InspectAPedia®   -   Search InspectApedia

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

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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Basic Septic System Design: Choose Septic Tank Size, Absorption System Size, Basic Design Notes

Photograph of  a conventional septic tank during installation.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:

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 key articles are shown in the at RECOMMENDED ARTICLES at end of this page.

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 SEPTIC SYSTEM DESIGN ALTERNATIVES - home

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 the WATER QUANTITY USAGE TABLES 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 septic 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:

Example Septic Leaching Trench Specification Code

DRAINFIELD LEACHING TRENCH SPECIFICATIONS [PDF] retrieved 2017/10/20, original source: http://www.odh.ohio.gov/

Design Manuals for Septic Systems

 




ADVERTISEMENT





Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-02-08 - by (mod) -

Damian

At GRAVELESS SEPTIC DESIGN CRITERIA

we discuss Infilrator septic systems as well as other gravelless or "no-rock" septic designs like yours.

There you'll also find free PDF-forms of downloadable installation instructions and design manuals if you don't already have those for your specific Infiltrator septic product.

In general, septic trenches do not have to all be on the same level as one another, but in the effluent distribution box or D-box you will want to take care not to overdose or over-load the lower trenches and to be sure effluent is distributed properly among all of the septic effluent absorption trenches you build.

When installing trenches at a sloped site the trenches generally run parallel to one another and along the fall line of the slope, as I'll illustrate below with an excerpt from the Infiltrator installation manual cited also herein;

Infiltrator provides a Contour Swivel Connection™ that is helpful on sloped sites.


On 2021-02-08 by Damian crumpton

We are in the process of digging out our Leach fields did I know the trench itself has to be level from front to back and was wondering if each parallel trench needs to be level with each other I am using a distribution box for speed levelers so it would seem that it wouldn’t have to beThe problem that I’m running into

The whole property is on a hill and if I make each trench level with each other I will be above the 18 inch mark at the top and below the 30 inch mark at the bottom I live in Texas am useing infiltrator panels and have around 6 ft of usable soil any help would be appreciated

On 2013-01-10 by steven buhl

Why is it so hard to find the minimum distance that the drain FIELD must be from the house?

On 2012-06-03 by Dave

Hello
Is a soil test that failed conclusive evidence that requires a new mound system? I have some clay in my drainfield. Good soil was found up to a 22 inch depth in a couple of areas. The requirement is for a 36 inch depth of good soil.

My septic system is working perfectly with no back up or odor issues. I cannot afford a new system. Are there options that I could utilize to possibly improve the drainfield? i.e. piping, more top soil, filling in low areas, etc. Need some help here. I live in Florence, Wisconsin. Thank you.

On 2012-04-16 - by (mod) -

Yes Dave, I've quoted an excellent table of septic system design flows titled "Table of Septic Drainfield Trench Lengths Determined by Soil Percolation Rate and Daily Wastewater Input Flow" - you could search InspectApedia for that phrase to see the article titled "Septic Drainfield Design: Septic Size Requirements Guide".

I put the table there because incoming wastewater flow + soil perc rate determines standard drainfield size. For the ClearStream and other alternative systems, you can use as input specifications the wastewater flow input assumptions from the same table.

For example: a 2 bedroom house assumes 260 to 300 gpd (a number I think is too small based on other sources who have opinions about water usage). A 5 bedroom house assumes 650 to 750 gallons per day.

In short the ClearStream should handle a 4 bedroom house.

These assumptions are very rough given that one 4 bedroom house may have say 5 occupants (2 parents and 3 kids) while another may jam in 10 or more occupants at 2 per room in North America. But code agencies in various U.S. states and provinces tend to specify by bedrooms in many cases because the number of occupants is less fixed.

I'll add some cross-reference links to soil percolation testing and drainfield sizing in the article above to help readers find that information - thanks for asking.

