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Recirculation

Recirculation

Recirculation
Aerobic Digester
Figure 12.7 Anoxic/aerobic digestion: a. intermittent feed; b. continuous feed without thickening; c. continuous feed with thickening.

with the exception of the addition of mixing equipment where necessary for anoxic operation.

Because of the need to recirculate nitrified mixed liquor for denitrification, as shown in Figure 12.7, complete denitrification will not generally be achieved in continuous feed A/AD processes. Consequently, the reductions in alkalinity destruction and oxygen utilization will not be as great as suggested by Eq. 12.9. This means that chemical addition for pH control may still be required for locations with very low alkalinity water. Nevertheless, the implementation of A/AD is still worthwhile because of the savings involved.

Research has demonstrated the practicality of the A/AD process. It has also indicated that solids destruction rates under anoxic conditions are less than the rates under aerobic conditions. This phenomenon can be explained adequately using ASM No. I with its reduced rate of hydrolysis of particulate substrate under anoxic conditions. However, application of this model also indicates that the overall stabilization of organic matter in the A/AD process will be the same as achieved in a CAD system of equal SRT. Although a build-up of particulate substrate occurs in the anoxic zone of an A/AD process, it is rapidly oxidized in the subsequent aerobic zone."'2" The rate of degradation of active biomass to particulate substrate, which is rate limiting, is the same in the anoxic and aerobic zones.

Autothermal Thermophilic Aerobic Digestion. As discussed in Section 10.2.7, heat is released when organic matter is oxidized, and Table 12.2 summarizes the amount associated with biomass destruction.42 Autothermal thermophilic aerobic digestion (ATAD) takes advantage of that heat to elevate the temperature of an aerobic digester. CAD does not experience a significant temperature increase because of the large mass of water flowing through the system, the conductive heat loss through the walls of the bioreactor, and the evaporative heat loss associated with operation of the oxygen transfer system. In ATAD elevated temperatures are achieved by thickening the feed solids to a concentration of 40,000-60,000 mg/L to minimize the mass of water that must be heated, covering and insulating the bioreactor to minimize conductive heat losses, and using a high efficiency oxygen transfer device to minimize evaporative heat losses. As a result, it is practical to achieve bioreactor temperatures in the thermophilic range (45° to 65°C) without external heating. Smaller bioreactors may be used for ATAD because of the smaller feed volumes resulting from the thickening of the feed solids and the higher digestion rates associated with the elevated temperature. The elevated temperatures also accelerate the inactivation rate of pathogens.">v4:

Figure 12.8 presents a schematic of ATAD. Two tanks in series are often used; significant digestion and heating occur in the first tank, with further digestion and heating to a temperature of about 55°C in the second tank. Feeding is often intermittent, with removal of digested solids from the second tank, transfer of digesting solids from the first to the second tank, and addition of feed solids to the first tank. This promotes temperature elevation and minimizes short-circuiting of feed solids to the digested solids, thereby giving better pathogen inactivation.

The bioreactors may be constructed of steel or concrete, but covers and insulation are generally required to control conductive heat losses. Proprietary oxygen transfer devices achieve the necessary oxygen transfer while minimizing associated heat losses. Bioreactor geometry is typically constrained by the requirements of these devices. High-purity oxygen may also be used for oxygen transfer, resulting in even

Table 12.2 Energy Released by Oxidation of Biomass

Units

COD basis

VSS basis

kJ/kg

13,200

18,800

kcal/kg

3,500

5,000

less heat loss." '* Foaming can be severe because of the high feed-solids concentrations and high temperatures. Consequently, specialized foam control devices are provided with many designs. Nitrification of the released ammonia-N typically does not occur because the elevated temperature minimizes the growth of nitrifying bacteria, which are mesophilic. Thus, oxygen requirements are reduced compared to CAD and pH depression is generally not a problem. In fact, the destruction of biomass in the absence of nitrification produces alkalinity, which results in pH values in the 7.5 to 8.0 range. This increased pH, coupled with elevated ammonia-N concentrations, can result in increased inactivation of viruses.1 Because of the accelerated destruction rates of VSS and pathogens, SRTs on the order of 5 to 6 days are often used. Although the concept of ATAD was originally developed in the U.S.,'"*1"" most applications are in Europe." 12

Autoheating can also occur in CAD systems if the feed-solids concentration exceeds 20,000 mg/L, the feed-solids are sufficiently biodegradable, and a high-efficiency oxygen transfer system is used.1*2" In such cases the temperature in the digester will depend on the heat loss characteristics of the bioreactor and the oxygen transfer device, along with ambient temperatures. Significant variations in digester operating temperature will occur in locations with significant seasonal ambient temperature variations. Procedures for computing the heat balance necessary to estimate temperatures in aerobic digesters have been presented elsewhere.'1

Another variation is the dual digestion process/ J: which uses a single-stage, high-rate ATAD system to heat solids for subsequent feeding to an anaerobic digester, as shown in Figure 12.9. Pure oxygen is typically used to provide oxygen to the ATAD unit, thereby minimizing heat losses from it. As a consequence, an HRT of about 1 day can be used to heat the solids to a temperature of 55 to 65°C. The short HRT minimizes the mass of biodegradable organic matter oxidized aerobically, thereby maximizing the mass fed to the downstream anaerobic digester. Oxygen feed is regulated to achieve the desired ATAD reactor temperature.'1 Significant solubilization of VSS occurs because of the high ATAD reactor loadings, and COD reduction, rather than VSS destruction, correlates best with heat generation. Excellent pathogen destruction occurs because of the elevated temperatures developed in the ATAD reactor.

12.1.3 Comparison of Process Options

Table 12.3 summarizes the benefits and drawbacks of the various aerobic digestion process options. CAD is a demonstrated, proven process. It is mechanically simple.

and it is simple to operate. It is possible to incorporate both solids thickening and digestion into a single vessel, and the supernatant is of reasonably good quality. In contrast, the power costs for CAD are relatively high, and the rates of pathogen destruction are low. Relatively long SRT's are required, resulting in relatively large tank volumes. The pH will drop because of nitrification, and the solids that are produced will generally not dewater readily by mechanical means. The benefits and drawbacks of A/AD are similar to those of CAD, except that it provides pH control and affords a moderate reduction in power requirements.

ATAD offers significantly reduced bioreactor volumes because of the smaller feed (low rates associated with the thickened solids that must be used and the much lower SRTs that result from the higher operating temperature. Nitrification does not occur, so the pH is generally above neutral and the power requirement is less. The higher temperature, elevated pH, and increased ammonia-N concentration result in greater rates of pathogen inactivation. The dewatering characteristics of the solids may be somewhat better than those obtained with CAD and A/AD, although further work is needed to verify this. In exchange for these benefits, ATAD is mechanically more complex and subject to severe foaming. It is a newer process, and engineers have significantly less experience with it than with CAD. Consequently, its performance and operational characteristics are less predictable. Although power costs are reduced compared to other aerobic digestion processes, they may still be relatively high. Separate thickening of the feed solids is required to achieve a sufficiently high concentration to achieve autothermal conditions. Successful operation also requires a sufficient fraction of biodegradable solids in the feed.

12.1.4 Typical Applications

Conventional aerobic digestion is widely used to stabilize the solids at small- to medium-sized wastewater treatment plants (less than 20,000 to 40,000 m'/day). Hundreds of examples exist. CAD is utilized in such circumstances because of its mechanical and process simplicity, and because solids thickening and stabilization can

Aerobic Digestion

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