vantageous to have a way to approximate the peak oxygen requirement for a single tank system like CMAS.

During a short-term transient loading, biodegradable organic matter will be oxidized to synthesize new cell mass, but little additional decay will occur because the decay rate is proportional to the active biomass concentration, which changes little during the transient. Consequently, the additional oxygen requirement associated with the short-term increase in loading will be proportional to (1 — YM |i()Xn.i), which is equivalent to (1 — Y,,). Thus, the fractional transient increase in oxygen requirement will be less than the fractional increase in the biodegradable organic matter loading.

Simulations conducted using wastewater characteristics and parameters similar to those in Tables 6.6 and 6.3, respectively, demonstrated that transient peak oxygen requirements correspond to oxidation of all of the additional readily biodegradable organic matter applied, but only a portion of the additional slowly biodegradable organic matter applied." This was true for a broad range of SRTs and load peaking factors. The load peaking factor is the peak mass loading divided by the average mass loading. Furthermore, the fraction of the additional slowly biodegradable organic matter oxidized, fXVH, decreased as the load peaking factor increased. Based on these considerations, the transient state oxygen requirement for the growth of heterotrophic bacteria (ROM ,s) may be estimated as follows:

ROm is = (1 - Ylui„Nl! l)[A(F-Ssl,) + fXs..iA(F-Xs„)] (10.12)

where A(F-Ssil) is the transient increase in the loading of readily biodegradable organic matter above the average loading, and A(F-XS(1) is the transient increase in the loading of slowly biodegradable organic matter above the average. The value of fxs ii will generally range from 0.5 to 1.0, with smaller values being associated with larger transient loading increases."

The peak oxygen requirement due to heterotrophic activity is the sum of the steady-state oxygen requirement, as given by Eq. 9.13, and the transient-state oxygen requirement as given by Eq. 10.12. The oxygen transfer rate to the system must be capable of meeting both requirements, in addition to any oxygen utilization by the autotrophic bacteria.

The transient increase in the oxygen requirement due to nitrification is more complicated for a number of reasons. The first is that SRT has a much stronger effect than it does on the heterotrophic oxygen requirement." At SRTs that are above the minimum SRT for the autotrophic bacteria, but below that required for full nitrification at steady-state, transient loadings will have no effect on the rate of nitrification, and hence on the oxygen consumption associated with it, because nitrification will already be occurring close to its maximum rate. At long SRTs where full nitrification can occur even during the transient, the increase in oxygen consumption rate will be proportional to the increase in loading, just as it is for heterotrophic bacteria, although the proportion oxidized will be different. At SRTs that are just sufficient to give full nitrification at steady-state, the ammonia nitrogen concentration may rise sufficiently during the transient to allow the rate of nitrification to reach its maximum value, thus causing the oxygen consumption rate to rise, but by a smaller amount than the loading increase. A second complicating factor is that not all nitrogen in the influent is in a form that is available to the autotrophic bacteria. Some will be in the form of biodegradable organic nitrogen. This nitrogen will become available only as the organic matter containing it undergoes decomposition. Based on the arguments in the preceding paragraph, we would expect all of the nitrogen associated with the readily biodegradable substrate to be made available as ammonia-N, but only the fraction fxs,, of that associated with the slowly biodegradable substrate. Finally, some of the ammonia-N entering during the transient will be incorporated into the extra biomass formed during the transient as the additional organic matter is removed. This, too, must be accounted for.

For the situation in which the SRT is sufficiently long to allow full nitrification during the peak loading period, the transient state oxygen requirement associated with the autotrophic bacteria, ROA TS can be calculated with an equation analogous to Eq. 10.12:

in which A(F Sn)j1S is the transient increase in ammonia-N available to the autotrophic bacteria. It is given by:

A(F-Sa„s = A(F-Sn1Ic) + A(F-Sns„) + fXSMA(F-XNSO)

- 0.087Y„.Tiox„T[A(F-Ssl,) + fx.s.iiA(F- Xso)] (10.14)

where A(F-SMu>), A(F'Snsu), and A(F-XNS(1) are the transient increases in the loadings of ammonia-N, soluble biodegradable organic nitrogen, and particulate biodegradable organic nitrogen above the average. The negative term in Eq. 10.14 accounts for the additional use of nitrogen associated with synthesis of the heterotrophic bacteria during the transient organic loading. The result from Eq. 10.14 must be added to the steady-state autotrophic oxygen requirement to determine the peak autotrophic requirement for this situation.

For the situation in which nitrification is not complete during the transient, causing the ammonia-N concentration to rise high enough to allow the autotrophic bacteria to grow at their maximal rate, the maximum autotrophic oxygen utilization rate, ROAn„,x, can be calculated from a modified form of Eq. 5.32:

YA Tio Xll'l

+ (1 - f„)bA (X„.A.T-V)i„XB.T I „ g I (10.15)

The mass of autotrophic bacteria in the system, Xl!AT-V, should be that associated with the average loading on the system. The last term is included because of the sensitivity of the autotrophic nitrifying bacteria to the dissolved oxygen concentration and the likelihood of that concentration falling during the transient. Consequently, the DO concentration used should be the concentration expected during the transient. In situations where this condition occurs, the peak autotrophic oxygen requirement will just be RO,Vm,,„ because that value cannot be exceeded. Consequently, the determination of which situation controls is made by comparing the two potential peak requirements. The smaller of the two controls.