On 2012-04-16 by Dave

In the new Septic Systems is there a FLOW RATE for the sewage Liquid. Eg. ClearStream 600N will handle 600 gallons of sewage per day. Does that mean 600/24hr = 25 gallons per hour. Or does it mean 600 gallons per day 200= 2 hr, 100 = next 4 hr. What systems does this apply to = ClearStream,EchoFlow, etc....

On 2011-11-06 by Gabe

can the leech field be replaced?and how to do it

On 2011-09-07 - by (mod) -

M Fleck I sure do agree; we used schedule 40 in a perforated drain line that had to run under a drive and even that had to be protected against crushing when the oil truck makes a delivery; Schedule 80 would have been better still.

On 2011-09-07 by m fleck

it is best to use a white perforated pvc. Typically a astm 30 or so. One that is not easily crushable if a vehicle, etc were to drive on it.

On 2011-06-25 by D Smith

Which is better to use for the septic drainfield/leachfield: The corrugated pipe that is PVC or the black flexible stuff that has the holes in it or the black flexible stuff with the slits cut around the entire circumference because they have all three types at my local lowes?

On 2020-09-06 - by (mod) -

1/8 - 1/4 inches per foot

On 2020-09-06 by Rob

What is the minimum slope required from dwelling outlet to septic tank inlet? Also, provided slope requirement can be met, is there a maximum overall distance allowed from dwelling to septic tank?

On 2020-07-21 - by (mod) -

Mark

22 and 45 are ok,
To make a 90 or a tee you use a sanitary or DWV fitting that makes a more gradual sweep rather than a sharp 90

On 2020-07-20 by MArk

What angles are acceptable for waste lines? 22 degree? 45 degree? Is a 90% too much ? What are the basic guidelines?

On 2020-01-08 - by (mod) -

Bill

Give the Town of Essex City building department a call directly to get the best answer:

Town of Essex, New York
2313 Main Street, Essex, New York 12936
P.O. Box 355
Email: townhall@essexnewyork.com
Website: www.essexnewyork.com

Phone: (518) 963-4287

On 2020-01-08 by Bill kelley

Engineered septic design plans are good for how long in Essex city ny

On 2019-12-05 - by (mod) -

jack said:

I love that you mentioned that the size of a septic tank should be determined by the expected usage of it. I hadn't thought of that before, but it would make a lot of sense.

We are looking to get a septic tank for our new home, and it will be important for us to know that we could get the right one. I will be sure to consider how much we will use it, so we could get the right size.


Moderator reply:

Jack, sizing the septic tank based on the anticipated daily wastewater flow, and sizing the drainfield capacity similarly, are the most-fundamental concepts of septic system design. If a tank is too small you end up pushing solids into the drainfield, ruining it.

In the ARTICLE INDEX you'll find articles on septic tank size guidelines.

Thanks for the comment.

On 2019-12-05 by jack

I love that you mentioned that the size of a septic tank should be determined by the expected usage of it. I hadn't thought of that before, but it would make a lot of sense. We are looking to get a septic tank for our new home, and it will be important for us to know that we could get the right one. I will be sure to consider how much we will use it, so we could get the right size. http://www.sizerwell.com/septic-systems

On 2019-06-18 - by (mod) -

Septic engineer would probably estimate the number of people likely to use the bathrooms in a given day and also the water usage per fixture to calculate the maximum daily in flow rate. If you have enough volume to give a 24-hour settling time you may be okay.

On 2019-06-18 - by (mod) -

The final legal Authority is of course your local building or plumbing inspector. Even though you don't have people living at the property 7 days a week the septic system needs to be able to handle surges in you.

If you have a septic tank that is too small the effect is there's not adequate settling time in the septic tank and solid waste ends up getting pushed into the drain field where it clogs and destroys the field. So I suspect 500 gallons is too small.

In the ARTICLE INDEX above you will find articles on septic tank size that should be helpful.

On 2019-06-17 by Lowell Jenson

I am trying to confirm if 500 gallon septic tank and leach field are large enough for small addition to 2350 SF church. Max. occupancy is 179 and 3 water closes, 3 sinks and 1 floor sink. Activity P15CP0485 in Vail, AZ.

