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SRT, days

Figure 13.23 Effect of SRT on the stabilization of primary solids and waste activated sludge. (From G. F. Parkin and W. F. Owen, Fundamentals of anaerobic digestion of wastewater sludges. Journal of the Environmental Engineering Division, ASCE 112:867-920, 1986. Copyright © American Society of Civil Engineers; reprinted with permission.)

a wide range of design procedures must be used to accommodate the wide range of anaerobic processes.

13.3.1 Anaerobic Digestion

The design of an anaerobic digester to stabilize solids is quite straightforward. Since anaerobic digesters are simple CSTRs, the SRT is equal to the HRT. Consequently, the process design consists simply of selecting an appropriate SRT and calculating the bioreactor volume directly from the solids flow rate and the definition of HRT, as given by Eq. 4.15. Principal concerns in choosing the SRT include the degree of stabilization and pathogen inactivation required, digester mixing efficiency, requirements for equipment and digester redundancy, and variations in solids flow rates.

Several factors must be considered when selecting the minimum acceptable SRT, including washout of methanogens, hydrolysis of particulate organic matter, and pathogen inactivation. As indicated in Figure 9.5, growth of aceticlastic methanogens can be maintained at SRTs as low as 5 days at 35°C, which is the most common digester operating temperature. While full-scale digesters have been sue-

cessfully operated at SRTs this low,75 it really is a lower limit and operation at such an SRT places the digester at risk for rapid washout of methanogens and process failure. Furthermore, the hydrolysis of particulate organic matter and its conversion to acetic acid will generally be the rate limiting steps when treating complex organic material. Consequently, longer SRTs are usually used.

A distinction must be made between the design of anaerobic digesters for treatment of primary solids and waste activated sludge, as discussed in Section 13.2.9. Figure 13.19 demonstrates that for municipal primary solids, an SRT of 8 to 10 days is needed at 35°C to ensure reasonably complete stabilization. Figure 13.24 presents o c o O

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