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During heavy rainstorms, the total flow coming to a sewage treatment plant may exceed its capacity, making it necessary to bypass the excess flow around the treatment plant, as shown in Figure 1.1.1a. Unfortunately, the storm overflow of untreated sewage causes pollution of the receiving body of water. A possible alternative, sketched in Figure 1.1.1b, is to screen most of the solids out of the overflow in some way and return them to the plant for treatment. Only the less objectionable screened overflow is discharged directly to the river.

To determine whether it was economical to construct and operate such a screening facility, the Federal Water Pollution Control Administration of the Department of the Interior sponsored a research project at the Sullivan Gulch pump station in Portland, Oregon. Usually, the flow to the pump station was 20 million gallons per day (mgd), but during a storm, the flow could exceed 50 mgd.

Figure 1.1.2a shows the original version of the experimental screening unit, which could handle approximately 1000 gallons per minute (gpm). Figure 1.1.2a is a perspective view, and Figure 1.1.2b is a simplified schematic diagram. A single unit was about seven ft high and seven ft in diameter. The flow of raw sewage struck a rotating collar screen at a velocity of five to 15 ft/s. This speed was a function of the flow rate into the unit and hence a function of the diameter of the influent pipe. Depending on the speed of the rotation of this screen and its fineness, up to 90% of the feed penetrated the collar screen. The rest of the feed dropped to the horizontal screen, which vibrated to remove excess water. The solids concentrate, which passed through neither screen, was sent to the sewage treatment plant. Unfortunately, during operation, the screens became clogged with solid matter, not only sewage but also oil, paint, and fish‐packing wastes. Backwash sprays were therefore installed for both screens to permit cleaning during operation.

Figure 1.1.1 Operation of the sewage treatment plant: (a) standard mode of operation and (b) modified mode of operation, with screening facility, F = flow; S = settleable solids.