On 2017-02-19 - by (mod) -

Ajayi

Watch out: I'm not sure a "mini" septic tank will be acceptable in most jurisdictions - it has to be big enough to properly process sewage and to allow clarified effluent to flow out into the soakaway bed or drainfield.

But if you search InspectApedia.com for SEPTIC TANK DEPTH you can read common depths - varying by site conditions and climate.

Putting the soakaway too deep into soil guarantees that it won't work: it will lack enough oxygen for necessary bacterial action.

On 2017-02-19 by ajayi joel

For a mini septic tank of 10" x 10" what depth is suitable for it and the Soakaway?

On 2017-01-25 by randy

in what order is lift station and septic tank installed is it sewage line from residence to lift station then to septic tank or is it sewage line to septic tank and then to lift station

On 2016-03-28 - by (mod) -

You need to both find the location of the soakbed or septic field - see inspectapedia.com/septic/Septic_Drainfield_Location.php

and look for filed plans or drawings too. Often we can spot the trench lines in the field area as long parallel depressions. Or you might have to make some test digs.

If you can find the septic tank you can find the Distribution box, from there it's usually possible to send a snake into lines, communicating with buried pipe mapping equipment.

On 2016-03-28 by Maggie allard

How do I find out the lay out of my septic field?

On 2015-09-08 - by (mod) -

Frank,
Looking at frost depth for northern WI via the U.S. National Weather Service finds:

Below is information about wisconsin soil temperatures at various depths as reported by weather observers with national weather service soil thermometers. Eqp means equipment problem.

- Depth Of Temp. Sensor Time/date
2 4 8 20 40 60 Of Last
Id County Location In. In. In. In. In. In. Report
Mfdw3 Wood Marshfield-uw 68 70 68 66 64 60 07 Am 09/08
Nelw3 Clark Neillsville 68 68 67 63 61 58 07 Am 09/08
Arx La Crosse La Crosse-nws 67 68 69 68 63 56 07 Am 09/08
Arlw3 Columbia Arlington-uw M 70 70 68 65 58 08 Am 09/04
Mkx Jefferson Sullivan-nws 71 70 70 68 65 64 07 Am 09/08
Ugrw3 Racine Union Grove M M 67 64 64 60 07 Am 09/08

Below Is Soil Temperature Information From Other Sources
- Depth Of Temp. Sensor Time/date
2 4 8 20 40 60 Of Last
Id County Location In. In. In. In. In. In. Report
Plow3 Portage Plover-south 71 73 71 65 65 67 07 Am 09/02
Mwow3 Waukesha Eagleville 72 71 71 70 66 63 07 Am 09/08


In Jefferson, Portage and Waukesha counties you're looking at 6 feet for the frost line depth.

Unless your design includes a drain-back system not just the pump but any liquid-filled piping such as a force-main pumping effluent up hill will be at risk of freezing.

On 2015-09-08 by Frank Norris

We live in northwestern Wisconsin. For a septic system with a large in-ground septic tank w/ filter plus a large in-ground affluent tank with pump to supply a large effluent field, how far below ground should a field supply pump pipe be placed underground to minimize freeze-up?

Do these pipes allow for back-flow of affluent to the affluent tank in between pumping cycles?

On 2015-07-27 - by (mod) -

Sure Terry, in the ARTICLE INDEX under SEPTIC TANKS see the article link titled

SEPTIC TANK SIZE: septic tank size and capacity vs. usage determine the required septic tank size, calculate size of an existing septic tank

On 2015-07-2 by terry

can you tell me how you figure out what size tank you need and how much drain line needed


...

Continue reading  at SEPTIC TANK SIZE or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

See SEPTIC SYSTEM DESIGN BASICS FAQs - questions & answers posted originally at this page

Or see these

Recommended Articles

Suggested citation for this web page

SEPTIC SYSTEM DESIGN BASICS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification
when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca

Comment Form is loading comments...

Citations & References

In addition to any citations in the article above, a full list is available on request.



ADVERTISEMENT