Figure 12.4 Effect of the SRT of an activated sludge system on the nonbiodegradable suspended solids content of the resulting waste solids. A. TSS basis; B. VSS basis. (From C. S. Reece, el 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.)

Feed Sludge

Supernatant (Optional)

Digested Sludge

Feed Sludge

Feed Sludge

Figure 12.5 Conventional aerobic digestion: a. Intermittent feed; b. continuous feed.

with biological wastewater treatment systems in which solids are wasted on a daily basis, usually over a relatively short time period. Digested solids are removed from the digester as necessary, depending on the downstream solids handling system. In some moderate size wastewater treatment facilities, solids are thickened prior to addition to the digester. In those cases, the SRT of the digester will be equal to its hydraulic retention time (HRT). When thickening is not used, the aerobic digester is operated like a sequencing batch reactor (SBR) to provide both solids thickening and digestion. Consequently, the SRT is greater than the HRT. The typical steps for operation of an SBR are shown in Figure 7.42. In this application, digested solids are withdrawn for further processing at the end of the supernatant draw period. A suspended solids concentration in the range of 12,500 to 17,500 mg/L can typically be achieved if waste activated sludge (WAS) is being digested. Somewhat higher concentrations can be achieved if a mixture of primary solids and WAS (15,000 to 25,000 mg/L) or primary solids alone (30,000 to 40,000 mg/L) is being digested. Although feeding is intermittent, feed is added many times during one SRT, making the bioreactor perform like a CSTR rather than like an SBR.

Solids may also be wasted from a biological wastewater treatment system on a more continuous basis, a practice often used in larger plants. Figure 12.5b illustrates an aerobic digestion system that receives feed on a continuous basis. It looks like an activated sludge system, with feed solids displacing digesting solids to a gravity thickener. Supernatant overflows the thickener, while thickened solids are withdrawn from its bottom and returned to the digester. Thickened solids are also periodically directed to solids handling, with the rate of thickened solids removal being adjusted to maintain the desired SRT. Suspended solids concentrations in the thickened solids are similar to those achieved with the intermittent feed process. Consequently, suspended solids concentrations within the continuous feed digester are lower.

Other operating modes are possible for the intermittent and the continuous feed systems. For example, because VSS destruction is first order, arrangement of aerobic digesters in series can increase the efficiency of the process, as discussed in Section 12.2.5.

Conventional aerobic digesters are constructed using facilities and equipment similar to those used for activated sludge systems. In fact, the aerobic digester may simply be one or more of the aeration basins provided in an activated sludge system. The bioreactors can be concrete or steel, or they can be lined earthen basins. Submerged aeration systems, such as diffused air, may be more desirable in cold climates than mechanical surface aerators because they minimize heat losses, which can be quite significant because of the relatively long HRTs. However, surface aeration systems can be used, particularly in warm climates. Decant devices are required for intermittently fed digesters and several approaches have been successfully utilized. SRTs on the order of 20 days will usually produce a significant VSS destruction efficiency, although longer SRTs may be required to achicve a SOUR less than 1 mg 0;/(g VSS-hr). Relatively long SRTs may also be required to meet pathogen destruction requirements, depending on the operating temperature.

Anoxic!Aerobic Digestion. One difficulty often experienced with the CAD process is destruction of alkalinity by nitrification, as discussed in Section 6.3.3. The destruction of organic matter, particularly active biomass, results in liberation of organic nitrogen as ammonia-N. Because of the long SRTs required to accomplish solids stabilization, nitrifying bacteria typically grow in the digester even if they are not present in the feed solids. Furthermore, because of the relatively high feed solids concentrations typically used, the ammonia-N concentrations that develop are high, causing nitrification to deplete the alkalinity in the system, dropping the pH. Figure 12.6 illustrates typical pH profiles during digestion of a WAS.1" Three operational modes and three temperatures were considered in this study. Two modes involved feeding of the digester, either on a continuous or a daily basis. The third was batch operation with only one feed addition. In all cases the pH dropped to relatively low values (4.5 to 5.5), which are typical of those that can be experienced in aerobic digesters without pH control. Although digestion will continue at these lower pH values, the rate will be reduced. The pH of the digester can be adjusted through the addition of bases such as lime, but this is an additional operating cost.

Just as is done with anoxic selectors (Section 11.3.1), A/AD incorporates an anoxic cycle to allow alkalinity production by denitrification to partially offset that consumed through nitrification.44 An additional benefit is a small reduction in process energy requirements since some of the organic matter is oxidized by using nitrateN generated through nitrification, rather than oxygen, as the electron acceptor.

As discussed above, digestion of cellular material using dissolved oxygen as the terminal electron acceptor, with concurrent oxidation of the ammonia-N to ni-trate-N, leads to the net destruction of alkalinity. When the production of autotrophic biomass is neglected (since it is small), the molar stoichiometry is:

Consequently, one mole of bicarbonate alkalinity is destroyed for each mole of biomass destroyed. Furthermore, seven moles of oxygen are used for each mole of biomass destroyed, which is equivalent to 1.98 g 0:/g VSS destroyed as shown in Table 12.1. However, if a situation could be devised so that all of the nitrogen released was

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