TM 5-814-3/AFM 88-11, Volume III
(3) Progressing-cavity. The progressing-cavity pump can be used successfully, particularly on concen-
trated sludge. The pump is composed of a single-threaded rotor that operates with a minimum of clearance
in a double-threaded helix of rubber. It is self-priming at suction lifts up to 28 feet, is available in capacities
up to 350 gallons per minute, and will pass solids up to 1.125 inches in diameter.
(3) Centrifugal. With centrifugal pumps, the objective is to obtain a large enough pump to pass solids
without clogging but with a small enough capacity to avoid pumping a sludge diluted by large quantities of
the overlying sewage. Centrifugal pumps of special design can be used for pumping primary sludge in large
plants (greater than 2 million gallons per day). Since the capacity of a centrifugal pump varies with the head,
which is usually specified great enough so that the pumps may assist in dewatering the tanks, the pumps have
considerable excess capacity under normal conditions. Throttling the discharge to reduce the capacity is im-
practical because of frequent stoppages, hence it is absolutely essential that these pumps be equippped with
variable-speed drives. Centrifugal pumps of the bladeless impeller type have been used to some extent and
in some cases have been deemed preferable to either the plunger or screw-feed types of pumps. Bladeless
pumps have approximately one-half the capacity of conventional non-clog pumps of the same nominal size
and consequently approach the hydraulic requirements more closely. The design of the pump makes clogging
at the suction of the impeller almost impossible.
(4) Torque-flow. This type of pump, which uses a fully recessed impeller, is very effective in conveying
sludge. The size of the particles that can be handled is limited only by the diameter of the suction or discharge
valves. The rotating impeller develops a vortex in the sludge so that the main propulsive force is the liquid
itself.
(5) Pump application. Types of sludge that will be pumped include primary, chemical, trickling-filter
and activated, elutriated, thickened, and concentrated. Scum that accumulates at various points in a treat--
ment plant must also be pumped.
(6) Primary sludge. Ordinarily, it is desirable to obtain as concentrated a sludge as practicable from
primary tanks. The character of primary raw sludge will vary considerably depending on the characteristics
of the solids in the wastewater, the types of units and their efficiency, and, where biological treatment follows,
the quantity of solids added from the following:
-- Overflow liquors from digestion tanks;
-- Waste activated sludge;
-- Humus sludge from settling tanks following trickling filters; and
-- Overflow liquors from sludge elutriation tanks.
The character of primary sludge is such that conventional non-clog pumps will not be used. Plunger pumps
may be used on primary sludge. Centrifugal pumps of the screw-feed and bladeless type, and torque-flow
pumps may also be used.
(7) Chemical precipitation sludge. Sludge from chemical precipitation processes can usually be handled
in the same manner as primary sludge.
(8) Trickling-filter and activated sIudge. Sludge from trickling filters is usually of such homogeneous
character that it can be easily pumped with either plunger or non-clog centrifugal pumps. Return activated
sludge is dilute and contains only fine solids so that it may be pumped readily with non-clog centrifugal
pumps which must operate at slow speed to help prevent the flocculent character of the sludge from being
broken up.
(9) Elutriated, thickened, and concentrated sludge. Plunger pumps may be used for concentrated
sludge to accommodate the high friction head losses in pump discharge lines. The progressing-cavity type
of positive displacement pump also may be used for dense sludges containing up to 20 percent solids.
Because these pumps have limited clearances, it is necessary to reduce all solids to small size.
(10) Scum pumping. Screw-feed pumps, plunger pumps, and pneumatic ejectors may be used for
pumping scum. Bladeless or torque-flow centrifugal pumps may also be used for this service.
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