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SEPTIC SYSTEM DESIGN BASICS - Choosing Septic Tank Size, Drain Field Size, Basic Septic Design InspectAPedia®  -     How to specify tye size of a septic tank How to specify the size of a septic drainfield Septic system design basics

This chapter 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. For more in-depth information about septic system design see our list of detailed septic design articles at Septic System Design.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References." This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

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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. Also take a look at the "Septic Publications" at "More Reading" below 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 page-left links under SEPTIC 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 the "Home Buyers Detailed Guide" listed under "More Reading".

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 Advice for Conventional Septic System Design

Septic Tank Sizing: Most jurisdictions want to see a minimum of 1000 gallons. 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 "Tank Size" article listed below to see the required tank size based on daily water volume usage and perhaps the "Water Usage" article which discusses water usage requirements to confirm that you're estimating the water volume reasonably.

Septic Tank Location:: Articles titled "Clearances for Septic Components" and :Clearances for Wells" 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 "Retention Time in Septic Tanks".

Septic leach field or drainfield size and layout planning: see our article "Absorption Field Design" which sizes the field based on its percolation rate and our article "Soil Percolation Tests" which describes performing a perc test, with perhaps a look at "Biomat Formation" to understand biomats that form below a drainfield and why a leach field in a wet area won't treat the effluent.

Leach Field Trench Specifications: Standard leach field trench specifications: 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:

• Leach line 4" perforated PVC pipe will slope at 1/4" per ten feet of length
• A good trench is 18" to 36" wide and 18 " to 30" deep (no deeper), and the pipe is put with holes down, and runs in a 12" fill (in the trench) of clean gravel 1 1/2-3" diameter stones.
• In the gravel in the trench, the perforated leach line pipe is placed on top of 6" of gravel and then is covered with additional gravel until the gravel is 2" over the pipe. With a 4" pvc pipe this is how I got the 12" fill of gravel.
• Remember to direct surface or subsurface runoff away from the septic system components. Otherwise you may flood the system in wet weather.
• "Fields: How Big Should the Leach Field Be?" has notes on calculating the number of linear feet of leach line trench for a conventional septic system leach field.

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• Additional technical contributors & reference sources for this article are listed below.

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Articles Describing Basic Design Specification in more detail

