HOW TO SIZE A LEACH FIELD

Calculating the size of a leach field is an important step in designing a wastewater disposal system. The following procedure outlines the key factors and steps involved in determining the appropriate leach field size:

1. Conduct a percolation test: A percolation test measures how quickly water drains into the soil and helps determine the soil percolation rate, expressed in minutes per inch (mpi). Lower mpi values indicate faster soil drainage, allowing for a smaller leach field size.

2. Determine the daily wastewater volume: Multiply the number of bedrooms by 150 gallons, which provides an average estimate of daily wastewater volume. For example, a three-bedroom house would have a daily wastewater volume of 3 x 150 = 450 gallons.

3. Consult local health codes: Check the local regulations and health codes to find the minimum required absorption area per gallon of wastewater. This requirement varies based on soil percolation rate and local guidelines. Reference the appropriate tables or guidelines provided by local authorities.

4. Calculate the total absorption area: Multiply the daily wastewater volume by the required absorption area to determine the total absorption area needed for the leach field. For example, if the daily wastewater volume is 450 gallons and the required absorption area is 2 square feet per gallon, then the total absorption area would be 450 x 2 = 900 square feet.

5. Determine trench length: Divide the total absorption area by the width of the trenches to calculate the total length of trenches required for the leach field. The trench width depends on factors like pipe size and gravel quantity. As a general estimate, assume a trench width of 2 feet. For instance, if the total absorption area is 900 square feet and the trench width is 2 feet, then the trench length required would be 900 / 2 = 450 feet.

6. Determine the number of trenches: Divide the total trench length by the maximum allowable length of a single trench, as prescribed by local health codes and land slope conditions. Typically, the maximum length limit is around 60 feet. For example, if the total trench length is 450 feet and the maximum allowable length is 60 feet, then you would require 450 / 60 = 7.5 trenches.

7. Finalize the number of trenches: Round up the calculated number of trenches to obtain the final number required for the leach field. Consider the spacing between the trenches, which should be equal to or greater than the trench width, based on local health codes and soil conditions. If you require 7.5 trenches and the trench width is 2 feet, you can round up to 8 trenches and space them at least 2 feet apart.

Remember, it is crucial to consult a professional septic system designer or installer before proceeding with the installation or replacement of a leach field. They will ensure compliance with local codes and consider site-specific conditions for an optimal and efficient wastewater disposal system.

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