Onsite Wastewater Management

An On-Site Wastewater Management System (OWMS) refers to any system used to treat and manage household wastewater on the property where it is generated, rather than sending it to a central sewer system.

OWMS includes a variety of treatment options, such as:

• Septic Tank Systems
• Aerated Wastewater Treatment Systems (AWTS) or Secondary Treatment Systems (STS)
• Composting Toilets
• Sand Filters
• Constructed Wetlands

These systems are designed to safely treat and dispose of wastewater within the boundaries of the property, protecting public health and the environment.

All approved on-site wastewater treatment systems receive a Certificate of Accreditation which is available on the CBOS website for reference.

The two main systems used with the Sorell Municipality are Septic Tank Systems and Aerated Wastewater Treatment Systems (AWTS).

Septic Tank System

A septic tank system is a type of on-site wastewater treatment system used when a property is not connected to a public sewer system. A septic tank is a watertight chamber filled with household wastewater made of concrete, fibreglass or plastic. It separates solids, grease, and oils from wastewater, then uses bacteria to break down the remaining liquid. The treated liquid, called effluent, is then discharged into a land application area (absorption trench, mound or sand filter) where it is further filtered by the soil.

Aerated Wastewater Treatment System (AWTS)

An Aerated Wastewater Treatment System (AWTS) or Secondary Treatment System (STS) takes household waste from toilets, basins, showers, baths, laundry and kitchen sinks.  An AWTS or STS treats the waste and distributes treated wastewater on-site to the land application area (irrigation or absorption area).

Unlike traditional septic systems, AWTS systems incorporate aeration, which uses oxygen to break down organic matter more effectively, resulting in cleaner wastewater. This treated wastewater can then be safely dispersed on the property, often for irrigation purposes, making it a more environmentally friendly and sustainable option for properties not connected to a public sewer system.

Note: The land application area is an important part of the on-site sewerage system.  It distributes the treated wastewater produced within the property.  This water must remain within the property boundary.

It may be harmful to eat anything that has grown from gardens watered with treated wastewater. Avoid skin and pet/livestock contact with treated wastewater.

AWTS are mechanical systems and must be regularly serviced (every 3 or 6 months) by a qualified plumbing contractor. An owner must ensure that they maintenance contract in place, particularly when they purchase a new property.

All on-site wastewater management systems (OWMS) require regular maintenance to function effectively and protect public health and the environment.

Maintenance Requirements

• Newly installed systems will have a maintenance schedule included in the Plumbing Permit.
• Your system designer will provide:
  • A maintenance report detailing ongoing requirements.
  • A Loading Certificate, which outlines the number of people the system is designed to accommodate.

Some systems, such as Aerated Wastewater Treatment Systems (AWTS), require servicing by a qualified person.  In such cases, owners must enter into a maintenance agreement with a licensed service agent.

Maintenance and Safety Advice

Please note: Your plumbing permit may contain advice on specific maintenance for your system.

General Safety

• Always wear appropriate personal protective equipment (PPE): gloves, waterproof overalls, boots, goggles.
• Never enter or lean into the septic tank—it may contain toxic or explosive gases.
• Avoid eating, drinking, or smoking while working near your system.
• Secure all access covers after maintenance to prevent accidents.

Warning Signs of System Failure

You may be experiencing a failing OWMS if you notice:

• Wet or soggy ground near trenches with visible effluent ponding.
• Sewage odours around the tank or disposal area.
• Slow-draining fixtures or wastewater backing up from the overflow relief gully.

Consequences of System Failure

A failed septic system poses serious health and environmental risks:

• Spread of infectious diseases.
• Breeding of mosquitoes, flies, and rodents.
• Offensive odours and nuisance.
• Pollution of waterways, beaches, and groundwater.
• Overgrowth of weeds and disruption of natural ecosystems.

Legal Responsibilities

Homeowners and occupiers are legally responsible for keeping their on-site wastewater system in good working order.

If you observe signs of failure, seek advice from a wastewater designer or plumber and if necessary, contact Council’s Environmental Health Officer to notify what rectification action you are taking.

Any alterations or repairs must be carried out by a Licensed Registered Plumber under a Plumbing Permit issued by Council, in accordance with the Plumbing Regulations.

Best Practices for Maintaining Your OWMS

Reduce Sludge Build-Up

• Scrape dishes to remove fats, oils, and food scraps before washing.
• Keep solids out of the system.
• Do not install a garbage grinder.
• Never flush sanitary products, wipes, or nappies.

Support Bacterial Activity in the System

• Use biodegradable, low-phosphorus soaps and detergents.
• Avoid strong chemical cleaners (bleach, disinfectants, paint, etc.).
• Use all cleaning products in moderation.

