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16 Chapter 1 Introduction to Water Systems
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5. Rapid filters operate at rates of 125 MGD/acre or 2 gpm/ft (1,170 MLD/ha or
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81 L/ min/m ), but rates run higher in modern works that include flocculating
chambers.
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6. Coke tricklers for aeration are rated at about 75 MGD/acre or 1.2 gpm/ft (700 MLD/
2
ha or 50 L/min/m ).

EXAMPLE 1.4 DETERMINATION OF THE CAPACITY OF TREATMENT PLANT UNITS
Estimate the capacity of the components of a rapid sand filtration plant (Fig. 1.5a) that is to deliver
10 MGD or 6,940 gpm (37.85 MLD or 26,268 L/min) of water to a city of 67,000 people.

Solution 1 (U.S. Customary System):
1. Two mixing basins, H 10 ft deep; number of mixing basins N 2.
(a) Assumed detention period t 2 min.
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(b) Volume V Qt>N 6,940 2>2 6,940 gal 928 ft each.
2
2
(c) Surface area A V>H 928>10 92.8 ft 0.785 D .
A * 4 92.8 * 4
(d) Diameter D = = = 10.9 ft.
A p A p
2. Two flocculating basins, H 10 ft deep.
(a) Assumed detention period t 30 min; number of flocculating basins N 2.
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(b) Volume V Qt>N 6,940 30>2 104,000 gal 13,900 ft .
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2
(c) Surface area A V>H (13,900 ft )>(10 ft) 1,390 ft each (such as 20 ft by
70 ft).
3. Two settling basins, H 10 ft deep, but allow for 2 ft of sludge; number of settling basins
N 2.
(a) Assumed detention period t 2 h.
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(b) Effective volume V Qt>N 6,940 2 60>2 416,000 gal 55,700 ft .
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(c) Surface area A V>H 55,700>(10 2)ft 6,960 ft (such as 35 ft by 200 ft).
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(d) Surface rating SR Q>A 6,940>6,960 1.0 gpm/ft .
4. Six rapid sand filters.
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(a) Assumed rating SR Q>A 3 gpm/ft ; number of filters N 6.
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(b) Area A Q>(N SR) 6,940>(6 3) 385 ft (such as 15 ft by 26 ft).
Solution 2 (SI System):
1. Two mixing basins, H 3.05 m deep; number of mixing basins N 2
(a) Assumed detention period t 2 min.
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(b) Volume V Qt>N (26,268 2)>2 26,268 L 26.27 m each.
2
(c) Surface area A V>H (26.27>3.048) 8.62 m .
A * 4 8.62 * 4
(d) Diameter D = = = 3.31 m.
A p A p
2. Two flocculating basins, H 3.05 m deep.
(a) Assumed detention period t 30 min; number of flocculating basins N 2.
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(b) Volume V Qt>N (26,268 L>min 30 min)>2 394,020 L 394 m .
2
3
(c) Surface area A V>H (394 m )>(3.05 m) 129.27 m each (such as 6.1 m
21.3 m).
3. Two settling basins, H 3.05 m deep, but allow for 0.61 m of sludge; number of settling
basins N 2.
(a) Assumed detention period t 2 h.
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(b) EffectivevolumeV Qt>N (26,268L>min 2 60min)>2 1,576,080L 1,576m .