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"Septic systems for the small shop or office can

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Septic Systems for the Small Shop or Office



The following article applies to the drawing template package sold here from 

 The drawing templates are in both standard PDF and CAD (What is a CAD Drawing?)        These drawings will enable you to prepare a set of construction drawings of your project for your property. Before beginning construction, every homeowner, excavator or designer in the septic industry must hand in clear, proper scale drawings to get approval to build a septic system for small offices in the country or away from municipal sewer and water. Important; In most areas the owner of property can prepare drawings and build his or her septic system. State and local laws tell you if you can do this yourself or if you must hire an expert. Read this disclaimer before getting started. If you are unclear about your rules, call your county or parish health department and ask for the rules in your area.

The Small Office above sits on a 40 acre parcel and is the management office for a rural gravel operation. Small offices are usually not required to have public restrooms as would a restaurant or a theater. Also the 2000 sq ft shop to the right is not expected to have customers stay more than a few minutes in the business. Health department regulations do not generally require that the public be included in the daily sewage calculations.

However there are several issues to be aware of when planning small septic systems for stores, offices, shops, industrial parks, studios or other uses with minimal flows;

Problem 1, Small Flows and How to Calculate: The basic office setup - one toilet and a hand sink for a few employees is about the smallest system there is. Because the double wide manufactured home above is used as an office the sewage load calculations for a 3 bedroom manufactured home do not apply. The building has only three full time employees with limited scope for expansion. The full bathroom with the former tub and shower has been converted to storage and only one sink and toilet remain. The smallest possible flow for any septic system is the result, less than 80 gallons per day (GPD) even when the flow rate is doubled in anticipation of unexpected expansion. This predicted flow rate is less than one quarter of the flow rate for the original 3 bedroom home that has now become an office.

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Most sewage flow rates are calculated using the USEPA Onsite Wastewater Treatment Systems Manual known in the trade as the EPA Purple Manual. This is not the only source for flow rates but it is the most used reference.  This manual has basically two methods to determine flow rates for the small shop or office;

1. Fixture Flow Rates in Table 3-3 on page 7 of the EPA Purple Manual, you will find various flow rates per hour for various domestic plumbing fixtures. Once a flow rate/ fixture per day or sometimes flow rate/ person/ fixture/ day is agreed upon with the health officials you can total up all fixtures and rates and arrive at a total gallons of sewage/ day (GPD.) The wide range of possible flow conditions usually tends to result in complex discussions and predictions and larger flows than would actually happen once the building is occupied. Flow rates based on occupancy or "Building Use" is generally a more accurate way to plan flow rates.

2. Building Use Flow Rates in Table 3-4 on page 9 of the EPA Purple Manual you can see all the possible commercial uses such as "Resort Cabin," "Theater," "Country Club/ guests and employees."  For example the standard sewage design flow for office workers or other daytime employees is 13 GPD. Customarily this flow rate is doubled to account for future expansion, occasional use by outside workers or visitors and to provide a margin of safety. Commercial spaces can change over time and the very small systems are less robust than larger systems for occasional overloads so make sure you predict your proposed rate carefully. 

The larger operation shown here to the right has 10 full time employees 10 x 13 GPD/ employee = 130 x 2 for expansion = 260 GPD.

It has a small lunch room with a sink and microwave. With minimal food prep and with no dish washer this on-site kitchenette uses the other domestic category = 1.6 GPD x 10 employees = 16 GPD x 2 for expansion = 32 GPD. The shower is intended for emergency wash down and occasional cleanup during certain annual operations. The flow rate for the shower 11.5 GPD per use x 3 uses per day = 34.5 GPD x 2 for expansion = 69 GPD. Total sewage load for this operation is therefore 260 + 32 + 69 = Total 361 GPD. Still, the total daily domestic (not including any commercial wastewater) design flow for this rural commercial operation is that of a 3 bedroom home.

Minimum System Size: Some new regulations outlaw very small systems. For some reason some regulators prefer a minimum size usually a 2 bedroom size. There is often no justification given for minimum system sizes although this is typical of the creeping new restrictions affecting rural development.

