Backwash velocity = design maximum washwater rate filter area conversion factor

[1000 L/m3 (449 gpm/cfs)]

The required backwash pipe area is determined by the maximum backwash flow and a

given design velocity, i.e., 1.2 m/s (4 fps):

Required backwash pipe area = {max. washwater flow [L/s (gpm)] / design velocity [m/s (fps)]}

conversion factor [103 m3/L (228 103 cfs/gpm)]

Similar to the influent line, the required backwash pipe diameter is chosen by using a

pipe diameter that provides a backwash velocity less than the given design velocity at

the above maximum backwash flow, while still being within the optimal washwater sup-

ply range of 1.53.0 m/s (510 fps) (Table 8-1).

8-3.3.3 ** Effluent Piping and Valve**. The effluent pipe required area can be deter-

mined using a similar methodology as the influent and backwash piping--the maximum

flow divided by a given design effluent velocity, i.e., 1.8 m/s (6 fps), and the appropriate

conversion factor. It is important to note that while the effluent maximum flow is the

same as the influent maximum flow, the effluent velocity may be different. The effluent

velocity usually exceeds the influent velocity.

8-3.3.4 ** Filter-to-waste (FTW) Piping and Valve**. The filter-to-waste (FTW) piping

should be determined using the maximum system flow and dividing by a given design

FTW velocity, i.e., 3.6 m/s (12 fps) multiplied by the appropriate conversion factor.

8-3.4

8-3.4.1 ** Pressure Filters**. Pressure filters operate with higher filtration rates than

gravity filters. Pressure filters can operate on filtration loading rates as high as 7 L/s m2

(10 gpm/ft2), and terminal head losses of up to 10 m (30 ft) without solids breakthrough.

Pressure filters are especially preferred over gravity filters when limited capital re-

sources and space constraints exist for a facility. There is also the advantage of longer

filter runs and lessened backwash requirements. Pressure filters, however, require more

energy to be run and more elaborate controls than gravity filters.

8-3.4.2 ** Traveling Bridge Gravity Filters**. Generally, the filter is sized based on a

suggested average and peak hydraulic loading. Typically, manufacturers have sug-

gested hydraulic loading rates of between 1.4 L/s m2 (2 gpm/ft2) and 3.4 L/s m2 (5

gpm/ft2). Increased hydraulic loading will result in increased head loss and rate of head

loss, possibility of solids penetration and breakthrough, and possibility of surface blind-

ing. If high peak flows in relation to average flow are expected or if peak flows are fre-

quent, peak flow size should govern. Where multiple units are desired, the total length

should be divided between the desired number of filter units.

8-3.4.2.1 Figure 8-1 is a sample curve of head loss increase versus influent solids load-

ing rate for both dual and single media filters. Designs with loading rates greater than 5

kg/ d m2 (1 lb/d ft2) should be approached with caution.

8-5