Domestic Wastewater Reuse in Remote Aboriginal Communities, through the use of Evapotranspiration Trenches

Adapted from a paper of the same name (Anda et al 1999)

Background
Evapotranspiration Systems
Use of Evapotranspiration systems to improve Environmental Health
References

Background

There are many instances of sewage ponding in Aboriginal settlements due to the inappropriate design of septic tanks and their associated leach drains. Over the past 10 years, in an attempt to alleviate this problem, reticulated sewer to lagoon systems have been installed as the funds become available. However these systems are not water wise and are expensive to install and maintain. Up to 85% of the cost involved can be in capital and maintenance costs associated with the pipes and pumps required to transport sewage to the treatment plant (Newman & Mouritz 1996). The potential to reuse treated effluent is limited as it is concentrated in one area.

Evapotranspiration systems offer an alternative to the above two approaches to domestic wastewater treatment. By using evapotranspiration trenches wastewater effluent can be reused for revegetation and food production, this allows for a holistic approach to service delivery in remote aboriginal communities.

Evapotranspiration Systems

Evapotranspiration systems can be designed for disposal of blackwater (domestic wastewater originating from the toilet and other sources) or greywater (domestic wastewater originating from the bathroom and laundry). For systems designed for blackwater the water must first pass through two septic tanks before passing into the evapotranspiration trench. The evapotranspiration trench is then used instead of a leach drain, with the added advantage that the water is used for growing trees.

Figure 1 shows a cross section of an evapotranspiration trench. A trench is dug and then lined with geotextile fabric, to prevent roots from the trees from clogging the pipes. A perforated pipe is layed along the length of the trench, then the trench is back filled with 15-50mm aggregate and then screened river sand is placed on top. This particular trench has been designed to cope with the large population fluctuations found in many aboriginal communities. The sloping walls of the trench mean that at times of low water use (when only small numbers of people are residing in the community or house) water will still reach the root level of the plants so they will still receive sufficient water. While, in times of high water use the trench is prevented from overflowing by the ever increasing volume of permeable aggregate and sand found at each successive level. It is also possible to interconnect the trenches of several houses or connect overflow to sewer (if present) in the event of extreme overcrowding in one of the houses as is the tendency in many communities for brief periods each year.

Figure1. Cross Section of an Evapotranspiration trench as designed by Halin Orion.

Diagram courtesy of PM&D

Use of Evapotranspiration systems to improve Environmental Health

Plate 1 depicts a typical remote aboriginal community in Western Australia. The arid nature of the landscape is clearly shown. Dust and associated health problems such as trachoma are one of the main concerns in these communities. Revegetation around houses to cut down on dust is a viable solution to this problem. However water resources are often scarce in remote communities. By installing evapotranspiration trenches as a means of wastewater disposal the wastewater from a house can be used to establish vegetation around the house. Careful design and placement of an ET system in a yard can make best use of the wastewater. Two examples of design methodologies which can enable the most appropriate placement of these wastewater reuse systems in the house yard or community landscape are:

hydroscoping (Colwill 1996) for water-sensitive garden aesthetic design
permaculture (Mollison 1988) for sustainable food production design.

Plate 1. Goodabinya, Marble Bar, East Pilbarra Western Australia.

Figures 2 demonstrates how an ET trench can be integrated with water harvesting swales and basins into a house yard design to maximise food production, shade, shelter and dust mitigation. This house is located at Irrungadji (Nullagine), which is on the edge of the Western Desert. Dense planting on the south-eastern side reduces the effect of the hot dry winds coming from the desert. The Evapotranspiration trench is also located on this side of the house, and the associated native plants offer some wind protection for the fruit trees which utilise the nutrient rich wastewater. Large shade trees on the western side offer the house protection from the hot afternoon sun.

Figure 2. Integration of ET trench in to holistic yard planning for maximum environmental benefit.

Design by Halin Orion, drawn by PM&D.

References

Anda M., Ho G. & Harris C. (1999) Evapotranspiration systems for domestic wastewater reuse in remote aboriginal communities. In Proceedings of On-Site '99 Conference: Making on-site wastewater systems work. University of New England 13-15 July 1999.

Colwill, J. (1996) The waterwise garden and how to create one, Water Authority of WA, Perth.

Mollison B. (1988) Permaculture: A designers manual. Tagari Publications, Tyalgum.