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'Adapted from Lawrence.'

'Adapted from Lawrence.'

10 20 30 40 50 Solids Retention Time (SRT), days

Figure 13.19 Effects of SRT and temperature on the anaerobic digestion of municipal primary solids. (From A. W. Lawrence, Application of process kinetics to design of anaerobic processes. In Anaerobic Biological Treatment Processes, ACS Advances in Chemistry Series 105:163-189, 1971. Copyright © American Chemical Society; reprinted with permission.)

10 20 30 40 50 Solids Retention Time (SRT), days

Figure 13.19 Effects of SRT and temperature on the anaerobic digestion of municipal primary solids. (From A. W. Lawrence, Application of process kinetics to design of anaerobic processes. In Anaerobic Biological Treatment Processes, ACS Advances in Chemistry Series 105:163-189, 1971. Copyright © American Chemical Society; reprinted with permission.)

increases to about 25 days when operating at a temperature of 20°C, and a higher residual VS concentration is observed. At 15°C an SRT of about 30 days is required to obtain stable operation, and only about one-half of the biodegradable volatile solids are destroyed at SRTs as long as 60 days. The curves showing the correspondence between VS destruction and methane production suggest that hydrolysis of the solids is generally the rate limiting step at these temperatures. Taken together, these data suggest that a temperature of about 25°C is the practical minimum for the anaerobic stabilization of municipal primary solids.

Temperature variations are also of concern, and it is typically recommended that systems be designed and operated to achieve variations of less than ± 1°C each day.14 717: 77 Some research indicates that anaerobic processes are capable of reacting successfully to temperature variations; although reaction rates decrease when the temperature is reduced, activity is restored quickly when the temperature returns to the optimum value. In contrast, experience with full-scale systems indicates that performance is adversely impacted by rapid temperature variations of as little as 2°C

to 3°C. This may be because of factors such as mixing and stratification within the bioreactor. Regardless of the mechanism, it appears prudent to adhere to recommended practice and to design and operate anaerobic processes to minimize short-term temperature variations.

Opinions vary concerning the benefits of operation under thermophilic conditions.* ^ ~75 Potential benefits include increased stabilization rates, resulting in smaller bioreactors; improved solids dewatering properties, which benefits downstream processing; and increased inactivation of pathogenic organisms, which increases the options for disposing of treated solids. Potential drawbacks include the increased energy required to achieve thermophilic operating temperatures and decreased process stability. Increased stabilization rates and increased methane production sufficient to meet the increased heating requirements have been demonstrated by some workers, but not by others. Likewise, improved solids dewatering properties have been observed by some, but not by others. However, increased pathogen inactivation is certain to be observed as a result of thermophilic operation. Decreased process stability because of increased VFA concentrations, increased sensitivity to temperature variations, increased ammonia toxicity, increased foaming, and increased odor potential are all areas of concern. Because of its uncertain benefits and numerous drawbacks, designs based on thermophilic operation should be approached with caution unless site-specific pilot test results and/or full-scale experience are available.

Like all biochemical operations, pH has a significant impact on the performance of anaerobic processes, with activity decreasing as the pH deviates from an optimum value. This effect is particularly significant for anaerobic processes because the meth-anogens are affected to a greater extent than the other microorganisms in the microbial community.:, _" J" ' >N I''"': As a consequence, there is a greater decrease in meth-anogenic activity as the pH deviates from their optimum value. A pH range of 6.8 to 7.4 generally provides optimum conditions for the methanogens, whereas a pH between 6.4 and 7.8 is considered necessary to maintain adequate activity. pH will also affect the activity of the acidogenic bacteria; however, the effect is less significant and primarily influences the nature of their products. A decrease in pH increases the production of higher molecular weight VFAs, particularly propionic and butyric acid, at the expense of acetic acid. As discussed in Section 2.3.2, one mechanism causing this is the buildup of H: in the system. As its utilization by methanogens is slowed, it begins to accumulate, which then slows down the production of acetic acid by the acidogens and shifts their metabolism toward other VFAs. The activity of the hydrolytic microorganisms is affected the least by pH deviations from neutrality.

