Abstract
Conventional water treatment processes use aluminium sulphate (alum) as a coagulant in the production of potable water. While alum is an inexpensive and reliable means of treating water, the process generates waste water containing dissolved Al. This waste water is primarily dealt with via on-site retention. In this study we investigate the cultivation of the freshwater macroalga Oedogonium as a means to sequester dissolved Al from waste water from a conventional water treatment plant. Furthermore, we examine the use of CO2 to manipulate the pH of cultivation as a means of enhancing the sequestration of Al by either increasing the productivity of Oedogonium or increasing the bioavailability of Al in the waste water. The relative bioavailability of Al under conditions of CO2 and no-CO2 provision was contrasted by comparing Al uptake by Diffusive Gradients in Thin Films (DGTs). Oedogonium was able to grow rapidly in the waste water (12 g dry weight m−2 day−1) while consistently sequestering Al. The Oedogonium-treated waste water had a sufficiently low Al concentration that it could be used in unrestricted irrigation in the surrounding region. When CO2 was added to the waste water containing concentrations of Al up to 8 mg L−1, there was a slight increase (~10%) in the rate of sequestration of Al by Oedogonium relative to waste water not receiving CO2. This was due to two concurrent processes. The provision of CO2 increased the productivity of Oedogonium by 15% and the bioavailability of Al by up to 200%, as measured by the DGTs. Despite this strong effect of CO2 on Al bioavailability, the increase in Al sequestration by Oedogonium when CO2 was provided was modest (~10%). Al was sequestered by Oedogonium to concentrations below permissible limits for discharge without the need for the addition CO2. The cultivation of Oedogonium in waste water from conventional treatments plants can simultaneously treat waste water for re-use and provide a biomass source for value-added applications.
Highlights
Conventional treatment of surface water is the dominant approach to producing potable water from surface water storages and supplies around the world and consists of five core steps; coagulation, flocculation, clarification, filtration and disinfection [1]
This study investigates the cultivation of Oedogonium in waste water from a water treatment residuals (WTR) detention basin in north Queensland, Australia
The Oedogonium cultivated in the waste water had a mean Al content of 467 mg kg−1 and an average of 5.4 g dry weight (DW) of biomass was harvested from each replicate each week (Table 1)
Summary
Conventional treatment of surface water is the dominant approach to producing potable water from surface water storages and supplies around the world and consists of five core steps; coagulation, flocculation, clarification, filtration and disinfection [1]. Alum is added to surface water and dissolves to release a number of Al(III) chemical species that will form flocs with negatively charged dissolved and colloidal impurities in the feed water [2]. These flocs are settled and removed from water through the subsequent clarification and filtration processes. Alum has been used for centuries in water treatment and remains the dominant means of coagulation in developing and developed nations alike [3,4]
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