For large drinking water
treatment plants the depth of the bed should be selected towards
the maximum and the water velocity around 5 m/hr, in any case not
more than 10 m/hr. This maximizes performance in removing iron and
manganese, reduces the frequency of backwashing, reduces power
consumption because average pressure drop is lower, and could
provide redundancy in case one of the filters is out of order and
higher flow rate has to be put through the remaining filters.
The upper limit of velocity,
up to 30 m/hr should be used for small bed depth and larger
allowed amount of residual iron and manganese in the filtered
water.
2) Backwash
mode
Total pressure drop
through the filter before backwashing is recommended to be maximum
100 kPa. The granules of DMX-065 filter media are porous.
The larger the pressure drop, the larger compaction forces are
applied to the filter media. The interaction between filter media
particles during alternated compaction under normal service and
expansion of the bed during backwashing leads in time to
deterioration of granules. Backwashing the filter when the
pressure drop has increased by 50 kPa from the initial clean
filter pressure drop is a good reference. Higher or lower values
could be set depending on the application and how long the filter
media has to last before changing it. Note that the filter media
would not loose significantly the effectiveness in removing the
iron and manganese but the pressure drop through clean filter bed
will increase.
Water velocity for
backwashing the filter is limited to 80 m/hr. This is the same as
recommended for ordinary sand filtration. Although it is possible
to use not filtered water for backwashing in general this is not a
good idea unless the water is relatively clean and the system is
set up with a rinse operating mode in addition to filtration and
backwashing. At low backwashing velocity longer backwashing time
is needed. In general backwashing velocity should be twice the
filtration velocity.
Backwashing time should
be determined by using a site glass on the discharge backwash line
or in some other way observe when the backwash water discharged is
satisfactorily clean. Backwashing time could vary from a few
minutes to 15 minutes.
3) Rinse mode
This mode follows backwashing
to remove the contaminant solids that would exit the filter before
the filter bed is compacted back and operates normally. This mode
is not necessary to be implemented in all water treatment systems.
Rinse time should be around 30
seconds for small bed depth and 1 minute or a little more for
upper limit of bed depth. Obviously the required time could be
found by checking the presence of contamination in the filtered
water when starting over normal filtration operation.
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