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Winter temperature gradients in circular clarifiers

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To understand the effect of temperature-induced density currents on circular clarifiers, a field study was performed on three uncovered circular tanks in Oregon during periods of low flow and high surface cooling. Temperatures were measured as a function of radial position and tank depth. Meteorological parameters were also measured to compute the surface heat flux from the water surface. Water temperatures in the tank were often 20 to 30 °C warmer than the equilibrium temperature.

These studies showed that density currents and unstable temperature gradients were common. Surface temperatures were often approximately 1 °C cooler than bottom temperatures. The momentum and suspended solids of the inflow caused a density current that moved along the clarifier bottom and then rose near the middle of the tank. In the winter, this density current rose as a result of the decrease of initial momentum of the inflow, the buoyancy of the inflow after suspended solids had settled, and the geometry of the tank. Also, vertical velocities induced by surface cooling were two orders of magnitude greater than the overflow rate, suggesting that particles could be kept in suspension by convective currents.


Document Type: Research Article


Publication date: 1998-11-01

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  • Water Environment Research® (WER®) publishes peer-reviewed research papers, research notes, state-of-the-art and critical reviews on original, fundamental and applied research in all scientific and technical areas related to water quality, pollution control, and management. An annual Literature Review provides a review of published books and articles on water quality topics from the previous year.

    Published as: Sewage Works Journal, 1928 - 1949; Sewage and Industrial Wastes, 1950 - 1959; Journal Water Pollution Control Federation, 1959 - Oct 1989; Research Journal Water Pollution Control Federation, Nov 1989 - 1991; Water Environment Research, 1992 - present.
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