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Septic System Design Basics
Choosing Septic Tank Size, Drain Field Size, Basic Septic Design

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The ABCs of septic tank and drainfield design:

This septic system design article outlines basic septic system design parameters such as finding the recommended septic tank volume and conventional recommended onsite wastewater soil absorption system (leach field or drainfield) size, along with some notes on how to calculate these from simple water usage and site conditions.



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SEPTIC SYSTEM DESIGN BASICS - Septic System Design:Choosing Septic Tank Size, Absorption System Size, Basic Design Notes

How to Use this Septic Design Guide

This document and the articles it recommends describe the planning and design of a basic or conventional septic system which uses a septic tank and soil absorption system. Septic system design parameters such as finding the recommended septic tank volume and conventional recommended leach field or drainfield size, along with some notes on how to calculate these from simple water usage and site conditions are discussed here and in our basic design articles.

For more in-depth information about septic system design see our list of detailed septic design articles at SEPTIC SYSTEM DESIGN. Also see the EPA design manual at SEPTIC SYSTEM DESIGN MANUAL, ONLINE. Also take a look at the septic publications listed at Technical Reviewers & References at the end of this article to see septic codes, septic design manuals, and onsite waste disposal standards links and articles.

The Basics of Septic System Design: The key articles shown in the at More Reading (at end of this article) under SEPTIC SYSTEM DESIGN BASICS will allow you to answer basic septic system design questions.

If you have no idea what the basic parts of a septic system are, you should take a look at the take a look at HOME BUYERS GUIDE to SEPTIC SYSTEMS.

The Details of Septic Designs: A more extensive list of detailed septic planning and design articles available online is provided after the "Basic Advice for Conventional Septic System Design" just below.

Difficult septic system installation sites which are rocky, steep, wet, having high groundwater or shallow bedrock, or sites limited in size are likely to require an alternative septic design using special methods and products. For those conditions see the "DESIGN ALTERNATIVES" links at the left of these pages.

Basic Specifications & Advice for Conventional Septic Tank & Drainfield System Design

Septic Tank Sizing: Most jurisdictions currently want to see a minimum of 1000 gallons.

Septic tanks may be approved constructed of concrete, fiberglass, or polyethylene. Site-built septic tanks may still be permitted in some jurisdictions. Any of these materials can work fine for a septic tank provided the tank is properly installed. (Lightweight tanks may need to be anchored to prevent floating up out of the ground when empty.)

But the right septic tank size depends on the anticipated usage not an arbitrary number.

To determine the required septic tank size you would need to look at the SEPTIC TANK SIZE to see the required tank size based on daily water volume usage and perhaps the WATER QUANTITY USAGE GUIDE article which discusses water usage requirements to confirm that you're estimating the water volume reasonably.

Septic Tank Location: Articles titled SEPTIC CLEARANCE DISTANCES and WELL CLEARANCES tell you where you can put the tank relative to other site features. Usually the tank is located close to the building for shorter waste line runs and because that's where there is often plenty of backfill to bury a septic tank during construction. Special tank designs are available for problem sites.

Septic Tank Retention Time if you just wanted to know our opinion about tank size as a function of effluent retention time, see EFFLUENT RETENTION TIME.

Septic leach field or drainfield size and layout planning: see our article SEPTIC DRAINFIELD SIZE which sizes the field based on its percolation rate and our article SEPTIC SOIL & PERC TESTS which describes performing a perc test, with perhaps a look at SEPTIC BIOMATS to understand biomats that form below a drainfield and why a leach field in a wet area won't treat the effluent.

Standard Leach Field Trench Specifications: conventional "two foot" gravel and perforated pipe absorption trench leach field trench specifications are given here. The typical numbers you'll see for trench design in typical soils with acceptable percolation rate and excluding special and problem sites will look something like this:

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