17 DEC 2003
systems treat throughput ranging from approximately 160 L/s (15950 gpm). Traveling
bridge systems are similarly sized. Because backwash is continuous there is no limiting
head loss or breakthrough condition that must be determined.
The shell of upflow and downflow continuous backwash systems generally
houses the media, media distribution system to re-inject the media into the bed after
washing, the media removal system to remove the media requiring cleaning from the
bed, and influent, effluent and reject wash water distribution systems, weirs, and lines.
Additionally, they will have a media cleaning system located either external to or within
the filter shell.
Backwash. Traveling bridge systems are designed such that individual
gravity filter cells may be backwashed while the remaining cells continue to filter influent
water. Backwashing occurs underneath a hood that is suspended below the bridge or
carriage. The hood moves slowly along the filter system. The backwash shoe frame
slowly blocks off filtering flow out of each cell through the effluent port. Concurrently,
backwash flow into the cell is slowly increased as the shoe moves over the cell. When
the shoe completely isolates the cell, full backwash flow occurs. Then, backwash flow is
slowly reduced as the shoe moves off the cell, and filtering resumes. The backwash
pump draws filtered water back through the effluent port to backwash the cell. Another
pump picks up wash water collected in the hood, and discharges it to a wash water
trough. These pumps are sized for the manufacturer's designed backflow rate, typically
1000 (L/min.)/m2 (25 gpm/ft2). Backwash may be triggered by head loss (water level
probes), automatically (timer), or manually. Typical reject rates range from 3 and 5% of
Continuous backwash systems allow the system to function continuously by
cleaning the used media in a washer unit located separate from the media. In downflow
and upflow continuous backwash systems, the media moves within the bed to a re-
moval port and are then washed via air scour or water, or both, before reinjection into
the bed. The media are re-injected using an eductor pipe, compressed air system, sand
washer chamber, and reject line. The turbulence within the tube scours the solids from
the media. The solids are then separated from the media grains in a separation or
washer chamber. Alternatively, the media may be washed in an external device to sepa-
rate out the solids. The media washer is basically a baffled chamber that uses gravity to
separate the solids from the media. The baffles allow for countercurrent washes. Up-
ward flowing water results in sluicing away low-density solids, and settling of the media.
After being washed, the media are returned to the filter shell. Reject rates for continuous
backwash systems typically range between 10 and 15% of the feed stream flow rate,
but may be up to 25%. Although reserve filtrate is generally used for backwash, in some
cases a potable water supply may be required.
Because continuous backwash systems have a continuous waste stream they
are not used as a primary or the only treatment process at HTRW sites. They are al-
most exclusively used in conjunction with an upstream clarification unit to handle solids
returned from the reject stream.