Mound Septic System Design: The Basics
Short answer: a mound is sized with three loading rates. The aggregate bed on top is sized from the design flow and the loading rate of the sand fill. The basal area, where the mound meets the native soil, must be large enough for that soil to take the flow. And on sloping or slowly permeable sites, the linear loading rate along the contour sets a minimum length, which makes the mound long and narrow. Sand depth gives the required separation above the water table or rock, and a pump doses the bed evenly through pressure laterals. The numbers come from your state's rules.
This guide covers the design basics for designers, installers and curious owners. It draws on the EPA's Mound Systems fact sheet (EPA 832-F-99-074), which describes the Wisconsin mound developed at the University of Wisconsin–Madison and adopted into many state codes, and on how landplan.io designs mounds.
When a mound is needed
A mound is used when the native soil can't give enough treatment before the effluent reaches a limiting condition. The EPA fact sheet lists sites with:
- slow or fast permeability soils,
- shallow soil cover over creviced or porous bedrock,
- a high water table.
The required depth of soil above the limiting condition "varies, but it is usually between 1 and 4 feet," the fact sheet says. When the soil test shows less than your state requires for trenches, the sand of a mound makes up the difference. On sites a little better than that, an at-grade bed may do; see Septic System Types.
The fact sheet also says mounds should not go in flood plains, drainage ways or depressions unless flood protection is provided, and that mounds can be built on slopes up to 25%, a limit set mainly by construction safety.
The parts of a mound
| Part | What it does |
|---|---|
| Septic tank (and any pretreatment) | Settles the solids |
| Dose tank and pump (or siphon) | Stores effluent and doses it in set volumes |
| Force main | Carries each dose to the mound |
| Pressure laterals | Small pipes with drilled orifices in the aggregate bed |
| Aggregate bed | Stone (or an approved product) that spreads the effluent over the sand |
| Sand fill | Treats the effluent and adds separation |
| Cap and cover | Soil over the bed and the sand, sloped to shed rain and be mowed |
| Observation ports | Let an inspector check for ponding |
Sizing a mound step by step
1. Design flow
For a home, the design flow comes from the bedrooms. The EPA fact sheet describes typical design flows of up to 150 gallons per day per bedroom; your state sets its own. See Septic Tank Size for how bedroom flows work.
2. The aggregate bed: flow ÷ sand loading rate
The bed sits on sand, so its bottom area is the design flow divided by the loading rate your rules give for the sand fill (the effluent is treated by the sand first). For a long, narrow bed on a slope, the bed's width is then limited so the length follows the contour.
Bed area (ft²) = design flow (gpd) ÷ sand fill loading rate (gpd/ft²)
3. The basal area: flow ÷ soil loading rate
Below the sand, the effluent has to enter the native soil. The basal area is the footprint where the sand meets the original ground, and it must be at least the design flow divided by the native soil's loading rate. On flat ground the whole footprint counts; on a slope, many methods count only the area under and downslope of the bed, because effluent moves downhill. The fact sheet notes that loading rates should be based on soil texture, structure and consistence, with a percolation test used only to confirm the soil description.
Basal area needed (ft²) = design flow (gpd) ÷ soil loading rate (gpd/ft²)
4. The length: flow ÷ linear loading rate
On sloping sites and slowly permeable soils, the effluent moves sideways along the slope as much as it moves down. The linear loading rate (gallons per day per foot of contour) caps how much flow each foot of mound may receive, which sets a minimum length. The fact sheet says research found mounds can work on difficult sites "if the system is designed using linear loading rates", and that absorption systems "should be long and narrow and should follow the contour". The more restrictive the site, the longer and narrower the mound.
Minimum length (ft) = design flow (gpd) ÷ linear loading rate (gpd/ft)
5. Sand depth
The sand under the bed, plus the native soil above the limiting condition, must equal at least the vertical separation your state requires. Most rules also set a minimum sand depth even on good sites. On a slope, the sand is deeper at the downslope edge of the bed than at the upslope edge, so the bed stays level.
