In a rock filter, pond effluent travels through a submerged porous rock bed, causing algae to settle out on the rock surfaces as the liquid flows through the void spaces. The accumulated algae are then biologically degraded. Algae removal with rock filters has been studied extensively at Eudora, Kansas; California; Missouri; and Veneta, Oregon (USEPA, 1983). Rock filters have been installed throughout the United States and the world, and performance has varied (Middlebrooks, 1988; Saidam et al., 1995; USEPA, 1983). A diagram of the Veneta rock filter can be seen in Figure 5.2. The West Monroe, Louisiana, rock filters were essentially the same as the one in Veneta, but the filters received higher loading rates than those employed at the Veneta system. Several rock filters of various designs have been constructed in Illinois with varied success. Many of the Illinois filters produced an excellent effluent, but the designs varied widely (Adam, 1986; Menninga, 1986); see Figure 5.3 for diagrams of the various types of rock filters applied in Illinois. Snider (1998) designed a rock filter for Prineville, Oregon, and knew of one built at Harrisburg, Oregon. Performance and design details are not available; however, Snider indicated that the systems were designed using information from the Veneta system. The principal advantages of the rock filter are its relatively low construction costs and simple operation. Odor problems can occur, and the design lives of the filters and the cleaning procedures have not yet been firmly established; however, several units have operated successfully for over 20 years.
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