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HIGH-RATE GRANULAR MEDIA FILTRATION 8.37

ter within the water treatment plant. State regulatory officials should be consulted during
the planning phase to determine recycle and/or discharge requirements for waste wash-
water. The Filter Backwash Rule places specific requirements on the recycle of backwash
water within the plant.

Standards Recommended by Others
The American Water Works Association (AWWA) has published various manuals and
handbooks that provide useful and up-to-date information for planning and designing fil-
tration facilities. The AWWA Research Foundation has published many research reports
and design manuals related to filtration, generally specific to a particular type of filtra-
tion or new advances in filtration technology.
The AWWA standards for filtering materials are a particularly useful tool because they
establish minimum quality standards for the purchase and installation of filter media of
various types.
NSF International has established a certification program for all direct and indirect
drinking water additives. Standard 60 relates to drinking water treatment chemicals, such
as coagulants and coagulant aids. Standard 61 relates to drinking water system compo-
nents to ensure that these additives are compatible with drinking water systems, includ-
ing filter media and coatings and linings for valves and piping systems.
Standards such as building, fire, and plumbing codes and other local ordinances must
be considered in the design of filters and their housing. However, most codes and ordi-
nances do not specifically mention water treatment plants, so local building code and fire
department officials should be consulted during the planning phase to obtain clarification
of their specific requirements. Occupational Safety and Health Administration (OSHA)
standards for worker safety also must be considered.

FILTER OPERA TION AND MAINTENANCE

When designing filtration equipment, the engineer must keep in mind how the facilities
will be operated and maintained, including common filter behavior and filter operating
problems.

Common Filter Behavior
In between filter backwashes, filters "ripen," improving their ability to remove floccu-
lated particulate matter from the water, until turbidity breakthrough, terminal head loss,
or the next scheduled filter wash occurs. Filters remove floc and larger particles by strain-
ing at the top surface of the filter media and smaller particles by the mechanisms of trans-
port (i.e., interception, sedimentation, and diffusion) and attachment to the deeper filter
media. In general, increasing the filtration rate tends to decrease the time to reach termi-
nal head loss or turbidity breakthrough. Increasing the ES of the filter media tends to de-
crease the time to reach breakthrough but increases the time to reach terminal head loss.
J. M. Montgomery (1985) includes a thorough discussion of filtration theory and mathe-
matical models for particle removal and hydraulics.
Filter Ripening. Clean filter media require a period of conditioning before particle at-
tachment mechanisms take effect. The mechanisms of particle transport apply during and