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134 6. CONDITIONING OF SALINE WATER 6.8.2 Example Calculations
This section presents the calculations associated with the design of the ferric chloride storage and feed system for 50,000 m3/day (13.2 MGD) seawater desalination plant designed for an intake flow of 127,910 m3/day (33.8 MGD) and feed average and maximum feed dosage of 15 and 50 mg/L, respectively.
6.8.2.1 Chemical Use
The daily amount of the needed chemical in kg/day is calculated using Eq. (6.2): Qdc 1⁄4 Concentration ðmg=LÞ  Flow m3day1000
For this specific example, the daily average and maximum chemical use is Qavgdc 1⁄4 15 mg=L  127; 910 m3 day1000 1⁄4 1919 kg=day;
Qmaxdc 1⁄4 50 mg=L  127; 910 m3 day1000 1⁄4 6395 kg=day; 6.8.2.2 Chemical Storage Tanks
(6.2)
The daily chemical use of 1919 kg/day is at 100% chemical concentration. Because the actual commercial product of liquid ferric chloride is delivered to the plant at 40% solution, the amount of this chemical that will need to be stored on site for 30 days is
Ast 1⁄4 1⁄2Qavgdc=ðstorage concentration %Þ  Storage time
1⁄4 1⁄21919 kg=day=ð0:4Þ  30 days 1⁄4 143; 925 kg of 40% liquid FeCl3 for 30 days (6.3)
Taking under consideration that the bulk density of this chemical Dd is 1.42 kg/L 1⁄4 1420 kg/m3, the actual storage volume is
Vst 1⁄4 Ast=ðDdÞ 1⁄4 143; 925 kg=1420 kgm3 1⁄4 101 m3
Typically, the actual storage tank volume is selected to be 10%e15% larger volume to allow the venting space/free board on the top and sediment accumulation at the bottom, which are inactive storage areas. As a result, the actual tank storage would be 101  1.15 1⁄4 116 m3. Assuming four individual storage tanks at 3 m (10 ft) diameter each, the depth of each tank will be (116m3/4 tanks)(3.143m3m/4)1⁄44.1m (13.5ft).
6.8.2.3 Water Dilution Flow
The average dilution flow (in L/h) needed to reduce the chemical concentration from its delivery concentration (Cd) to its application concentration (Ca) can be calculated by the following formula:
Qdavg 1⁄4 Qavgdc  1⁄2ðCd=CaÞ  ð1=DdÞ=24 (6.4)