Integrated land capability for ecological sustainability of on-site sewage treatment systems

The research project was formulated to solve serious environmental and possible public health problems in rural and regional areas caused by the common failure of soil disposal systems used for application of effluent from on-site domestic sewage treatment systems. On-site sewage treatment systems adopt a treatment train approach with the associated soil disposal area playing a crucial role. The most common on-site sewage treatment system that is used is the conventional septic tank and subsurface effluent disposal system. The subsurface effluent disposal area is given high priority by regulatory authorities due to the significant environmental and public health impacts that can result from their failure. There is generally very poor householder maintenance of the treatment system and this is compounded by the failure of the effluent disposal area resulting in unacceptable surface and groundwater contamination. This underlies the vital importance of employing reliable science-based site suitability assessment techniques for effluent disposal. The research undertaken investigated the role of soil physico-chemical characteristics influencing the behaviour of effluent disposal areas.\ud \ud \ud \ud The study was conducted within the Logan City Council area, Queensland State, Australia. About 50% of the Logan region is unsewered and the common type of on-site sewage treatment used is a septic tank with subsurface effluent disposal area. The work undertaken consisted of extensive field investigations, soil sampling and testing, laboratory studies and extensive data analysis.\ud \ud \ud \ud In the field study, forty-eight sites were investigated for their effluent application suitability. The sites were evaluated based on the soil physico-chemical characteristics. The field investigation indicated that there were nine soil orders in the study area. These soil orders were Dermosols, Chromosols, Kandosols, Kurosols, Vertosols, Sodosols, Tenosols, Rudosols and Anthrosols. The soils in all the investigated sites were acidic soils in the pH range between 5 and 6.5.\ud \ud \ud \ud The complexity of the large data matrix obtained from the analysis was overcome by multivariate analytical methods to assist in evaluating the soils' ability to treat effluent and to understand the importance of various parameters. The analytical methods selected to serve this purpose were PROMETHEE and GAIA. The analysis indicated that the most suitable soils for effluent renovation are the Kandosols whilst the most unsatisfactory soil order was found to be Podosol. The GAIA analysis was in agreement with quantitative analysis conducted earlier.\ud \ud \ud \ud An extensive laboratory column study lasting almost one year was undertaken to validate the results of the data analysis from the field investigation. The main objectives of this experiment were to examine the soil behaviour under practical effluent application and to investigate the long-term acceptance rate for these soils. Twelve representative soils were selected for the column experiment from the previously investigated sites and undisturbed soil cores were collected for this purpose. The results from the column study matched closely with the evaluation conducted at the earlier stages of the research. Soil physico-chemical analysis before and after effluent application indicated that the soils' acidity was improved toward neutrality after effluent application. The results indicated that soils have a greater ability to handle phosphorus than nitrogen. The most favorable cation exchange capacity for soils to treat and transmit effluent was between 15 and 40 meq/100g.\ud \ud \ud \ud Based on the results of the column study, the long-term acceptance rate (LTAR) was determined for the investigated twelve soil types. Eleven out of twelve soils reported specific LTAR values between 0.18-0.22 cm/day. For the duration of the laboratory study, the Podosol order did not reach its LTAR value due to the extremely sandy nature of the soil. The time required to achieve LTAR varied between different soils from 40 to 330 days. \ud \ud \ud \ud The outcomes of this research was integrated into a soil suitability map for on-site sewage treatment systems for Logan City Council. This will assist the authorities in providing sustainable solutions for on-site systems failure