These questions and answers about how to figure the size of large capacity or commercial septic tanks and wastewater systems can help point you in the right direction.

This article series 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?

FAQs on Septic Tank Design & Size for non-residential wastewater disposal systems

These questions and answers about commercial septic systems and septic tank sizes & designs were posted originally

at COMMERCIAL SEPTIC DESIGN - home - please be sure to review the advice given there.

What size septic drainfield will be needed for a small RV park with 15 sites and 2 restrooms?

Our drain field seems to be failing and we would like to replace it. We have a small RV park with a 3,200 gallon septic tank and three (known) 90 foot drain field lines (dome style) and one 40 foot drain field line.

My calculations are, if ALL of our 15 Full Hook-Up sites (sewer at the site) AND ALL of our 15 Partial Hook-Up sites (which have to pull up to the Dump Station to dump) dump each day, that is a total of 30 sites dumping X 50 gallons = 1,500 gallons a day. We also have 2 restrooms with 2 sinks, 2 showers and 2 toilets in each. Our soil is somewhat sandy with some rocks.

1. Is this max load Per DAY sufficient for our tank size?
AND
2. If we replace the drain field, how many 4" lines (up to 90'-100' each) would we need to adequately meet our needs? Thanks. On 2021-04-12 by Small 30 Site RV Park

by (mod) -

@Small 30 Site RV Park,

Thank you - You're asking good questions but not ones that anyone can nor should guess-at by a brief text inquiry.

An onsite septic engineer would look at the soil percolation rate to tell you how many feet of what type of absorption bed or trenches or chambers (not so reliable) you need.

A 3,200 gallon septic tank can support up to 2,500 gallons of wastewater a day. On that basis if your calculations are right, the tank is big enough. But I would re-visit the assumption of just 50 gallons of wastewater per day per hook-up site and add to that an estimate of wastewater from your restrooms, showers, etc.

Most people will use more than 50 gallons of water a day;

see WATER QUANTITY USAGE TABLES

But you could make an interesting calculation:

Take a look at the maximum RV size that you host and the maximum RV freshwater storage tank size and then at your own data on length of stay of your visitors and the frequency with which they fill-up their RV holding tanks. I've read assertions that an owner of a 35 ft RV carrying a 100 gallon supply of fresh water makes that last a week. That's just a little over 14 gallons a day.

An RV's freshwater tank size ranges from about 20 to 100 gallons depending on RV size. -various sources including https://camperreport.com/how-many-gallons-of-water-does-an-rv-usually-hold/The RV graywater tank in a typical RV might be just 30-40 gallons and blackwater tank about the same (and is emptied less-often than graywater tank)

Some RV publications say that RV'ers use very little water, perhaps 2-5 gallons per person per day - at least when NOT connected to a water supply hookup, but probably more when they're hooked up to a water supply; that'd support your assumption.

Some sources that looked more-realistic to me put the water consumption rate higher, at about half the at-home daily water usage, putting it (by my data) at 20-30 gallons/day) https://goodoldrvs.ning.com/profiles/blogs/water-usage-comparison

https://rvlife.com/daily-water-usage/ has a water usage determination page but offers no data, just usage measurement ideas.

https://musingsonentropy.com/2014/10/07/motorhome-living-one-month-in-daily-water-consumption/ has one opinion/reader describing using about 6-7 gallons a day per person (graywater).

Tell me, if you have it, your known fresh water consumption per hookup site per season and the number of season-days and we can get an average.

Also, we need to separate and report dumping of graywater vs dumping of blackwater as often those are in separate reservoir tanks and are accumulated and emptied at different frequencies - graywater more frequently than blackwater (sewage).

Remember the septic system has to be able to handle peak loads, not just the average load.

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.

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.

I need septic system design guidelines for a cafe

in Australia

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 On 2020-06-11 by Vicki

Answer by (mod) -

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

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.

How big does the septic tank need to be for a 250 person wedding facility?

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 2020-03-30 by Lucy

by (mod) - the tank and drainfield have to be able to handle the highest wastewater surge

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

Questions that Point to Need for a Septic Design Engineer:

What size should be for four storeys 1bhk apartments? (May 6, 2015) kunal suryawanshi

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? (June 20, 2015) Lori

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?
(July 21, 2015) Penny s 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.

What would be the septic tank and drainfield design for a 250 person event center?

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

Reply:

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

How bit will the septic tank be for martial arts school

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

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.

How big is a hotel septic tank?

Please can I know the size of hotel septic thanks On 2018-11-05 by Eric Agyemang

by (mod) - terms like "hotel" or "house" are not enough to know how big to make a septic tank or system

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"

Instead we need an actual estimate of the total daily wastewter flow. See the tables given

at COMMERCIAL SEPTIC DESIGN - home

Also take a look at the typical water usage information

at WATER USAGE REQUIREMENTS

Need septic design and safety specs for expanding a campground system with 100 new sites

we are planning on second phase of a growing campground. We are planning on installing 100 new campsite for RV and campers.

