Septic System Design - All you need to know

Posted by Steve Harmer on Friday, November 16th, 2018 at 9:15am.

BC septic systemsAn in-depth look at the foundation of a design for a typical septic system design in BC

With BC regulations, prior to moving forward with a septic system design BC, an authorized person, typically a Registered Onsite Wastewater Practitioner (ROWP), will perform what is called a site and soil evaluation.

Typical requirements in data are obtained to properly incorporate a septic system design BC, this will include checking the siting of the septic system to ensure that it will comply with any restrictions registered against the land title.

Identifying boundaries for horizontal separation from the system components to such things as property lines, water bodies drilled wells and break out points in the topography.Soil Permeability

Looking for potential site characteristics such as bedrock outcropping, vegetation and drainage, potential environmental constraints such as high rainfall, extreme cold, floodplains and other water bodies.

Soil conditions need to be investigated in the potential drain field areas and dispersal zones.  Test pits are excavated to determine and assess the underlying soil types, proper texturing and sampling techniques are conducted.

Percolation tests are performed to determine the rates at which the soil accepts water.

Determining the soil profiles and loading rates are a vital part of the site investigation, this is a must for designing any septic system BC.

Here is a basic look at some soil types and loading rates:

A ROWP will typically take and calculate the data from the soil, the environmental and topography information, the lot space, the size of the structure and the number of occupants into consideration when developing a septic system design BC.

In Canada there are 3 primary treatment types for a septic system, these are Type1, Type 2 and Type 3.

Typically Type 1 and Type 2 wastewater treatment systems are used on lots of properties with relatively ideal conditions for ground dispersal.

Type 1 systems are typically using only a septic tank as the primary treatment of the sewage, this only includes the separation of solids from fats and grease.

The effluent that travels to the drain field can be gravity fed or pumped.

Type 2 systems introduce further treatment to produce a higher quality effluent, and can often be discharged into a smaller field, this is especially important for smaller lot sizes or restricting elements such as setbacks.

Typically these Type 2 systems will use air to encourage the breakdown of bacteria in the septic tank that is done by producing aerobic bacteria, which are fantastic at breaking down solids in the septic tank.

Type 3 systems produce an even higher effluent than a type 2 system.  These systems can be referred to as a package treatment plant.

These plants typically employ aerobic treatment process, such as adding oxygen, disinfection using ultraviolet light can also be a method of treatment. Using chlorination/de-chlorination and ozone are other methods of this type of treatment plant.

A First Look At The Primary Treatment, The Septic Tank:

Septic Tank Canada

A typical home with a septic system will have a septic tank, it is either a concrete, plastic or fibreglass chamber that is used for the partial treatment of domestic wastewater and for retention. The septic tank design by itself will not achieve a high degree of sewage treatment.  The initial role is to prevent clogging in the field by conditioning the sewage through the separation of solids, oils and fats. This is done by the floating and settling of suspended solids which will anaerobically decompose in the septic tank.  The lighter weight matter such as fats and soap residue will stay on the top portions of the tank causing what is called a scum layer.

The heavier organic matter will then settle to the bottom and begin to decompose. The liquid in between the two layers that should be partially clarified will then flow through the outlet baffle towards the next component of the septic system. The initial step in designing a septic system in BC is identifying the proper sizing of the septic tank, and identifying the right type of tank; a concrete, plastic, one compartment, two compartments or even having multiple tanks. A septic tank design with a single compartment performs an acceptable household wastewater treatment, two compartment tanks should improve the biochemical oxygen demand or (BOD) and the (TSS) or total suspended solids removal.

 The septic tank is sized to the amount of household expected flow, it has to have sufficient volume for 3 days retention time.

If longer retention times are needed this will mean the volume of the septic tank will need to increase.

In BC the recommended septic tank capacity in the design is illustrated in this chart below:


Typically the sizing of the tank for each bedroom based on the average volume would be best described in this  chart: 

Septic System Design BC  requires a Registered Onsite Wastewater Practitioner (ROWP, Planner) to follow the current Standards Practice Manual.

