ABSTRACT

There are over 1000 drinking water treatment plants in the United States that use alum, Al2(SO4)3 14H2O as a coagulant for the efficient removal of particulate solids and colloids from surface water supplies [1,2]. For alum, the process of coagulation works best in the pH range 4.5-8.0. Similarly, for ferric chloride, another coagulant used in the water treatment plants, the optimum pH range is between 4 and 12. Alum is converted during the process into insoluble aluminum hydroxide, a major component (25%–50%) of solids in water treatment residuals (WTR). Water treatment plants in the United States produce over 2 million tons of aluminum-laden disposable WTR every day [3], which are bulky, gelatinous, and biologically inert slurry composed of suspended inorganic particles, natural organic matter (NOM), trace amounts of heavy metal precipitates, and aluminum hydroxide. The total solids content of the residuals normally ranges from 2% to 10% in mass per unit volume [4,5]. A typical composition of water treatment plant residuals is shown in Table 34.1 [6]. Due to regulatory changes in the recent past, WTR now have to be disposed in landfills or through land application [3,6]. And due to the magnitude and pervasiveness of the problem, the prospect of alum recovery from WTR has received considerable attention in recent years. The toxicity of free and complexed aluminum species toward various aquatic species and benthic organisms has been the focus of several studies [7,8]. Some researchers have linked aluminum ion’s contributory influence to occurrence of Alzheimer’s disease [9]. It is therefore recognized that the disposal of aluminum-laden solids from water treatment plants will receive a closer scrutiny in the coming years [10]. Ideally, a simple-to-operate process, which can recover alum selectively from the sludge, will significantly reduce the amount of disposable solids. However, the recovered alum has to be sufficiently pure to be reused as a coagulant at the front-end of water treatment. Such a process will truly combine pollution prevention with resource recovery, thus reducing stress on the environment.