The UASB reactor (Fig. 23.1) was initially developed for widespread use in the Netherlands, and is currently the most popular high-rate anaerobic system in the world, used, in particular, to treat different types of wastewaters (Lettinga and Hulshoff-Pol, 1991b; Lettinga, 1995). Unlike some systems, such as fluidized bed and biofilter systems, the UASB reactor does not require attachment material. Wastewater enters into a UASB reactor at the bottom and exits at the top, and the biomass is developed as a flocculent mass in an upward flowing water stream. The UASB reactor can be successfully applied for the anaerobic treatment of wastewater if well-settling biomass with high methanogenic activity is developed. Mixing in a UASB reactor is not necessary. Sufficient contact between the biomass and the substrate occurs because of the upflow velocity of the wastewater in the sludge blanket, as well as biogas production. Good distribution of the substrate is obtained by having more inlet points at the bottom of the reactor. A gas-solids separation system is used in this type of reactor to collect the biogas and to separate the biomass from the effluent. Biomass falls back in the reactor because of a decreased upflow velocity after biogas is separated in the settling section. A gas-solid separation system is even required for the treatment of much diluted wastewater (Hulshoff Pol and Lettinga, 1986; Lettinga et al, 1991).
A distinctive advantage of the UASB reactor is the very high loading rate achieved by the systems (Speece, 1996). UASB reactors can be used for high-strength wastewaters with a volatile suspended solids (VSS) : COD ratio of less than one and with COD concentrations between 500 and 20 000 mg/L (Mulligan, 2002). The UASB reactor is not therefore, an effective treatment process for wastewater containing high total suspended solids (TSS) concentration and organics in the wastewater must posses a proportionately high degree of solubility. The TSS should ideally be limited to around 500 mg/L (Verstraete et al., 1996). Some advantages and disadvantages of UASB digesters are shown in Table 23.3 (Weiland and Rozzi, 1991).
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