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CR1T RE>fOV.4i.
<br />the tank inlet. From this relationship it
<br />also can Ue sho�cn that the suUsidence
<br />velocity of the smallest particle to be re-
<br />moved must equal the surfnce settling rate
<br />ro oUtain a theoretical 100 percent remo��al
<br />oE particles equal or greater in size. For
<br />mnservative calculntions of particle re-
<br />moval, it may Ue assmned that there is no
<br />sedimentation in tUe influent and eRluent
<br />zones oF mrbulence.
<br />The previous information sim��ly points
<br />out the fact that in the aUsence of turbu-
<br />lence, detention nnd hydraulic suUsidence
<br />oE grit pnrticles are dependent on the sur-
<br />fnce nrea oE the grit chamUer.
<br />Turbulence
<br />�
<br />Coalescence
<br />Coalescence of particles probably does
<br />not affect sedimentation in grit chambers
<br />to am� appreciablc degree because of the
<br />relaHcely shallow depth and short deten-
<br />tion ;ime, and Uecause of die nature oE the
<br />material.
<br />TurUulence ef flo�v could Ue importmrt
<br />in reducing eff'iciencies oE grit chamUers.
<br />Care thereFore must Ue taken in designing
<br />transition sections, such as approach chan-
<br />nels, and inlet and out]et deaices to mini-
<br />mize this factor.
<br />Jt has been determined that Stokes ]a�v
<br />is applicable ��•hen the Revnolds number
<br />is less than 0.3; a slight modification of the
<br />]aw mxkes it af ealue for Re�Tolds num-
<br />bers that are greater than 1000. Ho��•ever,
<br />for the range Ueta�een these ]imits, a var��-
<br />ing coe6icient of drag must Ue inhoduced.
<br />Unfortunately, grit chamUers incol�'e Rey-
<br />nolds numbers in this range, namely 1 to
<br />10. A curvE has been de�•eloped Uy Fair
<br />and Geyer' diat allo�vs a relaH�•ely easy
<br />solution. For example, if a s��here of
<br />specific gracity 2.83, diameter 030 mm, and
<br />in a�easte��•ater at 40`C (6S`F) is allo��•ed
<br />ro setde, this wrve ��•ould gi�•e a settling
<br />�•elocity of 2.53 cm/s. This velocity cor-
<br />responds to an o��erAow rate oE 2 200
<br />nt"/m=•d (a3000 gpd/sq ft).
<br />Since non•aerated grit chamUers require
<br />]aminar flow (true streamline flow) for
<br />effective settling to be accomplished, care-
<br />ful consideration should be gi��en to inlet
<br />and outlet turbulence. If turbulence can-
<br />not be o�•errnme in the design of these
<br />]ocations, it shauld be assumed that there
<br />is no setding in tl�e influent and ef9uent
<br />zones, and the theoretical ]ength of the grit
<br />chamber should Ue increased accordingly.
<br />144
<br />Bottom Scour
<br />Bot:om scour is a pardcularly important
<br />factor affecting grit-chamber efficiency.
<br />Camp'� has sho�an that the scouring pro-
<br />cess itself determines the proper velocity
<br />oE flo��• through the unit. This may be ex-
<br />plained by the fact that there is a critica]
<br />flow �'e]ocih�, ���, o�•er �ahich partides of a
<br />particular size and density, once settled,
<br />may again he placed in motion and re-
<br />inhoduced into the stream 9ow, An ea-
<br />pression has been derived=P for the mo��e-.
<br />ment of granular materials by a flowing
<br />stream. The critical ��elocity at ���hich par-
<br />tides of a giren size and speciSc gravity
<br />start to scour is given by:
<br />1'. = C(Sk/.� � (C. — 1)d]o.s ;9)
<br />��•here;
<br />V, = critical velocity, cm/s
<br />f=(riction factor, (0.03 for grit cham-
<br />bers),
<br />k = eaperimental ccefixient, (0.06 for
<br />grit chambers),
<br />g = gra�•itational tonstant,
<br />C, = partide spccific gra�•ity, and
<br />d = particle diamecer, cm.
<br />Camp derived the fallo���ing formu]a
<br />from the aUo�•e Shields fonnula:
<br />V� = 1.3 [(s — 1) d]o.b (10)
<br />�chere;
<br />V� = critical celocity, fps
<br />s= particle speci5c gra�•ity, and
<br />d = particle diameter, mm.
<br />Particle diameter, d, in Equations 9 and
<br />10 refers to the diameter of the particle to
<br />be scoured.
<br />From Equations 9 and 10, the critical
<br />�•elocih•, ���, for grit partides 0.2 mm '�
<br />diameter and ��•ith a specific gravity of
<br />2.6�, is 0.�3 mis (0.7•
<br />practice, designs are c
<br />controlling the �•elocih�
<br />range of 02 to 0.4 m%s
<br />and as c]ose to 0.3
<br />possible.
<br />To appreciate the
<br />effect of bottom scour
<br />a grit chamber, it must
<br />there a•ill al�vays be a
<br />organic putrescible plrl
<br />aut ��'ith the grit. In :
<br />tl�is problem ���ill be
<br />than during periods of
<br />nance of velocities nt
<br />m/s (1 fps ), however,
<br />ing of the grit and th�
<br />� bed of substantial qc
<br />� putresciUle material.
<br />In some non-mecha�
<br />engineers have provid�
<br />tions across the stor:
<br />chamiel, or a removat
<br />t�aeen the storage sec
<br />� through section, in an
<br />scour. This plan �coul�
<br />as a means oE reduc
<br />, ��•here velocity contrc
<br />' flow is not satisfacrory
<br />the other hand, �rould
<br />the grit �vashing eHe
<br />normal flo��•s.
<br />Quantity and Con
<br />_ The quantity and a
<br />�ti�aste���ater streams �+•i
<br />]o���ing considerations:
<br />1, T��pe of collectie
<br />or combined).
<br />1 2. The percentage c
<br />the comUined categor�
<br />catch basins, and m�
<br />! and the amount of sto
<br />overHow points.
<br />3. T�•pes of street
<br />and maintenance proc
<br />4. Relati�•e area se�
<br />population ).
<br />5. Climatic condit:
<br />street sanding).
<br />6. Sewer grades.
<br />
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