For Estimating Onsite Wastewater System LTAR (2024)

July 3, 2008

Bouma Calculator

for Estimating Onsite Wastewater System LTAR

David Radcliffe[1] and Larry West[2]

Steady infiltration through an onsite wastewater system (OWS) trench bottom can be used to calculate the design hydraulic loading rate (HLRD) or long term acceptance rate (LTAR) for a soil treatment unit (Siegrist, 2007). In this document, we show how to use an Excel spreadsheet that implements a modified version of the equation developed by Bouma (1975) for steady flux through a trench bottom. Once the flux is known, it can be converted to a LTAR using a safety factor. We show here an example for a sand textural class. Open the Excel spreadsheet file 'Bouma Calculator' (Figure 1).

Figure 1. Bouma Calculator Excel spreadsheet.

In the yellow area are shown the user input values that must be provided for the soil of interest. These are the van Genuchten (1980) hydraulic parameters residual water content (Theta-r, cm3/cm3), saturated water content (Theta-s, cm3/cm3), two fitting parameters Alpha (1/cm) and n (unitless), and the saturated hydraulic conductivity (Ks, cm/day). The user has the option of specifying the depth of ponding in the trench (h0, cm), the saturated hydraulic conductivity of the biomat (Kbs, cm/day), and the thickness of the biomat (Zb, cm). The default values for the biomat parameters are shown in Figure 1. The van Genucthen (1980) parameters can be obtained from measurements of the moisture release curve (Theta-r, Theta-s, Alpha, and n) and saturated hydraulic conductivity (Ks) for a soil. Alternatively, one can use the mean values for textural classes shown in the Soil textural classes worksheet (see lower left corner of Figure 1). Click on the 'Soil textural classes' tab and the worksheet in Figure 2 will appear.

Figure 2 'Soil textural classes' worksheet.

These parameter values are taken from the Rosetta Lite database in HYDRUS (Šimůnek et al., 2006). To find the flux through a sand textural class, copy the values for the sand class from the Soil textural classes worksheet into the Calculator worksheet (Figure 3). We've also typed in that this is a 'Sand' in cell A4 to remind ourselves, but that's not required for the calculation.

Figure 3. Copy the parameter values for the sand from the 'Soil textural classes' worksheet into cells B4-F4 in the 'Calculator' worksheet.

Now the soil tension that results in an equal flow through the biomat and soil beneath the biomat must be found through a trial and error process. In the graph in Figure 3, the flux through the biomat (Qb) is shown as the gray curve and the unsaturated hydraulic conductivity function for the underling soil (K(h)), as the black curve. Both are plotted as a function of the soil tension beneath the biomat on the x-axis. Where the curves cross, flow through the biomat and underlying soil are the same. From the graph it can be seen that the soil tension value where the curves cross is about 40 cm, so enter a trial value for soil tension of 40 in cell A10.

Figure 4 Based on the graph, enter a trial value of 40 in cell A10 as an estimate of the soil tension where the curves cross (flux through the biomat and soil are equal).

The difference between Qb and K(h) is show in the 'Residual' cell A13. The residual is positive so you need to increase the trial value slightly. Try a value of 42 (Figure 5). This results in a smaller residual that is negative, so try a value slightly smaller than 42. By this process, you can find that the exact tension that produces a residual of 0.00 is 41.52 cm (Figure 6).

As an alternative to the trial and error approach, you can use the 'Goal Seek' tool in Excel. Put a guess value in cell A10 based on the graph (for example enter 40). Click on 'Tools' on the toolbar and then select 'Goal seek'. In the Goal Seek window, for 'Set cell:' enter A13, for 'To value:' enter zero, and for 'By changing cell:' enter A10. Then click OK. You should see the solution value in A10 of 41.52.

Figure 5. Put a trial value of 42 in cell A10. This results in a smaller residual that is slightly negative so decrease the trial value slightly.

Once the residual is zero, the "Flux' shown in cell B15 is an estimate of the steady flux through the trench bottom. For the sand textural class, it is 10.31 cm/day. The flux in gallons per day per square foot (gpd/ft2) is shown in cell B16. To convert this to a LTAR, you must decide on a safety factor. We recommend using a safety factor of 0.50 (cell A19). Using this value, you get an LTAR = 1.26 gpd/ft2 for the sand textural class (cell B21).

This approach is described in more detail in Radcliffe and West (2008a and 2008b).

Figure 6. By trial and error, you can find that the soil tension that results in a residual of 0.00 is 41.52 cm.


Bouma, J. 1975. Unsaturated flow during soil treatment of septic tank effluent. J. Environ. Eng. Div. Am. Soc. Civ. Eng. 101: 967-983.

Radcliffe, D.E., and L.T. West. 2008a. Design hydraulic loading rates for on-site wastewater systems. Vadose Zone Journal. In press.

Radcliffe, D.E., and L.T. West. 2008b. Spreadsheet for converting saturated hydraulic conductivity to long term acceptance rate for on-site wastewater systems. Soil Survey Horizons. Article submitted.

Siegrist, R.L. 2007. Engineering of a soil treatment unit as a unit operation in an onsite wastewater system. In K. Mancl (ed.) Eleventh individual and small community sewage systems conference proceedings. ASABE Publication 701P1107. St Joseph, MI.

