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Total HRT, days

Figure 14.8 Effects of total clear water zone HRT and the number of equal size completely mixed cells in series on algal growth in lagoons. Theoretical curves from Eq. 14.14. (From L. G. Rich, "Modification of design approach to aerated lagoons," Journal of Environmental Engineering, 122, 149-153. 1996. Copyright © American Society of Civil Engineers; reprinted with permission.)

where X,.N and X,,,, are the concentrations of photosynthetic microorganisms (algae) in the effluent from the last completely mixed cell and the influent to the first completely mixed cell, respectively, N is the number of equal-sized completely mixed cells, p.,. is the growth rate of the photosynthetic microorganisms (taken to be 0.48 day '), and t is the total HRT of the clear water zones of the series of cells. This equation is applicable for values of individual cell HRTs (t/N) less than 1/p.,., or 2.08 days. For values of individual cell HRTs greater than this, algal growth will not be controlled and XPS/X,,(I cannot be predicted by this equation. Based on experience with CMAL systems followed by benthal stabilization basins, Rich'5 recommended that X|,N/X,,,, ratios be kept less than 25. Equation 14.14 is plotted in Figure 14.8 where it illustrates that the total HRT in the clear water zone can be increased significantly when multiple cells in series are used while still minimizing the growth of algae. The effect is not linear, since an increase from one to two cells produces a much larger increase in allowable total basin clear water zone HRT than does an increase from two to three cells, and so on. The following example illustrates the use of this equation to configure a benthal stabilization basin.

Example 14.3.3.3

Determine what configuration should be used for the benthal stabilization basin sized in Examples 14.3.3.1 and 14.3.3.2 to minimize algal growth.

a. What are the HRTs in the basin and what do they suggest about the required configuration of the system?

Since the flow to be treated is 2,400 mVday, the HRT in the clear water zone is 7,600 + 2,400 = 3.1 days and the HRT based on the total basin volume is 13,700 -T- 2,400 = 5.7 days. Since both of these exceed a value of 2 days, excessive algal growth is likely if the basin is configured as a single cell.

b. What is the minimum number of cells in series required to control algal growth?

We saw in Section 14.2.1 that the minimum SRT for algal growth was around 2 days. This means that the maximum SRT per cell is around 2 days. Consequently, considering only the clear water zone, the stabilization basin would have to be divided into at least two equal size cells. Immediately after cleaning, light could penetrate throughout the basin, so the entire volume would be available for algal growth. Since the total HRT of the basin is 5.7 days, it would have be divided into at least three equal sized cells to keep the HRT in any given cell below 2 days.

c. Based on Eq. 14.14, how many equal sized cells in series should be used to prevent excessive algae growth?

An HRT of 2 days in each cell does not guarantee that the effluent algal concentration will be acceptable. Equation 14.14 must be used to evaluate the effect of the number of cells on algal growth potential for the full range of operating conditions. Consequently, use it to calculate the Xrs/X|.,, ratio based on the clear water zone and total basin volume for 3, 4, 5 and 6 cells in series. For example, using the clear water zone volume, which gives a total HRT of 3.1 days, and 3 cells in series:

which is acceptable. The values of X,.N/X,.<, are calculated for HRTs of 3.1 and 5.7 days and summarized in Table E14.2. The results shown there indicate that effective control of algae (X,,N/X,,,, less than 25) can be achieved using three cells if the clear water zone HRT is restricted to 3.1 days. Algae control, on the other hand, is quite difficult if the full basin volume is used, i.e., such as immediately after cleaning. Consequently, use three cells along with the capability to adjust the lagoon depth to control the clear water zone HRT. The combination of these features should provide ample flexibility to control algal growth.

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