17 DEC 2003
medium in any way. This may include any or all of the following, depending on the type
of filter used:
An initial gullet that disperses the velocity head.
A throttling control on the influent valve.
A distribution header, series of distribution troughs, and splash plates.
The effluent washwater supply and filter-to-waste piping is usually manifolded for a
common connection with the filter underdrain system. Piping, conduits, gates, and
valves are usually designed for the velocities and flows shown in Table 8-1. Using a
given hydraulic design flow of the system, the designer can determine the maximum
flow of the system as:
Maximum flow [L/s (gpm)] = hydraulic design flow [L/m2s(gpm/ft2)] filter area [m2 (ft2)].
Table 8-1. Design Velocities and Flow Volumes
Velocity, mps (fps)
Maximum Flow per Unit of Filter Area,
Ref: WEF Manual of Practice No. 8, ASCE Manual and Report on Engineering Practice No. 76, Water
Environment Federation and American Society of Civil Engineers, 1992.
* m/hr x 16.65 = L/(min. m2)
8-3.3.1 Influent Piping and Valve. The area of the influent pipe should be deter-
mined using the above maximum flow and a given design velocity i.e., 0.5 m/s (2 fps):
maxium flow [gpm (L/min)]
Required influent pipe area [m2 (ft2)] =
design influent velocity [fps (m/s)]
, 1.67 10
[2.28 10 10
A required pipe diameter is chosen such that the diameter provides for a velocity within
the optimum influent design velocity of 0.31.2 m/s (14 fps) (see Table 8-1) at the
above maximum flow.
8-3.3.2 Backwash Supply Piping and Valves. Using the given design maximum hy-
draulic washwater rate for the system and the given filter area, the designer can deter-
mine the maximum backwash flow as: