The 3 types of septic systems most commonly installed for residential properties are conventional septic systems, aerobic treatment systems, and advanced wastewater treatment systems. Each system treats household wastewater differently, making the right choice dependent on factors such as property size, soil conditions, local regulations, wastewater volume, and ongoing maintenance requirements.
Key Takeaways
- Conventional systems are the most common septic system.
- Aerobic systems provide a higher level of wastewater treatment.
- Advanced treatment systems suit more challenging sites.
- Soil conditions often influence system selection.
- Professional design helps maximise long-term performance.
Type 1: The Conventional Septic Tank with Absorption Trench
A conventional septic tank with absorption trench is the most common system on older Mornington Peninsula properties. It remains the standard installation for sites with suitable soil conditions.
How the System Works?
The system has two primary components.
The first is a sealed underground tank, made from concrete or polyethylene, that separates and partially treats all wastewater from the property through gravity settling and anaerobic bacterial digestion.
The second is a network of perforated pipes in gravel-filled trenches laid below the surface. These trenches receive the partially treated effluent and allow the surrounding native soil to complete the biological treatment process.
A complete explanation of what a septic tank is, how it is built, and why the Peninsula has so many of them covers the full background on the conventional system’s components, materials, and historical context across this region.
Soil Requirements
For a site to support a conventional system, three conditions must exist:
- Sufficient soil depth below the trench base to allow adequate treatment before effluent reaches groundwater
- Appropriate permeability to allow effluent to move through at a controlled rate — neither too fast nor too slow
- A water table that sits well below the base of the absorption trenches throughout the year
What This Means on the Mornington Peninsula?
Sandy coastal soils across parts of the lower Peninsula absorb effluent rapidly. This helps drainage but can allow partially treated liquid to reach groundwater before adequate biological treatment occurs.
This is why conventional systems on the Peninsula’s coastal fringe receive closer attention from environmental regulators than equivalent systems in areas with slower-draining soils.
Type 2: The Aerated Wastewater Treatment System (AWTS)
An aerated wastewater treatment system, referred to as an AWTS, is a more technically sophisticated wastewater treatment system that treats household sewage to a significantly higher standard than a conventional septic tank by introducing oxygen into the treatment process.
Where a conventional system relies on anaerobic bacteria that function in the absence of oxygen, an AWTS uses a blower or compressor to introduce air into the treatment chamber, which activates aerobic bacteria that break down organic waste faster and more completely.
The advantages an AWTS provides over a conventional system in the right application include:
- Higher treatment quality: Effluent leaving an AWTS is substantially cleaner than conventional septic effluent, with lower suspended solids, reduced biological oxygen demand, and lower pathogen concentrations.
- Suitability for challenging sites: Because the treated effluent is of higher quality, it can be dispersed to land that would not safely support conventional absorption, including smaller lots and sites closer to waterways or environmental sensitivities.
- Disinfection capability: Most AWTS configurations include a disinfection stage using chlorination or UV treatment before the effluent leaves the system, further reducing pathogen levels.
- Approved surface application: The treated effluent from a compliant AWTS can in some cases be approved for reuse as sub-surface or surface irrigation on the property, subject to council conditions and maintenance requirements.
- Compliance on difficult sites: On sites where a conventional system cannot meet the minimum separation distances or soil permeability requirements, an AWTS is often the system that can achieve council approval where a conventional installation cannot.
An AWTS requires more active maintenance than a conventional system, including a service contract with an accredited maintenance provider who inspects the unit on a regular schedule to confirm the aeration mechanism, treatment quality, and disinfection components are all functioning correctly.
Understanding how a conventional septic tank processes wastewater through its gravity-based two-chamber separation process clarifies the fundamental difference between the anaerobic conventional approach and the aerobic treatment achieved by an AWTS.
Type 3: Mound and Alternative Absorption Systems
A mound system is used on sites where the soil conditions, water table, or available land area make both a conventional absorption trench and a standard AWTS dispersal field impractical or unable to meet the minimum regulatory separation requirements.
The system works by constructing a raised mound of imported, well-draining sandy loam soil above the existing ground surface, into which treated effluent from the septic tank or treatment unit is pressure-dosed and dispersed through perforated pipes embedded in the mound material.
The imported soil provides the drainage depth and permeability characteristics that the underlying site cannot, allowing effluent dispersal to occur within a controlled, engineered soil profile rather than through the native soil that makes a conventional installation unsuitable.
On the Mornington Peninsula, mound systems are used in situations including:
- Shallow rock or clay subsoil: Where the native soil profile does not have sufficient depth before an impermeable layer is reached to accommodate standard absorption trenches.
- Seasonally high water table: In lower-lying areas of the Peninsula where the water table rises during winter and would otherwise saturate or overwhelm a conventional absorption field.
- Constrained lot area: Where the property does not have sufficient area for a conventional absorption trench system at the minimum required separation distances from buildings, boundaries, and water sources.
- Failed system replacement on difficult sites: Where a conventional system has failed on a site whose original conditions were marginal, and those same conditions prevent a like-for-like conventional replacement.
