Abstract: A process and apparatus for biological purification of wastewater resulting in reduction of biological oxygen demand, reduction of suspended solids, and for nitrification, wherein the process involves aerating wastewater in a treatment zone for reduction of biological oxygen demand, transferring the mixed liquor from this treatment zone on an alternate basis to a second or third treatment zone which undergoes an aeration/settle, fill and decant cycle. As the second or third zone is receiving influent, it is being decanted in a modified plug-flow fashion. The alternate treatment zones allow for a continuous discharge of treated effluent during the process. During aeration of the second or third treatment zone, mixed liquor is recycled to the first treatment zone. Sludge is retained long enough to achieve nitrification. This invention is particularly suited for modification of aerated lagoon systems to achieve nitrification with a high degree of efficiency.