INFLUENCE OF THE DILATANT PROPERTY OF
FILTER MEDIA ON THE EFFICIENCY OF AIR SCOUR
Gregory A. Pittner, Project Engineer
Met-Pro Systems, Inc.
Landsdale, Pennsylvania 19446
INTRODUCTION
The importance of granular media filters in water and wastewater treatment is well
recognized along with the convenient manner with which they can be cleaned for reuse.
As they have been applied more often to the tertiary treatment of biological and physical/chemical wastewater effluents though, it has been observed that their regular backwash and cleaning has become an important consideration if not a problem.   Many
investigators have dealt with filtration from the standpoint of solids removal, head loss
accumulation, and filter run length, tacitly assuming that the means of removing solids
from the filter bed were available and would be used.   Obviously, if adequate cleaning
does not occur, the design of a filter system could be seriously in error.
The objective of backwash is to thoroughly remove soil from the media and return
its original capacity to remove suspended solids.   Effective cleaning depends both upon
achieving an even distribution of backwash water and creating sufficient shear at the
particle surfaces to remove the accumulated solids.   Because filters recently applied to
tertiary treatment have not always been backwashed adequately with water alone, interest
has been generated in auxiliary techniques such as air scour and surface wash, which can
provide greater shear forces at the particle surfaces.
Cleasby, et al. [ 1 ], have investigated the effects of backwash with water alone, with
air scour followed by water backwash, and surface wash during water backwash in plant-
scale studies.   The wastewater source used was in one case settled trickling filter effluent.
and in a second case the same effluent coagulated with alum and settled.   During these
runs, media consisting of 12 in. of anthracite coal (ES = 0.94 mm, UC = 1.31) and 12 in.
of sand (ES = 0.38 mm, UC = 1.50) was used.   Their results indicate that water back-
washing alone at 40% expansion was incapable of adequately cleaning the media and
that in either of the above two applications some auxiliary method of backwash was
necessary.   Also, air scour prior to water backwash was shown to be an effective auxiliary method, but even this technique applied at a rate of 4 scfm/ft2 was not capable of
preventing mud ball formation, surface cracks, and the accumulation of a coating on the
media.
The purpose of this chapter is to discuss the advantage of using air scour and water
backwash concurrently to provide a great deal more agitation in the granular bed than
either process alone.   There is ample evidence in the literature that such agitation is
desirable [2] for improving the quality of backwash and indications are that this can
maintain a filter bed in good condition over a long term of many years in most situations.
To be discussed in connection with this backwash technique is dilatancy, an important
property of filter media which affects its capacity to be air scoured efficiently.   This
characteristic causes granular solids to resist fluid-like movement when sheared and to
settle into a very compact structure when air scoured, which then resists fluidization
during water backwash.   By maintaining flow through and expanding the bed, the effects
of dilatancy are counteracted, resulting in more fluid-like movement and more violent
particle contact.
To support the above, data were gathered during this investigation which show that
currents are established within an air-scoured filter bed, the velocity of which is affected
by both air and water flow rates.   The energy effect of backwash is also considered by
comparing the amount of work performed on media during water backwash only, air
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