TM 5-814-3/AFM 88-11, Volume III
(1) Filter yields. Filter yields vary from 2 to 15 pounds per square foot per hour for various types of
sludge. Vacuum filters for digested activated sludge will be designed for a yield of 2 pounds per square foot
per hour; while vacuum filters for raw primary sludge will be designed for a filter yield of 10 pounds per
square foot per hour. The design filter area will be for the peak sludge removal rate required plus 15 percent
area allowance for maintenance downtime. It will be assumed that the filter units will be operated 30 hours
per week.
(2) Filter sizes and equipment. Filter sizes cover a wide range and can be up to 12 feet in diameter,
with filtering areas up to 700 square feet. Vacuum filtration units are normally supplied with essential
auxiliary equipment from various manufacturers.
(3) Disposal of filtrate. Dewatering liquids will be returned to the head of the treatment plant. For this
reason, the solids concentrations of a vacuum filtrate must be kept as low as practical and can be assumed
to be about 10 percent.
(4) Design. Selection of vacuum filters is demonstrated in an example in appendix C.
d. Centrifugation. Centrifugal dewatering of sludge is a process which uses the force developed by fast
rotation of a cylindrical drum or bowl to separate the sludge solids from the liquid. In the basic process, when
a sludge slurry is introduced to the centrifuge, it is forced against the bowl's interior walls, forming a pool
of liquid. Density differences cause the sludge solids and the liquid to separate into two distinct layers. The
sludge solids "cake" and the liquid "centrate" are then separately discharged from the unit. The two types
of centrifuges used for municipal sludge dewatering, basket and solid bowl, both operate on these basic
principles. They are differentiated by the method of sludge feed, magnitude of applied centrifugal force,
method of solids and liquid discharge, cost, and performance.
(1) Basket centrifuge. The imperforate basket centrifuge is a semi-continuous feeding and solids
discharging unit that rotates about a vertical axis. A schematic diagram of a basket centrifuge in the sludge
feed and sludge plowing cycles is shown in figure 16-4. Sludge is fed into the bottom of the basket and sludge
solids form a cake on the bowl walls as the unit rotates. The liquid (centrate) is displaced over a baffle or weir
at the top of the unit. Sludge feed is either continued for a preset time or until the suspended solids in the
centrate reach a preset concentration. The ability to be used either for thickening or dewatering is an
advantage of the basket centrifuge. A basket centrifuge will typically dewater a 50:50 blend of anaerobically
digested primary and waste activated sludge to 10-15 percent solids.
16-11
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