e. What is the power requirement for mixing?

The volumetric power input for mixing can be calculated with Eq. 12.10. At 10°C, (i^ = 1 310 cp. Consequently,

II = (0.935)( 1.31"')(20,000" :"s) = 19.4 kW/1000 m'

Since the volume is 8,400 m\ the power requirement for mixing is (19.4)(8.4) = 163 kW. Since this exceeds the power required for oxygen transfer, the larger power for mixing must be provided.

f. Will pH control be required to maintain a neutral pH?

The destruction rate of VSS in the digester is (500)(6,000)(0.40) = 1,200,000 g/day. Since 0.44 g of alkalinity is destroyed for each gram of VSS destroyed when nitrification occurs, the mass rate of alkalinity destruction will be 528,000 g/day. The waste solids contain 150 mg/L of alkalinity, but 50 mg; L must be retained as a residual. Therefore the mass of alkalinity available per day is (500)(100) = 50,000 g/day. This is inadequate, so pH control must be used.

An alternative design approach involves the use of a batch reactor to characterize the solids to be digested, and it is commonly used (e.g., see Refs. 35 and 36). A sample of raw solids is placed in a well mixed vessel, aerobic conditions are maintained, and pH is controlled at 7. Data are then collected and used in Eq. 12.7 to determine the decay coefficient bMV and the nonbiodegradable VSS concentration, Xyt.v.no- To obtain the most accurate assessment of bMV, data should be collected on the oxygen uptake rate (OUR) and the TSS and VSS concentrations over time. Care

= 2,376.000 g/day = 99 kg/hr should be exercised to add distilled water to replace any evaporation losses and to scrape any solids that accumulate on the inside walls of the reactor back into the liquid to avoid changes in suspended solids concentrations not attributable to biological reaction. The batch reactor should be operated long enough to ensure that the majority of the biodegradable organic matter is destroyed, thereby allowing an accurate estimate of the nonbiodegradable VSS concentration to be made. This requires that the batch digestion time be greater than five times the reciprocal of the decay coefficient, bMV- To determine bMV the OUR data are analyzed in the same manner as described in Section 8.3.2 for the determination of bM. Once bMV is known, it can be substituted into Eq. 12.7 and the VSS data can be analyzed according to that equation for estimation of the nonbiodegradable VSS concentration. If OUR data cannot be collected, then bMV and XM v „,, can be determined simultaneously by fitting Eq. 12.7 to data on the VSS concentration over time. It should be recognized, however that an inaccurate estimate of the nonbiodegradable VSS concentration will result in an inaccurate estimate of bMV.

While measured parameter values are frequently used for sizing aerobic digesters, care must be exercised for the following reasons:

• Significant variation in the measured decay coefficient and the nonbiodegradable fraction of the waste solids may occur with time. Consequently, it is recommended that several batch digestion tests be conducted over time. Then a statistical approach can be used to select the design values for the decay coefficient and the nonbiodegradable proportion of the waste solids.'

• Conditions in the batch tests can differ significantly from those anticipated for the full-scale digester. Factors that may differ include pH, temperature, and suspended solids concentrations that can lead to oxygen transfer limitations, as discussed previously. While the data from lab-scale batch reactors can be used successfully to predict the performance of full-scale continuous How bioreactors, the batch results may significantly underestimate full-scale performance." This can be due to acclimation and/or to the maintenance of more favorable conditions in the full-scale bioreactor.

• If the waste solids to be studied come from a nonnitrifying activated sludge system, they may give erroneous results when the OUR technique is used to determine the decay coefficient. When the waste solids come from a fully nitrifying activated sludge system, ammonia-N will be nitrified as it is released during the batch digestion test.44 Consequently, the oxygen demand per unit of VSS destroyed will remain reasonably constant during the test and the decrease in OUR will be proportional to the destruction of biodegradable organic matter as assumed. On the other hand, if the waste solids are from a nonnitrifying system, the released ammonia-N will not be nitrified until a sufficient population of nitrifiers has developed/*111 In this situation, the oxygen demand per unit of VSS destroyed will not be constant and the change in OUR will not be proportional to the destruction of biodegradable organic matter. Ammonia-N concentrations should be monitored during the batch digestion test to detect whether consistent nitrification is occurring. If it is not, then either nitrification should be inhibited during the OUR measurements or nitrifiers should be added so that complete nitrification of ammonia-N occurs as it is released.

