The substrate inhibition difficulties associated with high-strength phenolic wastewaters can be overcome by strategies involving immobilization of bacterial cells (Keweloh et al., 1989). Cells that are immobilized onto various support materials are more resilient to chemical toxicity and can tolerate higher phenol concentrations than their suspended counterparts. For instance, cells of Pseudomonas putida immobilized in hollow fiber membranes degraded phenol at concentrations up to 3500 mg l-1, albeit at relatively low rates, while their suspended counterparts encountered complete substrate inhibition at the high phenol concentrations tested (Loh et al., 2000). Cells of Rhodococcus erythropolis UPV-1 immobilized on diatomaceous earth demonstrated enhanced respiratory activity and a shorter lag phase preceding phenol degradation, degrading phenol at a volumetric productivity of 11.5 g phenol l-1 d-1 (Prieto et al., 2002). These immobilizations require carrier materials for biofilm attachment, necessitating higher investment and operating costs.
Recent research efforts have focused on aerobic granulation as a new form of cell immobilization for exploitation in biological wastewater treatment (Morgenroth et al., 1997; Beun et al., 1999; Tay et al., 2001). Aerobic granulation technology can overcome the disadvantages associated with the use of carrier materials in traditional cell immobilization systems. Aerobic granules are self-immobilized microbial aggregates that are cultivated in SBRs without reliance on artificial surfaces for biofilm attachment, hence rendering carrier material and settling devices unnecessary. The basis for the formation of aerobic granules in the SBR is a repetitive selection for sludge particles such that denser components are retained in the system while lighter and dispersed particles are washed out. These aerobic granules have a strong, compact microbial structure, good settling ability and high biomass retention, with the ability to handle high organic loading rates (Moy et al., 2002). The initial studies have involved cultivation of aerobic granules on simple and relatively benign substrates such as glucose and acetate, using activated sludge as inoculum. However, aerobic granules should be suitable for application in degrading toxic chemicals such as phenol, as the aggregation of microorganisms into compact aerobic granules should confer additional benefits such as protection against predation and resistance to chemical toxicity.
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