Conserve Water to Reduce System Load

• Install water-saving fixtures (e.g., dual flush toilets, flow restrictors).
• Take short showers instead of baths.
• Wash only full loads of laundry or dishes.
• Spread water use throughout the week to avoid system overload.

Maintenance and Safety Advice

Please note: Your plumbing permit may contain advice on specific maintenance for your system.

General Safety

• Always wear appropriate personal protective equipment (PPE): gloves, waterproof overalls, boots, goggles.
• Never enter or lean into the septic tank—it may contain toxic or explosive gases.
• Avoid eating, drinking, or smoking while working near your system.
• Secure all access covers after maintenance to prevent accidents.

Septic Tank Care

• After desludging or moving into a home, temporary odours may occur—flushing a cup of agricultural lime weekly can help.
• Desludge your tank at least every 5 years.
• Keep the access cover visible and protected—do not bury it or drive over it. If the hatch is below the ground, install a riser to allow for easier access.
• Clean the outlet filter regularly as per manufacturer instructions.

AWTS Care

• Aerated waste water treatment systems require regular maintenance, usually 4 services per year.
• Servicing needs to be carried out by a qualified person (ask your local council for a list of suitable maintenance contractors).
• You will need to enter into a formal maintenance contract with the maintenance contractor.
• You will need to agree on the service costs with the maintenance contractor. This amount should be contained within a Maintenance Service Contract.
• The local council will require a copy of the formal contract once it’s been agreed to and signed by both parties.
• The local council will require a ‘receipt of servicing’ from your contractor after every service.
• You, as the owner, will also receive a copy of this service receipt.
• Generally a service of a typical aerated waste water treatment system takes between 0.5-1.0 hour to complete correctly
• Refer to your homeowner’s manual for the advice on care and maintenance. If you don’t have one, they are generally available on-line.

Effluent Absorption Area

• Prevent access by vehicles, livestock, or heavy foot traffic that may compact the soil or damage pipes.
• Avoid planting deep-rooted trees or shrubs nearby.
• Keep drainage channels clear to prevent surface water flooding the area.

What is a Septic Tank?

A septic tank is a watertight vessel usually made from concrete, plastic or fibreglass, which receives liquid wastes from water closet toilets and in some cases kitchens, laundries and bathrooms.

What does a Septic Tank do?

When the liquid wastes are received into the tank, any solid faecal matter heavier than water forms a sludge layer at the bottom of the septic tank. Solid matter lighter than water floats to the top to form a scum layer. Bacteria decompose or “digest” the solid matter in the tank to produce gas bubbles. As wastes enter the inlet of the tank, they displace the liquid effluent already treated in the tank. The displaced liquid flows out of the septic tank into the absorption trench. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to separate. The wastewater forms three layers inside the tank. Solids lighter than water (such as greases and oils) float to the top forming a layer of scum. Solids heavier than water settle at the bottom of the tank forming a layer of sludge. This leaves a middle layer of partially clarified wastewater. The layers of sludge and scum remain in the septic tank where bacteria found naturally in the wastewater work to break the solids down. The sludge and scum that cannot be broken down are retained I the tank until the tank is pumped. The layer of clarified liquid flows from the septic tank to the absorption trench.

How is Septic Tank effluent disposed of and treated?

The effluent leaving the septic tank flows into an “absorption trench”, the size and length of which is based on the various site factors such as the absorptive characteristics of the soil. The trench should be excavated parallel with the contour of the land. Some of the effluent is absorbed into the soil, some transpires through vegetation and some evaporates. The process of effluent filtering through the soil removes microorganisms and some nutrients.

What is an Absorption Trench?

Absorption trenches are used for distributing both sullage water and septic tank effluent into the ground for further treatment. A typical cross section of an absorption trench is shown below.

Absorption trenches will not last forever, eventually they ‘clog up’ and new absorption trenches are required. Please consult Council’s Environmental Health Officers for information on the process for installing new trenches. The length of time that absorption trenches lasts depends on:

• How much wastewater enters the trench;
• The absorptive capacity of the soil;
• Maintenance of the septic tank, e.g.: Pumping out;
• Ensuring that the trench isn’t damaged by vehicles or livestock;
• Use of sub soil drains to divert surface drainage away from the absorption area,
• how much your property is affected by seepage or ground water; and
• The amount of sodium salts in the wastewater.

Location of absorption trenches should be 1.5 metres from upslope boundaries and a sufficient distance from the side and down slope boundaries to prevent effluent discharging onto a neighbouring property. The slope of the land and soil conditions will significantly influence this distance.

How often should I empty my Septic Tank?

Generally, it is advisable to have your septic tank pumped out every three to five years. Obviously, the number of people living in the house and whether the dwelling is occupied permanently will influence the frequency of pumping required. Septic tanks should be filled with water after pumping.

What is the difference between single and dual purpose Septic Tanks?

