Nitrogen removal in the A-stage can reach 30–40%, as nitrogen of organic compounds is incorporated in upflow anaerobic sludge blanket (UASB) reactor sludge.
The sludge age of the B-stage is typically between 8 and 20 days promoting the growth of nitrifiers. Therefore, complete nitrification is usually achieved in the B-stage. Complete denitrification is difficult to achieve, because of the low C:N ratio in the influent of the B-stage. Insufficient carbon supply of carbon source to the B-stage occurs due to the high efficiency of organic matter removal in the A-stage. The problem can be solved by decreasing organic matter removal in the A-stage, external carbon source supply, intermittent aeration or decreased HRT of the A-stage and/or on-line control of certain operational parameters. To achieve biological nitrogen and phosphorus removal anaerobic and anoxic compartments are introduced before the aerated zone of the B-stage.
Phosphorus removal from the secondary effluent of the B-stage can be achieved by coagulation with ferric and aluminium salts, e.g. FeCl or Al(SO).