: EPA gives 10 to 20 years for many pumps and controls and 50 years or more for a concrete tank. The sand bed's useful life depends on siting, construction, wastewater loading, and care. Ask the inspector to assess the bed and the machinery separately.
EPA's 1999 mound fact sheet reports a success rate above 95 percent in Wisconsin, crediting careful siting, design, construction, and maintenance. That historical result shows what a well-run program can achieve. Tennessee's site requirements and the condition of your own system still guide decisions here.
Mounds can fail early for predictable reasons: a tank that never gets pumped and sends sludge into the pipes, a leaky toilet or water softener that overloads the bed, or a truck or skid steer that compacts the soil at the downhill toe. Avoiding those problems reduces the risk of early failure, but does not rule out eventual field repair or replacement.
| Choice | OK? | Why |
|---|---|---|
| Grass or turf | Yes, best choice | Holds the cover, sheds rain, and lets you spot wet areas |
| Shallow-rooted perennials or a low native grass mix | Yes, with care | Less mowing, but skip deep-rooted or water-loving plants |
| Trees and shrubs | No, keep them off the mound and its toe | Roots find the pipes; Ohio State Extension warns against them |
| Vegetable garden or raised beds | No | Digging breaks the cap, watering adds load, and you don't want food near effluent |
| Mulch, landscape fabric, or thick stone | No | Cuts off air, hides seepage, and adds weight |
| Sprinklers or irrigation | No | The mound already gets water from the house every day |
| What you notice | Likely cause | What to do |
|---|---|---|
| Alarm light or horn | Pump, float, power, or high inflow problem | Cut water use and call your service provider |
| Soggy or spongy toe or side slope | Overloading, uneven dosing, or clogging | Keep people and pets off; get the mound and dosing checked |
| Seepage or sewage smell | Breakout at the edge of the mound | Stop extra water use; treat it as a health problem |
| One strip greener than the rest | A leaking or overloaded lateral | Note the spot and ask for a pressure check |
| Slow drains or a backup | Tank, filter, pump, or line restriction | Have the whole path diagnosed, not just the tank pumped |
About our sources. Maury Septic uses the sources listed below, including EPA mound mechanics, Tennessee Rule 0400-48-01-.15(4) mound design minimums, Ohio State Extension sizing and planting guidance, national cost-guide ranges, and Maury County's mapped limestone geology. Follow the links for the underlying rules, fees, and guidance.
U.S. Environmental Protection Agency
Mound components, elevated sand treatment, site limitations, pressure dosing, construction risks, landscaping, failure signs, and maintenance practices.
U.S. Environmental Protection Agency
Failure signs, maintenance, pumping, water use, and drainfield protection.
U.S. Environmental Protection Agency
Functions, components, advantages, costs, and maintenance context for pumps and control panels used with LPP, ATU, and drip systems.
Tennessee Secretary of State
Official current chapter text governing Tennessee subsurface sewage disposal systems.
Tennessee Department of Environment and Conservation
Conventional, repair, and alternative-system applications, plus soil-map requirements.
Tennessee Department of Environment and Conservation
State mapping standards for soil depth, restrictive layers, drainage, absorption rates, slope, site features, and SSDS interpretations.
Tennessee Department of Environment and Conservation
Current state page for the Columbia office address, suite, main phone number, and statewide TDEC contact.
Tennessee Geological Survey
State-published geologic, unstable-materials, flood-prone-area, mineral-resource, and sinkhole maps for Maury County.
No. The height, sand depth, and side slopes are what make it work, and Tennessee's rule sets them. Cutting it down, piling on fill, or planting trees can ruin it. You can soften how it looks before approval by working the shape into the lawn and planting outside its footprint.
On paper, Tennessee's minimum layers add up to a little over 3 feet at the crest: 9 inches of gravel, 12 inches of sand, a 12-inch cap, and 6 inches of topsoil. Sloping lots and deeper sand make many mounds taller on the downhill side. Your engineered plan shows the actual heights.
A working mound shouldn't smell. Odor can come from a lid seal, a plumbing vent, or a treatment problem; smell alone does not identify the cause. A smell combined with wet ground, seepage, an alarm, or slow drains points to a real problem. Cut back on water, keep people away from any wet spots, and have the tank and mound checked.
Only briefly. The pump stops when the power goes out, and the dosing tank holds a limited reserve. Cut water use to the basics right away. If the alarm stays on after the power returns, or the breaker keeps tripping, call for service rather than resetting it over and over.
No. Tennessee still needs at least 24 inches of natural soil (20 for a modified mound), slopes of 12% or less, room for a duplicate area, and safe setbacks from sinkholes and wells. Some shallow lots fit LPP or drip instead, and some fit nothing. The soil map and permit review determine what qualifies.
Tell us about the property, access, and timing. Have your soil map, permit, and design ready for an installer to review. We can seek an estimate from a suitable installer, subject to availability.
Related: system types hub · engineered system cost · low-pressure pipe systems · drip systems · failed soil test options · septic landscaping · alarm guide
Keep your permit, system sketch, and service records together. They help the next professional understand your property.