A single purpose tank only receives toilet waste and has a minimum capacity of 1600 litres. Duel purpose tanks receive toilet wastes and all other wastewater from the kitchen, bathroom and laundry and are at least 3000 litres in capacity.

What is a AWTS?

An aerated wastewater treatment system is a system designed to treat your wastewater up to a secondary standard. This means that the treated water can be used to irrigate your property or be discharged into bodies of water.

While they often require more regular maintenance than other wastewater treatment systems, these model types are effective treaters of wastewater and can prove more environmentally friendly and efficient than other systems on the market.

What’s the difference between an AWTS and Septic Tank?

Both the aerated wastewater treatment system and the septic tank are the most common treatment systems available in Australia. They both use beneficial bacteria to break down and digest wastewater in order to treat it however septic systems are much less complex than the aerated wastewater treatment system.

This is because the aerated wastewater treatment system actually uses aerobic bacteria as well as anaerobic bacteria to achieve a greater level of treatment standard. The process means that effluent from an aerated wastewater treatment system can be discharged either above or below ground whereas effluent from septic tanks is still classed as a contaminant and must be discharged below ground.

How does an AWTS work?

All wastewater from your bathrooms, kitchen and laundry is collected in the Septic Tank (or Primary Chamber).  In this tank, the wastewater settle out into three separate layers:

• The fats, oils and grease (FOGs) float to the surface and create a thin layer of ‘scum’;
• The liquids or effluent in the middle; and
• The solids will eventually settle on the bottom of the tank as a layer of ‘sludge’

As more water enters the system from the house, the existing effluent that has undergone ‘Primary’ treatment, gets pushed into the second chamber or ‘Aeration Tank’. Here, the effluent is aerated with oxygen by an aeration pump in order to help the ‘live’ microbes in the water to digest and breakdown the breakdown the organic waste.

A third chamber is for additional clarification, as the solids are returned to either the septic chamber or the aeration chamber for further treatment before irrigation can happen.

In many systems the effluent undergoes disinfection by a Chlorinator which removes any remaining harmful bacteria, viruses and pathogens, producing clean, safe water.

When the final chamber fills up to a certain level, it will engage a float switch which will automatically engage an irrigation pump to send the final treated effluent out to your irrigation field.

Can I spray irrigate my sullage water?

No. Untreated sullage water should not be spray irrigated. Only wastewater that has been treated can be irrigated in this manner and only in controlled circumstances.

Spray irrigation of sullage water on residential size properties increases the risk of disease transmission by:

• Contaminating fruit and vegetables:
• Children may mistake sprinklers for spraying sullage water with drinking water sprinklers;
• Pets or children coming into contact with diseases after playing in areas where sullage water has recently been sprayed;
• Sullage water may also emit offensive odours and disturb surrounding neighbours.

Sullage water may contain high levels of sodium, which can affect the soil structure, particularly in clayey soils. Certain plants are also more sensitive to saline waters (such as sullage water).

I have problems with sewage smells in my backyard

Two common problems may cause this problem. Septic tank effluent absorption trenches may be ‘clogged’ and not functioning effectively causing pools of effluent to form on the surface of the ground. New absorption trenches will be required. Sewer vent pipes (SVP) that ventilate plumbing waste pipes may be the source of odours. Many factors may contribute to this problem such as local weather patterns, and the location and height of the SVP pipe. If this problem occurs frequently, increasing the height of the SVP may allow the prevailing winds to disperse the gases more effectively.

When I flush my toilet the water does not drain away or takes a long time to disperse.

The soil around the absorption trenches for the septic tank effluent become ‘clogged up’ after many years of use. This will cause water to either come to the surface of the ground or not allow wastewater from the septic tank to easily flow along the sewer pipe causing water in the toilet bowl to not drain or drain slowly. A new absorption trench may be required. Alternately, a simple blockage may have occurred in the pipe. Please contact your plumber for advice.

Sullage water is overflowing out of the overflow relief gully.

This may also be caused by the problems listed in item above.

Can I build a dwelling, shed or driveway over my septic tank or absorption trench?

No. Structures are not permitted over an absorption trench or the septic tank unless approval from Council has been obtained. Driveways or driving cars over areas where absorption trenches are situated will reduce soil permeability or damage the pipe work or PVC arch. All of which will cause the trench to be ineffective.

Subsoil/Cut-off/Agricultural Drains

Subsoil drains divert surface or subwater away from absorption trenches. These are vital in areas that are subject to runoff or seepage. An example is shown in the following diagram. Sub-soil drains eventually clop up with silt particularly in sandy/silty soils or where Geo-fabric is not used. A flushing point will allow the drains to be cleaned. Correct location, construction and maintenance of sub-soil drains may be the difference between absorption trenches working effectively or not.

