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Sizing A Septic System | Tank And Drain Field Factors

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Sizing a Septic System: What size septic system do I need?

A question that often arises with new septic system installations is: ‘What size septic system do I need?’

There is a common misconception that the size of the system is determined by the size of the home, but this is not entirely true. While the size of the home is certainly a factor, the size of the septic system is typically calculated taking the number of bedrooms into account, or more specifically, the number of anticipated occupants and the expected daily flow rate (litres per day). 

What goes in must at some point come out, water consumption is an important factor to consider when sizing a septic system. An accurate water reading detailing the household water consumption is an important tool to gauge current water usage habits and will help design an appropriately sized septic system.

When sizing a septic system, one has to ensure that the size of both the septic tank and the drain field will adequately cope with the wastewater generated by the occupants of the home.

 

Things to Consider when Sizing a Septic Tank

A septic tank needs to be adequately sized so that the retention time, — the length of time that wastewater effluent remains in the tank before flowing to the drain field — is sufficient enough to allow heavier solid particulates to settle to the bottom as sludge and lighter solids, such as fats and oils, to float up to the top of the tank to join the layer of scum above.

Septic Tank Installation

 

In order for this to be effective there needs to be a sufficient volume of liquid in the tank to facilitate the settling process. If this is not the case, solids will flow out with the wastewater into the drain field, where they can cause clogging that can eventually lead to system failure.

 

The minimum septic tank size for a three-bedroom home (or a home with less than three bedrooms) is typically 850-1000 gallons (3900 litres). This is based on an occupancy of 1.5 + people per bedroom which provides an estimate of expected water usage.

septic tank sizing
Septic Tank Sizing For Bedroom Count in BC

But there are a few additional things that should be taken into account. For example, if the kitchen is fitted with a garbage disposal unit, this is often counted with a minimum of a 50% increase to the daily flow because it generates organic waste that needs to be processed within the septic system.   

Also, note that oil and grease levels will increase when using garburators. A grease
interceptor could be needed.

If you have more people visiting on a regular basis, for example, if your teenager’s friends frequently hang out at your place, or if you have high volume fixtures such as a jacuzzi, you may need to increase the size of your septic tank and drain field accordingly.

water consumption

It is important to note, however, that the septic tank only partially treats the sewage; the rest of the treatment, together with the liquid effluent disposal, takes place in the drain field, which also needs to be adequately sized if it is to be effective.

 

Things to Consider when Sizing a Drain Field

Determining the most appropriate size of a drain field can be a bit more tricky, as one must not only take the household water usage and flow rate into consideration, but also the soil characteristics of the site where the drain field will be constructed, as well as the quality of the effluent entering the drain field.

sizing a septic system

Trenches can also be installed at a shallow depth — in which case trenches are partially below ground and partially covered, or “at grade.” In this case, the infiltrative surface is at the original grade
and the system is covered with cover soil.

All trench systems should be sized such that the horizontal basal area ONLY (NOT
including the sidewall area) is at least equal to the AIS (Daily Design Flow divided by the Hydraulic Loading Rate or HLR).

Distribution systems are designed to ensure even distribution over this area and to reduce
saturation of the basal area.

Trench infiltrative bottom area needed = Area of the Infiltrative surface (AIS)
Daily Design Flow ÷ Hydraulic Loading Rate = Area of the Infiltrative surface (AIThe total
the total length of trenches = AIS ÷ the trench width.

septic system sizing

Example:

Daily Design Flow for a 3 bedroom home of 1,300L/day, HLR of 30 L/day/m2 for a Loamy Sand (high sand content with small % of clay) and 0.6 m wide trenches.
1300L/D/m2 ÷ 30L/D/m2 = 43.33 m2 trench bottom area needed. Total length of trenches = 43.33 ÷ 0.6 = 72.2 m .

hydraulic loading rates

Since the soil needs to absorb the wastewater, we need to ascertain how quickly it can do this. The rate at which the soil can absorb water is known as the percolation rate.

