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SEPTIC SYSTEM INSPECT DIAGNOSE REPAIR
SEPTIC CARE INSTRUCTIONS
SEPTIC D-BOX INSPECTION
SEPTIC DRAINFIELD FAILURE DIAGNOSIS
SEPTIC DYE TEST PROCEDURE
SEPTIC FAILURE SIGNS
SEPTIC INSPECTION & TEST GUIDE
SEPTIC LIFE EXPECTANCY
SEPTIC SUPPLIES & PARTS
SEPTIC SYSTEM DESIGN ALTERNATIVES
SEPTIC SYSTEM DESIGN BASICS
SEPTIC SYSTEMS, HOME BUYERS GUIDE to
SEPTIC SYSTEM SAFETY WARNINGS
SEPTIC TREATMENTS & CHEMICALS
SEWAGE BACKUP PREVENTION
SEWAGE EJECTOR / GRINDER PUMPS
SEWER GAS ODORS
SEWER LINE REPLACEMENT
SINKHOLES, WARNING SIGNS
SOAKAWAY BED FAILURE DIAGNOSIS
SULPHUR & SEWER GAS SMELL SOURCES
TOILETS, INSPECT, INSTALL, REPAIR
TRAPS on PLUMBING FIXTURES
TREATMENTS & CHEMICALS, SEPTIC
VIDEO GUIDES: Septic Videos
WASHING MACHINES & SEPTIC SYSTEMS
WATER SOFTENERS & CONDITIONERS
WATER SUPPLY & DRAIN PIPING
WASTEWATER TREATMENT BASICS
WATER, WELLS, WATER TANKS: TESTING GUIDE
WINTERIZE A BUILDING
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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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:
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
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).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.
The tables of estimated wastewater flow volumes for non-residential designs are grouped into three large categories
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
Wastewater Flows from Institutional Sources
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.. 
Wastewater Flows from Recreational Facilities
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
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.
Questions & answers or comments about commercial or "non-residential" septic drainfield or soakaway bed size or capacity requirements & design
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