Šimůnek, J., M. Th. van Genuchten, M. Šejna. 2006. The HYDRUS software package for simulating two- and three-dimensional movement of water, heat, and multiple solutes in variably-saturated media. Version 1.0, technical manual, PC Progress, Prague.

van Genuchten, M.Th. (1980). A closed form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892-898.


[1]Crop and Soil Sciences Department, University of Georgia, Athens, GA. .

[2] National Soil Survey Center, USDA-NRCS, Lincoln, NE.

For Estimating Onsite Wastewater System LTAR (2024)


What is the approximate percentage (%) of Americans that use onsite septic systems for their wastewater disposal? ›

The remainder of the population — approximately 20% of Americans — rely on onsite wastewater systems such as septic tanks.

What is LTAR in a septic system? ›

What is LTAR ? The long-term acceptance rate (LTAR) is the amount of wastewater that can be applied each day over an indefinite period of time to a square foot of soil. Effluent from the onsite system is absorbed and properly treated.

What is a good perc test result? ›

Typically, a perc rate between 10 and 60 minutes per inch of drop is acceptable for a standard leach field. While a DIYer can conduct a perc test for their property, the test is relatively meaningless without knowing how to measure or understand the results.

How do you calculate wastewater treatment plant? ›

  1. Total domestic (raw) water demand = TWD.
  2. Estimated Sewage X = 90 % of TWD.
  3. Estimate Sewerage X = 90/100 * TWD For Eg If TWD = 1500 KLD.
  4. Capacity of S.T.P = 90 / 100 * 1500 KLD = 1350 KLD.
Sep 29, 2020

What is the average percent of the population across the US served by a septic system? ›

Nearly one in four households in the United States depends on an individual septic system or small community cluster system to treat its wastewater.

What percentage of Americans depend on wastewater treatment services? ›

More than 80 percent of the U.S. population receives their potable water from these drinking water systems, and about 75 percent of the U.S. population has its sanitary sewerage treated by these wastewater systems.

What does LTAR stand for? ›

The Long-Term Acceptance Rate (LTAR) or loading rate is one of the most significant metrics regarding septic design and installation that home builders and developers should understand. To oversimplify it, "Good soil" will have a higher LTAR while "bad soil" will have a lower LTAR.

What is the linear loading rate? ›

Linear loading rate is defined as the loading rate per linear foot of system (gallons per day per linear foot) along the contour. Linear loading rates are rarely a limiting factor in on-site system design when generous soil depths are present.

What is the rate of accumulation of sludge in septic tanks is recommended? ›

Rate of accumulation of sludge – 30 litres/person/year.

Sewage flow without sullage- 40-70 lcd. Cleaning period or removal of sludge: 6 to 12 months, mostly once in a year i.e. removal of sludge is less frequent.

What will fail a perc test? ›

Most often, when a building site fails a perc test, it's because the ground isn't permeable enough.

How do you calculate perc test? ›

The percolation rate is calculated for each test hole by dividing the time interval used between measurements by the magnitude of the last water level drop. This calculation results in a percolation rate in terms of minutes per inch.

How to calculate percolation rate? ›

The mpi, or percolation rate of a test hole (the time in minutes for the water level in the test hole to drop one inch), is determined by dividing the number of minutes elapsed by the water level drop in inches during the final measurement period.

How do you estimate wastewater flow? ›

Quantity= Per capita sewage contributed per day x Population

It has been observed that a small portion of spent water is lost in evaporation, seepage in ground, leakage, etc. Usually 80% of the water supply may be expected to reach the sewers.

How do you measure wastewater? ›

The traditional method of measuring wastewater flows is through the use of a flume and an ultrasonic flow meter in a flow manhole (concrete vault / manhole or fiberglass packaged metering manhole). The combination is reliable and usually requires low maintenance.

How do you calculate sewage waste? ›

Waste water calculation is based on algorithm for which the following parameters must be defined:
  1. specific water consumption divided per inhabitant [litres/day], for example 150 l/day.
  2. number of inhabitants, for example 1000,
  3. percent of population increase, for example 2%,
  4. project period, for example 20 years.

How many Americans use septic tanks? ›

More than 60 million people in the nation are served by septic systems. About one-third of all new development is served by septic or other decentralized treatment systems.

What percentage of wastewater is treated? ›

Proportion of safely treated domestic wastewater flows (%) Percentage of wastewater flows that are safely treated. Worldwide, the proportion of safely treated domestic wastewater flows has been 55.5% in 2020.

What percentage of the population is served with decentralized septic systems in the US? ›

Onsite/decentralized wastewater treatment systems – otherwise known as septic systems – serve approximately 25 percent of the U.S. population and one-third of new development. Liquid wastewater, called septic leachate, can leak into groundwater from a septic system.

What percentage of sewage sludge produced in the United States is spread on land? ›

Research on Evaluating Concentrations of Endocrine Disrupting Chemicals in Biosolids (Sewage Sludge) In the United States, about 60% of biosolids—solid residues produced by wastewater treatment—are applied to land as an agricultural amendment.

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