Fish Hatchery Management - fisheries & aquaculture

FISH HA'I'CHERY MANAGEMEN'I'R FORATED1OO FTa o-OaoUJFFUU)1006€ O 50 FT 1OO FTPROFILESOLID SETTLINGFIGURE 11. A characteristic settling profile for settleable waste solids isshown for a lJO ft x 100 ft tank with a 4-ft water depth and a watervelocity (V) of 0.056 ftlsecond. (Sorrtce' Jensen 1972.)their own weight under conditions of reduced water turbulence (Figure 1l).The design of settling basins should take four interrelated factors intoaccount: (1) retention time; (2) density of waste solids; (13) water velocityand flow distribution; (4) water depth.Retention time is the average period that a unit of water stays in thebasin before it is swept out. Depending on the quantity of wastes carriedby the water, retention time can be anywhere from l5 minutes to 2 hours.In general, retention time increases as the area and depth of the basinincrease. If flow currents are not managed correctly, however, some of thewater passes rapidly through even a larger structure while other waterlingers in backwater areas; the aaerage retention time may seem adequate,but much waste will still leave the basin. Therefore, it is important thatflow be directed evenly through the structure, and a system of baffles mayhave to be incorporated in the design. If water is too shallow, it constantlyscours the bottom, suspends wastes, and carries solids out to receivingwaters. Conversely, if water is too deep, solids do not have time to settlefrom top to bottom before water leaves the basin. A water depth of ii feetis a practical compromise in most circumstances.