Civil Engineering Reference
In-Depth Information
and have given excellent service. These areas include process water, wastewater treat-
ment, deep-well-injection water, API separator effluent, cooling water, and other sim-
ilar applications. Until a reliable means of restraining a dirty upflow bed is developed,
it appears likely that potable water applications of the upflow filter in the United States
will continue to be limited.
Biflow Filters
Biflow filters are an outgrowth of upflow filters, in that the divided flow (downward
from the top and upflow from the bottom (see Fig. 12-16) is an attempt to restrain
the bottom upflow portion of the bed by placing a downflow filter above it. Biflow
filters are used in Holland and Russia, but not to any extent in the United States. They
permit filtration in two opposite directions at the same time. Essentially, the top and
bottom halves are completely independent filters of equal capacity, which results in
some savings in structure and underdrains.
Unfortunately, the biflow filter has an inherent limitation that seems to preclude
development of a unit that will produce an exceptionally high-quality effluent. First
consider a single-medium biflow bed. The finest materials are at the top of the upper
downflow bed. This arrangement makes the top half of the bed a rapid-sand or surface-
type filter, and the quality of water produced at best cannot exceed that from a rapid-
sand filter. The bottom half of this same filter is a coarse-to-fine filter, but unfortunately
the finest material at the top outlet from the bed is somewhat coarser than the finest
material that can be successfully used in a rapid-sand filter. Obviously, the effluent
from this bed will be of lesser quality than that from the rapid-sand downflow filter
above. This situation has been recognized by researchers and revealed by pilot tests,
Influent
COAL
FINE SAND
Effluent
COARSE SAND
Influent
Fig. 12-16.
Cross section of dual-media biflow filter
