28 KISH HATCHERY MANAGEMENT 



Q 



100 FT 



^ 



PERFORATED SCREEN 



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^^ 



Q 



_i 

 O 



C/3 



CO 



100 



50 FT 

 PROFILE OF SOLIDS SETTLING 



85% 



100 FT 



Figure 11. A characteristic settling profile for settleable waste solids is 

 shown for a 30 ft X IGQ ft tank with a 4-ft water depth and a water 

 velocity (V) of 0.056 ft/second. (Source: Jensen 1972.) 



their own weight under conditions of reduced water turbulence (Figure ll). 

 The design of settling basins should take four interrelated factors into 

 account: (l) retention time; (2) density of waste solids; (3) water velocity 

 and flow distribution; (4) water depth. 



Retention time is the average period that a unit of water stays in the 

 basin before it is swept out. Depending on the quantity of wastes carried 

 by the water, retention time can be anywhere from 15 minutes to 2 hours. 

 In general, retention time increases as the area and depth of the basin 

 increase. If flow currents are not managed correctly, however, some of the 

 water passes rapidly through even a larger structure while other water 

 lingers in backwater areas; the average retention time may seem adequate, 

 but much waste will still leave the basin. Therefore, it is important that 

 flow be directed evenly through the structure, and a system of baffles may 

 have to be incorporated in the design. If water is too shallow, it constantly 

 scours the bottom, suspends wastes, and carries solids out to receiving 

 waters. Conversely, if water is too deep, solids do not have time to settle 

 from top to bottom before water leaves the basin. A water depth of 1^ feet 

 is a practical compromise in most circumstances. 



