The TPAD system was adapted to the two-stage nature of anaerobic metabolism, and the concept has been evaluated in detail by various researchers. The system is also referred to as 'phase separation' (Fox and Pohland, 1994).
Pohland and Ghosh (1971) first proposed the physical separation of acid formers and methane formers in two separate reactors where optimum environmental conditions for each group of bacterial communities would be provided to enhance the overall process performance. In order to accomplish phase separation, several techniques have been employed such as membrane separation, kinetic control and pH control. Currently, a combination of kinetic and pH control has proven to be most successful for application of the TPAD system (Ince, 1998). Low pH and short SRT limits the growth of methanogens in the acid-forming reactor. If a low pH in the proper range is maintained in the acid-forming reactor, then any of the process modifications can be used and acid-forming bacteria will dominate with negligible presence of a methanogenic population (Zoetemeyer et al., 1982; Kasapgil et al., 1995; Hwang and Hansen, 1998). Any type of reactor may be designed for the second reactor.
Zhang and Noike (1991) used both a single-stage reactor and TPAD processes to compare the characteristics of substrate degradation and reported that acetate-utilizing methanogens in the second phase of the TPAD system were established at 2-10 times higher concentrations than were present in the single-stage reactor. Some investigators (Ince and Ince, 2000) studied the changes in microbial populations in TPAD systems and found that they have several advantages over single-stage reactors, such as the facilitation of the selection and enrichment of different bacteria in each reactor, increased process stability and enhanced buffering of the metha-nogenic phase pH by the prior acidogenic phase. The major advantages and disadvantages of the TPAD system are described in Table 23.4 (Fox and Pohland, 1994; Ince, 1998).
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