Sorell Council does not endorse nor recommend any of the people/companies included in this list. The information is provided to assist developers identify those individuals/companies that have previously submitted site & soil evaluation reports and designed on-site systems. Other individuals/companies not included in these lists that have the appropriate skills and knowledge may also submit reports.

Council advises you confirm with the individual designer/consultant that they hold current accreditation under the Occupational Licensing Act 2005. This information is accessible on the CBOS website – Find a Licensed Tradeperson

List of On-Site WasteWater Site Soil Evaluators and System Designers

Many properties in areas such as Primrose Sands, Dodges Ferry, and other nearby locations were subdivided over 40 years ago—before regulations required assessing a lot’s suitability for septic systems as part of subdivision approval. Since then, both environmental and public health standards have evolved significantly, as has our understanding of how to manage on-site wastewater effectively.

Suitability of Land for On-Site Wastewater Management

Due to changes in land assessment standards and increased awareness of failing septic systems, some properties may not be suitable for any type of on-site wastewater management system. In such cases, it may be necessary to increase the property’s size—either by consolidating with neighboring blocks or through boundary adjustments—to provide enough space for safe wastewater disposal.

Important Considerations:

• All system designs must allow space for future absorption trenches, and must comply with setback and buffer distances from sensitive features like watercourses or bores.
• Designs must comply with the Australian Standard AS/NZS 1547:2012 – On-site Domestic Wastewater Management, which provides comprehensive guidance on system design, operation, and installation.
• It is strongly recommended to consult with a suitably qualified professional such as a Soil Scientist, Geologist, or Engineer for a site and soil evaluation.

Permit Requirements and the Application Process

A property will only be deemed suitable for an OWMS (such as a septic system) once an application has been lodged, and a permit has been issued by Council.

• If no prior application exists, the only way to assess suitability is to submit a new application, including a wastewater system design based on a detailed site and soil report.
• This can be a hypothetical design, used to determine whether the proposed development would meet wastewater disposal requirements.
• The Director of Building Control’s Guidelines for On-site Wastewater Management Systems should also be referred to when planning developments.

Factors Influencing System Design

Key determinants of system feasibility and design include:

• Property Size: Smaller lots (less than 1000m²) often have limited options for wastewater disposal.
• Number of Bedrooms: The number of bedrooms influences the estimated number of occupants, which in turn affects the design flow rate (volume of wastewater generated).
• Soil Type and Absorption Rate: These determine the required size of the absorption trenches or land application area.

For example, a 4-bedroom house will typically require much larger trenches than a 1–2-bedroom home, due to the higher expected wastewater output.

Alternative Systems and Approvals

If the site is not suitable for a conventional septic tank system, alternative wastewater solutions may be considered, such as:

• Composting toilets (waterless toilets)
• Aerated Wastewater Treatment Systems (AWTS)
• Modified land application systems (e.g., raised beds or mounds)

Note: Not all systems are approved for use in Tasmania. Before any OWMS can be installed, a permit must be obtained from the local Council.

Final Notes

Properties not connected to a reticulated sewer system must manage all wastewater within the property boundaries. Due to variations in land characteristics, no two sites are the same. For this reason, it’s not always possible to give a blanket statement about whether a property is suitable for a septic system—particularly for lots smaller than 1000m².

Key Takeaways for Buyers

• Always seek a site and soil assessment from a qualified consultant before purchasing.
• Understand that wastewater system approval is site-specific and based on the proposed development (e.g., number of bedrooms).
• Consult the Director of Building Control guidelines and AS/NZS 1547:2012 when planning.
• Ensure that a wastewater management permit is in place or can be obtained before proceeding with development.
• If you are purchasing a home with an AWTS, be sure to update the Service Agreement for the maintenance of the Onsite Wastewater System.

If you are planning to install a new or replacement on-site wastewater management system, including absorption trenches, you must first obtain a plumbing permit from your local council.

Step-by-Step Overview

1. Engage a Licensed Wastewater Designer
   A licensed wastewater system designer must be engaged to:
   • Assess your site
   • Prepare a suitable system design in accordance with Australian standards
   • Submit the design as part of your plumbing application
2. Submit a Plumbing Application
   You (or your representative) must lodge a plumbing application with your local council, including:
   • Site and soil evaluation report
   • Wastewater system design prepared by the licensed designer
3. System Installation
   Once your permit is approved, the OWMS must be installed by a licensed plumber who is qualified to carry out such work.

Maintenance and Repairs

• General Maintenance
  Routine maintenance, such as unblocking pipes, may be performed by a licensed plumber and does not require a permit.
• Septic Tank Pump-Outs
  This must be done by a licensed septic pumping contractor. These professionals are trained and authorized to handle and dispose of waste according to regulations.

 Important Reminders

• A permit is mandatory for all new and replacement OWMS installations.
• Only licensed professionals are permitted to design and install systems.
• Regular maintenance helps extend the life of your OWMS and reduce environmental risks.