(Flotsam and Jetsam: Coastal CRC newsletter
December 2005)
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Coastal researcher, Cara Beal |
Potentially hazardous pollution is escaping into Australia’s waterways from the nation’s ubiquitous septic tank systems. Thousands of homes and hundreds of shire councils may soon be directly affected by improved septic design standards which will reflect new research into the paths of poo – while a young scientist in Queensland has been up to her elbows in it.
“Limited science behind system design has been built in to the Australian and New Zealand Standards,” says Cara Beal of the Coastal CRC and the University of Queensland. “In the past we’ve given a lot of thought to the design of septic tanks, but surprisingly little to the next element in the system, the absorption trench.”
Cara has spent much of the past three years excavating and investigating trenches attached to working septic tanks, and her conclusions have been disturbing. Contaminated water - which should escape downwards into the soil - is in fact seeping sideways out of absorption trenches to a greater extent than first thought. “Effluent is assumed to mainly travel down into the soil through the floor of the trench,” she says. “However, in certain conditions, effluent is moving quite quickly outwards to potentially pollute groundwater.”
In most rural parts of Australia, household wastewater is processed through a septic tank. But, according to Ms Beal, few people are even aware of the covered trench where wastewater filters into the soil. “The soil is an excellent medium for filtering dirty water,” says Cara. “However it can only function as a filter when the dirty water flows through at a slow and constant rate. When an absorption trench is functioning correctly, the nutrients and germs in the wastewater are mostly trapped by a zone of bacteria which lines the trench floor. This ‘biomat’ is the key to minimising effluent polluting the groundwater,” she says. “We are now understanding that design and permeability of the biomat is at least as important as soil type itself.”
Current Australian and New Zealand design standards for trenches are based on the soil properties of any given site, says Cara, but her research has shown that this is inadequate.
“Irrespective of the soil type, the long-term vertical flow through a trench will be governed by the biomat, not by the soil,” she says.
Cara says that she combined state-of-the-art two-dimensional computer modelling of hydrologic flows with hands-on digging into real trenches and installing sensors, as well as a series of practical laboratory experiments to investigate flow rates of effluent through different soil types.
She was able to show that whenever a standard trench receives a heavy flow of effluent, the water takes the path of least resistance and escapes through the sides of the trench. Trench design and planning also takes into account the number of users of the system.
Cara presented her work to the major wastewater conference On-Site ’05, in Armidale (NSW), and was immediately invited to contribute to the 2005/06 review of the Australian and New Zealand Standards for trench design. “The most important outcome of my work is that it will be incorporated into Australian and New Zealand municipal standards, which are issued to all councils and consultancies,” she says. “This means that it will mean a real improvement for communities and individuals.” She recently won the Cooperative Research Centres ‘Young Water Scientist of the Year’ award. Further information contact c.beal@uq.edu.au
