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 septic tank & absorption field 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.

Septic 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

1. Wastewater flows from commercial sources 
   
   such as airports, service stations, bars, hotels, shopping centers
2. Wastewater flows from institutional sources 
   
   such as hospitals, prisons, rest homes, schools
3. 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

Notes: see footnotes in US EPA copy in our references; original source to EPA: Crites and Tchobanoglous, 1998

Wastewater Flows from Institutional Sources

Notes to the table above:

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

Notese to the table above

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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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-10-21 by (mod) - septic system, tank and drainfield, would be sized to handle the largest anticipated daily use

Teresa

Your septic system, tank and drainfield, would have to be sized to handle the largest anticipated daily use in people and wastewater volume (toilets, sinks, kitchen waste, other).

Ask your local septic or health approving board if the might approve a dosing system designed by your septic engineer. That might let you get away with a large septic tank that meters out the effluent more-slowly into a less extravagant septic drainfield - else you'd need one of the systems described on the page above.

On 2020-10-21 by Teresa

We are creating a septic system for a wedding venue. Venue can hold up to 450 people, we anticipate 8-10 days a month with that many people, and in reality many of those 8-10 days will have more like 100-300 people. The remaining 20 days will have 5 or less people. What size septic do we need?! Please advise.

On 2020-06-22 by Vicki

Thanks Dan, I appreciate you taking the time to respond. Will take your points on board, and see what I can sort out.

On 2020-06-17 - by (mod) -

Australia septic system design guidelines for a cafe

Vicki

In my opinion with a tank that small on The Daily Wastewater volume that you're probably projecting you can't produce a working septic system that will adequately treat the sewage and also quickly clogged with solids pushed into the drain field unless you go to a mini onsite wastewater treatment system and pool probably a larger tank.

It's useful to understand that there's an important distinction between successful disposal of a fluid and successful treatment.

Also means we don't see it again after the waste water goes down the drain or at least we don't see it for a few years and successful treatment means we're not discharging pathogens into the environment

On 2020-06-11 by Vicki

I am trying to get someone who can give me an answer to the following:

We have a Cafe which has been in business for 12 years.

We are trying to extend our hours, and for this reason council are demanding a complete new assessment on the septic system.
It has coped with 50-100 customers per day without any problems and often far more.
A whole new assessment will cost thousands.

I just need to know this:
We have a 3000L septic tank with biological effluent filter on the outlet, plus an internal effluent filter.

We have 3 evaporations-transpiration/absorption beds, each one being 20m x 2m and there is a 2m gap between each row.

How many Litres usage is acceptable per customer/per day?
With this size tank etc. what would be the maximum number of customers per day the system could handle?

Can anyone please help?

This is in Australia, but if it helps 3000L is 793 Gallons
Thanks in advance

On 2020-03-30 - by (mod) -

You may want to try the restaurant entry from the table above on this page but keep in mind the septic tank size is by no means going to be the limiting factor. You're probably going to need an onsite wastewater treatment system to handle the surge loads that are conventional drain field would not be able to accept

On 2020-03-30 by Lucy

I'm about to build a wedding venue facility for 250 persons which will be rented just the weekends.what size of septic tank I need to install?

On 2019-10-03 - by (mod) -

Cody

Because prices vary enormously by geographic area, site characteristics, and what your local code officials will permit, it's smart to get a LOCAL estimate from a local septic design engineer.

Your engineer will calculate the gallons per day of wastewater that must be treated and propose a system design and cost.

Example: an onsite wastewater treatment plant of modest size may cost between $200,000 and $1,500,000 depending on those variables.

On 2019-10-03 by Cody

I am about to open an RV Park with 200 park space. How much it costs for a septic system that side?

On 2018-11-28 - by (mod) -

The septic size needed would depend on the number of occupants more than the number of individual cottages. Best bet is to go through the table in the COMMERCIAL SEPTIC DESIGN article given here, then consult with a septic engineer, as most likely you'll need a type of onsite wastewater treatment system not just a simple tank and drainfield.

On 2018-11-28 by Boxcar

How big of septic field would a guy need to maintain 10 summer cottages...all plumbed in and two washrooms. ..two wash machines...and a small cooking kitchen

On 2018-11-05 by (mod) - size of hotel septic thanks

Eric

The required size of a septic system is not specified based just on "hotel" or "house" but rather on the anticipated daily volume of wastewater that it needs to handle. So there is no intelligent answer like "sure, 100,000. liters"

On 2018-11-05 by Eric Agyemang

Please can I know the size of hotel septic thanks

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 SEPTIC TANK 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.

Questions in search of a septic design engineer:

(May 6, 2015) kunal suryawanshi said:
what size should be for four storeys 1bhk apartments

(June 20, 2015) Lori said:
We're looking for a septic system that will take a 50 gal dump at once and not overflow the solids to the outlet side. Is there a special tank or way it should be installed?

(July 21, 2015) Penny said:
I own a 4 commercial washer laundromat. Also connected to the same septic is two washrooms and a small pizzeria open 4 days a week.

