Commercial Septic Tank & Drainfield Design Size Requirements

How to determine the septic drainfield size needed for commercial installations such as hotels, restaurants, businesses, hospitals, gas stations; - EPA & other design tables gives examples of commercial wastewater treatment system wastewater design flows
- Commercial leach field or soakaway field size requirements
- How big should a non-residential leach field be? How long should drainfield trenches be? How many trenches do we need for a commercial septic system?
- Design guide for commercial septic drainfields: field size, dimensions, depth, layout suggestions
- Typical wastewater flow rates from commercial sources
- Typical wastewater flow rates from institutional sources
- Typical wastewater flow rates from recreational facilities
- Questions & Answers about commercial or "non-residential" septic drainfield or soakaway bed size or capacity requirements & design
References

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This article describes the basic design approach to commercial wastewater treatment systems: how big should the septic tank be and how large should the drainfield be for non-residential installations like hotels, restaurants, gas stations, parks? Data is based on US EPA and other government wastewater disposal system design manuals and codes. 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. So wastewater volume design assumpations need to take into considerations different types of usage, visitor and visitor or occupant numbers when specifying a septic tank size or drainfield size.

Septic Tank Design & Size Recommendations for non-residential wastewater systems

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).

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).

The minimum permitted septic tank size at a property is regulated by local onsite codes (see NSFC, 1995) and should consider a conservative (safe) estimation of daily wastewater flow volume. Residential septic tank sizing tables are provided at SEPTIC TANK SIZE. Septic tank volume for a conventional tank and onsite effluent disposal system (such as a drainfield) is estimated at a minimum of 1000 gallons or 1.5 x average total daily wastewater flow. Quoting the EPA manual:

Most onsite code requirements for system design of residential dwellings call for estimating the flow on a per person or per bedroom basis. Codes typically specify design flows of 100 to 150 gallons/bedroom/day (378 to 568 liters/bedroom/ day), or 75 to 100 gallons/person/day (284 to 378 liters/person/day), with occupancy rates of between 1.5 and 2 persons/bedroom (NSFC, 1995).
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In lieu of using conservative design flows, a direct factor of safety (e.g., 2) may be applied to estimate the design flow from a residence or nonresidential establishment. Example: Multiplying a typical flow estimated (140 gallons/ day) by a safety factor of 2 yields a design flow of 280 gallons/day (1,058 liters/day). Factors of safety used for individual systems will usually be higher than those used for larger systems of 10 homes or more.
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Great care should be exercised in predicting wastewater characteristics so as not to accumulate multiple factors of safety that would yield unreasonably high design flows and result in unduly high capital costs. Conversely, underestimating flows should be avoided because the error will quickly become apparent if the system overloads and requires costly modification. . [1] chapter 3.

WATER USAGE TABLE provides companion data if you don't already know your daily wastewater volume

How is a septic drainfield size chosen?

In detail at SEPTIC DRAINFIELD SIZE we include tables that give the required septic system drainfield size based on

The soil percolation rate (in minutes per inch) and other soil properties
The number of building occupants (or number of bedrooms in a residential property x 2) and/or the assumed or design level in gallons per day of wastewater volume

to provide
The number of linear feet of conventional septic drainfield trench required to handle the wastewater daily flow volume

But these tables estimate wastewater volumes based on residential building occupancy - that is, for private homes. In order to obtain a more reliable estimate of wastewater volumes for non-residential properties, septic system design engineers construct tables that give ranges of estimated wastewater volume for quite a few different types of properties and users. A very common set of such tables is provided by the U.S. Environmental Protection Agency, first published in 1980, revised in 2002, in the US EPA "Onsite Wastewater Treatment and Disposal Systems Design Manual" (link to free copies of this document provided in this article's references).[1] And where a range of wastewater flow volumes is given, experts recommend that the designer use the larger volume number except for custom-engineered systems.[5]

The tables of estimated wastewater flow volumes for non-residential designs are grouped into three large categories

Wastewater flows from commercial sources such as airports, service stations, bars, hotels, shopping centers
Wastewater flows from institutional sources such as hospitals, prisons, rest homes, schools
Wastewater flows from recreational sources such as resort apartmeents, cafeterias, campgrounds, coffee shops, day camps, RV parks using a sewer hookup system, resort stores, theaters, visitor centers

But even within each of these categories you will see a very large range of wastewater flow estimates. For example, in category 1, commercial wastewaer sources, wastewater flows from a self-serve laundry are estimated by the number of washing machines, at 475 to 686 gallons per day, while at an airport wastewater flows are estimated per passenger at 2.1 to 4.0 gallons per day (about one toilet flush per passenger).

