Sizing A Septic System | Tank And Drain Field Factors


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Septic System Sizing Requirements: 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 needed 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 household water consumption is an important tool to gauge current water usage habits and will help design an appropriately sized septic system.

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

Things to Consider When Finding the Right Septic Tank Size

Different septic tank sizes will have different retention times. The retention time is the length of time that wastewater effluent remains in the tank before flowing to the drain field. This needs to be sufficient enough to allow heavier solid waste particulates to settle to the bottom as the sludge layer and lighter solids, such as fats and oils, to float up to the top of the septic tank to join the scum layer above.

In order for this to be effective there needs to be a sufficient volume of liquid in the septic 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

However, there are a few additional things that should be taken into account when choosing a new septic tank. 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 they 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 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 the 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 (AI The total the total length of trenches = AIS ÷ the trench width.

septic system sizing

Sizing a Septic Drain Field, Calculation 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 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 gravel-less systems, the 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 Size Dimensions

The intertrench spacing could be considered 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 are accessible above that level

Guideline minimum size/working volume = 1 day Daily Design Flow. Reserve volume (above pump on the 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 septic tank capacity of 50% of Daily Design Flow. This should permit dry access to the pump disconnect union, valve, etc. for service.


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 septic systems can be technical. The examples above relate to conventional-type systems, which are the easiest to calculate. Pressure distribution systems, lagoons, and aerobic systems for types 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 number 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 the 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.

Luis Goncalves, ROWP, IN, PL

I fell into this amazing trade quite by accident, I have always loved being around heavy machinery and moving earth and in contrast love working with designing things.  This trade in the septic field has led me down a wonderful path of exploring all of my skills and passions while working hard at achieving success.

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