Decantation underflow pumping

Control philosophies applied to continuous countercurrent decantation (CCD) thick eners are similar to those used for thickeners in other applications, but have emphasis on maintaining the CCD circuit in balance. It is important to prevent any one of the thickeners from pumping out too fast, otherwise an upstream unit could be stai ved of wash liquor while at the same too much underflow could be placed in a downstream unit too quickly, disrupting the operation of both units as well as reducing the circuit washing efficiency. Several control configurations have Been attempted, and the more successful schemes... 

Figure 1 presents the most widely used ASWWTP configurations for combined N and P removal. The A2/0 process presents a sequence of anaerobic reactors (to promote the growth of PAOs) followed by a sequence of anoxic to promote denitrification, and finally aerobic reactors. It has one internal and one external recycle stream. The internal recycle stream conducts a fraction of the nitrified liquor from the last aerobic to the P anoxic compartment, and the external recycle conducts a fraction of the sludge from the underflow of the sedimentation tank to the P compartment. In the UCT process, both recycle streams are feed to the anoxic zone and a second internal recycle stream is present from the anoxic to the anaerobic compartment. The modified UCT process has 2 internal recycles and 1 external one as in the original UCT process but the anoxic zone is divided into 2 zones. The external recycle is directed from the underflow of the decanter to the P anoxic zone. The P internal recycle stream conducts a fraction of the nitrified liquor from the aerobic to the 2 anoxic zone. Finally, the second internal recycle stream pumps a fraction of the mixed liquor from the P anoxic back to the anaerobic compartment. The Bardenpho process configuration has also an external... 

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