Wastewater Flows from Commercial Sources

Typical wastewater flow rates from commercial sources USA EPA Table 3-4 [1]
Facility	Unit	Wastewater Flow Gallons/Unit/Day Range	Wastewater Flow Gallons/Unit/Day Typical	Wastewater Flow Liters/Unit/Day Range	Wastewater Flow Liters/Unit/Day Typical
Airport	Passenger	2-4	3	8-15	11
Apartment house	Person	40-80
Automobile service station	Vehicle Served Employee	8-15 9-15
Bar	Customer Employee	1-5 10-16
Boarding House	Person	25-80
Department store	Toilet room Employee	400-600 8-15
Hotel	Guest Employee	40-60 8-13
Industrial Building (sanitary waste only)	Employee	7-16
Laundry (self-service)	Machine Wash	456-650 45-55
Office	Employee	7-16
Public lavatory	User	3-6
Restaurant (with toilet) Conventional Short order Bar/cocktail lounge	Meal Customer Customer Customer	2-4 8-10 3-8 2-4
Shopping Center	Employee Parking Space	7-13 1-3
Theater	Seat	2-4
Notes: see footnotes in US EPA copy in our references; original source to EPA: Crites and Tchobanoglous, 1998

Wastewater Flows from Institutional Sources

Typical wastewater flow rates from institutional sources - USA EPA Table 3-5 [1]
Facility	Unit	Wastewater Flow Gallons/Unit/Day Range	Wastewater Flow Gallons/Unit/Day Typical	Wastewater Flow Liters/Unit/Day Range	Wastewater Flow Liters/Unit/Day Typical
Assembly Hall	Seat	2-4	3	8-15	11
Hospital, medical	Bed Employee	125-240 5-15
Hospital, mental	Bed Employee	75-140 5-15
Prison	Inmate Employee	80-150 5-15
Rest Home	Resident Employee	50-120 5-15
School, day-only With cafeteria, gym, showers With cafeteria only Without cafeteria, gym, or showers	Student Student Student	15-30 10-20 5-17
School, boarding	Student	50-100
Notes: see footnotes in US EPA copy in our references; original source to EPA: Crites and Tchobanoglous, 1998. See www.epa.gov/safewater/uic.html for more information

Watch out: when designing non-residential wastewater systems keep in mind that the wastewater flow rate over a 24-hour period is probably not uniform and that very high maximum hourly flow rates may occur. For example even in residential facilities, maximum flow rate peaks twice a day between 9 and 10 AM and again between 6 and 8 PM.. [1]

Wastewater Flows from Recreational Facilities

Typical wastewater flow rates from recreational facilities - USA EPA Table 3-6 [1]
Facility	Unit	Wastewater Flow Gallons/Unit/Day Range	Wastewater Flow Gallons/Unit/Day Typical	Wastewater Flow Liters/Unit/Day Range	Wastewater Flow Liters/Unit/Day Typical
Apartment, resort	Passenger	50-70	60
Bowling Alley	Person	125-250	200 Note 2
Cabin, resort	Person	8-50	40
Cafeteria	Customer Employee	1-3 8-12	2 10
Camps Pioneer type Childrens, with central toilet/bath Day, with meals Day, without meals Luxury, private bath Trailer camp	Person Person Person Person Person Trailer	15-30 35-80 10-20 10-15 75-100 75-150	25 45 15 13 90 125
Campground, developed	Person	20-40	30
Cocktail lounge	Seat	12-25	20
Coffee Shop	Customer Employee	4-8 8-12	6 10
Country Club	Guests onsite Employee	60-130 10-15	100 13
Dining Hall	Meal served	4-10	7
Dormitory/bunkhouse	Person	20-50	40
Fairground	Visitor	1-2	2
Hotel, resort	Person	40-60	50
Picnic park, flush toilets	Visitor	5-10	8
Store, resort	Customer Employee	5-12 8-12	10 10
Swimming pool	Customer Employee	5-12 8-12	10 10
Theater	Seat	2-4	3
Visitor center	Visitor	4-8	5
Note: see footnotes in US EPA copy in our references; original source to EPA: Crites and Tchobanoglous, 1998 Note 2: seems strangely high for just bowling, is this an error? - DF

