Batchwise and continuous settling tests with raking action were made of several homogeneous flocculated slurries listed in Table 1, with experimental apparatus as shown in Fig.1 Performance results of continuous thickener are discussed in terms of the batchwise settling characteristics. Conclusions obtained may be summarized as follows
1) Continuous thickener with raking action should be designed on the information obtained from batchwise settling test made under the similarly stirred condition. Settling characteristics are markedly influenced by raking action.
2) Settling of such slurries with raking action is not the consolidation of their floc structures by compression as has been believed, but the mere settling due to their concentrations for the considerably wide range of concentration.
3) Relation between settling rate R and slurry concentration C may be computed from a few settling curves by the graphical method proposed by Kynch
2) and the authors
4) R-C relation under stirring condition may be represented with equation (3) for the range of high concentration.
4) Method of determining the settling area is discussed from two standpoints. One is based on the batchwise thickening operation as shown in Fig.6, and the other on the mass settling ve-, locity G [=C (R+Q
u/A)] of sludge layers of various concentrations. Thickening areas required from both the standpoints are proved to coincide with each other, provided that the settling curve and R-C relation of that slurry could be expressed by eq. (1) and (3) respectively.
5) Figure 7 shows the mass settling velocity G at certain Q
u/A as a function of slurry concentration. This curve shows the minimum value at C=C
mn which is the concentration of the capacity controlling layer in the lower part of the thickener. In this figure, C
fQ
f/A corresponds to the normal state of solid loading and in this case the underflow concentration to be obtained is C
u. If the solid loading per unit thickening area increases to C
fQ
f'/A, it becomes overloading and the part of the feed solid is carried out to overflow, but the underflow concentration remains constant at C
u. When the feed rate decreases to Q
f", it becomes underloading and the concentrated layer near the bottom of the thickener disappears and the underflow concentration will be diluted to C
u".
6) Sectional area of continuous thickener required to obtain the underflow concentration C
u when the feed rate is Q
f, may be determined graphically as shown in Fig.10. Product of C & R is plotted against C and the tangent PQ to CR-curve is drawn from the point S(C=C
u) on the C axis, which crosses the vertical axis at point Q. Ordinate of point Q represents the value of C
fQ
f/A, from which A may be computed.
7) It may be concluded from our experiments and considerations that the depth of thickened sludge or detention time has not the substantial effects on underflow concentration and thickening capacity, which contradicts the now prevailing conception of the effect of thickener height.
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