Wastewater/Septic Design

OSSF
On Site Sewerage Facility

If you are planning to purchase land for a new home, check before you buy. If a reticulated sewerage
system is not available you will need to consider an OSSF (On Site Sewerage Facility). Your first step
should be to obtain advice from Council or a registered site and soil evaluator.

Which system is best?

When choosing an OSSF it is important to consider where it will be used, how it will be used, and who will use it. An OSSF in a weekend holiday home, for example, will get far less use than a septic system in a large, permanently occupied family home. The OSSF you choose will depend on the suitability of the site for effluent absorption, how many people will live in the home, what area of land is available, what kind of lifestyle the family lives, and what heavy water-use appliances are in the home. A site and soil evaluator will advise you about what is best for your particular situation.

Soil type, salt content, local rainfall and the depth of the water table all need to be considered when deciding where to put an OSSF.  These decisions should be discussed with a site and soil evaluator.

If you are buying an existing home, ask the seller a few important questions, such as:

  • How old is the septic system
  • When was the tank last pumped out and de-sludged
  • How frequently was it pumped out
  • Have there been any signs of failure
  • Have there been any additions to the house that might make it necessary to increase the size of the system
  • It’s always a good idea to get a specialist to survey the septic system before you buy a property, or contact your local Council to carry out a compliance search.

How to protect groundwater

Groundwater (usually from bores) has been tapped for decades, but only recently have we started to understand how vulnerable it is to contamination from surface activities. Effluent from an OSSF can find its way into groundwater. It is vital to locate an OSSF a safe distance from wells, bores, creeks, lakes and houses, and to keep it well maintained.

A failing OSSF can leak chemicals such as medicines, pesticides, paints, varnishes and thinners into the local groundwater. Some chemicals, even in small amounts, can be dangerous to public health and the environment. Certain geological conditions, such as fractured bedrock and shallow groundwater tables may also allow bacteria and viruses to be transported very rapidly and to contaminate drinking water supplies. Groundwater is easily contaminated. Make sure your OSSF is located a safe distance from wells, bores, creeks, lakes and houses.

Septic Tanks

A healthy septic tank is a living ecosystem where the right bugs (bacteria) thrive in the right proportions to digest waste and treat the water (effluent). The septic tank produces effluent of primary quality that is further treated by the soil.

Health caution: Septic tanks do not kill pathogenic bacteria, viruses or parasites.  Septic tank effluent must be treated with extreme caution and contact with people, food, clothing and pets must be prevented!  Do wear protective clothing and wash your hands!!  The contents of a healthy septic tank should form three layers:

  • A layer of fats (called scum) which floats to the surface
  • A clear layer (called effluent)
  • A layer of solids (called sludge or bio-solids) which sinks to the bottom

The scum helps prevent odours escaping and stops air entering. The treated effluent flows out of the tank through an outlet pipe as new wastewater enters. In most septic systems, the effluent is discharged from the septic tank directly into the soil by pipes and trenches (an absorption field). In areas where soil is shallow or unsuitable, special absorption fields may be constructed (eg. raised earth mounds, evapotranspiration beds, or modified earth absorption fields). At this stage the effluent still contains large amounts of dissolved pollutants such as salts and nutrients (eg. compounds of nitrogen and phosphorus). It also contains disease-causing pathogens (eg. viruses, bacteria and worms).

In the absorption field, natural soil processes kill off more pathogens and break down some of the nutrients that cause pollution. This is a slow process, and soil bacteria need oxygen to work, so it is important not to overwhelm the soil with too much effluent. In time the effluent evaporates, is taken up by plants nearby or leaches into the groundwater zone. A hazard is created when effluent flows along surface or subsoil pathways into drainage channels, creeks or rivers. To start up a new or pumped out system, fill the tank with clean water and add a cupful of lime down the toilet every day for 7 days. The lime helps prevent odours and increases the pH (alkalinity), which encourages bacterial growth. This treatment can also be used if the septic becomes smelly.

Greywater tanks and greasetraps

Some septic systems have a separate tank for greywater, the wastewater that comes from the kitchen, laundry and bathroom. There may also be a greasetrap, for collecting oil and grease from the kitchen. The wastewater from the greasetrap eventually flows into the greywater tank where it is collected and pumped to the land application area via a hose and heavy droplet spray head. All greywater systems installed after 1998 must not discharge greywater above ground unless the wastewater has been treated to secondary quality.

Evapotranspiration beds

These are used where soil conditions are less suitable for absorption trenches and where evaporation and transpiration rates normally exceed rainfall. 

AWTS
Aerated Wastewater Treatment Systems

The effluent from an aerated septic tank is usually treated and disinfected to a secondary standard suitable for irrigated on to land. The AWTS consists of two tanks (sometimes within a single larger tank). The first is a basic septic tank where solids settle and anaerobic digestion occurs. In the second tank oxygen is bubbled through the effluent to encourage aerobic bacteria to digest the waste. Finally, the effluent is disinfected using chlorine or ultra-violet light before being pumped to an irrigation area. Fixed line drip irrigation systems are installed through garden beds and/or low throw spray irrigation systems for larger properties.  The extra treatment provided by an AWTS reduces pathogen levels, (and can sometimes reduce nutrients) as long as the system is kept well maintained and the disinfection unit is functioning properly. People using AWTS are required to enter into a regular maintenance contract for quarterly servicing, which is supervised by Council.

CQ Soil Testing pride ourselves in supplying
independent soil testing services.

Now Soil testing in Mackay and regions.