Civil Engineering Reference
In-Depth Information
over filter performance. The turbidity of filter effluent is instantly and continuously
determined, reported, and recorded. Turbidimeters are best used on individual filters
but can be of value to monitor the performance of a group of filters. When used on
individual filters, there is less opportunity for breakthrough in one filter to be masked
by the rest of the filters. The signal obtained from the instrument can be used to sound
alarms, or, if necessary, to shut down an improperly operating filter unit. This greatly
increases the reliability with which filters of all types may be operated, and may
broaden the range of safe application for pressure filters. The significance of filter
effluent turbidity is reflected in the observation that a turbidity increase of 0.1 NTU
was associated with an increase in amoebic cysts by a factor of 10 to 50 when the
filter had been operating at an equilibrium condition.
11
Turbidity standards, while more uniformly adopted than standards for particle coun-
ters, have become increasingly stringent. The 1986 SDWA amendments established
0.5 NTU as the benchmark for acceptable filter performance. Pending changes to the
regulations will lower the turbidity limit to 0.3 NTU, with the limit expected to even-
tually be lowered to 0.1 NTU. At these lower turbidities, the accuracy of turbidimeters
is challenged, which has drawn renewed interest in particle counters for use in mea-
suring filter performance.
Particle counters can measure the number of particles in selected size ranges (e.g.,
2-10
m, 10-20
m, etc.), enabling the operator to focus on particle size ranges of
particular concern for the filters. In some cases, the particle counter can provide in-
dication of a pending increase in filter effluent turbidity, as the initial filter break-
through will often occur with the smaller particles. Figure 12-6 shows an example of
how filter effluent particle count increases can often proceed increases in turbidity.
180
0.12
Turbidity (NTU)
Particle Counts (>2 um/ml)
160
0.1
Particle Counts (>5 um/ml)
140
120
0.08
100
0.06
80
60
0.04
40
0.02
20
0
0
Filter Run Hours
Fig. 12-6.
Example of differences in filter effluent particle counts and turbidity during a typical
filter run
