Septic drainfield design sketcyCommercial Septic Tank & Drainfield Design Size Requirements
     

  • COMMERCIAL SEPTIC DESIGN - CONTENTS: 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, from institutional sources, from recreational facilities
  • POST a QUESTION or READ FAQs 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.

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

Photograph of  a conventional septic tank during installation.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. 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).
...
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.
...
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

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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