Septic Drainfield Dimensions Q&A-2

FAQs about the required size of a septic soakbed, leachfield, or drainfield

Drainfield Size Requirements FAQs

How do I know how big the drainfield needs to be?

How do I know how big the drainfield needs to be ? On 2020-02-26 by terry dudley

by (mod) - drainfield size is based on wastewater flow volume per day and soil percolation rate, plus room for future drainfield replacement or repair

Terry

The two most-basic considerations in setting the size of a septic system drainfield are the daily wastewater flow in volume and the percolation rate of the soil. There are other variables such as the type of effluent disposal system used: sprays, gravel trenches, or no-rock gravel-less systems.

The basics of drainfield sizing are in the article above on this page. Please take a look and don't hesitate to follow up with specific questions if any of that is unclear as then I'll know what we need to make easier to understand.

How big (width and length) is a septic system drainfield?

What is the width and length of a drainfield ? On 2019-02-01 by Dorothy

Reply by (mod) -

Dot

Septic drainfield length and with sizes are given in tables in the article above - the answer to your general question is

.... it depends

1. on the soil properties and soil percolation rate

2. the type of septic system

3. the daily wastewater flow volume

4. other factors listed in the article SEPTIC DRAINFIELD SIZE

How do I calculate the size of our septic system leachbed?

How is leachbed calculated and sized ? On 2018-09-13 by Kenneth Mattison

Reply by (mod) -

Kenneth

To determine the required drainfield size for a property you find

1. the daily wastewater load volume in gallons - commonly based on number of bedrooms, or occupants for residential properties

2. the soil percolation rate - by performing a soil perc test - see SEPTIC SOIL & PERC TESTS at inspectapedia.com/septic/Soil-Percolation-Tests.php

3. Then you'd use the septic leachbed (drainfield, soakaway, soakbed, leachfield are synonyms) tables in the article above on this page to find the required drainfield size.

The septic field sizing tables, as you'll see in the data, translate the combination of daily wastewater volume and soil percolation (absorption) rate into the absorption area required.

In some tables you'll see that as "maximum application rate"

For example in our table titled

Absorption Bed Septic Bottom Area Size Required

Example: one of the data lines tells us that if the soil percolation rate is 8-10 minutes per inch then the maximum wastewater application rate for the field will be 0.70 gallons per day per square foot of absorption bed bottom area.

Let's calculate out a further example for the same data line.

If our septic absorption field bottom area is 100 sq.ft. then

Total Daily Wastewater Volume (that can be handled) = 0.7 gpd/sq/ft. x 100 sq.ft. = 70 gallons. Which is, obviously, not much when you consider that Americans use as much as 200 gallons of water per day in their homes, per person for bathing, toilets, cooking.

How do we figure out the size a drainfield for commercial properties?

What is the required size for a septic system for 70 units - RV Trailer Park 6/1/12

For commercial property with no bedrooms how is the size of the drainfield determined? - Anon 7/18/12

Reply:

Good question, Anon.

For an RV Trailer Park, using government design manuals [1][5] as a guide, we estimate that daily wastewater volume will be 75 to 125 gallons pe day per person, with a typical volume of 100 gpd. The drainfield will be designed to handle (100 gallons x number of visitors) per day.

For a commercial property with no bedrooms, the wastewater design volumes vary enormously depending on the type of facility. More details are

at COMMERCIAL SEPTIC DESIGN

Sizing for commercial drainfields and septic tanks) is more difficult than for residential installations. Residential designs start with a simple assumption of the number of occupants and asn average daily wastewater volume (common is 150 gallons/bedroom or 75 gallons per day per person, though some sources use larger numbers).

We often hear complaints from people who say "we are just two people living in a four-bedroom home, why does our septic system have to be designed to handle eight people (4 bedrooms x 2)?" A sensible answer is that the wastewater system should be designed to handle the number of occupants that the building is designed for.

But commercial installations vary widely in the wastewater volume used per person per day depending on the type of facility, the number of visitors to it, how long they stay there, and what activities they pursue.

For example a gas station at a turnpike may have thousands of visitors per day, many of whom use the toilet facilities - that's why we stop at a rest stop - even though the typical length of visit is relatively short.

