Contaminated water treatment in cold regions: an example of coagulation and dewatering modelling in Antarctica
Introduction
A major problem in cold regions contaminated site remediation is the potential for increased environmental impact as a result of disturbance of the site during excavation. Many cold regions contaminated sites are accessible only during the summer months, when snow and ice melts and the ground thaws sufficiently to enable excavation of contaminated material. As a result, clean-up activity often coincides with the brief summer thaw. Disturbance of contaminants by excavation at this time greatly increases the risk of a temporary increase in contaminant dispersal. To prevent widespread contaminant dispersal, a water management plan is often necessary, which ideally includes containment, collection and treatment of contaminated melt waters from the site.
As part of Australia's efforts to remediate abandoned waste disposal sites in Antarctica, a mobile, multi-stage water treatment process has been developed. It is envisaged that the system will also have application as part of a remediation scheme at other remote cold regions contaminated sites. The water treatment system, which is substantially based upon a potable water treatment process, is designed to remove contaminants associated with particles and colloids via coagulation and settling. Dissolved metals are removed using ion exchange, the details of which are the subject of another paper (Woodberry et al., 2003). Fig. 1 is a flow diagram of the system and the development and design is described in detail elsewhere (Northcott et al., 2003). In this investigation, we document the use of coagulation and dewatering characterisation techniques to optimise the operation of the water treatment system for contaminated waters in cold regions.
Characterisation of the coagulation process is based on a mechanistic approach that relates treated water quality to various operating parameters; chemical dose, water temperature and shear regime all have an impact on coagulation and the physico-chemical nature of the flocs produced. Floc strength and bonding influences clarified water quality, suspension dewaterability and effectiveness of dewatering operations.
Characterisation of the dewatering process is more difficult. There are inherent problems associated with design of dewatering equipment using empirical techniques, which can be overcome by basing process design on fundamental material properties. The theory of consolidation of concentrated suspensions developed by Buscall and White (1987) allows for prediction of dewatering behaviour for settling, thickening and filtration based upon two material properties; compressibility and permeability. Using a set of standard experimental techniques to determine these material properties, it is possible to describe a wide range of dewatering behaviours using this one theory.
Dewatering characterisation of sludges from the water treatment system is useful for determining the most appropriate means of handling and treating the sludges (Northcott et al., in press). The compressibility and permeability characteristics of the sludge can then be used to determine the rate and extent of dewatering achievable depending upon the solid–liquid separation technique. Thus far, process equipment which has been modelled using this theory include continuous flow gravity settlers (Landman et al., 1988), pressure filters (Landman et al., 1991, Landman and White, 1994, Eberl et al., 1995, Landman and White, 1997, Landman et al., 1999) and batch settling techniques (Howells et al., 1990, Landman and White, 1994). Our investigation is the first time that water treatment sludges have been characterised and dewatering processes modelled specifically for waste disposal sites in Antarctica. It is envisaged that the techniques outlined here will also be pertinent to environmental remediation and water treatment projects in other cold regions.
Section snippets
The Thala Valley case study
This investigation was conducted at an abandoned waste disposal site in the Thala Valley, near Casey station East Antarctica. The Thala Valley site is situated at the mouth of a major catchment. Each summer surface melt streams develop in the valley and form channels through the site. Water then entrains and dissolves contaminants before discharging into nearby Brown Bay. The waste disposal site is also subject to marine inundation during high tides (Snape et al., 2001).
Laboratory experiments
In this investigation, a
Effect of pH
For each batch of experiments, FeCl3 dose was held constant and caustic soda was used for pH adjustment to determine the impact of pH of the system on treated water quality. Caustic was added at 0, 1:3, 2:3 and 1:1 of FeCl3 concentration. All tests were conducted with mixing time and settling time set to 10 min.
The 5 °C coagulation experiments are presented in Fig. 2. The 10 mg L−1 FeCl3 dose has the smallest pH range between pH 5 and pH 9. The 20 and 30 mg L−1 tests have an almost identical pH
Laboratory experiments
Raw water turbidity, pH and coagulant dose were found to be important for coagulation control. There must be sufficient coagulant in solution in order to fully aggregate particles and facilitate precipitate formation for particle capture. Low temperature was found to have less influence on residual turbidity (the measure of treatment efficiency used in this investigation), than expected. In laboratory tests, temperature had little influence on water treatment in the 0–20 °C temperature range.
Conclusions
This investigation used laboratory and full-scale water treatment tests to predict coagulation behaviours of a water treatment system for cold regions contaminated site applications. Laboratory and water treatment system tests found that flowrate, coagulant dose, pH and raw water turbidity have a direct influence on final treated water turbidity, whilst bentonite and chitosan addition appeared to have little effect on water treatment. Temperature of coagulation was found to have no measurable
Acknowledgements
The authors would like to acknowledge the support of the Australian Antarctic Division and the Particulate Fluids Processing Australian Research Council's special research Centre, Department of Chemical and Biomolecular Engineering, University of Melbourne. Special thanks go to Dr. Shane Usher for his assistance with the dewatering characterisation analyses and modelling calculations.
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