What are the Different Types of Septic Systems? What’s the Difference Between a Type 1, Type 2 and Type 3 Septic System?
With many different types of septic systems choosing a septic system for your home can be quite a daunting task considering that there are several different types of septic systems available, and often a number of options to choose from within each type.
There are several factors which need to be considered: 1) It is important to select the most appropriate septic system for your soil conditions; 2) The system must adequately meet your household needs; and 3) The wastewater must be treated sufficiently enough to ensure the effluent that is discharged complies with local environmental health standards.
Different Types of Septic Systems
There are three types of septic systems, categorized according to the manner in which they treat wastewater that enters the system as follows:
- Type 1 Septic System: sewage is treated within a septic tank only before being discharged to a drain field where further treatment occurs naturally;
- Type 2 Septic System: treatment takes place within a septic tank plus an additional aerobic secondary treatment stage, typically within a small scale on-site mechanical biological packaged treatment plant, before being discharged to a drain field;
- Type 3 Septic System: this is a specially designed septic system that is capable of treating sewage to produce effluent of a higher quality standard, by including a disinfection process before being discharged into the environment via the drain field. Treatment takes place within an advanced on-site mechanical biological packaged treatment plant.
It is important to choose the most appropriate septic system for your property, as the wrong choice of system can end up being an expensive mistake.
Type 1 Septic System (Septic Tank System)
A Type 1 septic system is defined as an on-site septic system where a septic tank is the primary means of treating sewage.
The treated effluent can then be disposed of in a drainage field that can consist of seepage beds, sub-surface trenches or above-ground sand mounds. Effluent can either flow to the drain field via gravity or it can be pumped to the drain field under pressure.
Type 1 septic systems are a suitable choice for properties where the soil conditions are ideal for disposing the effluent into the ground. They consist of a septic tank installed underground, which receives the wastewater. Once the wastewater is in the tank the suspended solids settle down to the bottom forming a layer of sludge, while the less dense oil, grease and fat floats to the surface forming a layer of scum.
Because the tank is free of oxygen, it provides conditions that allow anaerobic bacteria to thrive. The community of anaerobic bacteria living in the septic tank break down the solids, reducing the volume by about 50-60%.
The watery effluent sandwiched between the layer of scum on the surface and the layer of sludge on the bottom flows out from the tank into a drain field where it is disposed of into the soil.
Naturally occurring microorganisms that live in the soil break down the effluent further, removing any harmful bacteria and pathogens before the effluent reaches the groundwater. The solids (scum and sludge) remaining in the tank need to be pumped out on a regular basis.
Type 2 Septic System (Septic Tank + Secondary Treatment)
The main difference between a Type 1 and a Type 2 septic system is that a Type 2 septic system includes an additional secondary wastewater treatment stage.
As with a Type 1 system, the anaerobic bacteria break down the solids in the oxygen-free septic tank. During the secondary treatment, stage oxygen is typically added to the wastewater which allows aerobic bacteria to flourish in the system.
The aerobic bacteria break down any suspended solids that may be present in the wastewater.
Extending Drain Field Life with Aerobic Septic Systems
Some experiments have suggested that retrofitting an aerobic septic system to an existing conventional gravity system may rejuvenate or extend the life expectancy of that system. Adding aeration or oxygen to an existing septic system may boost the aerobic bacterial action in the biomass, which can improve the septic field from a malfunctioned state. Although not quite that simple…
A settling compartment or additional tank may be required with an effluent filter because the agitation from the aerating can send suspended solids out into the septic field…this would lead to eventual clogging.
Type 2 septic systems are more efficient at treating wastewater than Type 1 septic systems.
As a result, the effluent that is discharged is cleaner and safer. This is an important factor to consider when a property has poor quality soils or there is insufficient depth between the surface and the constricting layer or water table to adequately treat the effluent.
Also, because this secondary treatment stage is more effective at breaking down the wastewater than a Type 1 septic system, the surface area of the drainage pipes in the drain field can be less than that used in a Type 1 septic system, and will, therefore, take up less space.
