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THE ROLE OF THE BIOMAT IN THE SUSTAINABLE PERFORMANCE OF SOIL ABSORPTION SYSTEMS IN AUSTRALIA: A REVIEW
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org
Citation: Paper number 701P0104, . (doi: 10.13031/2013.15781)Authors: C.D Beal, E.A Gardner , A. Vieritz, N.W Menzies
Keywords: biomat, LTAR, clogging zone, onsite soil absorption system, retardance
Septic tank - soil absorption systems (SAS) are the most common form of onsite disposal
technology in Australia (approximately 225,000 in Queensland alone). Hydraulic failure of SAS
is often reported. However, there is still a lack of substantiated knowledge on the key processes
of SAS in Australian soils. The onsite wastewater industry seems to have an empirical approach
to the design of absorption systems, not withstanding the best available advice contained within
AS/NZS 1547:2000 - a standard for onsite wastewater management jointly developed by
Australia and New Zealand.
In this paper, we describe the hydrology and biogeochemistry of soil absorption trenches from
basic soil physics principles. The hydraulic behaviour of SAS and role of the biological layer
(biomat) is discussed. We also show, using simple numerical modelling, how the interaction of
biomat resistance with soil hydraulic properties determines long-term acceptance rate (LTAR).
Our findings are similar to Bouma (1975), in that a two to three order of magnitude variation in
saturated soil hydraulic conductivity collapses to a one order of magnitude variation in LTAR.