Onsite Wastewater Management

Much of the Sorell Municipality is not connected to the reticulated sewerage system. Where this is the case all waste water must be treated and applied to the land within the confines of the property boundaries.

There are a number of different options available that can be utilised such as aerated waste water treatment, septic tanks, wastewater treatment plants and composting toilets.

For existing dwellings with on-site waste water systems Council keeps detailed drainage plans to assist in locating on-site wastewater management systems. It should be noted that these records do not exist for many older properties.

Please note that the installation of an on-site waste water system can only occur after a Plumbing Permit has been applied for and approved by Council.

An on-site wastewater management system (OWMS) is a generic term that includes all types of wastewater treatment systems such as:

  • Septic tank systems
  • Aerated wastewater treatment systems
  • Composting toilets
  • Sand filters
  • Wetlands

All on-site wastewater management systems (OWMS) require maintenance. Recently installed systems will have a schedule of maintenance included with the plumbing permit and the designer of your OWMS will normally include in their report details of required maintenance.

Your designer will also provide a loading certificate which details the number of people the system has been designed for and maximum number at any time the OWMS can accommodate to prevent failure or malfunction.

Some OWMS such as aerated wastewater treatment systems, require maintenance by a qualified person and the owner will be required to enter into a maintenance agreement with a service agent.

OWMS comprises a buried Septic Tank, where sewage solids are settled out and subjected to anaerobic digestion; a Septic Tank Outlet Filter, to prevent the carry-over of fine solids, which may otherwise damage and clog the effluent absorption system; and, a number of Effluent Absorption Trenches, which dispose of the effluent through absorption into the soil and by evapotranspiration into the atmosphere.

Microorganisms in the soil further breakdown organic matter in the wastewater and remove potentially harmful bacteria and viruses.

For your septic tank to work well

Reduce sludge build-up in the tank:

  • Scrape all dishes to remove fat, grease and food scraps before washing.
  • Keep all possible solids out of the system.
  • Do not install a garbage grinder.
  • Do not put sanitary napkins and other hygiene products into the system.

Keep bacteria working in the septic tank and effluent absorption trenches:

  • Use biodegradable soaps.
  • Use low phosphorus detergents and shampoos.
  • Use detergents and shampoos in the recommended quantities.
  • Do not use powerful bleaches, whiteners, nappy soakers, spot removers and disinfectants.
  • Do not put chemicals or paint down the drain.

Conserve water and reduce the volume of effluent requiring disposal, which will maintain the systems life and improve its performance by:

  • Installation of water conserving fittings (eg dual flush 6:3 litre toilets; shower flow restrictors; aerator taps and water conserving automatic washing machine).
  • Taking showers instead of baths.
  • Avoid excessive time in the shower.
  • Only washing clothes when there is a full load.
  • Only using the dishwasher when there is a full load.
  • Avoid overloading the system by spacing out water use as evenly as possible (e.g. do not do all the washing on one day; do not  run  the dishwasher and washing machine at the same time).

Advice on Maintenance 

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


  • Always wear appropriate personal protective equipment when inspecting or undertaking routine maintenance work on your septic system. (E.g. waterproof/washable/disposable gloves; waterproof overalls; waterproof boots; goggles or face shield.) Be especially careful to ensure any cuts or skin abrasions are protected by a waterproof dressing.
  • Never climb into, or put your head inside your septic tank, even when it has been emptied, as it may contain poisonous gases.
  • Do not eat or drink when carrying out work on your septic system.
  • Do not smoke or use naked flames near your septic tank when the access covers are removed as it may contain flammable/explosive gases.
  • Always ensure inspection openings and access covers are replaced and properly secured to prevent access by children etc.
  • Wash your hands (and any parts of your body which may have been splashed with septic effluent) thoroughly with soap and water after carrying out any inspection or maintenance on your septic system.

Septic Tank:

  • When you first move into your house, or after desludging the septic tank, you may notice a strong odour from the house vent pipes. This may be reduced by flushing a cupful of agricultural lime (eg “Limil”) down the toilet, each week until the odour subsides. The use of other products such as “Actizyme” may also be of use in this regard.
  • Desludge the septic tank at least once every three years.
  • Protect the septic tank from vehicles.
  • Keep the access cover exposed – do not bury it.
  • Clean the outlet filter regularly, (in accordance with the manufacturers instructions) by removing the tank access cover, pulling the filter outlet from the outlet dip pipe and using a hose pipe, rinsing the sludge layer off the filter and back into the septic tank through the open access opening. Do not forget to wear suitable gloves and wash your hands afterwards.

Effluent Absorption Area:

  • Discourage people from walking over the effluent absorption trenches.
  • Do not allow vehicles or livestock on or near to the effluent absorption trenches.
  • Do not grow deep rooting trees or shrubs over effluent absorption trenches or pipes.
  • Keep the cut-off drains above the effluent absorption area open and clear of debris to prevent rainwater flowing into the effluent absorption area.

