The process concept for sedimentation tanks has hardly changed over the past 80 years. Dimensioning these vessels according to existing guidelines guarantees safe operation. With ever tightening legislation, however, the question of expansion or upgrading of existing sewage treatment plants arises. Expansion is an expensive solution and impossible if the available space is scarce so that a new construction has to be built. The basis of upgrading consists in changed process concepts which are able to exploit the unused potential of existing tanks. An essential prerequisite for upgrading plants is the question whether the settling volume for activated sludge is sufficient. Beside the clarified water discharge, the feeding method for the sludge/water mix and the skimmer system have an essential influence on the separation efficiency in tanks. The inlet height is approx. 2/3 to 1/3 of the tank depth and the skimming direction in the counter flow. This concept is based on the empirical knowledge of normally minor turbulence in the tank. However, changed process concepts with a bottom-near inlet and concurrent skimming are able to minimize such turbulences to such an extent that the sludge load and thus the separation efficiency can be increased.
It is important to recognize that the process-engineering installations in rectangular sedimentation tanks have a great influence on the performance of this final treatment stage. Of particular importance is the design of the inlet section as turbulences are generated there by mixing with the wastewater inflow which may have an intense influence on the sedimentation process. The density of flows has a strong influence on the separation efficiency. The density flow sinks to the tank bottom during inflow and passes to the tank end. The rising density flow initiates back flow of the clarified water on the surface. To increase the separation efficiency of tanks, the density flow should be minimized or the engineering process modified in a way that the density flow will be integrated with the sedimentation process.
Inlet dimensions are important sizes The density flow can be substantially influenced by the inlet structure by minimizing the potential and kinetic energy of the wastewater stream with a suitable feed design. The inlet should have bottom-near feed openings to have an as small intermixing zone as possible between the activated sludge/water mix and the tank content. The velocity gradient in the inlet section should be small to avoid floe disturbance by shearing forces. The inlet section sludge scraping also influences the separation efficiency, most notably it impacts on the degree of thickening in the sedimentation tank. For minimisation of the turbulences in the secondary clarification tank and a consequently improved separation efficiency and for sludge thickening, practice has shown that scraping the sludge in the direction of the density flow works best. The scraping velocity should be low in order to prevent re-suspension of the activated sludge floes. To increase the degree of thickening and to minimize the volume flow of the return sludge, a minimum sludge residence time in the tank must be provided. Although a sludge hopper for thickening the activated sludge is not necessary, a sludge hopper at the tank end tends to increase the surface load of the tank.
The first mechanized rectangular sedimentation units in the United States were designed and installed by William M. Piatt in 1920 in Gastonia, North Carolina. Since then they have found widespread application in standard designs. Sludge removal equipment of this type usually consists of a pair of endless conveyer chains running over sprockets attached to shafts, one of the shafts is connected by chain and sprocket to a drive unit at one end of the tank. Attached to the chains at about 10 foot intervals are cross pieces of wood or flights, extending the width ' of the tank, linear conveyer speeds of 2-3 feet per minute are common with 1 foot per minute for activated sludge. A major problem is underwater maintenance and repair cost. The average life of the underwater equipment is about 8 years.
Further information on the operation and design basic for rectangular settling basins can be obtained from references 1 through 3.
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