The septic is located in a rural area. My septic area is wet, and smells of sewage. I was told that the soap doesn't break down and clogs the runs. What kind of system should I put in?
Thank you for your time, Penny
pcrowe@eagle.ca

Reply:

Lori,

Ask your septic designer about the design for a large holding tank and a dosing system that spreads the dumping of wastewater over time.

Penny

The determination of what kind, size, capacity, layout and location of septic system replacement you need, you'll need an on-site septic engineer or designer. That's because in addition to the daily wastewater volume and type that your laundromat produces, the engineer has to look at the soil properties at your site (for example the percolation rate), the space available, and the terrain shape.

Those will determine the type, size, and layout you need.

I speculate that your business produces a very large volume of soapy graywater that requires a very large wastewater absorption area (drainfield, leachfield, soakaway bed, drainage trenches seepage pits).

I would ask the septic engineer about graywater treatment options that might permit recycling and re-use of at least a portion of this water, both for environmental reasons (water usage reduction, reducing contamination of the envrionment) and economical reasons (reduced water usage, possibly construction of a smaller onsite wastewater treatment facility).

Let us know what your engineer recommends and we can discuss this further.

Question: event center septic tank and drainfield design

(Feb 13, 2016) Campbell said:
I'm curious about how big of a system I would need to operate and event center for 250 people each day.

Reply:

You cannot install a conventional septic tank and drainfield to support 250 people. You'll need an onsite waste treatment facility.

Question: septic for a martial arts school

(Apr 10, 2016) Chris said:
Will a 1000 gallon tank support 4 bathrooms, no showers, at a martial arts school?

Reply:

Chris:

1000 gallons gives us 250 toilet flushes with old toilets, or 666 flushes at 1.6 gpf modern toilets. I'd cut those numbers in half to allow for washing hands.

250 toilet flushes ought to handle a lot of martial artists.

But then you haven't said how often the tank is going to get refilled, nor how.

Without knowing how many people are actually using the system daily, or better, how many of them use the toilets daily, I can't give a more definite answer.

Question: septic tank size for a food store

(Apr 27, 2016) Anonymous said:
How big must a sewer tank be for a store that serves food ,.but no dine-in

We have 3 employees, private bathroom, not for public... can we do that

Reply:

Treat it as based on the number of employees in a food preparation facility - see the tables above.

Question: commercial building with one toilet and bath

(June 5, 2016) Anonymous said:
What size of septic tank should I use in a commercial building that has a 1 toilet and bath?

Reply:

I think your design engineer will say the answer depends on what the water usage is in the building - what "commercial activity" is going on, not just on number of employees.

Question: septic for a Nashville TN recording studio that hosts clients

Anonymous said:
We are planning to accommodate out of town clients for our recording studio on 4 acres. We also may use for personal events such as family or out of town guests when spaces are not used. Our goal is to use this as a mini RV hookup for about 8 spots. If affordable have 1 or 2 cabins such as koa in the far future. Regardless trying to plan with the future in mind, dont want to redo later as we grow. How do we determine the size and type of septic do we need for this? No city sewers are here.
Davidson county nashville tn

Reply:

I would hire a local septic design engineer. She will consider all of the design factors involved - more than I can know from just your question's text - for example in even for occasional use the system has to be sized and designed to handle the maximum number of simultaneous occupants; that doesn't mean that the disposal field has to be "oversized" as a dosing system and large septic tank might be considered part of the design for intermittent use.

But the designer will consider

- the soil properties: percolation rate

- terrain properties: slope, size of area available, laoyout, piping distances

- ground water and high water table

- presence of nearby waterways and water supply well that require clearance distances from septic components

- tank location and size

- local regulations

On 2012-07-18 by (mod) -

Good question, Anon. We have posted a reply at the end of the FAQs section just above so that we can include links to more detailed information.

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 an average daily wastewater volume (common is 150 gallons/person per day 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 has 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).

See this article COMMERCIAL SEPTIC DESIGN - above on this page.

On 2012-07-18 by Anonymous

For commercial property with no bedrooms how is the size of the drainfield determined?

This Q&A were posted originally

at SEPTIC DRAINFIELD SIZE FAQs

...

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

Or see COMMERCIAL SEPTIC DESIGN FAQs - questions and answers posted originally on this page.

Or see these

Recommended Articles

• COMMERCIAL SEPTIC DESIGN
• DISINFECTION SYSTEMS, SEPTIC EFFLUENT
• DRIVING or PARKING OVER SEPTIC
• SEPTIC SYSTEM DESIGN SPECIFICATIONS for ALTERNATIVE SYSTEMS
• SEPTIC DRAINFIELD INSPECTION & TEST - home
• SEPTIC DRAINFIELD LOCATION for help in finding an existing septic soakaway bed, leach field or drainfield
• SEPTIC DRAINFIELD SIZE
• SEPTIC DRAINFIELD SHAPE
• SEPTIC DRAWINGS
• SEPTIC SOIL & PERC TESTS
• SEPTIC SYSTEM DESIGN SPECIFICATIONS for ALTERNATIVE SYSTEMS
• SEPTIC TANK SIZE
• SEWAGE TREATMENT PLANTS, RESIDENTIAL / LIGHT COMMERCIAL

Suggested citation for this web page

COMMERCIAL SEPTIC DESIGN 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

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