Handbook of Civil Engineering Calculations

(singke) #1
considerations. Operating at higher speeds helps achieve optimum performance which is
weighed against somewhat greater operating and maintenance costs. Increasing bowl
speed usually increases solids recovery and cake dryness. Today most centrifuges used in
wastewater applications can provide good clarity and solids concentration at G levels be-
tween 1,800 and 2,500 times the force of gravity.


  1. Compute the centrifugal force in the centrifuge
    The centrifugal acceleration force (G), defined as multiples of gravity, is a function of the
    rotational speed of the bowl and the distance of the particle from the axis of rotation. In
    the centrifuge, the centrifugal force, G, is calculated as follows:


(2TrN)^2 R
32.2 ft/s^2

where TV= Rotational speed of centrifuge (rev/s)
R = Bowl radius, ft (cm)


The rotational speed and bowl diameter of the centrifuge will vary depending upon the
manufacturer. However, a rotational speed of 2,450 r/min and a bowl diameter of 30 inch-
es (72.6 cm) are common for this type of sludge dewatering operation.
Therefore, the centrifugal force is


^ ((27r)(2450 r/min/60 s/min))
2
(30 in/12 in/ft)(0.5) 0 fff ^
G = -• = 2,555 Gs
32.2 ft/s^2


  1. Find the polymer feed rate for the centrifuge
    The major difficulty encountered in the operation of centrifuges is the disposal of the cen-
    trate, which is relatively high in suspended, non-settling solids. The return of these solids
    to the influent of the wastewater treatment facility can result in the passage of fine solids
    through the treatment system, reducing effluent quality. Two methods are used to control
    the fine solids discharge and increase the capture. These are: (1) increased residence time
    in the centrifuge, and (2) polymer addition. Longer residence time of the liquid is accom-
    plished by reducing the feed rate or by using a centrifuge with a larger bowl volume. Bet-
    ter clarification of the centrate is achieved by coagulating the sludge prior to centrifuga-
    tion through polymer addition. Solids capture may be increased from a range of 50 to 80
    percent to a range of 80 to 95 percent by longer residence time and chemical conditioning
    through polymer addition.
    In order to obtain a cake solids concentration of 20 to 28 percent for an aerobically di-
    gested sludge, 5 to 20 pounds of dry polymer per ton of dry sludge feed (2.27 to 9.08
    kg/ton) is required. 15 Ib/ton (6.81 kg/ton) will be used for this example. Usually this val-
    ue is determined through pilot testing or plant operator trial and error.
    The polymer feed rate in Ib/h of dry polymer is calculated using the following:


„ , H (polymer dosage, lb/ton)(dry sludge feed, Ib/h)
Polymer feed rate (Ib/h) = ^^

Using values defined previously, the polymer feed rate is:

,„ „ , (15 lb/ton)(670 Ib/h)
Polymer feed rate (IbTh)=V ^0n

= 5.0 Ib/h of dry polymer (2.27 kg/h)
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