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We often discuss water treatment systems as complete-mix (CSTR), plug flow (PFTR), or hybrid (this includes multiple CSTR). In the real world of waste treatment systems we rarely find pure complete mix or plug flow systems. An Batch Reactor (SBR) is a common true CSTR, along with the large single, highly mixed tank based activated sludge units found in chemical industries where influent dilution is important.
Systems such as oxidation ditches, aerated basins, and systems like the diagram above (left) are called plug flow which is not usually the correct model. For a true plug flow system, the water and solids move in one direction, without back-mixing. The action of mechanical aeration, diffusers, mixers, and even friction with walls creates back-mixing in most of these systems. This back-mixing creates a flow pattern simulated by multiple CSTRs (the area around each mixing zone becomes an in-basin CSTR). As the number of CSTRs increases (smaller CSTRs in series), tracer studies show more of a plug flow peak for effluent tracer concentration.
Systems such as oxidation ditches, aerated basins, and systems like the diagram above (left) are called plug flow which is not usually the correct model. For a true plug flow system, the water and solids move in one direction, without back-mixing. The action of mechanical aeration, diffusers, mixers, and even friction with walls creates back-mixing in most of these systems. This back-mixing creates a flow pattern simulated by multiple CSTRs (the area around each mixing zone becomes an in-basin CSTR). As the number of CSTRs increases (smaller CSTRs in series), tracer studies show more of a plug flow peak for effluent tracer concentration.
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