the scum airlift pump, and shall be supported from the tank wall or scum
baffles. The inclined approach ramp leading to the scum trough shall be
shaped to contain the scum as it is moved up the incline to the trough by
the scum skimmer.
3.2.3.3
Scum Collection in Rectangular Settling Tanks
**************************************************************************
NOTE: Delete paragraph for extended aeration type
below 6.57 L/s 150,000 PGD capacity and all step
aeration type and extended aeration type when
eventual conversion to step aeration type is
contemplated.
**************************************************************************
Scum Collection in Rectangular Setting Tanks With Mechanical Sludge
Collectors: Scum collection shall be accomplished by means of traveling
blade or paddle skimmer or by suction skimmer. Traveling blade or paddle
skimmer shall be attached to the traveling sludge collector at surface
level and shall be designed to sweep the surface of the settling tank in
one direction only with each pass of the sludge collector. Collected scum
shall be discharged into a trough, collector pipe or suction skimmer. Scum
trough shall be steel, 6 mm 1/4 inch minimum thickness, with approach ramp
and shall have a connection to the scum air lift pipe and shall be
supported from the tank wall. Collector pipe shall be steel pipe with a
1.05 rad 60 degree wide slot cut symmetrically above the vertical axis. At
maximum intervals of 750 mm 30 inches, a 50 mm 2 inch wide band of full
circumference shall be left for stiffness. End supports shall include a
rolled steel collar welded to an adjustable steel end plate. A readily
renewable grease-resistant, watertight seal shall be provided and so
constructed that it will allow smooth action of the revolving pipe. Seal
shall be readily renewable without removing pipe. End supports shall be
secured to the concrete walls by stainless steel anchor bolts having a
minimum diameter of 16 mm 5/8 inch. Collector pipe operator shall be
manual, lever type mounted on the collector pipe. Operator shall be steel
pipe having a minimum diameter of 31 mm 1 1/4 inches and shall be secured
to the collector pipe with a bolted connection. Operator shall extend at
least 900 mm 3 feet above the top of tank and shall permit rotation of the
collector pipe at least 0.52 rad 30 degrees each side of the vertical axis.
Collector pipe shall have suitable means of discharge to the scum airlift
pump.
3.2.3.4
Scum Transfer
**************************************************************************
NOTE: Delete requirement for solenoid valve when
automatic operation is not required. Automatic
operation is to preferred especially in plants where
minimal operator attendance would be anticipated.
**************************************************************************
Scum transfer shall be accomplished by means of an airlift pump and piping
to the aeration tank. Airlift pump shall be as specified in paragraph
entitled "Sludge Transfer Pump(s)," except that it shall be 50 mm 2 inch
size. [A percentage-timer-controlled solenoid valve with valved bypass
shall be provided on the airlift pump air supply.] Materials for piping
shall be as specified in paragraph entitled "Piping."
SECTION 11375N
Page 101
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