The MLE process is commonly used to retrofit existing activated sludge facilities for
biological nitrogen removal. In the MLE process, the reactor is configured to have an
anoxic zone prior to the aerobic zone. The reasoning for this configuration is to allow the
alkalinity produced by denitrification in the anoxic zone to be available for the
nitrification step in the aerobic zone. Due to the internal recycle, the nitrate removal of
the process is limited by the internal recycle rate. This can potentially lead to low nitrate
removal efficiencies. An additional limitation of the MLE process is that it can generally
only achieve a total nitrogen effluent concentration of 5 to 8 mg/L, which is
comparatively high compared to other systems (Metcalf & Eddy et al., 2002)