Civil Engineering Reference
In-Depth Information
clarifier. This sample stream is then passed through a small (usually 4
1
⁄
2
-inch-diameter)
pilot filter to determine whether the coagulant dose is proper, by continuously moni-
toring the pilot filter effluent turbidity. This technique provides a continuous, direct
measurement of the turbidity, which is achieved by filtration of water that has been
coagulated in the actual plant. Thus, no extrapolation from small-scale laboratory co-
agulation experiments is required. The only purpose of this test is to determine the
proper coagulant dose; it is not to predict the length of filter run, nor to determine the
optimum filter aid dose, nor to predict the rate of headloss buildup. Although pilot
filter columns have been used for other purposes as mentioned above, the technique
of interest here is for monitoring the coagulation process.
The pilot filter technique has the advantages of offering a continuous monitoring
of the plant-scale coagulation process with a minimum lag time. Filtering the water
through the pilot filter yields immediate information about the adequacy of the coag-
ulant dose. In a typical situation, correctness of coagulant dose is determined within
10 to 15 minutes after the raw water enters the plant. Experience at many locations
shows that the pilot filter effluent turbidity is a very accurate prediction of the plant-
scale filter effluent turbidity.
The mixed-media filter bed design described in Chapter 12 is often used in the
pilot filter. The mixed-media design has the ability to accept the high solids load
associated with most unsettled, coagulated waters without excessive headloss buildup.
The turbidity of the pilot filter is monitored continuously and recorded. High turbidity
in the filter effluent could result from either an improper coagulant dose or a break-
through of floc from a properly coagulated water. To ensure that breakthrough does
not occur, supplemental polymers are injected into the pilot filter influent line. These
additional polymer doses may shorten the pilot filter run times but can prevent break-
through. Typically, it is desirable to backwash the filter every 1 to 3 hours. To provide
a continuous monitoring of the coagulation process, two pilot filters are used in par-
allel. The system is equipped so that the filter is automatically backwashed on a high
headloss signal. When one pilot filter enters the backwash cycle (which requires only
about 10 minutes), the other pilot filter is automatically placed in service. Where very
high raw-water turbidities can be expected, a miniature flocculator-tube settler device
can be installed ahead of the pilot filters to ensure reasonable filter run times even
during periods of high turbidity. Typically, the pilot filters are contained in a console
unit or in the plant control panel, which may also house turbidimeters for monitoring
the plant-scale filter effluent turbidity.
At least two manufacturers (Neptune Microfloc and Turbitrol Co.) offer commercial
pilot filter systems, both based on the same principle but differing somewhat in me-
chanical aspects.
The pilot filter systems can be used to accurately indicate coagulation conditions
with a minimum lag time, so that the operator can make the necessary adjustments in
the plant chemical feed. Alternatively, the coagulant feed can be controlled automat-
ically.
Design for Automated Coagulant Control
This system is used in conjunction with
a pilot filter system and automatically varies the plant coagulant dosage to maintain
the effluent turbidity from the pilot filter at the desired set point value regardless of
variations in raw-water quality and other related factors. Other chemicals can be ad-
justed automatically by this system in direct relation to the coagulant dosage. The
coagulant control system normally consists of a pilot filter system, an automatic control
unit, and switches and controls for the chemical feed equipment. The output signal
