Effect of diurnal temperature variation on the hydraulics of clarifiers

This study was carried out to obtain data on the behaviour of thermally induced density currents in primary rectangular clarifiers so that better models can be developed for these units. This note deals with the case when the influent is cooler than the ambient temperature in the tank. The experiments were made in a model with a scale of about 1:20 compared to the typical full-scale clarifier. Temperature surveys and dye tests were carried out for turbulent flow and temperature differences between influent and effluent that were equivalent to ±0.2 °C in the prototype on a diurnal basis. The results indicate six flow regimes that follow a decrease in influent temperature: (i) denser wall jet; (ii) splash at the end wall; (iii) moving internal hydraulic jump; (iv) submerged internal hydraulic jump; (v) splash at the influent baffle; and (vi) stratified flow. A comparison of the test data with those available in the literature showed that the entrainment equations involving the Richardson number are adequate for modelling, but the classical hydraulic jump equations need modifications for the effect of entrainment. Key words: clarifiers, rectangular, primary, model, density currents, internal hydraulic jumps, unsteady flow, denser wall jets.