peaked after about 24 and 23 days respectively (ref 28). In the case of I-131 this time represents about 3 half lives.
Initial concentrations of radioactivity in the reservoir can be predicted from measurements of ground deposition. (Ref 28). The specific activity of the reservoir water is, to a first approximation, given by:
Specific Activity
=
Ground Deposition (Bq m ̄2) × Surface Area of Reservoir (m2)
Vol of Reservoir (m3) × 103
Bq 1-1
As time passes the radioactive contamination in the reservoir will tend to be deposited at the mud/water surface of the reservoir bottom by sedimentation and exchange processes (ref 29) and the concentration of radioactivity in the water will fall fairly rapidly.
Water treatment
Hong Kong's water supply is treated in a number of stages. Firstly sulphate of alumina is added to the water which is then allowed to settle in large clarifiers or accelerators. The sulphate of alumina assists the suspended solids in the water to coagulate into large particles and settle on the floor of the tanks as sludge. Following this coagulation and settlement process the water is filtered through sand filters. Hydrated lime is then added to the water to neutralise the acidity caused by the addition of sulphate of alumina. Finally the water is sterilised by the addition of chlorine solution and a fluoride compound is added for the protection of teeth.
The coagulation and settlement process and the filtration will both remove large amounts of radioactivity from the water supply. The removal efficacy is dependent upon the particular radionuclide, but 50-75% removal is typical. Table 9.1 gives the typical ranges of removal efficacy for a number of water treatment processes, and Table 9.2 gives the typical removal efficacy by settlement.
TABLE 9.1
REMOVAL OF
RADIOACTIVITY BY
WATER TREATMENT
PROCESS (REF 30)
Process
Fraction of Radioactivity Removed
Soluble
Suspended
Alum coagulation and sand filters
25-85%
50-90%
Coagulation with iron salts and sand filters
25-85%
50-90%
Lime-soda softening
30-90%
50-98%
Retreatment with clay slurries
50-80%
50-95%
TABLE 9.2 REMOVAL OF
RADIOACTIVITY BY
SETTLIING* (REF 30)
Coagulation and filters
Settling Time
12 hours
24 hours
*As measured at about 3 m depth
in a sedimentation tank.
2 days
3 days
4 days
5 days
6 days
7 days
8 days
9 days
10 days
Fraction of Radioactivity Removal
80%
89%
94.5%
96.5%
97.5%
98%
98.4%
98.6%
98.9%
99%
greater than 99%
Further information on the removal of radioactivity from water is given in reference 31.
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