0.20b 0.14 0.10 0.07 0.055 0.049 0.044 0.037 0.033 0.031 0.029 0.025 0.022 0.018 0.008

0.41 = (0.37 + 0.48 + 0.38)/3 0.20 = (0.24 + 0.16)/2

It is not necessary to interpolate the x corresponding to vp = 0.16 m/min. From the table, x = xo = 0.52.


R = 1-0.52 + -1- [0.14(0.07) + 0.1(0.04) + 0.07 (0.1) + 0.055(0.02) 0.16

+ 0.049 (0.01) + 0.044 (0.02) + 0.037(0.03) + 0.033(0.02) + 0.031 (0.01) + 0.029 (0.01) + 0.025 (0.02) + 0.022 (0.05) + 0.018(0.028) + 0.008(0.092)]

5.2.5 Primary Settling and Water-Treatment Sedimentation Basins

The primary sedimentation tank used in the treatment of sewage and the sedimentation basin used in the treatment of raw water for drinking purposes are two of the units in the physical treatment of water and wastewater that use the concept of flocculent settling. These units are either of circular or rectangular (flow-through) design; however, they differ in one important respect: the amount of scum produced. Whereas in water treatment there is practically no scum, in wastewater treatment, a large amount of scum is produced and an elaborate scum-skimming device is used.

The primary sedimentation basins used in wastewater treatment and water treatment also differ in another respect: the length of detention time. Although longer detention times tend to effect more solids removal in water treatment, in primary sedimentation, a longer detention time can cause severe septic conditions. Septicity, because of formation of gases, makes solids rise resulting in inefficiency of the basin. Thus, in practice, there is a practical range of values of 1.5 to 2.5 h based on the average flow for primary settling detention times in wastewater treatment. This range of figures, although stated in terms of the average, really means that it takes an average of 1.5 to 2.5 h for a particle of sewage to become septic whether or not the flow is average.

Both water and wastewater treatment also need to maintain the flow-through velocity so as not to scour the sludge that has already deposited at the bottom of the settling tank. They also need properly designed overflow weirs, an example of which is shown in Figure 5.7b. The particles in both these units are flocculent, so the flow-through velocity should be maintained at no greater than 9.0 m/h and the overflow weir loading rate at no greater than 6-8 m /h per meter of weir length, as mentioned before. Some design criteria for primary sedimentation tanks are shown in Table 5.2. Except for the detention time, the criteria values may also be used for settling tanks in water treatment.

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