Operating problems

Problems can occur with systems which have not been maintained and where absorption areas have become blocked or clogged. The warning signs are obvious and include:

  • The effluent absorption area is wet or soggy with wastewater ponding on the surface of the ground.
  • “Sewage” smells near the septic tank or effluent absorption area.
  • Drains and toilet run slowly or wastewater runs out from the overflow relief gully.

Advice on the consequences of septic system failure

A failed septic tank and/or effluent absorption system is a serious health and environmental hazard and can lead to:

  • The spread of infectious diseases to your family and neighbours.
  • Breeding of mosquitos and attraction of flies and rodents.
  • Nuisance and unpleasant odours.
  • Pollution of waterways, streams, beaches and shellfish beds.
  • Contamination of bores, wells and groundwater.
  • Excessive and unsightly weed growth.
  • Alteration of local ecology.

Advice on home owner/occupier responsibilities

Homeowners and occupiers are legally responsible for keeping their on-site wastewater system in good working order. If any of the before mentioned warning signs are evident, you (the home owner or occupier) should contact Council’s Environmental Health Officer.

Any required alteration or modification to the Septic System must be undertaken by an appropriately qualified Registered Plumber, in accordance with a Plumbing Permit, issued by Council under the Plumbing Regulations.

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

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

Much of the land subdivided in Primrose Sands and Dodges Ferry (and other areas) was subdivided over 40 years ago before the time when consideration of the lots suitability for septic tank systems was part of the criteria used for determination of whether subdivisions would be approved or not. In addition, standards used for assessment of a properties suitability for wastewater disposal has continued to change over years in response to the number of septic tank systems that are not working effectively and changing community public health and environmental standards.

It is therefore possible that some properties may not be suitable for any type of on-site waste management system. In these cases increasing the land area (if possible) by adhering one or more blocks or a boundary adjustment to purchase some land off a neighbour may provide sufficient area for wastewater disposal. Please remember that provision for future absorption trenches and setback/buffer distances to sensitive features must be included within all designs for wastewater disposal systems.

Australian Standard AS/NZS 1547-2012 On-site Domestic Wastewater Management includes considerable information on the design, operation and construction of wastewater disposal systems. It is recommended that this standard be consulted when designing systems. Several consultants in Tasmania have specialized skills in designing on-site disposal systems.

A property will only be suitable for a septic tank system or other type of on-site wastewater management if an application has been lodged and a permit issued. If an application has not been made previously then the only way a decision can be made is for an application to be submitted. This application must include a design of an on-site wastewater management system that is based on the site conditions i.e. from the site and soil evaluation report prepared by a consultant such as a: Geologist, Soil Scientist or Engineer. The plan submitted may be a hypothetical design but at least if a permit is issued for this proposal it will indicate that wastewater disposal from the proposed type of development is acceptable or not. For advice on land required for wastewater treatment refer to Director of Building Control — Guidelines for On-site Wastewater Management Systems. On small properties (less than 1000m2) the size and number of bedrooms may be a significant determinant in whether a permit will or will not be issued.

The greater the number of bedrooms the higher the potential occupancy and in turn the higher the design wastewater flow rate will be (the amount of wastewater generated). Absorption trenches (and other disposal areas) are sized mainly on the absorptive capacity of the soil and the volume of wastewater generated. For this reason the size of trenches for a 1-2 bedroom house may vary considerably compared with a 4 bedroom house. Therefore, the number of occupants is determined by the number of bedrooms in the house and the volume of wastewater is calculated based on the number of occupants. It is important to recognize that approval to install a septic tank is not a generic approval, but a specific approval for a proposed development (house). If the site is not suitable for a septic tank system other alternatives such as a waterless toilet (e.g.: composting toilets), aerated wastewater treatment system (AWTS) or septic tanks with modified land application areas (above ground beds or mounds) may be suitable. Technologies continue to be developed, however, not all of these new systems have sought approval for use in Tasmania. Before any on-site system can be installed a Permit must be obtained from Council.

Suitable onsite waste water systems

Properties that are not located within areas serviced by reticulated sewerage must treat and dispose of wastewater within the boundaries of the property that generates the wastewater. No individual properties are ever the same and the location and size of buildings also varies. For these reasons it is often difficult (particularly on properties smaller in area than 1000m2) to state that the property is “suitable for septic tank system.”

Installing a new or replacement on-site wastewater management system (OWMS) requires a plumbing permit, including new absorption trenches.

You will need to engage a licensed wastewater designer to prepare a suitable design and submit that design with your plumbing application.

An OWMS must be installed by a licensed plumber.

Maintenance work such as unblocking pipes maybe done by a plumber without needing a permit. Other maintenance work including septic tank pump outs may only be undertaken by a licensed pumping contractor.