How the Numbers Work

These sections explain the engineering behind each step of Septic Design: how landplan.io sizes the system from the flow and the soil, and what it checks.

Wastewater flow

Every size in the design comes from the daily design flow: the volume of wastewater the building makes in a day, in gallons per day (gpd) or litres per day.

For a home, the flow usually comes from the number of bedrooms. Areas use one of these methods:

• Flow per bedroom per day. The most common. landplan.io uses 120 gpd per bedroom unless your area sets another value, such as 150 gpd in many states or 100 gpd in Pennsylvania and Florida.
• A table by bedrooms, as in Texas.
• Building classifications, as in Pennsylvania, Minnesota, Connecticut and British Columbia.
• Fixture count, where your area allows it (Arizona, for example). See Fixture count.

Some areas allow a lower flow when the home has low-flow fixtures. The US EPA WaterSense program lists fixtures that qualify.

A business is sized by its use: a flow per unit, such as 2 gpd per seat for a church. See Commercial flow.

You set the bedrooms or the flow on the house with the Building with Flow tool. Details shows the flow in step 1, Flow & Sizing.

Soil and loading rates

The soil where the system goes absorbs and treats the effluent. A soil evaluation tells you how much effluent it can take and how deep the system can go:

• A profile or horizon description, which records the texture, structure and colour of each layer.
• A percolation test, which times how fast water soaks away.

Your area may use one or both. The soil test gives:

• The limiting condition. The depth of a layer that stops water moving down, such as a seasonal high water table, bedrock or a restrictive layer. Every system must keep a vertical separation above it.
• The infiltrative loading rate (ILR). The flow each square foot of soil can take, in gpd/ft². Many areas give a higher rate for effluent from an advanced treatment unit (a pretreated rate).
• The linear loading rate (LLR). The flow each foot along the contour can take, in gpd/ft. It is used in Ohio and British Columbia, among others.

landplan.io sizes the system from the soil test nearest to it. Without a soil test, it uses a default loading rate and marks the design as preliminary. Add the soil test first: see Soil Tools.

Alabama. The rates come from ADPH 420-3-1, Tables 2 and 3. A percolation rate gives the rate for its minutes per inch: 0.75 gpd/ft² at 5 to 15 MPI, 0.60 at 16 to 30, 0.50 at 31 to 60, then lower down to 0.109 at 240 MPI. Under 5 MPI and over 240 there is no rate in the tables, so enter the one your health department approves. A soil profile gets the rate of its texture group (Table 21); a loam or clay without its clay content and mineralogy gets the lower of its groups' rates. The pretreated rate is Column VI, for effluent with advanced treatment. A rate you enter on the soil test is kept.

Sizing the absorption area

The absorption area is usually the daily design flow divided by the loading rate:

• Area needed (ft²) = design flow (gpd) ÷ ILR (gpd/ft²).
• Length along the contour (ft) = design flow (gpd) ÷ LLR (gpd/ft), where your area uses a linear loading rate.

Your area may then change the area:

• Area reductions for a gravelless product or for pretreatment. Ohio allows 25 % for each, Colorado 30 % for a gravelless product.
• A resting area, kept in addition, such as 25 % in Ohio for trench systems.
• Product rules, such as a minimum pipe length for ATL and AES systems.

The Contour tool's editor compares the Total Computed Area with the Required Trench Area as you design, and Details compares Area needed with Area provided. See Absorption area.

Elevations and the hydraulic profile

The effluent must flow downhill from the house to the tanks, and on to the soil by gravity or by a pump. landplan.io reads an elevation at each part you draw: the building sewer, the tanks, the distribution method and the absorption area. The elevations start from the site's public elevation data. Replace them with your own readings from a benchmark when you have them.

From the elevations, landplan.io draws the hydraulic profile (sheet HP-1): every part seen from the side, with its inverts, the grade and the cover. It shows the tank dig depths, the slope of each pipe and the height the pump must lift the effluent. See Elevations and Elevation Points.

Treatment levels

Raw wastewater from a home is measured by its biochemical oxygen demand (BOD, the organic matter bacteria will consume) and its total suspended solids (TSS). The septic tank settles out most solids. Further treatment lowers both, and many areas then allow a smaller or shallower absorption area.

Treatment units use three main methods:

1. Aeration. Air mixes the wastewater and gives oxygen to the bacteria in it. With oxygen, the bacteria break down the organic waste many times faster. See Aerobic Treatment Units.
2. Filtration. The effluent passes through media, from gravel and sand to peat, coconut husk, textile or membranes. Bacteria grow on the media and treat the effluent. See Filtration Systems.
3. Disinfection. Ultraviolet light or chlorine kills the pathogens left in highly treated effluent, as for a spray system. See Spray Distribution Systems.

Add a treatment unit with the Tank tool as an Advanced Treatment Unit. See Tank Tools.

Distribution media

The media is what the effluent soaks through in the trench or bed before it reaches the soil:

• Aggregate, such as washed gravel or #57 stone.
• Gravelless products: plastic chambers, bundles of polystyrene aggregate (EZ Flow), geotextile sand filter blocks (Eljen GFS) and large diameter pipe (Advanced Enviro-Septic).
• Sand fill, under the bed of a mound, an at-grade system or an ATL or AES bed.

You pick the media in the contour tool's Design form, for example Distribution Trench Media. The list comes from your configuration. See Contour Systems.

Distribution methods

The effluent reaches the absorption area by gravity or by a pump.

By gravity, a distribution box splits the flow between the trenches. The trenches can be fed:

1. In parallel. Every trench gets effluent at the same time.
2. Sequentially. The first trench fills, then the effluent moves on to the next one.
3. In series (serial). The effluent flows from one trench into the next, as if they were one long trench.

Under pressure, a pump doses the effluent through a force main to the laterals. A distribution valve or a headworks with solenoid valves sends each dose to one zone at a time. A dosing siphon or a dosing flout doses without a pump.

Place the distribution with the Distribution tool. See Distribution Method.

Hydraulics

Moving effluent under pressure, often uphill, takes a pump sized for the job. Hydraulics accounts for the height the pump lifts the effluent, the friction losses in the pipes, valves and fittings, and the pressure the far end of each lateral needs (the distal head). Together they give the total dynamic head (TDH) the pump must deliver at the required flow.

A pressure network must also spread the effluent evenly: the orifices must be sized and spaced so that the first and last orifice of a lateral discharge nearly the same flow. Details works this out in step 3, Pressure & Pump. See Pressure and Pump.

Setbacks

Every area requires the system to keep a distance from what it could harm, or what could harm it: wells, springs, streams, wetlands, the property line, the house, water lines and more. These are the setbacks, or isolation distances.

• Mark each feature with the Isolation tool. It draws the setback buffer around it.
• Smart design and Auto design place every part outside the setbacks of your configuration and your state's setback rule, and keep the absorption area off ground steeper than your state allows (25 % unless your state sets another limit).
• When a distance comes from neither, they use a fallback, such as 50 ft from a private well, and list it under Check these so you can confirm it.
• The review lists every setback the design breaks as Must fix. See Suggestions and checks.