Biological and engineering considerations in the fine-screening of small organisms from cooling water intakes

Fine-screening of small organisms can be grouped into two concepts: impingement and deterrent. Impingement concepts involve the collection and removal of organisms with through-flow, center-flow, and dual-flow traveling screens. In evaluating the biological effectiveness of these screens, consideration should be given to species, approach and through-mesh velocities, mesh opening, impingement duration, debris loading, and type of organism wash system. Engineering considerations include potential for clogging, need for continuous operation of the screen, and screening material. Furthermore, each type of traveling screen has advantages and disadvantages relating to the potential for debris and organism carryover, screening area, screenwell layout, velocity distribution along screen face, and potential effect on pump performance. Impingement and diversion systems require that collected organisms be returned to the source water body. Transport systems can utilize either gravity flow or pumping units. The impingement concept appears to be the most practicable solution for once-through cooling systems. Deterrent concepts, in which the organisms passively drift by a screening device, include infiltration systems such as radial wells, porous dikes, and wedge-wire screens. Biologically, the radial wells and wedge-wire screens will cause little impact to small organisms. The porous dike has not been employed for the protection of small organisms to date. Engineering considerations presently limit the use of wedge-wire screens and radial wells to makeup water intakes with small withdrawals