UFC 3-280-04
17 DEC 2003
5-1.1.5 The depth of the filter is established based on the underdrain selection, sup-
port gravel requirements, depth of the filter media, and the operating water depth above
the filter media.
5-1.1.6 There are many different styles of filter underdrains available. The designer
must evaluate them on the basis of their hydraulic distribution capabilities, head loss
characteristics, materials of construction, and the associated support gravel require-
ments. In selecting the underdrain system, the designer would normally contact various
filter equipment and underdrain suppliers to discuss the process application with them.
The available products, options suitable for the application, and the relative costs can
be established. Once this information is obtained, the designer would use his judgment
in selecting what type of underdrain is best suited for the project and is to be used as
the basis of design. Support gravel requirements are dictated by the underdrain selec-
tion. Gravel gradations for the various support gravel layers are provided by the under-
drain manufacturer. The gravel is used to prevent plugging of the underdrain with the
media and loss of media. The filter media depth is established based on the process re-
quirements and is set on the basis of experience with similar types of applications or
pilot testing for a particular application. The last item to establish is the operating water
depth over the filter. The depth over the media should be selected to provide an ade-
quate operating range for the filter. The operating range depends on the method of flow
control selected and the terminal head loss desired. In the case of constant rate filtra-
tion, the method of control most commonly employed, the depth should be set to pro-
vide adequate submergence to protect against air binding problems. The range of oper-
ating depth above the filter can vary greatly. Normal ranges are from 30 to 45 cm (12 to
18 in.). Additional depth should be provided above the high water level, based on plant
hydraulics and overflow considerations and to maintain adequate freeboard from the
operating level of the filter.
5-1.1.7 The filter backwashing requirements must be considered in the sizing of the
filter, as the filter size influences the sizing of facilities and equipment required for
backwash. The size of the filter will dictate the required flow rate and, if applicable, the
storage volume required for a filter backwash. The rate of backwash affects the sizing of
the wastewater troughs, wash water gullets, backwash supply piping, and waste back-
wash drain piping from the filter.
5-1.1.8 In the design of filtration facilities, the designer must consider each of the fea-
tures discussed above to develop the facility layout and must have an understanding of
the impacts of the various features on one another. In selecting the number and sizing
of the filters, the designer must evaluate and consider the operational flexibility of the
facility with regard to the plant flow anticipated and the impacts on the auxiliary systems,
such as holding tanks, air compressors, and backwash pumps, required for the filter
backwashing process.
5-1.2
Media. Most package systems are pre-engineered by the manufacturer with
optimum media sizes, loadings, and backwash flow rates already determined. There
may be options available to the filtration system designer but these are often limited. If
the designer does specify system parameters outside the manufacturer's available op-
5-2
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