COMMUNICATIONS
Device to Aid Pilot Plant Final Settlement Edwin F. Barth U. S. Department of the Interior, Federal Water Pollution Control Administration, Cincinnati Water Research Laboratory, Cincinnati, Ohio 45226
A mechanical-hydraulic system controlled by a cycle timer has been helpful in sludge return on pilot size activated sludge plants.
A
ctivated sludge pilot plants operating in the 300- to 1000-liter-per-day range usually experience difficulty in providing rapid return of sludge to the aerator. To provide adequate detention times in final settlement, several factors inherent in the design result in holding the sludge in the clarifier longer than would be encountered in full-scale operation. Sidewall surface effects are very significant in the pilot scale. Longitudinal velocity is almost nonexistent; scraper mechanisms are not satisfactory. Bridging of the solids and draw-off through a hole in the blanket is common. All of these factors lead to a considerable sludge blanket in the final settler and effectively prevent the solids from recycling to the aerator. Various scraper and propeller mechanisms were tried to move the solids to the recycle pump inlet. Because of sidewall effects, scrapers tend to pass through the sludge rather than to move it. Propeller devices in the low-speed range act in a similar manner. High-speed propellers disturb the sludge to an extent that reduced settlement efficiency occurs. A mechanical-hydraulic system controlled by a cycle timer has been helpful (Figure 1). The outlet from the settler is connected to the intake side of the pump. The rubber tubing used for this connection passes through a solenoid-operated hose clamp. The clamp is of the normally open variety. The clamp is controlled by a I-minute cycle timer. On signal from the timer, the solenoid clamp closes, causing a vacuum to form in the tubing from the clamp to the pump (Figure 2). At the end of the on-cycle, the solenoid clamp opens rapidly, causing a hydraulic surge from settler to pump. This surge jars the sludge in the settler. The cycle is repeated automatically every minute. An on-cycle of 5 to 10 seconds is sufficient. Repetition of the jarring cycle compacts the sludge into the settler outlet. The characteristics of the hydraulic surge can be controlled to some extent by the suction developed by the pump, the inside bore diameter of the rubber tubing, and the length of the tubing between the clamp and pump. Use of this device has greatly improved the underflow solids concentration, and bridging has been almost completely eliminated.
Figure 1. Solenoid clamp arrangement
Ackno wfedgment Photographs are by Robert Bloomhuff. Receiced.for reriew Noremher 24, 1967. Accepted December 29, 1967.
Figure2.
Clampclosed
Volume 2 , Number 2, Februar) 1968
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