6. Side slopes and cover
The sides of the sand and of the finished mound are graded to a set ratio, commonly 3:1 (three feet out for each foot up), so the mound sheds water and can be mowed. The soil cover runs over the whole sand fill to the toe. The final footprint, from toe to toe, is what must fit inside the setbacks.
The sand fill
The sand does the treatment, so its quality matters. The EPA fact sheet says suitable sand should contain 20% or less material greater than 2.0 mm and 5% or less finer than 0.053 mm, or meet sieve specifications such as ASTM C-33. Many states name a specific sand; ask the supplier for a gradation sheet. A mound takes many truckloads, so the source and delivery distance are a large part of the cost.
Pressure dosing
A mound is always pressure dosed. Each dose pressurizes the laterals, and each orifice delivers nearly the same flow, so the effluent spreads over the whole bed instead of overloading one end. The designer sizes:
- the laterals and orifices: diameter, spacing and the head at the far end, so flow along each lateral stays within the even-distribution limit your rules set;
- the force main: its diameter, so velocity is high enough to keep it clean;
- the dose volume and frequency: often several doses a day, sized from the pipe volume and the flow;
- the pump: from the static lift, the friction losses and the distal head, the total dynamic head at the required flow.
A distributing valve can split a long mound into zones dosed in turn. See Pressure and Pump.
Construction points that affect the design
- Don't damage the topsoil. The fact sheet stresses that the native topsoil is usually the most permeable layer: it is plowed or broken up, never removed, and kept free of construction traffic.
- Stake the contour. The centre line follows the natural contour.
- Clear trees properly. Trees are cut at the ground, not pulled, and the mound may need to be larger where stumps and boulders take up area.
- Keep water away. Roof drains and surface runoff are directed away from the mound.
See the build sequence, with photos, in Mounded Leaching Systems.
Operation and maintenance
The fact sheet says maintenance of a mound is generally minimal, but the septic tank and dose tank should be checked and pumped as needed, the dose tank, pump and floats checked annually, and laterals flushed annually. Grass on the mound, no traffic and no deep-rooted shrubs help it last.
How landplan.io designs a mound
In landplan.io you draw the mound's centre line along the contour with the Trench tool, then pick Mounded Leaching System in the Contour tool. Its design form starts from your area's configuration: bed media, sand, bed width and height, sand treatment depth, cover, upslope and downslope depths, side slope ratio, lateral layout, orifice size and spacing. As you change them, it shows the Required Basal Width, the Estimated Sand Ton, the lateral length and the estimated flow, and checks:
- vertical separation to the limiting condition,
- in-situ soil above the limiting condition,
- sand depth under the media,
- cover over the whole sand fill,
- side slopes against the configured ratio,
- the drawn length against the design length.
Smart design and Auto design can do it all for you: size the mound from the soil test, find a spot on the contour clear of every setback, add the dose tank and force main, and size the pump. The report prints the cross section, top view, hydraulic profile and dosing detail.
Frequently asked questions
How big is a septic mound?
It depends on the design flow, the soil and the slope. The bed itself may be only a few feet wide, but with the sand and the side slopes, the footprint from toe to toe is much larger, and on slow soils the mound gets longer. Your designer can tell you once the soil test is done.
Why are mounds so expensive?
The EPA fact sheet notes that construction costs are typically much higher than conventional systems. The sand fill, the pump and controls, the pressure network and the extra design all add up. See Septic System Cost.
How long does a mound last?
The EPA fact sheet says a properly installed and maintained mound should last a long time. Overloading with water, poor dosing and compaction are the common causes of early failure.
Can a mound go on a slope?
Yes. The fact sheet says mounds can be built on slopes up to 25%, with the limit mainly for construction safety. On steep sites the mound is narrower, longer and deeper at its downslope side. Your state may set a lower limit.
What sand is used in a mound?
A coarse sand that meets your state's specification, often concrete sand or a sand meeting ASTM C-33. Ask for a gradation sheet.
Design it from your address
Create a free account, type the address, add your soil test, and landplan.io designs the mound on the contour, inside your county's setbacks, with the pump sized and the report drawn.