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8.14 CHAPTER EIGHT

unsuitable for filters removing large quantities of suspended solids or for applications
where polymers are used. Optimum expansion rate is influenced by the media size and
porosity. Kawamura (1999) expresses the optimal expansion rate as
(0.1)°.22 _ _ f
Optimal expansion rate = 1 - (0.1) 0.22

0.6 -f
0.4
where f = porosity ratio.
High-rate water wash tends to stratify granular media. In multimedia beds, this action
is essential and beneficial, but it is not required for uniformly graded single-medium beds.
In single-medium beds, high-rate water wash results in movement of the fine grains to
the top of the bed, which has a negative effect on head loss and filter run length.
Upflow Water Wash with Air Scour. There are numerous approaches to using auxil-
iary air scour in backwashing filters. Air scour has been used alone and with low-rate wa-
ter backwash in an unexpanded bed or slightly expanded bed. Each procedure takes place
before either low- or high-rate water wash.
Air scour provides effective cleaning action, especially if used simultaneously with
water wash. Cleaning is attributable to high interstitial velocities and abrasion between
grains. On the other hand, air wash has substantial potential for media loss and gravel dis-
ruption if not properly controlled. Use of air scour can significantly reduce the quantity
of water required for backwashing filters.
If more than one filtering medium is used and stratification of the bed is desired, high-
rate water wash must follow air scour. In a single-medium bed, if a low-rate wash can
adequately remove scoured solids, high-rate wash can be avoided.
If air scour occurs simultaneously with water wash, airflow must usually be stopped
before washwater overflow into the washwater collection troughs to prevent media loss.
For this reason, the permissible duration of air washing is short unless the concurrent wa-
ter wash rate is low or the filter box is very deep.
Experience indicates that air scour essentially eliminates mudball formation. Difficul-
ties have arisen, however, from failure to remove scoured solids from filter surfaces. Con-
tributing factors probably include low water-washing rates, long horizontal-travel dis-
tances to backwash troughs, and a necessary lag between termination of air scour and
initiation of higher-rate water wash.
Air scour complicates and increases the cost of wash systems as air blowers, air pip-
ing, and air and water backwash controls are required. Air backwash can also cause up-
sets to underdrain systems because of the potential to compress air in the underdrains.
Pressure relief systems should be considered with air scour.
Upflow Water Wash with Surface Wash. Surface wash systems have been widely
used for many years. Fixed systems distribute auxiliary high-pressure washwater from
equally spaced nozzles in a pipe grid. Rotary systems have pipe arms that swivel on cen-
tral bearings. Nozzles are placed on opposite sides of the pipes on either side of the bear-
ing, and the force of the water jets provides the thrust required to rotate the pipe arms.
Rotary systems are more often utilized, and they generally provide better cleaning ac-
tion, lower water requirements, and less obstruction for filter access. Possible problems
with rotating surface wash units include failure to rotate, nozzle clogging, failure to clean
in comers, abrasion of concrete walls near the point of closest passage of the ann, and
locally high velocities caused when passing under washwater collection troughs. Either
type of system may fail to provide auxiliary scour where it is most needed. This can be
especially true in multimedia beds if substantial removals are occurring at media inter-
faces.