a.
The return sludge shall be mixed by a mixer in a mixing chamber
prior to entering the aeration tank influent distribution channel;
mixer shall be as specified under paragraph entitled
"Appurtenances and Accessories."
b.
The return sludge shall be discharged directly into the aeration
tank from a traveling pump traversing almost the full length of
the aeration tank.
3.2.3
Scum Collection and Transfer
3.2.3.1
Scum Collection in Hopper Bottom Settling Tanks
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NOTE: Delete the paragraph for extended aeration
type above 4.38 L/s 100,000 GPD capacity, step
aeration type above 2.96 L/s 20,000 GPD and for all
sizes for complete mixing type. For plants below
0.66 L/s 15,000 GPD capacity, hopper bottom only
should be specified for settling tanks. For plants
above 4.38 L/s 100,000 GPD capacity, mechanical
sludge collectors only should be used in settling
tanks. For plants between 0.66 and 4.38 L/s1 5,000
and 100,000 GPD capacity, either hopper bottom or
mechanical sludge collector should be allowed as a
Contractor's option. However, of the various types
of mechanical sludge collectors, rotating sludge
collectors appear to be available for plants of 1.31
L/s 30,000 GPD and above; chain-and-flight,1.75 L/s
40,000 GPD and above; traveling reciprocating type,
0.66 L/s 15,000 GPD and above.
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Scum collection shall be accomplished by means of a suction skimmer.
Suction skimmer shall be a 50 mm 2 inch diameter drop pipe supported by a
structural steel member and so mounted that it can be raised or lowered
with respect to the liquid surface by means of stainless steel adjusting
screws with hand knobs or by corrosion-resisting turnbuckles located above
the liquid surface. Lower end of drop pipe shall be attached to a 50 mm 2
inch air lift by means of a flexible hose of chloroprene or other material
suitable for use in sewage.
3.2.3.2
Scum Collection in Circular Settling Tanks
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NOTE: Delete paragraph for extended aeration type
below 1.31 L/s 30,000 GPD capacity.
**************************************************************************
Scum Collection in Circular Setting Tanks With Mechanical Sludge
Collectors: Scum collector assembly shall include a blade skimmer and a
scum trough. Assembly shall continuously move surface scum to the scum
trough. The assembly shall discharge scum with a minimum discharge of
water. Blade skimmer shall include an arm fabricated of structural steel
shapes to which is attached a steel blade skimmer or floating redwood
skimmer, with adjustable chloroprene wiper. Scum skimmer shall be
supported by center shaft and one scraper arm. Scum trough shall be welded
structural steel, minimum thickness 6 mm 1/4 inch, shall have connection to
SECTION 11375N
Page 100
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