indicated.
Valves shall be closed.
3.5
UNDERDRAIN
**************************************************************************
NOTE: Prefabricated package systems may have the
underdrain installed. This paragraph should only be
included where the underdrain is to be installed in
the field.
**************************************************************************
The underdrain shall be installed in accordance with approved shop drawings
and requirements of the manufacturer, including instructions of the
manufacturer's representative. The filter tank shall be cleaned and washed
prior to installation of the underdrain. Broken or defective components
shall be replaced. The manufacturer-supplied underdrain shall not be
modified. Following installation, the underdrain system shall be
completely cleaned and washed free of loose materials and debris. Any
damaged surfaces shall be restored to the original or equivalent coating.
Internal water pressure or testing of the system shall not be performed for
a minimum of [_____] days after completion of installation. The underdrain
system shall be maintained in clean condition until installation of filter
media.
3.6
SUPPORT MEDIA
**************************************************************************
NOTE: The need for a supporting layer of gravel
will depend on the selected underdrain. If the
underdrain does not require a gravel layer, delete
this paragraph. Also, delete this paragraph for
travelling bridge, continuous backwash, cartridge
and bag filters.
**************************************************************************
Gravel size and layer thickness shall be as follows:
Gravel Layer
Layer Thickness
Size Limits
____________
_______________
___________
Bottom
[_____]
[_____]
Second
[_____]
[_____]
Third
[_____]
[_____]
[_____]
[_____]
[_____]
Top layer
[_____]
[_____]
The gravel shall be placed by hand to avoid movement to the underdrain
system and to ensure free passage of water from the underdrain. Each gravel
layer shall be completed before the next layer above is started. Gravel
that becomes mixed shall be removed and shall be replaced in layers as
specified. The correct thickness of each layer shall be obtained as
follows: Before the gravel is placed, the top of each layer shall be
marked on the side of the filter. Then the top of each layer shall be
leveled against a water surface held at the appropriate mark. None of the
particles shall be less than half submerged, and there shall be no places
SECTION 11393
Page 36
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