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Temperature °C x SRT, days

Figure 12.11 Effect of the temperature-SRT product on the VSS destruction efficiency during aerobic digestion. Reference numbers refer to original source. (From Process Design Manual for Sludge Treatment and Disposal.1")

Relatively long aerobic digester SRTs are required for operation at lower temperatures (10°C), while much shorter SRTs arc required at higher temperatures, particularly when they are in the thermophilic range. This observation provided one justification for development of ATAD.

Figure 12.12 Effect of temperature and aerobic digester SRT on the SOUR of the digested solids (From N. R. Ahlberg and B I. Boyko, Evaluation and design of aerobic digesters. Journal, Water Pollution Control Federation 44:634-643, 1972. Copyright ® Water Environment Federation; reprinted with permission.)

SRT, days

Figure 12.12 Effect of temperature and aerobic digester SRT on the SOUR of the digested solids (From N. R. Ahlberg and B I. Boyko, Evaluation and design of aerobic digesters. Journal, Water Pollution Control Federation 44:634-643, 1972. Copyright ® Water Environment Federation; reprinted with permission.)

Figure 12.13 Effect of temperature on the inactivation ol total coliforms during aerobic digestion in batch reactors. (From J. T. Novak et al.. Stabilization of sludge from an oxidation ditch. Journal Water Pollution Control Federation 56:950-954. 1984. Copyright c Water Environment Federation; reprinted with permission.)

Aeration Time, days

Figure 12.13 Effect of temperature on the inactivation ol total coliforms during aerobic digestion in batch reactors. (From J. T. Novak et al.. Stabilization of sludge from an oxidation ditch. Journal Water Pollution Control Federation 56:950-954. 1984. Copyright c Water Environment Federation; reprinted with permission.)

Since the inactivation of pathogens during aerobic digestion is a function of both temperature and SRT, regulatory approaches have specified the required digester SRT to achieve compliance with pathogen control requirements as a function of digester temperature."' 41 The available data indicate that operating temperature has a stronger effect on pathogen destruction than the digester SRT. Consequently, the requirements for pathogen destruction cannot be expressed as a simple temperature-SRT product.

As discussed in Section 11.3.1, a residual alkalinity of around 50 mg/L as CaCO, is required to maintain a stable pH near neutrality. However, alkalinity is destroyed if the ammonia-N released during aerobic digestion is nitrified. As shown in Hq. 12.8, the amount is one mole of HCO, per mole of biomass destroyed. This corresponds to 0.44 g of alkalinity (expressed as CaCO,) per g VSS destroyed. Consequently, as illustrated in Figure 12.6, unless pH control is practiced, the pH will decrease during CAD if the released ammonia-N is nitrified. pH values of 5 or below are routinely observed in operating aerobic digesters without pH control. Although the destruction of biodegradable organic matter proceeds at these low pH values, the rate is reduced compared to the rate at pH 7." More rapid digestion has been demonstrated through the addition of lime and other pH control chemicals to maintain the pH near neutrality.14

12.2.3 Mixing

Adequate mixing energy must be provided in aerobic digesters to maintain solids in suspension. Solids settlement will reduce the effective volume of the bioreactor and result in anaerobic conditions in the settled solids. The provision of adequate mixing can be a challenge because of the high suspended solids concentrations commonly maintained. Design and operational manuals typically recommend an air input rate to aerobic digesters of 20 to 40 mV(min-1,000 m1) when diffused air systems are used.4'4" Reynolds1" has provided the following equation to calculate the volumetric power input (IT, expressed in units of kW/1000 m1 of bioreactor volume) required to maintain solids in suspension during aerobic digestion:

where is the viscosity of water in centipoise and XM , is the MLSS concentration in the digester, expressed as mg/L as TSS. At 25°C and a MLSS concentration of 10,000 mg/L, Eq. 12.10 gives a volumetric power input of 13.8 kW/1000 m\ which is near the minimum recommended for mechanical aeration devices in activated sludge systems, as discussed in Section 10.2.5. While Eq. 12.10 can be used to estimate required mixing energy inputs for CAD applications, care should be exercised when applying it to the high suspended solids concentrations used in ATAD. For example, mixing energy inputs in the range of 85 to 105 kW/1.000 nr are typically used with ATAD,42 whereas Eq. 12.10 would give lower values.

Evidence suggests the difficulty in maintaining aerobic conditions throughout an aerobic digester. For example, several researchers have indicated that the specific decay rate of a variety of waste solids declined as the suspended solids concentration was increased.1"""1h Since there is no biological basis for such an observation, the most likely explanation is the increasing difficulty of transferring oxygen and maintaining aerobic conditions throughout the digesting solids particles. Furthermore, a loss of nitrogen that can be attributed to denitrification has been observed in aerobic digesters, even though measurable dissolved oxygen concentrations were maintained on a continuous basis.These observations suggest that it may not be possible to maintain fully aerobic conditions in many full-scale aerobic digesters.

12.2.4 Solids Type

Waste solids being sent to aerobic digestion vary in the proportions of their biodegradable and nonbiodegradable components. For example, if waste activated sludge is being digested, one factor that can influence this is the SRT of the activated sludge system from which it came. This can be seen in Figure 5.7, where the active fraction, which is numerically equivalent to the biodegradable fraction for the situation simulated, decreases as the activated sludge SRT is increased. It can also be seen in Figure 12.4, where experimental determinations of the nonbiodegradable fraction arc shown as a function of the SRT of the activated sludge system from which the solids came. The implication of this is that it will be more difficult to achieve a given percent VSS destruction in solids from an activated sludge system with a long SRT, simply because a smaller fraction of the solids is biodegradable. The nature of the influent to an activated sludge system will also influence the biodegradability of the solids wasted from it. This can also be seen in Figure 5.7, where two cases are compared, one with and one without inert, i.e., nonbiodegradable, suspended solids in the influent. Clearly, the MLSS in the system receiving inert solids has a lower active, i.e., biodegradable, fraction, which will make it more difficult to achieve a given percent VSS destruction. For other solids, the biodegradable content will depend on their source. On the order of 60 to 80% of the VSS in domestic primary solids will be biodegradable,but no such generalization can be made for solids produced during the treatment of industrial wastewaters. Their biodegradable content must be measured.

Waste solids also differ with respect to the nature of their biodegradable component, depending on their source. This is important because the nature of that biodegradable component determines the rate at which it is stabilized. Theoretically the rate of degradation of waste biomass will be nearly independent of the solids source." 41 The data presented in Figure 12.11 suggest that this may be true, since they came from several sources. Likewise, the data in Figure 12.3 show that the rate coefficient for destruction of waste activated sludge is independent of the SRT of the system from which it came." In general, the rate of destruction of domestic primary solids will be lower than the rate of destruction of waste biomass. ' This occurs because of the need to first convert the particulate organic matter contained in the primary solids to active biomass that is subsequently oxidized.

12.2.5 Bioreactor Configuration

Because the destruction of biodegradable organic matter can be characterized as a first order reaction, the efficiency of an aerobic digester can be improved by configuring it as a series of CSTRs, provided that there is no solids recycle around the reactor chain. The impacts of bioreactor configuration on VSS destruction efficiency can be estimated with:

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