There are several factors that affect the percolation rate, including soil type and texture (sandy soils drain faster than clay soils), the depth of the soil, and a high water table or seasonal changes to the level of the groundwater that could hamper the drainage efficiency of the soil.

soil permeablility

If the percolation rate of the soil is too slow, sewage can rise up and pool on the surface, creating an unsavoury and unhealthy environment; if the percolation rate is too fast, the effluent will not be properly treated before it filters into the groundwater. In cases such as these, it will not be feasible to install a drain field in the naturally occurring soil on the site, but rather an above ground sand mound needs to be constructed to facilitate adequate percolation, which will ensure effective treatment of wastewater in the drain field.

For single and multiple pipe gravelless systems, effective trench width is taken to be the
outside diameter of the pipe or pipe bundle.
For gravelless chamber systems, the effective trench width is taken to be, at a maximum,
the outside dimensional width of the chamber in contact with the bottom of the trench or
bed. A more conservative approach could be taken by using the actual exposed interior
dimensional width of the chamber at the trench or bed bottom.
For geocomposite systems, the effective trench width is taken to be the outside
dimension(s) of the bundle(s) in contact with the trench or bed base (or sand layer, where
used).

Trench Dimensions

The inter trench spacing could be considered as a potential system reserve area.
Trench width should not be less than 30.5 cm (1′) and not greater than 90 cm (3′).
Trench length should not be greater than 15 m (50′) for any one lateral in a gravity
distribution system. Non-dosed gravity systems should preferably use shorter laterals
(less than 50′).
Spacing should not be less than 1.8 m (6′) from centre line to centre line, except in the
case of pressurized shallow narrow drain fields.

GRAVITY TRENCH DISTRIBUTION DESIGN CONSIDERATIONS

Gravity flow should not be used for distribution areas exceeding 152 linear metres of
610 mm wide trench (500 lineal feet/2 foot wide trench) or for distribution systems
greater than 93 m2 (1,000 ft2) infiltrative surface.
Gravity systems larger than this size should only be constructed if DOSED. These
systems should use dosing to sequential distribution, pressure manifold distribution or
dose to Distribution Box (D-Box only for slopes below 15%). Serial distribution should
not be used for these larger systems. Dosing systems should be designed and constructed
per the standards of this manual (linked standard).

Pump Tank Sizing

The type of pumping configuration that will be used determines tank sizing. Guideline
volumes for chamber selection are set out in the following sections.
The working volume of a pump tank is from the inside bottom of the tank to the invert of
the inlet pipe. Where the pump tank is installed at an appropriate elevation (use
worksheet in Appendix P) in relation to the preceding tank (for example, a septic tank),
then the alarm reserve volume could include the depth from the invert of the inlet to the
underside of the tank lid, as long as the valve and union is accessible above that level

Guideline minimum size/working volume = 1 day Daily Design Flow. 
Reserve volume (above pump on float to alarm float on) a minimum of 15% of Daily
Design Flow.

Alarm reserve volume (above alarm float on, to the maximum permitted effluent level) a
minimum of 50% of Daily Design Flow. This should permit dry access to pump
disconnect union and valve, etc. for service.

Summary:

The type of septic system (whether it is a type-1, type-2 or type-3 system) will affect the quality of the effluent flowing into the drain field from the septic tank. Because cleaner effluent will require less treatment in the drain field, a drain field for a type-2 system can be smaller than that processing effluent from a type-1 system, and a drain field for a type-3 system can be smaller still.

As we can see, accurately sizing a septic system can be technical, the examples above are relating to conventional type systems which are the easiest to calculate.  Pressure distribution systems, lagoons, aerobic systems for type 2 and 3 can get very technical.

How to size a septic system depends on the hydraulic loading rates of both the soils and the wastewater treatment level.  The amount of occupants, how many bedrooms a home has and the overall size of a home play important factors in assessing the proper size for both septic tank and drain field.

There is a heavy onus to conduct through site investigations to determine the verticle separation in the soils from any restrictive elements and to input data on the hydraulic load rates through percolation testing and soil texturing.

High volume fixtures and garburators will affect a septic system by adding large quantities of organics that don’t properly break down as well as high volumes of water usage and thus have to be sized accordingly.

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