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The effectiveness of evapotranspiration systems in disposal of wastewater in remote Aboriginal communities in Northwest Australia

McGrath, D. (1989) The effectiveness of evapotranspiration systems in disposal of wastewater in remote Aboriginal communities in Northwest Australia. Honours thesis, Murdoch University.

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The outstation movement has resulted in the development of many new Aboriginal communities, ranging in size from small family groups to collectives of 100 or more people in remote areas of Australia. Health problems arising from ineffective waste disposal systems and lack of maintenance of existing systems are prevalent in many of the communities.

In some areas of the Pilbara, the soil is extremely impermeable with high clay and silt contents causing water applied to the soil to pond on the surface. In such situations conventional septic tank/soil absorption systems frequently fail: effluent moving into the disposal field is not absorbed by the soil sufficiently rapidly to prevent rise to the surface and hence system failure occurs. The only alternative currently used in such situations is a reticulated sewerage collection system which is very costly and frequently fails due to lack of maintenance.

Evapotranspiration (ET) systems could be used instead, being considerably less in cost and potentially requiring less maintenance than reticulated systems. In this study the effectiveness of ET systems in disposing of wastewater and the application of these systems to remote Aboriginal outstations was investigated.

Experiments were carried out between August 1988 and March 1989 at Murdoch University. Monitoring of daily changes in water levels in watertight tanks (constructed of either concrete drainage pipes with set in bases or polyethylene drums) containing soil, grass, gravel or trees (Eucalyptus camaldulensis). The gravel tanks were filled to the brim with 40mm gauge crushed granite. The tanks with soil, grass and trees were filled with graded gravel above which was placed a 40cm depth of Bassendean sand. As much as possible, the water table in each tank was kept within the sand layer to allow capillarity of water to the surface to occur. The bulk density and average particle density of the sand was measured to determine the porosity, from which the actual ET from the tanks was calculated. Calibration experiments were conducted to measure the time taken for the water level to stabilise when water was added or removed from a tank.

The bulk density of sand used in the study was 1.44g/cc and the particle density was 2.11 glee. From this the porosity was calculated as 0.32. Calibration experiments showed that the water level took up to six days to stabilise (to a water content of 0.91 L/cm sand in tank) when water was added down the standpipe to the tanks containing sand and even longer when added to the surface simulating rainfall. This placed the accuracy of ET determined for each lysimeter in some doubt over short periods as water was frequently added at intervals smaller than six days. Over a larger period however, water balances with an approximately constant water level in the tank gave a good estimate of ET rates.

ET from bare soil and grass followed similar trends to pan evaporation rates for the same period, ranging from 30-60% of pan evaporation for soil and from 60-80% of pan evaporation for grass. ET rates increased in the tree lysimeters as the plants grew and exceeded pan evaporation rates in December and February. Evaporation from gravel filled lysimeters was low, being as little as 10% of pan evaporation at a depth to water table of 28cm. The results showed that the presence of vegetation increased water loss, indicating that soil capillarity may not be important in the functioning of an ET system that has plants growing. Gravel was not a suitable medium for sustained high ET rates.

The water balance equation was used to determine the ET system sizing requirements for a given locality, where system depth was dependent upon the total storage of wastewater needed in a year and on the storage capacity of the fill material used in the disposal field. Application of the equation in designing an ET system for Punmu showed that the information that was available was pan evaporation rates, rainfall and average wastewater production figures. Information that was not available and would require collection on-site was seepage rates of the soil, and likely numbers of people present throughout the year at the outstation. Similarly, the ET rates for trees could not be determined from pan evaporation data and required further investigation. The acceptance by residents at an outstation of ET systems should also be examined - fear of snakes hiding in vegetation may perhaps cause people to pull out shrubs planted on a disposal field.

In conclusion, ET systems would appear to be a suitable alternative to reticulated systems in remote communities. The arid conditions of the Pilbara favour the successful implementation of such a disposal system, provided sufficient wastewater is generated all year round to maintain vegetative growth. There should be no public health risk involved in the functioning of a system if it is designed to maintain a depth to water level in the bed of at least 20cm.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Environmental Science
Notes: A digital copy of this thesis is not available. Your library can request a copy from Murdoch University Library via Document Delivery. A fee applies to this service.
UNSD Goals: Goal 6: Clean Water and Sanitation
Supervisor: Ho, Goen and Mathew, Kuruvilla
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