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8.30 CHAPTER EIGHT

Filtration Rate. Flow-measuring devices are recommended for monitoring flow through
individual filters. Filtration rates can be monitored and controlled by comparing the me-
tered flow rate through a filter to its surface area. Meters can be combined with modu-
lating valves to automatically control filtration rates. The most common type of measur-
ing device is a venturi tube because it can be easily checked in the field with manometers.
It is often impossible to provide the ideal length of pipe preceding the flow-measuring
device, particularly when it is mounted in the filter's effluent piping. However, this is
usually not a concern if the total plant flow is being split equally between all operating
filters. Local and remote indicating and recording devices are usually included. Orifice
plates can also be placed in filter effluent lines to limit maximum filtration rates. If flow
splitting by weirs or other such device is employed, the filtration rate may be determined
by dividing the total plant flow by the number of operating filters.

Head Loss. Head loss in a filter bed is a valuable indicator of filter bed condition and
may be used to automatically activate filter washing. Head loss through the filter media
is normally monitored by differential pressure-cell devices that measure the water pres-
sure above and below the filter media.
Aside from head loss developed by particle retention within the filter media, operat-
ing head loss depends on the filtration rate, the clean bed head loss through the filter me-
dia, and head losses through the filter underdrain system and the effluent rate controller.
Terminal head loss is the difference between the static head "available" between the wa-
ter elevations in the filters and the filtered water effluent control weir, less the operating
head losses through the clean media, underdrain system, effluent piping connections and
bends, and the effluent rate controller. Modern plants typically have a terminal head loss
of 8 to 10 ft (2.4 to 3 m), while many older plants have significantly less. Clean bed head
losses range from 1 to 2 ft (0.3 to 0.6 m) depending on media specifications and filtra-
tion rate.
Filters should be washed when terminal head loss is reached; otherwise, turbidity break-
through may occur. Also, a vacuum can result if head loss at any level in the filter bed
exceeds static head. This situation is referred to as negative head and can cause air bind-
ing of the filter media. When pressure in the filter bed drops below atmospheric levels,
dissolved gases are released from the water being filtered. Gas bubbles trapped in the bed
further increase head loss and aggravate the problem. They may also result in media dis-
placement during filter washing.
This problem is particularly acute when filtering is done with insufficient water depth
over the media or when surface waters are saturated with atmospheric gases because of
rising temperatures in the spring. Remedies for air binding in gravity filters include in-
creasing washing frequency, maintaining adequate static head above the media surface,
and keeping the clearwell water level above the top of the filter media to keep it sub-
merged. Pressure filters normally discharge well above atmospheric pressure and are not
subject to air binding.
The head loss sensor connection to the filter box should be located approximately 4
in. (10 cm) above the top of the washwater collection trough to prevent washwater from
entering the sensor. A sediment trap with drain installed on the sensor line will capture
any sediment that may enter the line. The end of the sensor should be turned up, keeping
a full column of water in the line at all times to minimize air entrainment. A fine-mesh
stainless steel screen installed on the end of the sensor will prevent clogging with filter
media. However, this screen requires periodic cleaning to prevent a buildup of material
that may cause a false head loss reading.
Another valuable monitoring method is to measure head loss at points within the fil-
ter bed by installing several pressure taps at various depths of the filter bed. These pres-
sure taps can be connected to transparent tubes, creating a piezometer board. The pres-
sure taps can be monitored and recorded continuously for better observation and control