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A design chart and a simple design procedure have been developed for applying the critical shear stress approach for the hydraulic design of self-cleaning circular sewers. Studies indicate that the possible region of deposition is limited to the lower relatively flat part of the sewer and the average boundary shear stress can be used safely as the basis of design for partially full flow. Suitable critical shear stresses for design uses are suggested. The conventional minimum velocity approach tends to either overdesign smaller sewers or underdesign larger sewers. A numerical example is included to illustrate the application of the chart and the procedure.

Extended plain sedimentation could be a possible alternative for pretreatment of surface waters in water purification. It does not add pollution to the environment and its sludge is less objectionable for disposal. Theoretical investigation shows that gravitational effect dominates settling down to the particle size of 1 μ\N. Experimental results indicate that extended plain sedimentation is technically feasible to remove a substantial amount of turbidity with residual turbidity down to less than 10 FTU. An 88% turbidity removal could be achieved in a settling tank of 7.5-day detention. Suspended solids concentration appears to be more appropriate as an indicator of the performance of settling of natural waters. From the viewpoint of functioning of settling tanks, extended plain sedimentation should mean extension towards a lower design overflow rate rather than towards longer detention periods.

16. Rubin, A.J. & Kovac, T.W. Effect of Aluminum(III) Hydrolysis on Alum Coagulation, Chemistry of Water Supply, Treatment and Distribution (A.J. Rubin, editor). Ann Arbor Science, Ann Arbor, Mich. (1974). 17. Cohen, J.M. Improved Jar Test Procedure. Jour. AWWA, 49:1425 (1957). 18. Rubin, A.J. Aggregation and Restabilization of Clays with Aluminum Hydroxide. Proc. 49th National Colloid Symposium, Clarkson College, Potsdam, N.Y., (June 1975). 19. Swartzen-Allen, S.L. & MatijeviC, E. Colloid Surface Properties of Clay Suspensions. II. Electrophoresis and Cation Adsorption of Montmorillonite. Jour. Colloid Interface Sci., 50:143 (1975). 20. Singh, S.S. & Brydon, J.E. Solubility of Basic Aluminum Sulfates at Equilibrium in Solution and in the Presence of Montmorillonite. Soil Sci., 107:12 (1969). 21. Black, A.P. & Hannah, S.A. Electrophoretic Studies of Turbidity Removal by Coagulation with Aluminum Sulfate. Jour. AWWA, 53:438 (1961). 22. Hall, E.S. Electrokinetic Study of Dispersions of Clay in Hydrolyzed Aluminum Solutions. Discussions Faraday Soc., 42:197 (1966). 23. Kruyt, H.R. Colloid Science, Vol. 1. Elsevier, New York (1952). 24. Tezak, B.; Matijevi6, E.; & Schultz, K.F. Coagulation of Hydrophobic Sols in Statu Nascendi. III. The Influence of the Ionic Size and Valency of the Counterion. Jour. Phys. Che m., 59:769 (1955). 25. Matijevi^, E. Colloid Stability and Complex Chemistry. Jour. Colloid Interface Sci., 43:217 (1973). 26. MatijeviC, E.; Janauer, G.E.; & Kerker, M. Reversal of Charge of Lyophobic Colloids by Hydrolyzed Metal Ions. I. Aluminum Nitrate. Jour. Colloid Interface Sci. 19:333 (1964). 27. MatijeviC, E. & Stryker, L. J. Coagulation and Reversal of Charge of Lyophobic Colloids by Hydrolyzed Metal Ions. III. Aluminum Sulfate. Jour. Colloid Interface Sci. 22:68 (1966).

The result of a comparison indicates that downflow settlers may have better performance provided the effluent collection system presents no problem. If the design criteria on overflow rate for conventional settling tanks are used for designing high-rate settlers, the latter will give better performance within the normal range of practical design. It appears that a high-rate settler system can handle considerable overloading without serious reduction in its turbidity removal efficiency. A settler with its flow through velocity less than 0.6 fpm (18.3 cm/min) tends to perform better than that with a higher flow through velocity even though the design overflow rate is kept the same. The theoretical design equation is found to incorporate the effects of angle of inclination and relative settler length reasonably well.

A mathematical model adaptable to digital computer applications is developed for estimating the total annual cost of a newly constructed or expanded public water supply or wastewater disposal system to provide cost information for rate studies. One unique feature is the inclusion of the effects of inflation and construction cost increases. The estimated annual costs will be in the form of a time series useful for long-range financial planning. A numerical example is presented to illustrate the application of the model. The results of the example indicate that inflation has pronounced effects on the estimated annual costs, and its omission in making cost estimates may result in insufficient revenue for the proper operation of the system. A sample computer program is presented to show the programming technique required to obtain correct results. The model is also a good tool for comparing alternative projects in water resources planning.

The purpose herein is to present a solution chart for obtaining critical depths in trapezoidal channels without involving any complex supplementary mathematical operation.

analyzer at their disposal, most of the pipe network analysis problems in the water supply engineering field are solved by the Hardy Cross method.1 The method is easy to apply and yields very good results. One of the objections one might have against the method is its monotonous, time-consuming, iterative procedure. The availability of electronic digital computers has greatly changed the picture. The computer takes over the tedious arithmetic operations and obtains the desired results in a small fraction of

NONUNIFORM FLOW IS POSSIBLE IN FLAT OPEN CHANNELS AND THE SURFACE PROFILE OF FLOW IN SUCH A CHANNEL IS ALWAYS IN THE FORM OF A BACKWATER CURVE. THE PRESENT PRACTICE FOR BACKWATER PROFILE COMPUTATIONS EITHER NEEDS EXPENSIVE COMPUTER TIME OR IT EMPLOYS A VERY TEDIOUS MANUAL TRIAL AND ERROR METHOD. AN EASY-TO-USE ALINEMENT CHART IS DEVELOPED FOR DETERMINING SURFACE PROFILE OF SUBCRITICAL, GRADUALLY VARIED FLOW IN FLAT RECTANGULAR OPEN CHANNELS. THE ONLY PARAMETER NEEDED IN APPLYING THE CHART IS THE CRITICAL DEPTH WHICH CAN ALSO BE READ FROM AN AUXILIARY SCALE IN THE CHART. KNOWLEDGE OF NONUNIFORM FLOW IS NOT ESSENTIAL FOR USING THE CHART. NUMERICAL EXAMPLES ARE INCLUDED. /AUTHOR/

Hydraulic control devices are extensively used in connection with open dispersion channels to achieve uniform flow distribution in treatment plant design. A general equation applicable to all types of control devices has been developed for design uses. Study results indicate that, for plants with no appreciable flow variations, orifices and ports need much less control head than other types of devices to obtain the same control efficiency. Rectangular weirs and V-notches give equal or even better control efficiency at flow rates lower than the design flow. Orifices and ports, on the other hand, have lower control efficiency at low flow rates. This can be improved by providing appropriate initial flow depths in the channel. If plant operation requires to maintain certain minimum velocity in the dispersion channel, rectangular weirs with no initial flow depth in the channel represent the best choice.