NOVELTY - The wastewater purification method to denitrify nitrogenous compounds, comprises admitting wastewater into a first vessel for holding and digesting wastewater with a facultative heterotrophic bacteria, removing a predetermined portion of fluid from the first vessel, admitting a part of the predetermined portion of fluid into second vessel for the growth of autotrophic ammonia to oxidize bacteria as the dominant microbial population, supplying restricted fluid communication between the first and second vessel, and fostering the growth of autotrophic ammonia oxidizing bacteria.    USE - The method is useful to biologically denitrify nitrogenous compounds in wastewater using a simultaneous reaction under aerobic conditions.    ADVANTAGE - The method can consistently reduce nitrates below the drinking water standard, does not depends on the action of bacteria in a soil leach field/engineered mound septic system to remove ammonia, and increases the capacity of bacterial digestion to process the materials at a higher rate.    DETAILED DESCRIPTION - The wastewater purification method comprises admitting wastewater into the first vessel for holding and digesting wastewater with a facultative heterotrophic bacteria as the dominant microbial population, removing a predetermined portion of fluid from the first vessel, admitting a part of the predetermined portion of fluid into second vessel for the growth of autotrophic ammonia to oxidize bacteria as the dominant microbial population, supplying restricted fluid communication between the first and second vessel, and fostering the growth of autotrophic ammonia oxidizing bacteria in the partially isolated zone. The portion of fluid is admitted to the second vessel remain in the second vessel for a sufficient amount of time to replenish the volume of the first vessel with autotrophic ammonia oxidizing bacteria in which the facultative heterotrophic bacteria are the dominant microbial population. The second vessel is disposed within the first vessel. The second vessel is provided with a repository for a starting culture of autotrophic ammonia oxidizing bacteria. The autotrophic ammonia oxidizing bacteria convert ammonia to nitrite in the first vessel. The facultative bacteria in the first vessel are used to strip the oxygen from the nitrite and convert it to gaseous nitrogen by denitrifying the effluent. The second vessel for the production of the ammonia oxidizing bacteria is created by partitioning a portion of the first vessel to close off a water column from the rest of the liquid in the entire first vessel. The partition extends upward above the fluid surface to preclude fluid passing over the partition. The bottom of the column or side partitions is closed limiting flow from the outer vessel into the first vessel. A pump is used to remove the portion of fluid from the first vessel. The portion of fluid is introduced into the second vessel containing sufficient ammonia to support a colony of ammonia oxidizing bacteria. The second vessel has a reserve of inorganic carbon. An organic carbon is depleted in the first vessel due to the prolonged aerobic treatment time by inhibiting the growth of facultative heterotrophs. Digestion of the carbon creates mineral carbonates fostering the growth of autotrophic ammonia oxidizing bacteria in the second vessel. A part of liquid admitted to the second vessels displaces liquid from the second vessel forcing it out into the second vessel. The displaced liquid is enriched with autotrophic ammonia oxidizing bacteria. The denitrifying reaction is carried out by the joint action of the autotrophic ammonia oxidizing. The facultative heterotrophic bacteria are sustained through the steady inoculation of the more sensitive autotrophs into the portion of the first vessel that is outside of the partition forming the second vessel. A third treatment vessel is aerated for sustaining a colony of facultative heterotrophic bacteria. The flow of fluid between the first vessel and the third vessel is less restrictive than the first and second vessel. The fluid admitted has a mean residence time of less than 4 days and greater than 2 days. An INDEPENDENT CLAIM is included for an apparatus for wastewater treatment.    DESCRIPTION OF DRAWING(S) - The diagram shows an apparatus:    Arrow; (10)    Vessel; (101)    Sub-chamber; (102)    Inlet port. (102a)