MIL-HDBK-1005/16
return/waste activated sludge and digested solids, cement-lined
ductile iron pipe is recommended.
10.3.3
Head Loss Determination. At concentrations above about
1.5 percent solids, wastewater solids streams act as
non-Newtonian fluids. As a result, head losses encountered in
the pumping of thick solids are greater than those for water
under similar conditions. Head losses increase with increased
solids content, increased volatile content, and lower
temperatures. Higher pressures are also required to overcome
resistance and start flow, especially after the discharge piping
has been shut down for several hours or more. In primary solids
systems, grease coatings that reduce the effective diameter of
the pipeline may also cause heads greatly in excess of the
theoretical head. As a result of these factors, head losses
should be determined with great care. Guidelines for head loss
calculations are provided in WEF MOP 8.
10.3.4
System Layout. Make provisions for cleaning suction
and discharge piping (normally by pigging). Pigs operate best at
3 to 5 fps (0.9 to 1.5 m/s) and require pressures higher than
those supplied by most solids pumps. A flushing connection
should be provided on suction lines and, in some cases, on
discharge lines. Use of plant effluent for flushing water at a
flow rate of 160 gallons/minute (10 L/s) and a pressure of at
least 70 psi (483 kPa) has been recommended (refer to Metcalf &
Eddy,1991). Hot water is preferable for flushing where grease
buildup has occurred.
Use long sweep elbows or 45-degree bends rather than
90-degree elbows wherever possible. Because erosion of elbows is
likely, locate them where replacement will not be too difficult.
10.4
Pump Selection. This paragraph introduces pump types
and guides the designer in pump selection. Par. 10.5 details the
design of pumping systems. Additional information pertinent to
designers can be found in WEF MOP 8.
Pumps used to convey wastewater solids and scum include
the centrifugal, plunger, progressing cavity, and diaphragm
types. Pump selection from among these types depends on the type
of solids to be pumped. Refer to Table 17.
Descriptions of these pumps are provided in the
following two publications: Metcalf & Eddy, 1991, and
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