Key Points

1. Aerobic digestion has two primary objectives: (1) the destruction of biodegradable particulate organic matter, and (2) the inactivation of pathogens present in waste solids.

2. Aerobic digestion is most applicable to the stabilization of waste biological solids, such as those generated by activated sludge and trickling filter facilities. It can also be used to stabilize primary solids, but aerobic digestion of such solids is often less economic than anaerobic digestion.

3. The influent to an aerobic digester contains both biodegradable and nonbiodegradable particulate organic matter. The relative proportions of each depends on the loading and operating characteristics of the process producing the solids.

4. The destruction of biodegradable particulate organic matter can be characterized as a first order reaction.

5. Both volatile and fixed suspended solids are destroyed during aerobic digestion, although the relative proportions destroyed may not be the same. Fixed suspended solids are lost as they are solubilized and released from the biodegradable particulate organic matter destroyed.

6. Solids stabilization is typically quantified as either the percent VSS destruction achieved during digestion or the specific oxygen uptake rate (SOUR) of the digested solids.

7. In conventional aerobic digestion (CAD) the solids are maintained under aerobic conditions at the ambient temperature for a period of time adequate to achieve the desired degree of solids stabilization and pathogen inactivation. Both intermittent and continuous feed options are available. Nitrification of released ammonia-N typically occurs, resulting in the destruction of alkalinity and depression of the pH.

8. Anoxic/aerobic digestion (A/AD) includes an anoxic and aerobic sequence in the digestion process. Alkalinity produced through denitrifica-tion can offset that consumed in the nitrification of the ammonia-N released. Oxygen requirements are also reduced in comparison to CAD.

9. Autoheating of the digester can be achieved if the solids are thickened prior to digestion and the vessel is designed to minimize heat loss. In autothermal thermophilic aerobic digestion (ATAD), such approaches are used to achieve bioreactor temperatures in the 45 to 65°C range. This results in increased rates of solids stabilization and pathogen inactivation. Because nitrification does not occur under thermophilic conditions, pH depression is avoided and oxygen requirements are reduced.

10. The destruction of biodegradable organic matter in an aerobic digester can be characterized using a variety of approaches. Mathematical approaches include a first-order decay model, the simplified model presented in Chapter 5, and IAWQ ASM No. 1. Another approach uses empirical correlations, such as those that relate percent VSS destruction to the operating temperature-SRT product.

11. Aerobic digestion is most efficient at neutral pH. Maintenance of a neutral pH can be accomplished by use of A/AD to denitrify any nitrate-N generated, ATAD to eliminate nitrification, or chemical pH control in CAD.

12. The mixing energy required to maintain solids in suspension increases as the suspended solids concentration in the aerobic digester is increased.

13. The performance of an aerobic digester can be improved by designing and operating it as a series of CSTRs rather than as a single CSTR.

14. Data collected using batch tests can provide the basis for the design of aerobic digesters. Because of the variability associated with the solids' characteristics, a series of tests should be run and a statistical approach used as the basis for the design. The batch tests must be conducted under conditions reflective of those anticipated in the full-scale system.

15. The physical design of an aerobic digester can significantly influence its operation and performance. Heat loss can be a particularly significant problem in colder climates.

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