Volatile solids digestion is time dependent. Therefore, the most important consideration in sizing a digester is that the bacteria be given sufficient time to reproduce. To provide sufficient time, the key design parameter in determining the digester volume is the solids retention time (SRT), which is the average time the microbial mass is retained in the digester. It can be defined operationally as the total mass of solids in the digester divided by the mass of solids withdrawn daily. In anaerobic digesters without recycle or supernatant draw-off (e.g., single-stage high-rate digestion), the SRT is equivalent to the hydraulic detention time.
The relationship between SRT and process performance, and critical SRT, which is the lowest SRT beyond which the process fails, have been described in Section 5.2.1. The design SRT should be selected with care. A margin of safety should be provided because the critical SRT values shown in Figure 5.2 were determined in bench-scale digester studies maintained with ideal conditions of temperature, complete mixing, and uniform feeding and withdrawal rates. A minimum safety factor of 2.5 is recommended.
The recommended minimum design SRT is 10 days for systems operating at 35°C (95°F). Values for systems operating at other temperatures are shown in Table 5.4. These design values must be met at all conditions expected, including peak sustained loading, and must account for additional volume for grit accumulations and for differences in the rate of feeding and withdrawal. Taking all these into account, SRT values in the range 15 to 20 days are recommended for design.
TABLE 5.4 Suggested Solids Retention Time for a High-Rate Digester
Critical SRT (days)
Minimum Design SRT (days)
18 24 30 35 40
65 75 85 95 105
28 20 14 10 10
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