Settling Tank

where necessary to achieve this performance. A stilling baffle shall be

provided in the aeration tank before the outlet into the settling tank.

2.7.3

Settling Tank

**************************************************************************

NOTE: Delete the first and fourth sentences for

extended aeration type plants above 4.38 L/s 100,000

GPD capacity, step aeration type plants above 3.0 L/s

67,500 GPD capacity, and for all complete mixing

type plants.

**************************************************************************

NOTE: Delete reference to mechanical sludge

collection equipment for extended aeration type below

0.66 L/s 15,000 GPD capacity. 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 L/s and 4.38 L/s

15,000 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.

**************************************************************************

NOTE: Settling tanks shall have a freeboard of not

less than 300 mm one foot for plants of 0.88 L/s

20,000 GPD capacity and below, and not less than 450

mm one foot 6 inches for plants of more than 0.88 L/s

20,000 GPDcapacity.

**************************************************************************

NOTE: In all plants (except circular plants with

concretric inner settling tank) where the design

flow exceeds 4.38 L/s 100,000 GPD, the settling tank

should be in multiple units each capable of

independent operation and whose combined capacity

equals the design flow rate. In fourth sentence,

delete first and third optional wordings when design

flow is 4.38 L/s 100,000 GPD and below; delete

second optional wording when design flow exceeds

4.38 L/s 100,000 GPD.

**************************************************************************

NOTE: Delete the first and fourth sentences for

extended aeration type plants above 4.38 L/s 100,000

GPD capacity, step aeration type plants above 3.0 L/s

67,500 GPD capacity, and for all complete mixing

SECTION 11375N

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