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HIGH-RATE GRANULAR MEDIA FILTRATION 8.31

(Monk and Gagnon, 1985). Although this unit is a good performance monitoring tool, it
is not necessary to install one on each filter.
Filter Wash Rate. Because filter wash flow requirements may vary with the seasons
owing to differences in water temperature and pretreated water quality, operator knowl-
edge of the filter washwater rate in use at a particular time is essential. Flow tubes are
usually employed as the monitoring device and are usually matched with a downstream
flow control device to control the washing rate. Recording meters are generally not nec-
essary, but a totalizing device is desirable to determine the overall volume of water used
in washing. An alarm can be provided that is actuated if the wash rate exceeds a prede-
termined maximum.
Length of Filter Run. Filter performance is often judged by the length of the filter run,
but too long a filter run may not be good for filter operation. Long filter runs make wash-
ing a filter much more difficult because of particulate matter compaction in the filter me-
dia. In addition, a long filter run indicates that the filter is not working at its most cost-
effective capacity.
Rapid sand filters are generally operated with run lengths between 12 and 72 h, typi-
cally with 24-h runs. Pressure filters may have somewhat longer filter runs than gravity
filters if they can be operated at higher head losses without turbidity breakthrough.

Pilot Filters. Pilot filters are bench-scale models of full-scale plant filters that can be
used to determine optimum coagulant dosage. Coagulated water is diverted to pilot filters
from the full-scale pretreatment units. Monitoring the pilot filter effluent turbidity pro-
vides an indication of the adequacy of coagulant feed. Pilot filters greatly reduce the lag
time in coagulant feed system adjustment and improve plant performance. Parallel pilot
filters are usually provided to ensure continuous control. Because of the nature of con-
necting piping to pilot filters, higher filter aid polymer dosages are usually required than
in the full-scale plant. The effect of increasing the polymer dose is to shorten filter run
times. Consequently, pilot filters are generally not used to predict run lengths or polymer
dosage. In most cases, however, these variations do not affect determination of optimum
coagulant dosage.

Filter Operation and Control
Several types of filtration operation and control are used. Many plants have filter control
consoles located immediately adjacent to the filters they serve to observe filters for mal-
function during backwashing. Remote operation from the plant's central control console
is also practiced in some plants. This allows a single operator to wash filters and still ob-
serve other plant processes.
In the past, all major valves were controlled by individual manual controls, and all fil-
ter operations were operator-directed. However, advances in sensing and control equip-
ment have made the use of remote automatic or semiautomatic control commonplace. In
semiautomatic operation, filter washing is initiated by the operator but consists of a pre-
determined sequence that requires no additional attention.
Fully automatic filters are washed without operator input on the basis of loss of head
in the filter bed, filtered water turbidity, or a fixed maximum run time. Automatic systems
permit operation of all filters from a central location, reducing personnel requirements.
However, remote automatic operation may not permit the operator to observe the wash cy-
cle directly. Automatic systems should allow easy modification of the washwater and aux-
iliary scour rates and sequence by the operator. Process instrumentation for monitoring and
alarming to indicate when problems arise is essential for all modes of operation.