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z� <br />be such that nll particles u•ith the design celocity V� �cill settle to the <br />bottom oE the tsnk. The design celocitp, detection time, and basin <br />depth are related as follo�cs: <br />depih ($•17) <br />v` detention time <br />In nctual prnctice, design factors have to be included to allow for <br />the eHects of inlet and outlet turbulence, ehort circuiting, sludge stor- <br />age, end velocity gradients due to the operntion of sludge-removal <br />equipment.. These factors n•ill be discussed in Chap. 11. The discussion <br />in this chapter refers to ideal settliug in �chich the tactors are omitted. <br />Type-1 �ettling in an ideal settling bnsin is sho�cn in F�g. S•5. Par- <br />ticles that have a velocity of fell less than V� n•ill not all be removzd <br />during the time provided for settling. Assuming that the particles of <br />various sizes are uniformly distributed over the eutire depth of the <br />basin at the inlet, it can be seen from an analysis of the particle trajec- <br />tory in Fig. 3•5 that particles �cith a settling velocit,y less than i�� �eill <br />be removed in the ratio <br />g, a V` (3•13) <br />where X, is the fraction of the particles with settling celocity V, that <br />are remoced. <br />In a typicul su5pension of particulate matter, a lurge gradation <br />of particle sises occurs. To determine the eSiciency of removal for a <br />given settling time, it is necessary to consider the entire runge of <br />settling velocities present in the system. This can be accomplished in <br />t�co �r•ays: (1) by use of siet•e anal}•ses and h}•drometer tests combined <br />ticith Eq. 8•12 or (2) by use ot a settling column. Nith either method, <br />a settling-velocity analysis curce can be consUucted from the data. <br />Such a curve is shon•n in �g. 8•6. <br />F16. 8•5 Type•1 settling in en <br />We�l settling basfn. <br />T <br />D <br />1 <br />�Disuete senling�---I <br />G � <br />;x <br />C; <br />$B <br />'o� <br />og <br />� � <br />LL - <br />v� v, <br />Senliny v�: <br />For s given clarific <br />Q=V,A <br />onlp those particles ���iti <br />removed. Th� remainin <br />The total fraction of pa <br />Frnction removed <br />�chere 1 = X, = frnctic <br />�X� v'dx = fractic <br />The follon•ing example <br />EXAMPLE i4 � <br />A particle size <br />of sand partic <br />celocity has b <br />SettlinQ valoeity <br />teet per minu• <br />Weight fraction <br />1Vhat ia the <br />gpd/sqft7 <br />