Process Description

Aerobic digestion has been used for several decades to stabilize the waste solids produced at municipal and industrial wastewater treatment plants.4" Its popularity increased throughout the 1960s and into the 1970s because of its simplicity and lower capital cost relative to anaerobic digestion. Although it had previously been used primarily in small wastewater treatment plants, during this period it was also used in medium to large facilities. This trend was halted in the mid 1970s as rapidly escalating energy costs adversely impacted its overall cost-effectiveness relative to other solids stabilization options. Then, in 1979, federal regulations governing the management of solids from municipal wastewater treatment plants were issued that set new requirements controlling pathogens when solids are to be reused."41u This further decreased the attractiveness of aerobic digestion since its rates of pathogen inactivation are generally lower than anaerobic digestion. Nevertheless, aerobic digestion remained a popular option for small wastewater treatment plants because of its simplicity. In addition, thermophilic aerobic digestion processes, which have higher solids stabilization and pathogen inactivation rates, were developed in the late 1970s and early 1980s.414: Consequently, aerobic digestion remains a viable option for the stabilization of waste solids at many wastewater treatment plants.

12.1.1 General Description

Figure 12.1 summarizes the biochemical transformations occurring in an aerobic digester. Biodegradable particulate organic matter is hydrolyzed and converted into biodegradable soluble organic matter, releasing nutrients such as ammonia-N and phosphate. The biodegradable soluble organic matter is then converted into carbon dioxide, water, and active biomass through the action of heterotrophic bacteria. The

active biomass, in turn, undergoes decay, resulting in the generation of additional carbon dioxide and water, along with inactive biomass, i.e., debris. Nonbiodegradable particulate organic matter in the influent is not affected by the digestion process and becomes a portion of the digested solids. Figure 12.1 is based on the traditional decay model for biomass destruction, as discussed in Section 3.3.1. The lysis:re-growth model, described in Section 3.3.2, is equally applicable and, in fact, International Association on Water Quality activated sludge model (IAWQ ASM) No. 1, with its explicit treatment of hydrolysis, nitrification, and denitrification, provides a more accurate description for some aerobic digestion process options. Nevertheless, the simplified models often used to design aerobic digesters are directly related to the traditional decay model, and thus it is emphasized herein.

Observations of aerobic digestion processes provide the following conceptual framework upon which design models are based:

• The suspended solids in the influent stream can be segregated into biodegradable and nonbiodegradable components.14 ~ The biodegradable components include particulate organic matter, XM and active biomass, both heterotrophic and autotroph ic, X|( n and X^ A. The nonbiodegradable component consists of particulate inert organic matter, X„ and biomass debris, X„.

• A nonbiodegradable residue will result from aerobic digestion, even if no nonbiodegradable particulate matter is present in the influent solids stream because biomass debris results from the decay of active biomass. 14

• Aerobic digestion results in the destruction of both volatile suspended solids (VSS) and fixed suspended solids (FSS).7 0 This occurs because both the organic and inorganic materials in the biodegradable suspended solids are solubilized and/or oxidized as the solids are digested. However, the volatile and fixed components of the biodegradable and nonbiodegradable suspended solids are not equal. Consequently, VSS and FSS will not generally be destroyed in the same proportion. However, in spite of the loss of fixed solids during aerobic digestion, most designers focus on loss of VSS.

• The biodegradable fraction of solids is a function of their source. ~ " " This is clearly illustrated by the models discussed throughout previous chapters. For example, both primary solids and waste activated sludge from a system with a short SRT will contain relatively high fractions of biodegradable material, whereas waste activated sludge from a system with a long SRT will contain a low fraction of biodegradable material and a high fraction of biomass debris.

• The destruction of biodegradable suspended solids can be characterized as a first order reaction.14 " This occurs because the decay of active biomass is a first order reaction. Biodegradable particulate organic matter is rapidly converted to active biomass. Then that biomass, as well as any active biomass present in the influent, decays in a first order manner, resulting in an overall first order reaction for loss of biodegradable suspended solids. As a result of this relationship, the destruction of biodegradable suspended solids is often referred to as decay, and the first-order reaction rate coefficient is called a decay coefficient.

• For solids containing a relatively high proportion of active biomass. the value of the decay coefficient for biodegradable suspended solids is relatively independent of the SRT at which the waste solids were produced. " ' 14 This is because the decay coefficient for the biodegradable suspended solids will be influenced strongly by the decay coefficient for heterotrophic bacteria, which is relatively constant.

Mathematically, these relationships can be summarized as follows. Because design of aerobic digesters is typically concerned with VSS destruction, the concentrations will be expressed as VSS, although it should be recognized that they could also be expressed as total suspended solid (TSS) or chemical oxygen demand (COD). The VSS undergoing aerobic digestion, XMV, can be subdivided into biodegradable and nonbiodegradable components, XN1 Vl, and XS1A„, respectively:

where bsiv is the first-order decay coefficient based on the loss of VSS. Decay coefficients can also be determined on the basis of COD or TSS, depending upon how the solids concentration is expressed. Because the COD/VSS ratio can be considered to be constant, the decay coefficients based on VSS and COD loss have the same numerical values. However, because VSS and FSS are not generally destroyed in proportion to each other, the decay coefficient on a TSS basis will have a different

XM.V — XM v 1, + XMA.„ Furthermore, loss of biodegradable VSS occurs in a first-order manner:

numerical value. The nonbiodegradable solids are considered to be totally inert so that nothing happens to them during aerobic digestion.