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Frequently Asked Questions (FAQs) about commercial septic systems design, cleaning, maintenance schedules, and repair

Question: how often should I clean the restaurant external grease trap & aerobic septic system?

I have a year old aerobic commercial system servicing two restaurants the service contract expiring,

1. A 2,000 external grease trap for a 2,000 sf Mexican food medium volume, two commodes, with grease pots external from the system to dump most of the grease and oils.

2. A 3,000 external grease trap servicing a 4,225 Italian food low volume, two commodes little grease and oils.

When this restaurant was in a different location with 4 other businesses on a 3,000 tank it was pumped every six months.

I have three quotes all within $ 200.00 of each other and all say their scheduling is the best. Either tank has been pumped since startup or according to all 3 pumpers are not at manitory levels yet. Quote 1 pump every six months, quote 2 pump every 6-8 months, and the other 8-12 months. I know you can not issue a concrete decision without additional information but a best guesstimation would be helpful. Thank you in advance. The company with the best price and the every six month time frame does not have the best reputation. What would suggest? - J.R. 7/21/12

Reply:

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem, evaluate the condition of the system, and thus give more specific advice - which I realize you understand from your question. That said,

Because expert sources have made clear the number one cause of failiure in Aerobic systems was inadequate maintenance, and because the cost of system failure remedy can be high, it makes sense to me to err on the "safe" side. You could use the commecial wastewater flow estimates in the article just above to see what the standard design parameters should have been for your system.

The true economics of managing onsite wastewater treatment systems have to include not only the periodic cleaning costs, but an allowance for repair or replacement of system components. And that latter figure has to be increased to allow for risks of costly damage if we wait too long between cleaning intervals.

But more immediately, I would have each system opened and pumped & cleaned immediately.

But as part of that cleaning process, ask the service company to make measurements of the actual levels of sludge, scum, grease, in each system.

By comparing the measurements to the allowable or desired net free area in the treatment tank, or to our own recommended sludge and scum levels found at MEASURE SCUM & SLUDGE you can calibrate your actual system usage and accretion of waste products that need cleaning against the time since last cleanout.

With that data you can then schedule cleaning more accurately and economically.

Let me know what you find when the systems are actually measured - what we learn will surely help other restaurant operators.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

