Heavy metals are often present in a wide variety of industrial wastewater. Heavy metals are non-biodegradable and accumulative in the environment and affect human health when they enter into the food chain. So far, stringent limits on metal concentration have been established due to the relatively high toxicity of heavy metals to environmental receptors. In environmental engineering, more and more research has focused on the removal of heavy metals due to their toxicity to human beings and aquatic life even at relatively low concentrations.
The conventional methods for heavy metal removal from aqueous solution include precipitation with lime or other chemicals, chemical oxidation and reduction, ion-exchange, filtration, electro-chemical treatment, reverse osmosis filtration, evaporative recovery, and solvent extraction. However, when the heavy metal concentrations in the wastewater are low, these processes would have some problems, such as incomplete heavy metal removal, high reagent or energy consumption, generation of toxic sludge or other wastes. Recently, adsorption by activated carbon was applied to remove low-level soluble heavy metals from aqueous solution (Kadirvelu et al., 2001; Mohan and Singh, 2002). While the versatility of activated carbon as adsorbent is obvious in water treatment, it might be costly.
Biosorption is one of the current research focuses looking for inexpensive technology for the removal of heavy metals from aqueous solution. Under this circumstance, a vast array of biomaterials had been tested as biosorbents for heavy metal removal, such as marine algae, fungi, hairy roots of Thlaspi caertulescens, wasted activated sludge, digested sludge, and so on (Lodi et al., 1998; Zhou, 1999; Valdman and Leite, 2000; Taniguchi et al., 2002). Most biosorbents used today are suspended microorganisms in forms of bioflocs. One of the major operation problems associated with the suspended flocs is post-separation of biosorbent from the treated effluent. To overcome this drawback, cell immobilization technique is deployed, but the deployment of immobilization procedure is expensive and complex. It should be realized that the disadvantages of conventional biosorbents in the form of bioflocs have seriously limited the application of biological process for the purpose of removal of metals from wastewater.
Aerobic granulation is an innovative biotechnology developed recently (Liu and Tay, 2004a). Aerobic granules are microbial aggregates with strong and compact microbial structure, and settling velocity and density of aerobic granules are much higher than conventional bioflocs (Liu et al., 2005). When selecting appropriate biosorbents for the removal of heavy metals from industrial wastewater, three criteria have to be seriously taken into account, i.e. effectiveness, robustness, and reliability of biosorbents. It appears that the characteristics of aerobic granules may satisfy these requirements for biosorbents (Liu et al., 2002, 2003a,b, 2004b; Xu et al., 2004, 2005). Therefore, this chapter looked into some up-to-date progress of the biosorption of soluble heavy metals by aerobic granules.
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