Trench Absorption vs Mound Systems: Which Suits Your Hastings Property?

When a conventional septic tank treats wastewater and sends the clarified effluent to a dispersal area, that effluent needs somewhere to go. The two most common dispersal methods in the Port Macquarie-Hastings region are trench absorption systems (in-ground channels in the existing soil) and raised mound systems (engineered fill placed above the natural ground surface). Choosing the right one, or having the right one chosen for your site during the design phase, significantly affects both performance and longevity.

Quick answer (BLUF)

Trench absorption systems are suitable for sites with adequate soil permeability and sufficient separation to the water table. Raised mound systems are required where the natural soil is unsuitable, typically high water tables, shallow restrictive layers, low permeability clay soils, or steep terrain, and allow on-site sewage management where trenches would fail. In the Hastings region, mound systems are common in coastal areas with high water tables and on the Comboyne Plateau where clay soils predominate.

Trench absorption systems explained

A trench system consists of gravel-filled trenches dug into the native soil, with perforated pipe running through the gravel. Effluent from the tank flows into the trenches and percolates through the gravel and then into the native soil, where the remaining biological treatment occurs.

Advantages:

• Lower construction cost (no fill material, no mound construction)
• Largely invisible in the landscape, no raised earthwork
• Lower maintenance requirements than more complex systems
• Well-understood by contractors, approved practitioners, and PMHC

Disadvantages:

• Requires adequate native soil, percolation testing must confirm the soil can accept effluent at the required rate
• Soil must have adequate depth to the water table (typically at least 600 mm unsaturated soil below the trench base)
• Susceptible to hydraulic overloading in the Hastings region’s wet season when soils are already saturated
• Subject to biomat formation, which progressively reduces capacity over time
• Not suitable for sites with shallow rock, hardpan, or high water tables

Where trenches work well in the Hastings region:

• Properties on free-draining sandy or loamy soils in the coastal fringe (some parts of Laurieton, Port Macquarie elevated areas)
• Rural blocks with deep, well-structured soils with moderate percolation rates
• Sites above the seasonal water table with adequate depth to any restrictive layer

Raised mound systems explained

A raised mound system involves importing clean, coarse sand (the “fill material”) and constructing an elevated bed above the natural ground surface. The septic tank effluent is pumped or gravity-fed into perforated pipes within the sand fill, where it disperses and undergoes treatment as it moves downward through the mound and into the native soil.

The mound provides additional treatment depth and separation from the water table or restrictive soil layer that the native soil cannot provide.

Advantages:

• Can be installed on sites where trenches would fail, high water tables, impermeable soils, shallow rock
• The engineered fill material provides consistent, known hydraulic properties
• Better hydraulic performance in wet conditions, the mound drains freely regardless of the native soil water table
• Can be sized more precisely for the required hydraulic load

Disadvantages:

• Higher construction cost, importing suitable fill material to rural Hastings sites can significantly increase cost
• Visible above-ground structure, aesthetically intrusive, requires landscaping management
• Requires more site area than an equivalent trench system
• The pump (if required to distribute effluent to the mound) adds an electrical and mechanical component that needs maintenance
• PMHC approval and a more detailed design are required

Where mounds are required in the Hastings region:

• Coastal low-lying areas (Lake Cathie, Camden Haven, Bonny Hills) with high seasonal water tables
• Properties with shallow clay or rock layers preventing adequate trench depth
• Sites on the Comboyne Plateau with poorly structured, slowly permeable soils
• Flood-affected sites where the mound can be elevated above flood levels
• Properties where normal trench setbacks cannot be achieved but a mound can be located with adequate separation

AWTS with drip/subsurface irrigation: a third option

Both trench and mound systems treat primary-quality effluent. An Aerated Wastewater Treatment System (AWTS) produces secondary or tertiary quality effluent, which can be dispersed via smaller-area systems including subsurface drip irrigation or spray irrigation.

This is increasingly the preferred solution in the Hastings region for constrained sites because:

• The smaller absorption footprint is easier to fit on lots where space is limited
• Higher-quality effluent requires less soil treatment, meaning setbacks to water sources can sometimes be reduced
• Subsurface drip irrigation can be installed on the same area as an existing failed trench, provided adequate separation from the saturated zone is maintained

The tradeoff is cost (AWTS unit plus irrigation infrastructure) and the mandatory quarterly service contract.

Cost comparison in the Hastings region

Costs vary significantly with lot access, distance from Port Macquarie, soil conditions, and required system size. Remote sites (Comboyne, upper Wauchope) typically attract higher costs due to travel time and material delivery.

How the site assessment determines which you need

A soil and site assessment by a qualified practitioner will determine which system type is appropriate for your site. Key factors:

1. Percolation rate (perc rate): Fast (sandy) soils may actually be too permeable, effluent may not be treated adequately before reaching groundwater, requiring an AWTS upgrade rather than standard trenches. Moderate permeability is ideal for trenches.

2. Depth to restrictive layer: Rock, hardpan, or perched water table within 600-900 mm of the proposed trench base typically requires a mound or AWTS solution.

3. Seasonal water table: The assessment should consider the wet season maximum, not just current conditions. In the Hastings region’s high-rainfall environment, the wet season water table can be several hundred millimetres higher than at the time of assessment.

4. Available land area: Mounds require more surface area than equivalent trenches because the fill is shallower than in-ground trench depth.

FAQs

Can I use my existing trench area for a mound if the trenches have failed?

Not usually in the same footprint, the existing ground is likely saturated and the old trenches contain biomat. A new mound should be located on previously undisturbed ground. Your contractor will assess whether the existing trench area could be rehabilitated or whether it must be decommissioned.

How big is a typical raised mound system?

A mound for a 4-bedroom home typically measures 15-25 m long by 8-12 m wide and 600-900 mm high at the crest. Exact dimensions depend on effluent volume, fill material specification, and PMHC’s design requirements. The mound footprint plus buffers can require a clear area of 30 m × 20 m or more.

Does a mound system smell more than trenches?

Not inherently. Both systems disperse effluent through the soil, which provides natural odour control. If a mound system produces odours, it usually indicates the effluent is surfacing (overflow) or the fill material has become waterlogged. Properly functioning mounds are generally odour-free.

Do mound systems need electricity?

Only if a pump is used to distribute effluent to the mound (required when gravity distribution isn’t possible due to elevation). Many mound systems on gently sloping sites can use gravity distribution, avoiding the pump and its associated running costs and maintenance requirements.