There will be circumstances, particularly during preliminary design, where insufficient information is available to allow the procedures above to be used. In that case it may be satisfactory to multiply the heterotrophic and autotrophic steady-state oxygen requirements by an oxygen peaking factor to arrive at the transient state oxygen requirements. Figure 10.18 provides oxygen peaking factors as a function of the load peaking factor. It was generated from simulations conducted using wastewater characteristics and parameters similar to those used in Tables 6.6 and 6.3, respectively. Because the oxygen peaking factor for the removal of organic matter was not influenced strongly by SRT, the curve for carbon oxidation should be safe for a broad range of SRTs. The curve for nitrification, on the other hand, is only valid for SRTs above 10 days.

A final point about transient loadings concerns their impact on mixing energy input and floe shear. It will be recalled from Section 10.2.5 that there is both a lower and an upper limit on the volumetric power input to an activated sludge bioreactor, with the lower limit being the minimum energy required to keep the MLSS in suspension, and the upper limit being set to prevent floe shear. The ratio of the upper to the lower limit is around 4.5. It is not unusual, however, for the ratio of the maximum loading to the minimum loading within a day at a wastewater treatment plant to be greater than 4.5, particularly for small plants.: This suggests that the ratio of the maximum to minimum oxygen requirements associated with diurnal loadings can be greater than 4.5. In that situation, since the volumetric power input required for oxygen transfer is directly proportional to the oxygen requirement, it would be impossible to meet both the upper and lower limits on power input. Consequently, most designers use the upper limit and the peak oxygen requirement during sizing of the bioreactor and then limit the turn-down on the aeration system to meet the lower limit during low loading, recognizing that the DO concentration in the bioreactor will be higher than needed then. The other alternative is to include flow equalization in the process flow diagram.

Load Peaking Factor

Figure 10.18 Effect of the load peaking factor on the oxygen peaking factor for a CMAS system receiving a diurnally varying input. Data from Amalan.

Load Peaking Factor

Figure 10.18 Effect of the load peaking factor on the oxygen peaking factor for a CMAS system receiving a diurnally varying input. Data from Amalan.

Distribution of Volume, Mixed Liquor Suspended Solids, and Oxygen in Nonuniform Systems. As indicated by guiding principle 4 in Table 9.1, the total mass of biomass and the total oxygen requirement in the various alternative activated sludge systems will all be essentially the same, provided they all have the same SRT. Thus, they can be calculated by the simple model of Chapter 5 as modified in Section 9.4.2. However, when the design involves the distribution of flows or volumes into reactors in series, the biomass and oxygen requirement must also be distributed appropriately. This can be done for the steady-state case by the application of mass balances and appropriate heuristics. Distribution of the transient-state oxygen requirement is more difficult. Because the procedures involved are unique to each activated sludge variation, they will be considered individually in the sections that follow.

10.3.3 Design of a Completely Mixed Activated Sludge System—The General Case

Because it is the simplest, the basic design process will be outlined for a CMAS system. All examples will be developed for the wastewater characteristics given in Table E8.4 and the kinetic and stoichiometric parameters given in Table E8.5. Those values are for a temperature of 20°C. As indicated earlier, all organic substrate concentrations will be expressed as biodegradable COD and all MLSS concentrations will be expressed as TSS. Thus, the parameters on the right side of Table E8.5 apply. Two situations will be considered. First, to illustrate basic principles, the case of full equalization, i.e., the steady-state case, will be considered. Then we will consider the impacts of diurnal variations in loading, i.e., the case without equalization.

Basic Process Design for the Steady-State Case. The first task in a process design is to establish the maximum and minimum sustained temperatures likely to be encountered in the activated sludge system. The stoichiometric and kinetic parameters are then adjusted to those temperatures using Eq. 3.95, as discussed in Section 10.3.2. The temperature adjusted parameters are used in selection of the design SRT. Because we have already discussed the selection of the SRT, it will not be considered further here. Rather, we will assume that the decision has already been made.


A CMAS system is to be designed to remove organic matter from a wastewater with the characteristics given in Table E8.4. Removal of ammonia-N is not required, so the system does not have to nitrify. Consequently, an SRT of three days has been chosen for the design. The average design wastewater How rate is 40,000 m'/day and full equalization will maintain the loading at the average value throughout the day The oxygen transfer system will be sized to maintain the DO concentration above 1.5 mg/L under all conditions. The parameter values characterizing the wastewater at 20°C are given in Table E8.5. However, the lowest sustained temperature anticipated is 15CC and the highest is 25°C. Prepare a table of temperature adjusted parameter values by using the temperature coefficients in Table El0.1.

All temperature adjustments are made with Eq. 3.95. in which k: is the value of the parameter at reference temperature T-. Using bH as an example.

bM,n = (0.18 day ') 1.04"" = 0.15 day 1 b„.,s = (0.18 day ') 1.04' " '"' = 0.22 day ' The values of the other parameters are given in Table El0.2.

The next step in the process design is to calculate the oxygen requirement for the system. As mentioned in Section 10.3.2, that should be done for the highest expected sustained temperature because that is when the highest oxygen requirement will occur. The information has two uses. First, it provides the base requirement for design of the oxygen transfer system. That aspect of design will not be covered in this book, so the reader is referred to other sources for it.4'1 17 Second, the maximum oxygen requirement will be used with the upper limit on the volumetric power input, 11,, to select the lower feasible reactor volume based on floe shear, V, , s, as given by Eq. 10.4. The maximum oxygen requirement will also be used in Eq 10.5 to calculate the lower limit on bioreactor volume based on oxygen transfer, V, ,,,-. The minimum oxygen requirement, which will occur at the lowest, sustained operating temperature, must also be calculated. It will be used with the lower limit on the volumetric power input, II,., to select the upper feasible bioreactor volume, Vt, as given by Eq. 10.3. Those volume limits will then be used to make the final selection of the bioreactor volume and the associated MLSS concentration.

The oxygen requirement for removal of organic matter can be calculated with Eq. 9.13:

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