[1] US EPA Onsite Wastewater Treatment Systems Manual [online copy, free] Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, revised 2002, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm Onsite wastewater treatment and disposal systems, Richard J Otis, published by the US EPA. Although it's more than 20 years old, this book remains a useful reference for septic system designers. U.S. Environmental Protection Agency, Office of Water Program Operations; Office of Research and Development, Municipal Environmental Research Laboratory; (1980)
[2] "International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
[3] "Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.
[4] Manual of Septic Tank Practice, US Public Health Service's 1959.
[5] "Installers Manual for Conventional Onsite Domestic Wastewater Treatment and Disposal Systems", Department of Environmental Conservation, Division of Environmental Health Drinking Water and Domestic Wastewater Program, Alaska Department of Environmental Conservation, 1 Aug 2000, Anchorage Offices, 555 Coredova, Anchorage AK 99501, Tel: 907-269-7500. retrieved 17 July 2012, original source: http://dec.alaska.gov/water/wwdp/onsite/pdf/Certified_Installer%27s_Manual.pdf [Copy on file as Alaska_Certified_Installer's_Manual.pdf].
Notice: [Quoting]
This document contains information regarding the installation of onsite se3wer systems for single-family and duplex residences. It must be used by Certified Installers and homeowners who are subject to 18 AAC 72. Additional requirements are included in 18 AAC 72. If there is a conflict between the provisions of this manual and 18 AAC 72, 18 AAC 80, or other state regulations, the regulations language controls. AEDC offices.
The regulations cited above for Alaskans can be found at http://www.alaska.gov/dec./deh/water/ci.htm
[6] Soil Percolation Tests soil perc testing guide and instructions
[7] Percolation Testing Manual, CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950
[8] Test Pit Preparation for Onsite Sewage Evaluations, State of Oregon Department of Environmental Quality, Portland OR, 800 452-4011. PDF document. We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design. Readers should also see Soil Percolation Tests and for testing an existing septic system, also see Dye Tests
[9] Wells and Septic Systems, Alth, Max and Charlet, Rev. by S. Blackwell Duncan, $ 18.95; Tab Books 1992. We have found this text very useful for conventional well and septic systems design and maintenance --DF. Quoting an Amazon description:Here's all the information you need to build a well or septic system yourself - and save a lot of time, money, and frustration. S. Blackwell Duncan has thoroughly revised and updated this second edition of Wells and Septic Systems to conform to current codes and requirements. He also has expanded this national bestseller to include new material on well and septic installation, water storage and distribution, water treatment, ecological considerations, and septic systems for problem building sites.
[10] Design Basics for Septic Systems: Choosing Septic Tank Size, Leach Field Size - basic septic system volume and absorption system design guides.
[11] How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
[12] HOW BIG SHOULD THE LEACH FIELD BE? - an engineer's view of leach field sizing
[13] Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems
[14] Sketches of the Septic System Components Private Sewage Disposal Systems - Septic Drawing Library
[15] Percolation Testing Manual, CNMI Division of Environmental Quality, Gualo Rai, Saipan provides an excellent English Language manual guide for soil percolation testing. Original source: www.deq.gov.mp/artdoc/Sec6art108ID255.pdf
[16] Soil Test Pit Preparation, fact sheet, Oregon DEQ Department of Environmental Quality, original source www.deq.state.or.us/wq/pubs/factsheets/onsite/testpitprep.pdf The Oregon DEQ onsite water quality program can be contacted at 811 South Ave, Portland OR 97204, 800-452-4011 or see http://www.oregon.gov/DEQ/
[17] Thanks to reader Michael Roth for technical link editing 6/29/09.
[18] Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
[19] Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf.
[20] Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
[21] Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
[22] Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

[23] The Plumbers's Handbook, Howard C. Massey, Craftsman Book Company; Rev Sub edition (April 1998), ISBN-13: 978-1572180567 includes septic system design basic sketches and specifications.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd

Builder's Guide to Wells and Septic Systems, Woodson, R. Dodge: $ 24.95; MCGRAW HILL B; TP; Quoting from Amazon's description: For the homebuilder, one mistake in estimating or installing wells and septic systems can cost thousands of dollars. This comprehensive guide filled with case studies can prevent that. Master plumber R. Dodge Woodson packs this reader-friendly guide with guidance and information, including details on new techniques and materials that can economize and expedite jobs and advice on how to avoid mistakes in both estimating and construction. Chapters cover virtually every aspect of wells and septic systems, including on-site evaluations; site limitations; bidding; soil studies, septic designs, and code-related issues; drilled and dug wells, gravel and pipe, chamber-type, and gravity septic systems; pump stations; common problems with well installation; and remedies for poor septic situations. Woodson also discusses ways to increase profits by avoiding cost overruns.
Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP; Quoting an Amazon reviewer's comment, with which we agree--DF:This book is informative as far as it goes and might be most useful for someone with an older system. But it was written in the early 1980s. A lot has changed since then. In particular, the book doesn't cover any of the newer systems that are used more and more nowadays in some parts of the country -- sand mounds, aeration systems, lagoons, etc.

The NSFC Products List has an excellent list of design manuals/modules available from their website or by telephone 800-624-8301
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