After the values of bMV and XM v ,1(, have been determined, the SRT required to achieve either a desired percent VSS destruction or a desired SOUR can be calculated. The SRT required for a given percent VSS destruction in a single-stage digester can be obtained with a rearranged form of Eq. 12.4:

Similarly, the SRT required to achieve a given SOUR in a single-stage digester can be obtained with a rearranged form of Eq. 12.6:

Q _ 1000 • io xm.V(XM vo — XM y „o)__XM vn 2 20)

Consideration must be given to the SRT required to meet each criterion when deciding on the design SRT. If both are reasonable, then the larger of the two should be used. On the other hand, if one is inordinately high, then the SRT should be selected from consideration of both criteria.

Once the required SRT has been determined, the remainder of the design proceeds in exactly the same manner as described in Section 12.3.2.

Batch tests with a waste activated sludge have revealed that the decay coefficient for its aerobic decomposition has a value of 0.216 day ' at 20°C. The solids to be digested are wasted from the bottom of the final settler at a concentration of 12,000 mg/L as VSS. Solids at that concentration were used to run the batch tests, revealing that the nonbiodegradable VSS concentration was 5,400 mg/L. If the lowest temperature expected in the digester is 12°C, what SRT would be required to achieve at least 38% VSS destruction and reduce the SOUR to 1.0 mg O^ig VSS • hr) or less? Is it realistic to meet both criteria? Assume that the temperature coefficient for bM% has a value of 1.029.

a. What is the value of the decay coefficient at 12°C?

The decay coefficient can be corrected for temperature with Eq. 3.95:

b. What SRT is required to meet the percent VSS destruction criterion at

This may be determined with Eq. 12.19:

c. What SRT is required to meet the SOUR criterion at 12°C?

This may be determined with Eq. 12.20. Note that SOUR and bMS must have consistent time units. To be conservative, assume that nitrification occurs, making the value of i,,XMA equal to 1.98.

(J. Is il realistic to meet both criteria?

An SRT of 88 days is very long and may not be realistic unless the waste activated sludge How is very small. The reason that it is easy to meet the VSS destruction criterion but not the SOUR criterion is that the waste activated sludge has a fairly high percentage of biodegradable solids. A reasonable compromise would be to choose an SRT between the two values.

If an activated sludge system is being designed with the simple model of Chapter 5 or with IAWQ ASM No. 1, the output may be used directly in the design of an aerobic digester by simulation. In either case, the characteristics of the waste solids are first determined from the simulations used in the activated sludge design. For the simple model of Chapter 5 the components will include active biomass, biomass debris, and inert organic matter. Only the active biomass will be degraded in the digester, leading to additional debris. The concentrations of active biomass and debris in a completely mixed digester can be calculated with Eqs. 5.64 and 5.65, respectively, while the MLSS concentration can be calculated with Eq. 5.66. The values of XHH1, and X,,,, in those equations are the concentrations in the waste activated sludge entering the digester. If the waste solids also contain inert organic matter (IOM) that originated in the influent to the activated sludge system, its concentration should be added within the bracket of Eq. 5.66 to reflect its presence in the digester. The concentrations of the various components as given by Eqs. 5.64-5.66 are in COD units. They can be converted to TSS or VSS units by using appropriate conversion coefficient, i.

When using ASM No. 1, the output from the activated sludge simulation will provide the concentrations of active biomass, biomass debris, slowly biodegradable substrate, and IOM in the waste activated sludge. These components can be used directly as inputs into a model for the aerobic digester. This is particularly useful when A/AD is being considered since ASM No. 1 can handle both nitrification and denitrification. Simulations conducted with different bioreactor configurations and different recirculation ratios will allow the designer to select a system capable of optimal performance.

Even if the activated sludge system has been designed by simulation, the aerobic digester can be designed with the simple first order model presented earlier in this chapter. In that case the total VSS concentration entering the digester would be calculated with Eq. 12.3, whereas the influent nonbiodegradable VSS concentration would be calculated as:

X\| Y.„o = f|)'Xn |{A() + XI)A(, + X| Y(> (12.21)

The biodegradable solids can then be calculated with Eq. 12.5. Once those terms are known, everything can proceed exactly as presented in Sections 12.3.1-12.3.3.

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