Environmental Engineering Reference
In-Depth Information
Solution:
3.7%
100%
8.34 lb
1 gal
850 gal
×
×
=
262.3lb
lb
120 min
262.3
60 min
1hr
×
=
131.2lb/hr
131.2lb/
hr
2
=
0.94 lb/hr/ft
2
140 ft
10.7.4 Centrifugation
Centrifuges
of various types have been used in dewatering operations
for at lease 30 years and appear to be gaining in popularity. Depending on
the type of centrifuge used, in addition to centrifuge pumping equipment
for solids feed and centrate removal, chemical makeup and feed equip-
ment and support systems for removal of dewatered solids are required.
10.7.4.1 Operational Considerations
The centrifuge spins at a very high speed. The centrifugal force it
creates throws the solids out of the water. Chemically conditioned sol-
ids are pumped into the centrifuge. The spinning action throws the sol-
ids to the outer wall of the centrifuge. The centrate (water) flows inside
the unit to a discharge point. The solids held against the outer wall are
scraped to a discharge point by an internal scroll moving slightly faster
or slower than the centrifuge speed of rotation. In the operation of the
continuous-feed, solid-bowl, conveyor-type centrifuge (this is the most
common type currently used), as well as in other commonly used centri-
fuges, solids/liquid separation occurs as a result of rotating the liquid at
high speeds to cause separation by gravity.
The solid-bowl centrifuge is a rotating unit with a bowl and a con-
veyor. The unit has a conical section at one end that acts as a drain-
age device. The screw conveyor pushes the sludge solids to outlet ports
and the cake to a discharge hopper. The sludge slurry enters the rotat-
ing bowl through a feed pipe leading into the hollow shaft of the rotat-
ing screw conveyor. The sludge is distributed through ports into a pool
inside the rotating bowl. As the liquid sludge flows through the hollow
shaft toward the overflow device, the fine solids settle to the wall of the
rotating bowl. The screw conveyor pushes the solids to the conical sec-
tion, where the solids are forced out of the water and the water drains
back in the pool. Expected percent solids for centrifuge dewatered sludge
are in the range of 10 to 15%. The expected performance is dependent on
the type of sludge being dewatered, as shown in Table 10.7. Centrifuge
operation is dependent on various performance factors:
•
Bowl design (length/diameter ratio, flow pattern)
•
Bowl speed
•
Pool volume