Most campers are 2 person ownership since this is over 55 age, 6 month seasonal facility.

Currently local ordinance restricts one acre lots to one home. There is nothing on the books for CAMPGROUNDS/RV. but your EPA water stats clearly reflect difference in single family homes.

Our goal is to max our facility not min. its potential safely. On 2018-10-14 by joetuz

by (mod) - no conventional residential septic tank and drainfield would ever suffice. You need and onsite sewage treatment system

Joe

The EPA as well as your local state or municipal septic regulations are definitely not intended to "minimize" a septic system nor to minimize the use that you can make of a site.

Rather, those regulations are intended to tell us the *maximum* use that can be made of a site without creating unsafe or unsanitary conditions such as, in essence, peeing into the well or contaminating nearby lakes, ponds, streams or waterways.

If we exceed or violate those rules we're doing just those things: adding space to a campground is wonderful - I'm a camper myself and enjoy more space and less crowding. But I'd not return to a campground that was sending open sewage into the stream where I swam or into the water I drank or into the fish I eat.

A separate issue is the zoning one. A local ordinance that restricts one acre lots to one home does not address campground use where a far greater number of people may be on the site at one time than would ever possibly work with a conventional home septic system.

it makes no difference if that occupancy is intermittent: the septic system has to be able to handle the peak occupancy: It’s not as if it’s ok to poop in the well just sometimes but not always.

Before giving up on your expansion you want to get help from a septic design engineer who is familiar with local regulations, who knows local code officials, and who can help you with a design that works for the site, including the available space, soil properties, distances to streams etc., and thus that can meet code.

For example once resolving the zoning issue and thus the site usage issue, f there isn’t room for a conventional septic drainfield and tank system to handle your occupancy, there may be alternative septic designs such as a small on-site wastwater treatment facilitiy that can handle the wastewater.

Certainly for a 100 to 200 occupancy site, that’s likely to be what’s needed, as no conventional residential septic tank and drainfield would ever suffice.

What's the septic tank size for an adult daycare facility?

I'm opening an Adult Day Care with a possible capacity of 20 - 25 clients daily, our hours of operation would be 8 hrs daily 5 days a week.

I wanted to know what size septic tank would be necessary if sewer connection not available. On 2018-08-23 by Terry

by (mod) - septic TANK size depends on the total daily wastewater volume not just the number of occupants

Terry the septic TANK size depends on the total daily wastewater volume not just the number of occupants (though that's a key factor as you'll see in the table on this page).

Some septic tank sizes are at SEPTIC TANK SIZE https://inspectapedia.com/septic/Septic_Tank_Size_Tables.php

BUT that too is very incomplete: the critical factor for you is less tank size and more what is going to happen to the wastewater effluent that leaves the septic tank ( a tank is normally always full so 100 gallons in = 100 gallons of effluent out into the drainfield)

Rather it's then the drainfield size that will be critical for you - you need space to fit a field that can handle the daily wastewater flow OR if you don't have enough space for a conventional soakaway bed your local building or health department may accept a proposal for a small onsite wastewater treatment plant that discharges acceptably clean water to the environment.

To answer that question you need an onsite septic engineer who examines the space at your site, the setback requirements for tank and field, and who does soil tests to get the soil percolation rate - a slow perc soil needs more areas of soakaway

Septic tank capacity for a winery with tasting room

I am trying to estimate septic tank capacity for a small winery with a tasting room. At peak (once or twice a week) we may have 75 patrons.

There are 3 stools and 1 urinal as well as a few sinks. The soils perks VERY well. Thoughts? On 2018-04-04 by Zgraywine dog

by (mod) - calculate the estimated daily wastewater flow

If you assume each patron uses a toilet or urinal at least once during a visit you can calculate the estimated daily wastewater flow. Both tank and drainfield need to accommodate that.

I might select coffee shop or dining hall from the table at COMMERCIAL SEPTIC DESIGN

What's the septic size for a school with 50 students and 10 adults?

I am director of elementary school with 50 students and 10 adults. School in session 5 days/wk, 9 months/yr.

Looking to move to a property with septic system. How big system do I need for my school?

Property currently used as three bed house with out buildings. Thank you! On 2018-02-17 by Kimberly

Answer by (mod) -

School estimates of septic wastewater loads are in a table on the article COMMERCIAL SEPTIC DESIGN. You messy find that a wastewater treatment plant iswhast will fit your needs.

What kind of septic system is needed for 200 person wedding facility with 6 toilets?

We are building a 200 person barn to use for weddings and other events with around 6 toilets in total.

