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solids were produced was varied to produce waste solids with varying proportions of active and inactive biomass. As indicated in Figure 12.2, both VSS and FSS (referred to in the figure as nonvolatile suspended solids [NVSS]) were destroyed during aerobic digestion. In fact, the proportions of VSS and FSS destroyed were similar. However, Figure 12.3 shows that the decay coefficient was relatively constant and independent of the SRT of the activated sludge system in which the waste solids were generated, although its numerical value depended on whether it was quantifying the loss of TSS or VSS. In contrast, the nonbiodegradable suspended solids content of the waste solids increased as the SRT of the activated sludge system was increased, as illustrated in Figure 12.4. Similar effects have been observed by others. ~ JJ

12.1.2 Process Options

Three basic aerobic digestion process options exist. They are conventional aerobic digestion (CAD), anoxic/aerobic digestion (A/AD), and autothermal thermophilic aerobic digestion (ATAD).

Conventional Aerobic Digestion. Conventional aerobic digestion is quite simple. It consists of the addition of solids to an aerated vessel and their retention there for a period of time equal to the SRT. Figure 12.5 illustrates two CAD bioreactors, one with intermittent and one with continuous addition of feed solids.

In the intermittent process, Figure 12.5a, solids are added and removed from the digester periodically, usually once per day. This process is used in conjunction

Time, days

Figure 12.2 Destruction of TSS, VSS, and FSS (called NVSS) during batch aerobic digestion of waste activated sludge. (From C. S. Recce, et al.. Aerobic digestion of waste activated sludge. Journal of the Environmental Engineering Division, ASCE 105:261-272, 1979. Copyright © American Society of Civil Engineers; reprinted with permission.)

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