Problem 2, Finding Protected Spots for the System: As with our two buildings shown above, most commercial, light industrial buildings, warehouses and farm buildings are surrounded by parking spaces and driveways. Finding an open spot, close to the building for the tanks and drainfield sometimes is not easy.

The tanks can be made traffic bearing and placed under driveways and parking lots (see photo left.) The small inset photo shows the standard tank with PVC (plastic) risers and PVC lids (green.) The standard non-traffic bearing tank also usually has a 3 inch thick concrete top. The traffic bearing tanks usually have a 6 or 7 inch thick top with extra steel reinforcing, concrete risers and cast iron lids as shown. The concrete risers are built up with pre-cast rings of various thicknesses from 2 inches to 6 inches. The rings are stacked to put the lid a few inches above finish grade to prevent surface water intrusion into the tank. The concrete grade rings and the cast iron frame are sealed to each other and to the top of the tank with a watertight tar/rope gasket. Water intrusion into the system would upset the calculated capacity of the drainfield. Excess water into the system can be almost as hard on some systems as excess sewage.

PVC risers are either cast into the tank top by the manufacturer in the factory or sealed onto the tank top with epoxy by the excavator during construction.

Surprisingly traffic bearing tanks and risers with cast iron lids only cost one or two hundred dollars more than non-traffic bearing standard models with standard concrete tops and PVC risers and lids.

Drainfield Protection: There are several strategies to locate the drainfield and a drainfield replacement area for the small shop or office where it will be protected from being driven or parked on by vehicles (usually a rule.)

The drainfield although small must be at least ten feet from a building in most counties. The tank is usually fitted in between the drainfield and the tank. The example to the right shows this standard situation for a roadside small commercial building. The drainfield is given a spot along the least traffic accessible side of the building. The tank and D-box are between the building and the trenches. Trenches can be seen in the foreground going right and left from the sand bedded and covered effluent line.

It can be impossible to find a clear spot for the drainfield close to the building. If the site is dead level flat like this one, a traffic bearing tank could be placed in a driveway with a drainfield remotely located away from paved areas. A pumping tank is the usual solution to deliver effluent to a remote location on a flat site or when the drainfield area must be upslope of the septic tank. The tank elevation depends on the lowest drain in the building.

Alternatively a small diameter inverted siphon line can be trenched across the site below the frost zone to a remote D-box and drainfield almost any distance from the septic tank. We show how these systems are designed in the drawing package.

Paving over the drainfield is never allowed because the drainfield must remain aerobic to work. The soil must have oxygen to destroy the viruses and bacteria. Paving and driving on the drainfield and replacement area will compact the soil and destroy soil structure permanently reducing its capacity to treat sewage effluent.   

Pressure distribution can be routinely required on sites in the area due to excessively coarse or shallow soil and a range of other conditions common to difficult sites. Small commercial septic systems may not be given exemptions that would apply to single family homes. This seems unfair except remember commercial water and septic systems compared to home systems are more public in every respect. 

For example in some areas commercial pressure septic systems no matter how small may be required in some states to have explosion proof pumping and float control to meet state electrical codes. This makes the idea in suitable soils and areas of no special environmental concern of exempting very small commercial systems from some stringent rules because of the economic hardship of meeting the rules when the flows are almost too low to mater. An explosion proof panel can cost hundreds of dollars more than a standard small pumping panel. A gravity system is rarely advised in areas where pressure systems are in general use except in high voltage enclosures and very small flows. In any case direct the health official to this article if it will help and their department has discretion over small commercial septic systems.

What Does the Drawing Package Contain? The package shows a variety of designs for septic systems for the small shop or office plus examples of several sites of this type. They show you how to prepare the drawings you will need for your project. Learn how to show such things as water well placement, building location, drainfield design, site contours, landscaping, construction and pipe details and all the other notes and instructions required to fully describe your project. The package contains both PDF and editable DXF drawings as well as instructions and photos to illustrate the work.          return to top


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Septic Systems for the Small Office  - Last Revised: 11/11/2016
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