If the total amount of occupants residing in the home are expected to be higher than the calculated minimum occupancy per bedroom, then the ROWP will use the following table to identify the Daily Design Flow:

The fluid leaving the septic tank (effluent) will then need to be further treated through a sub-surface disposal to the soil.

Septic Tank Volume

The removal of pathogens, solids and nutrients are achieved as the effluent travels through the soils in the dispersal location or (field).

There are varying degrees of treatment which are obtained as a function of the climate, vegetation and the amount of water uptake by the grass or plant life, characteristics of the soils and the depth of aerated soils above a water table. Wastewater treatment is also influenced by the actual flow rate coming from a home as compared to the DDF from our Standard Practices Manual.

In the septic system design BC, the actual size of the disposal field is in proportionate to the anticipated flow and characteristics of the soil type on the lot. The depth of the trench is determined from the soil characteristics, the amount of water table and climate has a variable.

“Site investigations are a must prior to any septic system design or installation.  Often septic system failures occur when the drain fields are poorly designed.”

Septic System Design BC requires the use of a distribution method that ensures an even flow is achieved from the septic tank to the drain field. If an uneven distribution occurs then overloading of certain sections of the drain field can occur, which can lead to its eventual failure.

Distribution methods that are common will include:

A Distribution Box – A containment box that captures the effluent flows from the septic tank, these can be made from concrete, plastic or fibreglass and can come in various shapes and sizes.

The effluent is then dispersed evenly through various outlets in the distribution box that lead to the network sections of the drain field.

Here is a typical gravity system where the wastewater from the septic tank leads to the distribution box for even distribution into the drain field:

septic distribution

The DropBox – Similar to the Distribution Box in composition and is used on sloped lots using a gravity distribution.

The inlet pipe is set higher than the outlets which allow the effluent to travel downward to the drain field trenches.

A series of drop boxes are situated on a sloping drain field.  After the saturation of the highest field section, the flow of effluent continues to travel to the next drop box in sequence and the trench below.

The drop box outlets can be capped if there is a need to control the direction of effluent in order to give the upper trenches a rest.

Pressure Distribution by Pump – Electric pumps will also deliver controlled effluent amount (doses) to the drain field.dropbox

Each dose of effluent will then be delivered in a uniform distribution to the network piping in the drain field.  This can help achieve an improved performance to any drain field because the effluent disperses into the soil simultaneously through orifices in the network piping.

This is extremely advantageous for drain fields with poor soil or shallow conditions such as clay, heavy silts or high water table.

There are many variables in determining the type of system that would be required for the lot, this article only covers the basic idea of designing

Sand mound

a septic system in British Columbia and does not go too far into the depth of systems such as a sand mound.

Above ground septic systems such as the sand mound are used on properties where soil has a shallow depth, high water table or restrictive soil types such as clay and silts.

The electric pump will require maintenance and can be subject to failure with power outages.  This type of system is more expensive than the gravity system due to the extra components and tank required.

Here is a typical electric pump set up in one of the septic tank chambers, a separate pump tank can also be used and required in certain cases.

Septic pump discharge

Each dose of effluent to the drain field can be controlled through a time dosing panel as pictured or through demand dosing.  This can improve the drain field performance by not allowing oversaturation.

Instead of the drain field being dosed as water enters the tank, the dose is now set to certain time segments.

Syphons are also used to evenly distribute effluent to the network piping.

This is typically done in a large area, normally over a long length of force main pipe that leads from the pump or syphon station down to the drain field area. The effluent travels from a dose tank to the syphon, then it flows to the drain field in pressurized doses.  This allows a uniform distribution. Syphons work by using air and water pressure along with gravity.

A poorly maintained or designed septic system can negatively impact the environment and the economic health of a community.

Septic systems that are failing can reduce the value of a property and add to the pollution of water bodies used for recreation and commercial activities. In British Columbia, a homeowner’s septic system design bc must meet legal requirements prior to being installed, repaired or upgraded. In BC, the Ministry of Planning is in charge of Septic Systems.

Here is some helpful regulatory information about the government’s role in wastewater management and septic systems concerning property owners.

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