The pH sensitivity of the methanogens, coupled with the fact that VFAs are intermediates in the stabilization of organic matter, can result in an unstable response by anaerobic systems to a decrease in pH.:,5K 7'7" The unstable response may be triggered by a high VOL that results in an increase in the production of VFAs by the acidogenic bacteria. If the increased VFA production rate exceeds the maximum capacity of the methanogens to use acetic acid and H:, excess VFAs will begin to accumulate, decreasing the pH. The decreased pH will reduce the activity of the methanogens, thereby decreasing their use of acetic acid and H,, causing a further accumulation of VFAs and a further decrease in the pH. If this situation is left uncorrected, the result is a precipitous decrease in the pH, the accumulation of higher molecular weight VFAs, and a near cessation of methanogenic activity. This condition is known as a "sour" or "stuck" anaerobic process. It can be corrected in its early stages by resolving the environmental factors causing the imbalance between the acidogenic bacteria and the methanogens. In the case considered above, this could be accomplished by reducing the VOL to the point where the VFA production rate is less than their maximum consumption rate. This will allow consumption of the excess VFAs in the system, thereby causing the pH to return to neutrality and the activity of the methanogens to increase. The VOL can then be increased as the process recovers until the full loading capability is utilized. In extreme cases, decreases in loading must be coupled with the addition of chemicals for pH adjustment, as discussed below.

For an anaerobic process functioning within the acceptable pH range, the pH is controlled primarily by the bicarbonate buffering system. Bicarbonate alkalinity is produced by the destruction of nitrogen-containing organic matter and the reaction of the released ammonia-N with the carbon dioxide produced in the reaction. This is illustrated by Eq. 13.6 for the conversion of primary solids (represented as C,(,H|yO,N) to methane, carbon dioxide, biomass, and ammonium bicarbonate"*:

As illustrated, bicarbonate alkalinity is produced in direct relation to the ammonia-N released. A strong base is needed to react with the carbon dioxide produced in the system to form the bicarbonate. In most instances, ammonia is the strong base, although the cations associated with soaps or the salts of organic acids can also serve to maintain electroneutrality in the reaction with carbon dioxide.

The concentration of bicarbonate alkalinity in solution is related to the carbon dioxide content of the gas space in the bioreactor and the bioreactor pH:

where S|iA,k is the bicarbonate alkalinity expressed as mg/L as CaCO, and jv„. is the partial pressure of carbon dioxide in the gas space expressed in atmospheres."1 Jv5'1 This relationship is presented in Figure 13.20 and illustrates that typical anaerobic processes operate with bicarbonate alkalinities in the range of 1,000 to 5,000 mg/L as CaCO, and carbon dioxide partial pressures of 25 to 45%.

When VFAs begin to accumulate in an anaerobic process, they are neutralized by the bicarbonate alkalinity present. Consider for example, acetic acid. Acetic acid is released by the acidogenic bacteria in nonionized form, but exists as acetate ion at neutral pH. The reaction of acetic acid with bicarbonate alkalinity to convert it to acetate is:

C,„H,X>,N 4- 4.69 H;0 -» 5.74 CH., + 2.45 CO: + 0.2 QH70:N + 0.8 NHjHCOi

where HAc represents nonionized acetic acid and Ac represents acetate ion. When a pH end point of 4.0 is used in the alkalinity analysis, acctate will be partially converted to acetic acid and will, therefore, register as alkalinity. Thus, if VFAs are present, the total alkalinity will represent the concentration of both bicarbonate ion and VFAs. If the concentration of VFAs is known and is expressed as acetic acid, the bicarbonate alkalinity can be calculated from the total alkalinity as:

where STAlk is the total alkalinity expressed as CaCO, and SV|A is the concentration of VFAs expressed as acetic acid. The factor 0.71 converts the VFA concentration expressed as acetic acid to CaCO, and corrects for the fact that approximately 85% of the VFA anions are titrated to the acid form at a pH of ^O.48 ^ Other organic and inorganic bases, such as sulfides, can also be titrated to their acid form and, consequently, measured as alkalinity. The concentrations of these anions are typically small relative to the bicarbonate concentration, but the potential for such interferences with bicarbonate alkalinity measurement should be recognized.

As discussed above, under stable operating conditions, bicarbonate is the primary form of alkalinity in anaerobic processes. However, under unstable operating conditions VFAs will react with bicarbonate alkalinity, both reducing its concentration and producing carbon dioxide (see Eq. 13.8), which increases the carbon dioxide content of the gas space. Reference to Figure 13.20 illustrates that both of these changes act to decrease the pH in the bioreactor. Stable operation of anaerobic processes is generally achieved by the maintenance of a relatively high concentration of bicarbonate alkalinity so that increased VFA production can be tolerated with a minimal decrease in bioreactor pH.

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