The US EPA Wastewater manual as well as some U.S. state DEC/DEP wastewater specifications guidelines have published a series of tables of ranges of wastewater production for different types of facilities per visitor or user along with other sources of possible usage volume (such as number of parking spaces).

In a separate article COMMERCIAL SEPTIC DESIGN we provide excerpts from that larger body of information.

How do I find out the size of an existing drainfield?

I am trying to determine the size of an existing conventional drainfield. how do i go about that? - Alan H. 1/17/12

Reply:

Alan, on determining the size of an existing septic drainfield, if you are unable to find as-built plans or a site map for the septic fields, you will want to review the advice given at SEPTIC DRAINFIELD LOCATION (article links listed at the ARTICLE INDEX the bottom of this article ) on how to find the location of the drainfield.

The rough size and shape of that area will be as close as you'll get without some digging and probing.

Do-it-Yourself Septic Leachfield Redesign and Repair FAQs

We're repairing our own failed septic leach field constructed in desert sand. Will our new design exceed the original system capacity?

We are installing a new leach field in between a semi-failed field designed as follows: At the centered outlet T, a 10' space to the right and 10' space to the left is followed by 90 degree elbows that form two 50' leach lines.

The original design is TRENCH AND GRAVEL - about 2' wide, and 4' of gravel under the leach pipe.

Soil is southwest desert sand. We are hoping to install a BED design that will be approximately 10 to 12 feet wide by approximately 50 feet long, filled to about the same depth of gravel as the original trenches.

3 leach lines will connected to the original left line just near the T connection from the septic tank outlet line. From there, the 3 lines (with no distribution box) will be interconnected at the other end as well. The final design will be a rectangular shaped "ring" in order to use as much of the bed as possible. No - we don't have a pressurized system with a pump nor are we wanting to do this.

Our system performed without issue for 19 years of very heavy peak period usage. We are a 4 bedroom house - 3 rooms used for a B&B. Our issue was surely caused by limited access to the septic tank itself when regular 3 year pump out couldn't completely remove the solids.

Eventually the leach lines got clogged. We water snake jetted the leach lines and then pressure forced fresh water with a bladder from the tank outlet pipe in hopes of reviving the drain field but this didn't prove completely successful.

This was after full access was made to the tank - something that should have been done years ago.

I strongly feel the original leach line trenches would have continued to perform had they not been clogged.

This was ultimately made worse by an errant toilet running for over 4 hours and packing the solids.

Note: The existing lines will still remain connected.

My We are doing this project ourselves - today we have a backhoe rented and will begin the dig. Will this new design EFFECTIVELY EXCEED THE ORIGINAL DESIGN?

I realize we are losing side wall absorption (hence interconnecting the other end of the 3 leach lines - with leach pipe of course), but the intention is to hopefully have an overbuilt design that will perform better and longer.

Consequently, we had to dig a relief pit to survive the last 3 weeks of business season. We reopen in March. Hope the vision is clear for the question? Thanks On 2019-01-04 by eric ingvardsen

Reply by (mod) -

Eric

Thanks for the question and septic history.

Watch out: doing your own septic drainfield design and repair without advice from a septic engineer who knows your local laws and soil properties risks spending a lot of time and money on a failure.

About jetting to restore your failed septic

I agree that there is virtually NO independent, un-biased data or research that would claim that jetting or other drainfield rejuvenation approaches produce a lasting result (though there are one or two that somewhat work but at the cost of contaminating the environment with caustics).

I would like to offer a more reliable answer to your "will our new septic design be ok and exceed the original" but I can't.

What you need to proceed with your septic rebuild

- an onsite inspection of the site for drainage, space, location, and implementation details such as trench location, dimension, materials, gravel, drainage

combined with

- soil perc testing that tells us the soil properties.

It is the combination of soil perc rate and the anticipated daily wastewater flow that give a septic designer a guide to just how much leach area is needed to adequately both dispose of and treat the sewage effluent.

Finally: I imagine you're quite correct that the inability to pump the septic tank on a regular schedule (again based on usage levels) is what ultimately did in the fields; while I've seen an occasional (rare) septic system that lasted nearly 100 years, more usually 20 years of life isn't bad for a septic field.