Civil Engineering Reference
In-Depth Information
be used when modeling the age of water because there is no way to measure this
parameter directly.
The other approach is to start the model simulation with arbitrary initial values and
run it for a sufficiently long period under a repeating hydraulic loading pattern until
the system's water-quality behavior settles into a periodic pattern. Results from the
last period are taken to represent the system's response to the imposed hydraulic load-
ing.
In addition to initial conditions, the quality of all external inflows into the system
must be known. These data can be obtained from existing records when simulating
existing operations or could be set to specific values when investigating operational
changes.
Reaction-Rate Data
The specific reaction-rate data needed depends on the constit-
uent being modeled. It is essential that these data be developed on a site-specific basis,
because reaction rates can differ by orders of magnitude for different water sources,
treatment methods, and pipeline conditions.
First-order rate constants for chlorine decay in the bulk flow can be estimated by
performing a bottle test in the laboratory. Water samples are stored in several amber
bottles and kept at constant temperature. At several periods of time, a bottle is selected
and analyzed for free chlorine. At the end of the test, the natural logarithms of the
measured chlorine values are plotted against time. The rate constant is the slope of
the straight line through these points. There is currently no similar direct test to esti-
mate wall-reaction rate constants. Instead, calibration against measured field data is
used.
A bottle test can also be used to estimate first-order growth rates for THMs. The
test should be run long enough so that the THM concentration plateaus out to a
constant level. This value becomes the estimate of the maximum potential of THM
formation. A plot is then made of the natural logarithm of the difference in the for-
mation potential and measured THM level versus time. The slope of the line through
these points is the growth-rate constant.
Water-quality models depend on hydraulic models to provide information on flows
and velocities in pipes. An acceptably calibrated hydraulic model is a requirement.
Frequently, hydraulic models are calibrated to match pressures measured in the field.
Since calibration only for pressure does not assure that flows and velocity are accu-
rately predicted, additional hydraulic calibration may be required when performing
water-quality modeling.
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Although most water-quality models can be used to represent both conservative and
nonconservative substances, the use of conservative substances is more appropriate for
calibration of hydraulic models.
The calibration process can be summarized as follows:
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1. Conservative tracer is identified for distribution system. The tracer can be a
chemical that is added to the flow at an appropriate location or, in the situation where
there are multiple sources of water, can be a naturally occurring difference in the water
sources, such as hardness. Chemicals that typically are used include fluoride, calcium
chloride, sodium chloride, and lithium chloride. Selection of the tracer geneally de-
pends upon government regulations (e.g., some localities will not allow the use of