There will be a kitchen area with sinks and a dishwasher. Mostly used on the weekends.

What type of system would this require? On 2018-02-08 by Glenn

Answer by (mod) -

The challenge, Glenn, is that your system has to be able to handle the peak usage loads - 200 people over a day or two.

For that you may need a mini onsite wastewater treatment system rather than a conventional septic tank and soakaway bed.

by theplumbingengineer

@Glenn,
The tank needs to be sized to handle the peak load. The leach field can be sized for average flow. In other words, you have an event where 200 people use the facility for several hours so the wastewater needs to be contained in the tank.

Then you have about a week before the next event for the leach field to absorb it.

You should hire an engineer to do the calculations based on occupancy, use, drainage field absorption rate, and local code requirements.

Answer by (mod) -

Glenn and Plumbing engineer

Thank you plumbingengineer for helping out with Glenn's question about a septic system for handling a peak-load

Here are some details that clarify the problem and explain why a typical septic tank and drainfield designed with capacity to handle a building that normally just has say 4-10 occupants cannot handle a surge of 200 people without risk of failure.

The problem here is not simply the septic tank size, it's the discharge of a large volume of wastewater into the absorption field over a short time period - during the event.

If we assume, generously, that during an event attended by 200 guests and say 20 staff, or 220 people total, we're looking at a total waste volume of no less than 2,200 gallons of water flowing into the septic tank during the event for toilets and hand-washing alone, with no other water usage such as for cleanup or cooking.

IF we had a 2200 gallon septic tank

and

IF the septic tank were pumped absolutely empty before the event

THEN yes the tank could absorb that wastewater surge without flooding the drainfield

You would have to pump the septic tank out completely before every event. And I'm doubtful that local health authorities will approve such a system.

That is not the normal situation.

Normally a septic tank is completely full, all the time.

In the normal case, then, during the event, 2,200 gallons of wastewater (or more if there is kitchen waste, cleaning wastewater, etc) is pushed into the absorption field.

There is no conventional residential-type septic drainfield or absorption field design that handles that volume of wastewater during a short burst of activity. The system will be flooded and will fail.

What's the septic tank size for a university with 6000 students?

size of septic tank for an university with around 6k students On 2018-01-17 by riya

by (mod) - no conventional septic tank and drainfield would work - you need an onsite treatment facility

Riya

A conventional septic tank and absorption field will not handle 6,000 occupants. You will need an onsite wastewater treatment facility.

How many septic tanks do we need for a labor camp with 650 people?

For a forced labor camp with 626 people how many septic tank required On 2017-06-01 by Anonymous

by (mod) - you would need a local on-site septic Waste Water Treatment Plant.

Anon,

The answer to your question is not going to be the number of septic tanks alone, but how the Wastewater will be finally processed and disposed of. You would need such a large physical area for any conventional septic system for the number of people that you describe that it does not seem reasonable to me.

More likely you would need a local on-site septic Waste Water Treatment Plant.

How to calculate the septic size needed for a bar with 150-200 occupants?

We are trying to figure out what a septic field would look like if it were to support a bar with an occupancy ranging from 150-200 people.

Is there a way to figure out some general information about a septic field of this magnitude? Is it even possible? On 2016-07-14 by Blake

Answer by (mod) -

Blake, our table at COMMERCIAL SEPTIC DESIGN gives typical wastewater flow for various occupancies including a bar.

You can multiply the Bar flow rates given for employees and customers by your numbers to get a reasonable guess at the total wastewater flow.

Example: using your 200 people number, that's 200 to 1000 gallons of wastewater per day - before adding water use for employees.

Your septic engineer will give you a design spec for that volume of wastewater.

Yes it's possible, but not cheap. You may need more than a conventional tank and drainfield, perhaps a small wastewater treatment plant.

Is 2400 sqft enough to put in a septic system at Lake Granbury?

Am looking at property at lake granbury, has no septic system, is about 2,400 Sq ft property, is it large enough if you can put a system in. On 2016-07-12 by Donna Lass

Answer by (mod) -

Donna that's a question to take to your local septic engineer and building department. You'll have to review the intended property use, required septic size, septic system type, space needed for that septic system type, local property line setback regulations, and of course what else will be located on the property.

Other considerations include soil properties and presence of nearby waterways. In sum, no competent honest answer can be offered that will be right for all property circumstances, but even knowing nothing about your specific property, it sounds rather doubtful.

Contact your Granbury (Texas) Building Department Building and Permits and Inspections

What the septic system designer will consider in setting septic tank and field size

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

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.

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• COMMERCIAL SEPTIC DESIGN - topic home
• DISINFECTION SYSTEMS, SEPTIC EFFLUENT
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• SEPTIC DRAINFIELD SIZE
• SEPTIC TANK SIZE
• WATER QUANTITY USAGE TABLES

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