The ability of bacteria to degrade phenol has many practical applications, such as the biological treatment of phenol-containing industrial waste-water and the bioremediation of sites polluted with phenolic compounds. The biological treatment of phenol wastewater has been mostly based on conventional continuous aerobic activated sludge systems. Activated sludge is a suspended growth process that began in England at the turn of the last century and has been widely used in municipal and industrial wastewater treatment. This process essentially consists of an aerobic treatment that oxidizes organic matter and other wastewater contaminants to carbon dioxide, water, and new cell biomass. Air is supplied by diffused or mechanical aeration and the microbial cells form activated sludge flocs that are allowed to settle in a secondary clarifier.
Although phenol removal has been carried out for many years by activated sludge systems, the treatment process has been known to break down because of the toxicity effects of high phenol concentrations encountered during episodes of fluctuations in phenol loads and of high phenol loading rates in excess of 1 kg phenol m "3 d-1 (Watanabe et al., 1996, 1999; Kibret et al., 2000). Phenol toxicity can cause inhibition of the degradation processes, decrease in the settleability and washout of sludge biomass, high phenol concentrations in the effluent, and lead to the unrecoverable failure of activated sludge systems, often rapidly.
The sequencing batch reactor (SBR) represents a promising form of biological wastewater treatment technology belonging to the group of so-called fill and draw reactors (Wilderer et al., 2001). The SBR process is a variable volume, suspended growth, biological wastewater treatment technology that is characterized by a repetitive batch cycle consisting of several successive phases (usually fill, react, settle, decant, and idle), each lasting for a defined period. Each phase can be adjusted according to its position and function within the batch cycle to satisfy specific treatment objectives. Unlike activated sludge systems, aeration and sedimentation-clarification occur sequentially in the same vessel in SBR technology. Because of its applicability to simple automation, the ease with which its operation can be modified, its single-tank design and the ability to select robust microbial communities, SBR technology has been gaining widespread acceptance within the engineering community (Mace and Mata-Alvarez, 2002). However, in comparison with continuous flow activated sludge systems, the knowledge base for SBR performance during practical situations has not been fully developed, and there are few reports on the use of SBR for treatment of phenol. Although an SBR had been recently reported to treat phenol wastewater at a high phenol loading rate of 3.1 g phenol l-3d-1, the settling ability of flocculated sludge in that reactor was generally poor, even at a low phenol loading rate of 0.52 g phenol l-3 d-1 (Yoong et al., 2000).
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