ELIMINATING BACKFLOW IN RETROFIT BNR SYSTEMS

The retrofit of an existing activated sludge system with a biological nitrogen removal system requires the installation of a baffle wall to separate the biological nitrogen removal anoxic and oxic zones. The ba ffle wall consists of a weir and a submerged orifice. An experimental investigation of the hydraulics of the anoxic/oxic baffle wall is pres ented. It is shown that backflow from the oxic zone to the anoxic zone can occur if the baffle wall is not designed properly. This backflow will reduce the efficiency of the biological nitrogen removal process. Based on the experimental data, a design procedure that eliminates ba ckflow over the anoxic/oxic baffle wall is developed. The design proce dure introduces a backflow parameter that is a dimensionless number de pendent on the total discharge, orifice area, tank depth, and oxic zon e air concentration. The resulting design offers an economical alterna tive to designs implementing a valve option.