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.