Land capability assessment for on-site sewage treatment systems
Al-Shiekh Khalil, Wael, Goonetilleke, Ashantha, Carroll, Steven P., & Dawes, Les A. (2005) Land capability assessment for on-site sewage treatment systems. Faculty of Built Environment and Engineering.
Background The research project was formulated to solve possible environmental and public health impacts which can result from the failure of subsurface dispersal systems used for the application of effluent from on-site sewage treatment systems. On-site sewage treatment systems adopt a treatment train approach with the associated soil dispersal area playing a crucial role. The most common on-site sewage treatment system that is used is the conventional septic tank and subsurface effluent dispersal system. The subsurface effluent dispersal area is given high priority as it is the ‘last line of defence’ before effluent enters a groundwater or surface water source. This underlies the vital importance of employing reliable science-based site suitability assessment techniques for effluent dispersal.
The Project The study was conducted within the Logan City Council area. About 50% of the Logan region is unsewered and the common type of on-site sewage treatment used is a septic tank with a subsurface effluent dispersal area. The work undertaken consisted of extensive field investigations, soil sampling and testing, laboratory experiments and extensive data analysis.
The overall study consisted of three stages of soil evaluation, namely (1) a theoretical soil evaluation; (2) multivariate chemometrics analysis; and (3) soil column experiments conducted under laboratory conditions. The theoretical soil evaluation was based on physico-chemical data derived from the extensive field sampling and testing regime undertaken during the study. This evaluation allowed a preliminary assessment of the effluent renovation ability of the different soil groups found within the study area. However a shortcoming of this evaluation was that only a limited number of physico-chemical parameters could be taken into consideration. Unfortunately, due to the large amount of data generated during extensive field investigations, manipulating or analysing the entire dataset in order to derive meaningful outcomes was difficult. Through this study several other important physico-chemical parameters, although investigated, but did not play a major role in the initial evaluation. The twin challenges of manipulating the large volumes of data collected, and the need to develop reliable methodology for soil evaluation, was overcome by using multicriteria decision making methods of PROMETHEE and GAIA. The use of multivariate data analysis techniques allowed the assessment of all key soil physico-chemical characteristics to be conducted and to identify the interactions and correlations between multiple parameters that are difficult to achieve using simple statistical analysis. This approach helped to derive a better understanding of soil behaviour on the basis of its physico-chemical characteristics, and further enhance the initial soil evaluation based on the collected physico-chemical information.
Subsequently, the conclusions derived from the evaluation of collected physico-chemical data were validated using experimental soil columns. The three evaluation stages as formulated provided valuable information regarding soil performance under sewage effluent application by helping to link theoretical evaluations to field conditions. The integration of the extensive data and knowledge generated through the various stages of soil evaluation made it possible to develop a ‘Soil treatment ability map’ for on-site sewage treatment. This map was then integrated with the flood risk map to develop a ‘Risk map for on-site sewage treatment’ for the Logan region, thus providing information on the land capability for on-site sewage treatment. A further important outcome was that the data generated through the laboratory column experiment enabled the determination of the long term acceptance rate (LTAR) for the common soil types in the study region.
The study outcomes enabled the achievement of the primary aims of the project. It is expected that this research will contribute to achieving the following objectives: • Provide a rational basis for strengthening management strategies governing on-site sewage treatment in the Logan City Council area. • The continuous improvement and/or development of relevant design criteria based on currently available ‘state of the art’ research outcomes. • Ensuring that the land capability to treat sewage effluent is one of the primary factors taken into consideration in decision making relating to land development.
The key findings from the study and recommendations made are listed separately
Impact and interest:
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