This makes them ideal for use on properties where space is limited. Treatment prior to discharging into the drain field greatly reduces the likelihood that the drain field will clog up with continual use, allowing its size to be reduced.
However, the one drawback is that treatment costs associated with a Type 2 septic system will be higher than that of a Type 1 septic system.
As per our Standards Practice, here is a subsection of our code: III- 18.104.22.168.(d) Type 2 effluent hydraulic loading rate and vertical separation
i) Vertical separation for Type 2 hydraulic loading rates
The use of a Type 2 treatment method is a viable option on many projects and sites (particularly if available area is limited), although this will seldom be the only viable option.
The Volume II standards indicate a larger vertical separation for Type 2 effluent hydraulic loading rates (HLR) in some cases, particularly for gravity distribution and for demand dosing.
A larger soil depth is specified to provide for adequate soil based treatment (particularly pathogen removal) in these cases because:
• Type 2 effluent may have pathogen levels nearly as high as in Type 1 effluent.
• Type 2 effluent hydraulic loading rates are higher than for Type 1, and so:
• risk of bypass or saturated flow from the infiltrative surface is higher,
• there is a higher risk of soil saturation if multiple doses are applied in a short time, and
• total number of pathogens applied will be correspondingly higher at the higher HLR.
• Type 2 systems typically have less or no biomat, resulting in:
• less effective distribution with gravity dispersal,
• less moderation of effluent flow to the soil from the infiltrative surface, and
• less removal of pathogens by the biomat.
Since Type 2 treatment systems are often used as a solution for a small site, the standards of this manual allow for high HLR with Type 2 effluent. It is considered that vertical separation can be adjusted, but site area is generally fixed.
So, the standards prioritize higher HLR for Type 2 rather than smaller VS. However, if Type 2 effluent is applied using Type 1 hydraulic loading rates (HLR) then the standards allow for use of Type 1 vertical separation standards and Type 1 dosing and distribution standards.
This is because at the lower HLR the shallower VS will be adequate for treatment. For example, for gravity distribution to Fine Sand Table II- 14 (page II-26) specifies 120 cm VS for Type 2 effluent HLR, but if the effluent was applied at Type 1 HLR, then 90 cm VS could be used.
ii) Type 2 effluent with uniform distribution
The Volume II standards allow smaller vertical separation when timed or micro-dosing options are used with Type 2 HLR. This is because these options lead to lower hydraulic application rates (HAR), more similar to those for Type 1 HLR. They also lead to lower risk of soil saturation by spacing doses apart. Timed and micro-dosing, therefore, reduce the risk of bypass flows and poor pathogen removal posed by high Type 2 HLR. See Section III- 22.214.171.124.(a) for discussion of hydraulic application rate (HAR).
Timed dosing standards for Type 2 HLR result in higher HAR than for Type 1 HLR. For this reason, on some soils, the standards specify slightly larger VS for Type 2 HLR with timed dosing.
Since micro-dosing ensures the same HAR regardless of hydraulic loading rate, the VS standards are the same for Type 1 and Type 2 HLR on all soil types if micro-dosing is used.
Standards for sand mound and sand lined trench or bed systems allow very high HLR to the sand media with Type 2 effluent, for that reason micro-dosing is required where this high HLR are used.
How do Aerobic Septic Treatment Units Work?
“An aerobic treatment unit is basically an “oxidizer” which uses extra oxygen dissolved in the wastewater to support aerobic microorganisms which in turn decompose dissolved organic and nitrogen compounds into simple CO2 or into inorganic compounds.
As microorganisms die off they accumulate as a sludge of biological material, some of which supports the development of new cells or microorganisms to keep the system working. ATU’s separate solid waste first in the “trash tank” and later, additional solids are separated in the clarifier or settlement tank from which they may be returned to the primary tank for more treatment. (See the sketch at the top of this page.)
“Typical organic materials that are found in residential strength wastewater include carbohydrates, fats, proteins, urea, soaps and detergents. All of these compounds contain carbon, hydrogen, and oxygen. Domestic wastewater also includes organically bound nitrogen, sulfur and phosphorus.
