As discussed in Section 5.1.2, anaerobic digestion involves two major phases: hydrolysis and acid formation together, and methane production. In the three preceding high-rate digestion processes, both phases take place in a single reactor. In two-phase digestion, the two major phases are divided into separate tanks coupled in series. Pilot studies (Ghosh et al., 1975; Lee et al., 1989) and data from two full-scale facilities (Ghosh et al., 1995) have shown that better sludge digestion can be achieved by optimizing the two phases separately.
Figure 5.8 Two-phase anaerobic digestion.
Figure 5.8 illustrates a two-phase anaerobic digestion system. The first reactor, known as an acid-phase digester, is for hydrolysis and acidogenesis and is designed for a 1- to 2-day detention time. This phase can be operated in either the mesophilic or thermophilic regime. pH in the reactor is between 5.5 and 6.5. Methane generation is negligible in this reactor. The second reactor, known as a methane-phase digester, is designed for about 10 days of detention time and operates in the mesophilic temperature range. Advantages cited for two-phase anaerobic digestion compared to single-phase digestion include:
• Higher volatile solids reduction because it allows the creation of an optimum environment for the acid formers
• Increased production of gases
• Higher content of methane in the final product gas
• Higher pathogen reduction
• Fewer foaming problems
• Better stability of the digestion process
Recent studies (Schafer and Farrell, 2000b) have produced several variations of the two-phase anaerobic digestion system, including staged meso-philic digestion, temperature-phased digestion (TPAD), acid/gas-phase digestion (as described above), and staged thermophilic digestion. These variations use different solids retention times in the two reactors and operate either reactor in the mesophilic or thermophilic regimes. Some of these processes are reported to be capable of meeting class A biosolids requirements.
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