Other alternative absorption configurations used in specific circumstances include drip irrigation systems, where treated effluent is released at low and controlled rates through subsurface drip lines, and constructed wetland or reed bed systems that use wetland vegetation to provide additional biological treatment before dispersal.
How Septic System Design Is Determined for a Property?
The septic system design appropriate for any site is not a decision made by the homeowner or the plumber independently.
It is determined through a formal assessment and permit process that begins with site investigation and concludes with a council-issued installation permit specifying the approved system type, its required configuration, and the ongoing operating conditions that apply.
The factors that the assessment considers and that determine which system type is approved are:
- Soil permeability testing: A percolation or hydraulic conductivity test confirms whether the site’s native soil can accept effluent at the rate the household generates it, and at what depth this is achievable.
- Soil profile depth: The depth to any impermeable clay layer, rock, or seasonally saturated zone determines whether conventional absorption trenches can be installed at the minimum required depth below the inlet.
- Seasonal water table level: The high water table level must sit a defined minimum distance below the base of the absorption system throughout the year, and seasonal assessments confirm this.
- Site topography and gradient: The land’s gradient affects whether gravity distribution is achievable across the absorption field or whether pressure dosing is required to distribute effluent evenly.
- Proximity to water features and boundaries: Mandatory separation distances from bores, waterways, drainage lines, property boundaries, and buildings limit where the system can be positioned and how large the absorption area can be.
- Household design flow: The daily volume of wastewater the property is expected to generate, based on bedroom count and occupancy, determines the minimum tank capacity and absorption area required.
Installing or altering any onsite wastewater system in Victoria requires an A20 permit from the relevant local council. The EPA Victoria’s A20 permit requirements for onsite wastewater management systems set out the conditions that apply to the permit application and installation process, including what documentation and site assessment information the council requires before a permit is issued.
Which Type Is Most Common on the Mornington Peninsula?
The conventional septic tank with absorption trench is by far the most prevalent sewage treatment system type across the Mornington Peninsula, a direct result of the region’s development through the mid-twentieth century when conventional gravity-fed systems were the standard and often only approved option available.
The majority of the approximately 22,000 Peninsula properties not connected to the reticulated sewer network operate on conventional systems, many of which are now between 30 and 60 years old and functioning well beyond their original design period.
AWTS installations are more common in properties developed from the 1990s onward, or where a conventional system was refused at the permit stage due to site conditions, or where a failing conventional system has been replaced with an AWTS on a site that cannot support a conventional replacement.
Mound systems are present across the Peninsula but less common, appearing predominantly on hillside or coastal properties where topography, shallow rock, or high seasonal water tables ruled out both conventional and AWTS dispersal fields.
The EPA Victoria’s guidelines for onsite wastewater management provide the regulatory framework that governs which system types are approved for which site conditions across Victoria, and councils on the Peninsula apply these guidelines when assessing each permit application.
When to Consider Upgrading from Septic to Sewer
Alt Text: Upgrading From Septic To Sewer At A Mornington Peninsula Property
For homeowners whose existing septic system is approaching or has passed the end of its useful life, or where a failed system cannot be cost-effectively replaced on the existing site, upgrading to the reticulated mains sewer is often the most practical long-term outcome.
This pattern is playing out across the lower Peninsula as older conventional systems age out of service and the expanding sewer network brings connection within practical and financial reach of more properties. A full breakdown of why many Rosebud homes are upgrading from septic to sewer covers the specific drivers behind the trend, what the process involves, and what motivates homeowners to make the transition.
The financial case for a sewer upgrade rather than septic replacement depends on the distance from the property to the nearest sewer main, the difficulty of the connection relative to the cost of replacing the existing system, and whether the ongoing maintenance obligations of a new septic system represent good value over its expected life.
DCG Plumbing’s septic to sewer upgrade service covers the full connection process across the Mornington Peninsula, from initial site assessment and feasibility through to drain installation, tank decommissioning, and compliance certification.
Need Help with Your Septic System on the Mornington Peninsula?
DCG Plumbing provides septic system inspections, maintenance, and pump-out coordination across the Mornington Peninsula for conventional, AWTS, and mound system types, with all work carried out by licensed plumbers experienced with the Peninsula’s soil conditions and ageing onsite wastewater infrastructure.
Call 0401 266 656 or get in touch through the website to arrange a site visit or discuss your system’s current condition and options.
Final Thoughts on What Are the 3 Types of Septic Systems
What are the 3 types of septic systems in common residential use are the conventional septic tank with absorption trench, the aerated wastewater treatment system, and the mound or alternative absorption system.
Each addresses a different set of site conditions and household requirements, and the appropriate choice is always determined through soil testing, site assessment, and a council permit process rather than by preference or cost alone.
For the majority of older Mornington Peninsula properties, the conventional system is what is in the ground, and understanding what type of system is present, what its current condition indicates, and what the options are when it reaches the end of its service life is the most useful starting point for any homeowner managing an ageing onsite wastewater system.
