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Publication Abstract

Test Results of a Settling Basin Hydraulic Model Humphreys, Harold W., J. Rodger Adams, and Nani G. Bhowmik, 1974  Illinois State Water Survey, Champaign, IL,  ISWS RI-77    Full Text Available

The Illinois State Water Survey in cooperation with the City of Chicago studied the flow characteristics in a hydraulic model of one of the sixteen settling basins in the Central Water Filtration Plant (CWFP). The study objective was improvement of flow conditions in the prototype settling basins in order to improve the quality of the settled water applied to filters.

A l-to-24 scale model of one of the prototype settling basins was constructed. The model includes the raw water supply channel, the mixing basin with four end around mixing channels including paddle flocculators in each channel, the upper and lower settling basins and the settled water collector.

The model performance was evaluated by making slug tracer studies, obtaining velocity distributions in the upper settling basin and taking time-lapse color photographs of the advance of dye in the upper settling basin.

The original model and several modifications were tested. Modifications tested included turning vanes at a bend in the mixing channels, reduced open area in the slotted baffle wall, various types and locations of screen grids in the upper settling basin, additional outlet ports from the lower settling basin and varied numbers of paddle flocculators in the fourth mixing channel.

Test results are discussed for all modifications. Specific test results are presented for the original model and selected modifications. Results are also given for a dispersion test and for an investigation of turbulent velocity fluctuations in the upper settling basin.

Model test results showed that slug tracers were not sensitive to most of the modifications investigated. Paddle flocculators prevent short circuiting in the mixing channels as well as promote flocculation. An inclined screen at the downstream end of the upper settling basin was not advantageous and can not be recommended for use.

Recommendations for installation in prototype basins are: 1) turning vanes at abrupt channel bends, 2) open area less than 5 percent in slotted baffle walls, 3) full depth screen baffles if three are used in series and if sludge removal equipment is properly designed and 4) training walls in wide basins.

Modifications recommended for trial in CWPP are turning vanes in the bend between mixing channels 3 and 4, and reduction of open area in the slotted baffle wall to 2.6 percent. Four additional outlet valves from the lower settling basin are suggested for reduction of head loss only.

A deaeration system was installed to supply partially degasified water to the model. Deaerated water was required for operation of the thermistor velocity meter which was developed to measure the very low velocity flow in the settling basin.

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