Septic System Design & Installation Details

• Absorption Field Design size and specifications for leach fields, absorption beds, gravelless systems, seepage pits
• Absorption Field Layout: septic drainfield or leaching bed shape and placement considerations
• Absorption Fields Trench Length has an example of calculating leach line trench length.
• Alternating Bed Septic Systems may provide a retrofit repair for a failing drainfield or a means of extending drainfield life
• Authorities, Septic Design Specifications by Federal and State Regulators
• Aerobic Septic Systems: Design Alternatives for Difficult Sites, specifications, photos, sketches, product sources, suppliers, designers
• Books & Building Codes for Septic System Design. Septic System Education Materials
• Baffles & Tank Tees: septic tank baffles and tees, design to prevent system clogging
• Capacity: Determining Needed Septic Capacity - Septic Tank Size vs. Usage: What septic size is required?
• Capacity: Determining Needed Septic Capacity - Based on Water Usage: Typical residential water usage for people, appliances, and activities.
• Cesspools: explained, design specifications, safety, photos, sketches
• Clearances & Distances from septic system components to other site features, property boundaries, waterways, etc.
• Contaminants in sewage which may be released from sewage spills from residential septic systems and wastewater disposal systems
• Drywells Home Buyer's or Owner's Advice
• Composting Toilets Are They A Septic System Design Alternative for Difficult Sites?
• Design Alternatives for Septic Systems: Advanced & Alternative Septic System Designs for Onsite Wastewater Disposal - design engineers, consultants, products, books for special problem situations, difficult soils, old system repairs. Aerobic septic systems, chemical, composting, incinerating & waterless toilets, Evaporation-Transpiration (ET) Septic Systems, septic media filters, greywater systems, holding tank septic systems, mound septics, raised bed septics, pressure dosing septic systems, sand bed filters, peat beds, constructed wetlands, septic disinfection systems.
• Design Basics for Septic Systems: choosing septic tank size, leach field Size - basic septic system volume and absorption system design guides.
• Disinfection for Aerobic Septic Systems - use of calcium hypochlorite tablets versus swimming pool chorine tablets
• Disinfection for Aerobic Septic Systems - warning about use of swimming pool chorine tablets
• Dosing Systems, Gravity, Siphons, Floats - to control movement of septic effluent to the absorption system
• Dosing Systems, Pressure - Pressure dosing, using mainfold systems, rigid pipe systems, and drip irrigation systems to control movement of septic effluent to the absorption system
• Drainfield Design size and specifications for septic drain fields, septic leaching beds or leach fields, absorption beds, gravelless systems, seepage pits
• Drawings and sketches of general septic system designs and septic system types - septic plans
• Drywells design specifications for drywells, seepage pits, leaching pits
• Evaporation/Transpiration Septic Systems - septic system design alternatives for difficult sites
• Filters for Septic Tanks & Graywater to protect septic drainfields and reduce drainfield clogging
• Gravelless Septic Systems - design and installation specifications
• Greywater Systems or Graywater systems for disposal of onsite wastewater: designs, products, advice, books.
• Global Warming Effects on Septic Systems and Onsite Wastewater Treatment or Disposal
• Lagoon Systems for human and animal waste and wastewater treatment
• Leach Field Size specifications for leach fields, absorption beds, gravelless systems, seepage pits
• Media Filter Septic Systems as components of alternative septic systems for difficult sites
• Mound Septic Systems as components of alternative septic systems for difficult Sites
• Percolation Tests septic soil tests, deep hole tests
• Pressure Dosing Septic Systems: explanation, design alternatives, photos, sketches, product sources, suppliers, designers
• Products - for Alternative Septic System Designs aerobic system pumps, media filters, gravelless systems, other advanced wastewater treatment products
• Pumps Septic pumps, sewage ejector pumps, grinder pumps, effluent pumps, sump pumps, & septic pumping stations compared; pump alarms. Advice.
• Raised Bed Septic Systems raised bed effluent disposal system as a component of alternative septic systems for difficult sites
• Retention Time in Septic Tanks Septic Tank Pumping Guide discusses retention time and "net free area" in a tank (allowing for sludge and scum layers)
• Sand Bed Septic Systems for effluent disposal as a component of alternative septic system design for difficult sites
• Seepage Pits for onsite wastewater disposal - design specifications. For more detail see Drywells design specifications
• Septic Tank Types: characteristics and inspection guide for steel, concrete, plastic, fiberglass, and home made septic tanks
• Seepage Pit Collapse or Failure - stages in the life of a drywell, seepage pit, cesspool, and what to do if the pit is damaged
• Sewage Contaminants which may be released from sewage spills from residential septic systems and wastewater disposal systems
• Steep Slope Septic Designs - how to install sewer lines from building to septic tank; how to install septic drainfields, leach field systems at steep or sloped sites.
• Sewage Treatment Systems for onsite wastewater treatment & disposal, small residential and community system designs
• Tank Size: septic tank size and capacity vs. usage determine the required septic tank size, calculate size of an existing septic tank - 2005 Ed.
• Tank Tees & Baffles: septic tank baffles and tees, design to prevent system clogging
• Toilets, Chemical, Composting, Incinerating: chemical toilets, composting toilets, greywater systems, holding tanks, incinerating toilets
• Wastewater Treatment Basics describes the basic processes that occur in wastewater treatment processes
• Waterless & Low Water Septic Systems, chemical toilets, composting toilets, greywater systems, holding tanks, incinerating toilets
• Wetland Septic Systems

More Reading:
NYS Wastewater Treatment Standards for Individual Households detailed design specifications for septic systems, both conventional tank and leach field and several common alternative designs such as mound systems, raised bed septics, sand filter bed septics, evaporation/transpiration septic systems.
Septic System Design Manuals
Septic Disposal Standards
Septic Publications
Home Buyer's Detailed Guide to Septic Systems - Buying a Home With a Septic Tank

Septic Tank Soil Absorption Systems - Decentralized Sysetms Technology Fact Sheet, US EPA, EPA 932-F-99-075, September 1999 - original citation www.epa.gov

Link Exchange & Book, Consultant, Product Listing Policy

Septic design consultants and alternative septic design engineers, septic pumping and repair contractors, academics, researchers, and other onsite wastewater industry participants can be listed here at no charge beyond exchanging web links by contacting me. Technical reference books on septic system inspection, design, repair, maintenance, and alternative onsite waste disposal designs, technical documents, and other online resources are listed at this website at no charge as well.

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More Information on Building Diagnostic Inspections and Repairs

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank

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