Risk-based approach to on-site wastewater treatment system siting design and management

The use of on-site wastewater treatment systems (OWTS) for the treatment and dispersal of domestic effluent is common in urban fringe areas which are not serviced by centralised wastewater collection systems. However, due to inappropriate siting and soil characteristics, the failure of these systems has become a common scenario. The current standards and guidelines adopted by many local authorities for assessing suitable site and soil conditions for OWTS are increasingly coming under scrutiny due to the public health and environmental impacts caused by poorly performing systems, in particular septic tank-soil adsorption systems. In order to achieve sustainable on-site wastewater treatment with minimal impacts on the environment and public health, more appropriate means of assessment are required.\ud \ud The research described in the thesis details the processes adopted for the development and implementation of an integrated risk based approach to OWTS siting, design and management. This involved detailed investigations into resolution of some of the inherent deficiencies identified in the existing OWTS codes and guidelines, including more thorough site and soil assessment and data analysis, integration of the key risk facets of OWTS siting and design, environmental and public health, and the incorporation of scientific knowledge into the assessment processes. The research undertaken focused on four key research areas; (i) assessment of soil suitability for providing adequate treatment and dispersal of domestic wastewater; (ii) contamination of ground and surface waters as a result of OWTS failure and the major factors influencing contaminant fate and transport; (iii) assessment of environmental and public health risks due to poor OWTS performance; and (iv) the development of an integrated risk assessment framework for OWTS siting, design and management. \ud \ud The research conducted was multidisciplinary in nature, with detailed investigations of the physical, chemical and biological processes involved in on-site wastewater treatment and dispersal. This involved extensive field investigations, sampling, laboratory testing and detailed data analysis across the fields of soil science, groundwater and surface water quality, chemical and microbiological contamination, and contaminant fate and transport processes. The interactions between these different disciplines can be complex, resulting in large amounts of data being generated from the numerous field investigations and sampling processes undertaken. In order to understand the complex relationships that can occur, multivariate statistical techniques were utilised. The use of these techniques were extremely beneficial, as not only were the respective relationships between investigated parameters identified, but adequate decisions based on the respective correlations were formulated. This allowed a more appropriate assessment of the influential factors, and subsequently the inherent hazards related to OWTS, to be conducted.\ud \ud The primary research objectives for this research were investigated through a series of scientific papers centred on these key research disciplines. The assessment of soil suitability was achieved through extensive soil sampling throughout the study area and detailed laboratory testing and data analysis. The studies undertaken are described in Chapters 3, 4 and 5. Paper 1 (Framework for soil suitability evaluation for sewage effluent renovation) outlines a framework for assessing the renovation ability of the major soil groups located throughout Southeast Queensland. This framework formed the basis for the assessment of OWTS siting and design risks employed in the developed risk framework. Paper 2 (Use of Chemometric Methods and Multicriteria Decision-Making for Site Selection for Sustainable On-site Sewage Effluent Disposal) details and justifies the multivariate data analysis techniques used in establishing the soil framework. Paper 3 (Assessment of soil suitability for effluent renovation using undisturbed soil columns) describes investigations of the use of undisturbed soil cores for the assessment of long term soil renovation ability. This study was undertaken to validate the research outcomes achieved through the established framework developed in Paper 1. \ud \ud Papers 4, 5 and 6 (Chapters 6 - 8) focus on contamination issues of ground and surface waters resulting from poor OWTS treatment performance, and the different factors that influence pollutant fate and transport. The investigation of ground and surface water contamination, detailed in Paper 4 (Assessment of High Density of Onsite Wastewater Treatment Systems on a Shallow Groundwater Coastal Aquifer using PCA) and Paper 5 (Environmental and anthropogenic factors affecting fecal coliforms and E. coli in ground and surface waters in a coastal environment) was achieved through extensive ground and surface water sampling and testing from several monitored study sites. Analysis of the resulting data indicated that several key factors, including rainfall, site and soil conditions and on-site system density can significantly influence the fate and transportation of pollutants emitted from OWTS. An additional issue found during this research in assessing faecal contamination of water resources was the necessity to ensure that the respective sources of contamination were actually OWTS. The inherent difficulty in identifying the actual source of contamination was resolved by employing a source tracking method, namely antibiotic resistance analysis of faecal coliforms (Paper 6; Sourcing fecal pollution from onsite wastewater treatment systems in surface waters using antibiotic resistance analysis). Finally, Paper 7 (Integrated Risk Framework for On-site Wastewater Treatment Systems) describes the development of the final generic integrated risk assessment framework and how the outcomes, as discussed through the previous 6 papers, were combined to assess the environmental and public health risks inherent in OWTS siting and design.\ud \ud The outcomes of this research have significantly contributed to knowledge of best practice in OWTS siting, design and management. The developed soil suitability framework allows more appropriate assessment of soil characteristics for providing effluent renovation. This is generally not done in the current assessment techniques for OWTS. Additionally, the use of this framework incorporates scientific knowledge into the assessment of OWTS, allowing a more rigorous and scientifically robust assessment process. The processes and techniques used in the soil suitability framework, although based on the common soil types typical of South East Queensland, can be implemented in other regions, provided appropriate soil information is collected for the area of interest.\ud \ud The integrated risk framework has also been developed on a generic level, allowing easy implementation into most assessment processes. This gives the framework the flexibility to be developed for other areas specifically targeting the most influential OWTS siting and design factors, and the potential environmental and public health hazards within those regions. The resulting research outcomes achieved through the studies undertaken were subsequently applied to a case study for the development of the integrated risk framework for the Gold Coast region. The developed framework, based on scientific research, has allowed a more appropriate means of assessing site suitability for OWTS and appropriate management and mitigation of the environmental and public health risks inherent with poor OWTS performanc