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Solids Retention Time (SRT), days

Figure 13.24 Fate of various components of municipal primary solids during anaerobic digestion at 35°C. (From A. W. Lawrence, Application of process kinetics to design of anaerobic processes. In Anacrotic Biological Treatment Processes, ACS Advances in Chemistry Series 105:163-189, 1971. Copyright © American Chemical Society; reprinted with permission.)

Solids Retention Time (SRT), days

Figure 13.24 Fate of various components of municipal primary solids during anaerobic digestion at 35°C. (From A. W. Lawrence, Application of process kinetics to design of anaerobic processes. In Anacrotic Biological Treatment Processes, ACS Advances in Chemistry Series 105:163-189, 1971. Copyright © American Chemical Society; reprinted with permission.)

information about the degradation of the various components of those solids and shows that the overall performance is limited by the degradation of lipids. This is consistent with Figure 9.5, which shows that anaerobic oxidation of long and short chain VFAs requires an SRT of about 10 days. The data presented in Figures 13.19 and 13.24 are also consistent with Figure 13.23, where it is observed that an SRT of 10 days results in reasonably complete stabilization of primary solids. Thus, an SRT of at least 10 days is needed to stabilize primary solids at 35°C. However, the hydrolysis of the biomass in waste activated sludge occurs at a slower rate than the hydrolysis of primary solids. As a consequence, a longer SRT is required if waste activated sludge is to be stabilized. Using Figure 13.23 as a guide, an SRT on the order of 15 to 20 days is required to achieve substantial stabilization of waste activated sludge. These conclusions are consistent with observations at full-scale plants.7275

Pathogen control is a relatively new requirement for anaerobic digesters. It has been known for some time that digestion reduces the concentration of indicator organisms. In fact, that is one reason anaerobic digestion has been used. However, the purposeful design of digesters to achieve a specific degree of pathogen control is new, and few data exist upon which to base such a design. Currently, U.S. regulations require a minimum SRT of 15 days for anaerobic digesters operating at 35°C to ensure effective reduction of pathogens in municipal wastewater solids.7" Operation of anaerobic digesters in series also increases pathogen destruction, just as it does in aerobic digesters. Continued evolution of procedures to design anaerobic digesters to control pathogens is expected.

Once the SRT, i.e., HRT, has been selected, the effective volume of the digester is calculated by multiplying the design solids flow rate by the SRT. The design solids flow rate should be for the month or week in which the highest volume of solids is produced to allow the digester to function properly under all reasonable operating conditions. Variations in both the mass of solids produced and the performance of upstream solids thickening devices should be considered in choosing that flow rate. The total volume is then calculated considering the relationship between the effective and total volumes. The effective volume is less than the total volume because of ineffective mixing, leading to the accumulation of grit in the bottom and scum at the top of the digester. In digesters with older style mixing systems, the effective volume can be less than 50% of the total volume, but in digesters with modern mixing systems the effective volume is generally at least 90% of the total volume. Volume should also be allocated to grit and scum accumulations. Typical designs allocate the volume of the floor cone (see Figures 13.2 and 13.3) to grit accumulation and the top 0.6 m of the digester to scum accumulation.5"72 5 Minimal grit and scum accumulations occur in egg shaped digesters (Figure 13.4), so the total and effective volumes can generally be assumed to be the same.

Once the total bioreactor volume has been determined, the number of individual units and their dimensions must be selected. Provisions must be made for units to be removed from service for maintenance, so a minimum of two units should be provided. The impacts on performance of having a unit out of service must also be considered, and this may dictate the number of units provided and/or the total volume. In doing so, it may be assumed that the unit will be removed from service during average, rather than peak, solids production. The gas production rate is estimated based on the mass of volatile solids stabilized and the conversion factor of

0.7 standard m' of methane produced/kg of VS destroyed, as presented in Section 13.1.1. The process design for an anaerobic digester to stabilize the waste solids produced at a municipal wastewater treatment plant is illustrated in the following example.

Example 13.3.1.1

An anaerobic digestion system is to be designed to stabilize the solids produced by a municipal wastewater treatment plant. It must be capable of destroying pathogens, implying that the SRT must be at least 15 days at 35°C. The estimated masses of primary solids and waste activated sludge to be produced daily under various conditions are given in Table E13.1. After blending and thickening, the solids concentration entering the digestion system is expected to average 60 g'L (kg/m ) and to range from 50 to 70 g/L. The volatile solids concentration is 75'v of the total solids concentration. Design the system with multiple digesters, but assume that one will be taken out of service for cleaning only under average loading conditions.

a. What solids How rates must be processed by the system?

The mass flow rates of dry solids under various conditions are given in Table El3.1. These may be converted to volumetric flow rates by assuming solids concentrations. It is likely that the thickener can maintain the average solids concentration under average and maximum month conditions, but that performance will deteriorate during the maximum solids production week. Consequently, the average solids concentration is used to calculate the average and maximum month volumetric flow rates but the minimum solids concentration is used to calculate the maximum week volumetric flow rate. The results are summarized in Table El3.2.

b. What SRT should be used in the design?

Because an excellent degree of solids stabilization is desired under average loading conditions, an SRT of 20 days is appropriate, based on Figure 13.23 This value should be attained even during the maximum solids production month, but it is unrealistic to maintain it during the maximum week. However, to ensure pathogen destruction under all conditions, an SRT of at least 15 days must be maintained even during the maximum week.

c. What effective lotal digester volume must be provided?

The required effective digester volume must be calculated in two steps. First the volume required for each flow rate must be calculated based on the assumption that all units are in service. Since the SRT is the same as the

Table E13.1 Solids Production Rates for Design of the Anaerobic Digester in Example 13.3.1.1

Type of solids

Mass of dry solids, kg/day

Average

Maximum month

Maximum week

WAS Total

Primary

22,500 20,000 42,500

27,000 24,000 51,000

Table E13.2 Anticipated Volumetric Solids Flow Rates Under Various Conditions for Example 13.3.1.1
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