Civil Engineering Reference
In-Depth Information
be conducted for the range of mixing used in the process. Air or oxygen should be
added in lieu of ozone to prevent degradation of the tracer. The flow rate of air or
oxygen used for the contactor should be applied during the study to simulate actual
operation. Tracer studies should then be conducted at several air / oxygen-to-water
ratios to provide data for the complete range of ratios used at the plant.
Tracer Selection
Chloride and fluoride are the most common tracers used in drinking water plants
(nontoxic and approved for potable water use). Rhodamine WT can be used, but it
has several restrictions. However, Rhodamine B is not recommended by the EPA.
Drawbacks of chloride and fluoride are that salts can induce density currents. Fluoride
has the following limitations: It is difficult to detect at low levels, and many states
impose a finished water limitation of 1 mg / L. The federal secondary and primary
MCLS are 2 and 4 mg / L, respectively. EPA recommends that fluoride be used only
in cases where feed equipment is already in place, for safety reasons.
Tracer Study Methods
It is recommended that at least 90 percent mass recovery for both methods be accom-
plished. One of the following two methods are generally used to perform tracer
studies:
28
Step-Dose Method
. Introduce tracer at constant dose until the concentration at the
endpoint is steady-state. Frequently employed for drinking water applications. The
results are directly used to determine the
T
90
value. Feed equipment is available
•
1
2.0
0.9
1.8
Tracer Injection Rate
0.8
1.6
0.7
1.4
0.6
1.2
0.5
1.0
Observed C/C
Data
0.4
0.8
0.3
0.6
0.2
0.4
0.1
0.2
T
10
= 11 minutes
0
0.0
0
5
10
15
20
25
30
35
40
Time Since Tracer Injec
tion (minutes)
Fig. 26-43.
Step-feed tracer curve
