A study by a University of Queensland PhD student is challenging the current design recommendations for septic trenches - the most common on-site treatment system in Australia.
Cara Beal, from UQ`s School of Land and Food Sciences, has been examining the key water and pollutant pathways in septic trenches and how they vary with soil type. Pollutants include nutrients, bacteria, viruses and oxygen-demanding substances.
“Pathways and flow rates in trenches determine how efficiently the effluent is treated. By understanding this interaction we can reduce the potential for water pollution from these systems,” Ms Beal said.
“Until recently septic trenches were designed largely on local knowledge and rule of thumb. The Australian and New Zealand Standard 1547:2000 now provides a framework but the science behind the design recommendations remains thin.”
Ms Beal`s research has focused on the biomat zone, a biologically active layer on the bottom and lower sidewalls of a trench.
“My work shows the resistance of the biomat zone creates a narrow range of flow rates through a trench, regardless of whether the soil is sand or clay,” she said.
“I have found that the unsaturated soil characteristics are more important than the saturated characteristics as water flow occurs more slowly in these drier soil conditions.
“But the current standard is based on the saturated characteristics of a soil.”
Ms Beal`s work is believed to be the first to predict a range of long-term acceptance rates for Australian soils using the biomat zone as the governing factor in effluent flow.
The critical condition for adequate treatment in septic trenches is a slow passage of effluent through an unsaturated zone (~100cm) of soil. Ms Beal said the biomat zone promoted these conditions irrespective of soil type.
“The underlying assumption is that effluent will be travelling through the bottom of the trench system,” Ms Beal said.
“But I am finding that in permeable soils, effluent flows sideways at high rates, particularly during peak trench usage or heavy rainfall, which in turn limits the frequency of the effluent surcharging from the trench.”
“However, this creates conditions where poorly treated effluent may enter groundwater.”
Ms Beal conducted her research by building miniature septic trenches, looking at those in the field and using a two-dimensional computer model to predict water pathways and flow rates.
She is supervised by UQ`s Associate Professor Neal Menzies, Adjunct Associate Professor Ted Gardner from the Department of Natural Resources and Mines and Dr Gunnar Kirchhof from UQ.
Over one million households in Australia rely on on-site wastewater treatment and dispersal systems. Septic systems comprise more than 80 percent of these.
Ms Beal, in association with the Department of Natural Resources and Mines, has also conducted a review of on-site wastewater treatment systems for the Moreton Bay Waterways and Catchments Partnership.
She said most septic systems were poorly regulated because about 75 percent of them were installed before any guidelines were introduced.
“It is very hard to quantify the impacts in non-sewered areas when we don`t really know where the systems are and how they are performing,” she said.
The review, which involved a survey of 19 Councils in Southeast Queensland, concluded that every local council should conduct a house-by-house audit of septic systems.
A paper from the study received the Australian Water Association Best Poster Award at the OzWater Conference in May. Ms Beal is also a finalist in the CRC Young Water Scientist of the Year Award to be held at the River Symposium in Brisbane in September.
Media: For more information, contact Cara Beal (telephone 07 3346 9546 or 0418 789 420, email: c.beal@uq.edu.au) or Ted Gardner (telephone 07 3896 9488) or Chris Saxby at UQ Communications (telephone 07 3365 2479).