Sometimes it is desirable to consider the solids in terms of the constituents used in the simplified model of Chapter 5 and in ASM No. 1. In those terms, the VSS consists of:

XMV = Xs.v + X|t.|| v + X|, AA + X|) y + X| v (12.3)

All symbols have been defined previously; the subscript V simply indicates that they are expressed on a VSS basis. The particulate substrate and the biomass are considered to be biodegradable, although decay of biomass leads to debris. The debris and the inert material are, of course, nonbiodegradable. The fate of these components can be modeled in ASM No. 1, just as the various activated sludge systems are modeled in Chapter 7. This has been done by Marais and coworkers "" using the model upon which much of ASM No. 1 was based. The simplified model of Chapter 5 does not contain a term for particulate substrate and it is not as accurate as ASM No. 1. Nevertheless, it has been used to model aerobic digestion by lumping the particulate substrate with the biomass and considering the system to be a bioreactor receiving only biomass and nonbiodegradable solids. In that situation, the autotrophic biomass is generally neglected as being insignificant. This is done in Section 5.2.3 for COD units, with Eqs. 5.64 and 6.65 expressing the heterotrophic biomass and debris concentrations, respectively, in a single continuous stirred tank reactor (CSTR). Equation 5.68 gives the oxygen requirement for such a bioreactor when nitrification is not considered. Figure 5.10 shows the effects of solids retention time (SRT) on the theoretical performance of a single CSTR receiving only active heterotrophic biomass and debris. There it can be seen that a point of diminishing returns is reached at which further increases in the SRT have little effect. When that point is reached, most of the suspended solids will be nonbiodegradable, with only a small fraction of active biomass.

Since the purpose of aerobic digestion is to stabilize the biodegradable organic matter in an influent waste solids stream, criteria must be available for quantifying the degree of stabilization. Although several could be proposed, two frequently used ones are the VSS destruction efficiency (expressed as the percent VSS reduction) and the specific oxygen uptake rate (SOUR) of the digested solids (typically expressed as mg 0:/(g VSS • hr)).': A VSS reduction of 38% and a SOUR of 1.0 to 1.5 mg OV(g VSS hr) are values typically used to represent stabilized solids/1"'

The simple first-order model presented above can be used to estimate the effect of the size of a completely mixed aerobic digester on the degree of solids stabilization achieved. The VSS destruction efficiency, EVMV, in a CSTR can be calculated as a function of its SRT:

Exsn = 100

where the subscript O represents the influent concentrations. From Eq. 12.1:

Thus, it can be seen that the term XMA(, - XM v.„o is just the concentration of biodegradable VSS entering the digester. Consideration of Eq. 12.4 in the limit as the SRT becomes very large reveals that the first bracketed term on the right side rep-

resents the highest possible VSS destruction efficiency since it is just the fraction of the influent VSS that can be degraded biologically. It is an important determinant of whether a target destruction efficiency of 38% can be economically achieved.

Calculation of the actual VSS destruction efficiency in an operating digester should be based on the mass flow rates of VSS entering and leaving it. While some sources'" use the percent VSS content of the feed and effluent solids to make this calculation, such a procedure can give an inaccurate measure because of changes in the FSS as discussed above. Consequently, it should not be used.

The SOUR for a completely mixed aerobic digester can also be calculated as a function of the SRT:

SOUR = 1000

where i„ xm.v is the conversion factor from VSS units to oxygen units. The numerical value for ioxw.v depends on whether the ammonia-N released through digestion of biodegradable VSS is nitrified in the aerobic digester. If the biodegradable VSS can be assumed to have the same elemental composition as biomass, i.e., QH70;N, then the values of ioxvt.v can be determined theoretically. If the released ammonia-N is not oxidized to nitrate-N by autotrophic biomass, the value will be that given in Table 3.1, or 1.42 g 02/g VSS. If, on the other hand, the released ammonia-N is nitrified, the value of i<>xvi.v will be increased by 40%, to 1.98 g O^/g VSS. Table 12.1 summarizes these values as well as the values the conversion factors would have if the solids concentrations were expressed in COD or TSS units. If the biodegradable solids cannot be assumed to have the same elemental composition as biomass, the values of the conversion factor must be determined experimentally.

Although Eqs. 12.4 and 12.6 are for a CSTR, many aerobic digester studies are done in batch reactors. The VSS concentration in a batch reactor at any time t can be estimated by writing the mass balance equation on biodegradable VSS with Eq. 12.2 as the loss term, integrating it, and adding the initial nonbiodegradable VSS concentration to give the total VSS concentration:

XM.v = XMV.no + (XMvo ~ XMv.„o)exp(-bMv-t) (12.7)

This equation indicates that the VSS concentration will decline exponentially over time, but will approach a residual concentration equal to the nonbiodegradable solids.

The effects described in this section have been observed experimentally, as illustrated by the data of Reece, et al.'5 who performed aerobic digestion studies in batch reactors using solids produced in an activated sludge system. Since the wastewater was totally biodegradable, the waste solids consisted primarily of active biomass and biomass debris. The SRT of the activated sludge system in which the waste

Table 12.1 Oxygen Mass Equivalents for Biomass
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