During biochemical degradation, these three elements are biologically transformed from organic forms to mineralized forms (i.e., NH3, NH4, NO3, SO4, and PO4).” In contrast, an example of an anaerobic process involved in the breakdown of wastewater would be fermentation, the exothermic, enzymatic breakdown of soluble organic compounds which does not depend on the presence of dissolved oxygen. Methane and CO2 are both products of wastewater fermentation”. — InspectApedia (Guide to Aerobic Septic Systems)
Type 3 Septic System (Septic Tank + Secondary Treatment + Disinfection/Filtration)
Type 3 septic systems are custom designed sewage treatment plants that are capable of treating wastewater to a very high standard, producing clear, odourless effluent that has a much higher water quality than Type 1 or 2 systems.
A Type 3 septic system is required to include a method of disinfection that is capable of removing pathogenic organisms before the effluent is discharged to the drain field.
A Type 3 septic system typically consists of the following components: 1) a septic tank where wastewater is broken down in an oxygen-free environment by anaerobic bacteria; 2) a secondary treatment stage, where oxygen is usually added to facilitate aerobic treatment as with a Type 2 septic system; 3) the addition of a third stage, where disinfection takes place.
The disinfection process can incorporate chemical treatment with chlorine, or disinfection with ozone or ultraviolet light, and can include the addition of a filter for more effective removal of pathogens. Finally, the treated effluent is discharged to a drain field, usually under pressure (i.e. pumped rather than gravity fed).
Type 3 septic systems are a good choice for properties where soil conditions are very poor and conditions are not suitable for a Type 1 or Type 2 septic system. This includes sites with the following parameters:
- Soil depth is less than 0.5 feet (15 cm)
- Soil percolation rates are very slow or very fast
- Poor soil structure
- Limited space to install a more conventional septic system.
In cases where conditions are appropriate for a Type 1 or Type 2 system, but space is extremely limited, a Type 3 system may be a better option as a much smaller drain field is required due to the high quality of the effluent discharged.
One may very well ask ‘why do we need different types of septic systems when they all perform the same job?’
This is true to a certain degree, however, the efficiency of treatment and the water quality of the effluent discharged at the end of the treatment process is higher in a Type 2 system than a Type 1 system, and higher still in a Type 3 system.
So, ultimately the choice of septic system will hinge on the soil conditions at the site, the amount of space available for a drain field, and/or the need for a higher quality effluent that is cleaner and safer, and which will not pose a risk to the environment or to human health.
It is best to have an authorized qualified professional conduct a thorough site analysis to assess the conditions on site and to determine the amount of wastewater that the household is likely to generate each day.
This will enable them to design an appropriate septic system that is able to cope with the family’s output, taking full cognizance of any limiting factors on site to ensure that the system is not overloaded and environmental health is not compromised.
The Sewerage System Regulation Process:
‘The Ministry of Health provides policy support for land use management, which includes both onsite sewage disposal and subdivision activities. The fundamental goal is to minimize, mitigate and/or prevent potential risks to human health. This is achieved through legislative requirements, supporting policies and guidelines.’ – Govt of Canada Website
Standards for Seasonal use as per Standards Practice Manual: III- 2.2.1 SEASONAL USE
Seasonal use systems should be installed in compliance with the Sewerage System Standard Practice Manual.
Type 2 or Type 3 systems may not be suitable for some seasonal dwellings, such as a vacation cottage,
because these treatment systems typically run on electricity and may rely on biological processes that cannot be sustained under seasonal use. When specifying a treatment system at a seasonal dwelling, provide measures to maintain the operation of the system during periods of intermittent power supply and limited water use.
Here is an explainer video on different types of septic systems from Washington State. Although the video is from the US, BC regulations are quite similar to Washington standards.
In Conclusion, some of these systems are certainly more complex than others, to be certain of the type of septic system you may require for your lot a more thorough analysis should be done.
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