Energy efficient treatment of A-stage effluent: pilot-scale experiences with shortcut nitrogen removal.

Energy autarky of sewage treatment plants, while reaching chemical oxygen demand (COD) and N discharge limits, can be achieved by means of shortcut N-removal. This study presents the results of a shortcut N-removal pilot, located at the biological two-'stage (high/low rate) wastewater treatment plant of Breda, The Netherlands. The pilot treated real effluent of a high-rate activated sludge (COD/N = 3), fed in a continuous mode at realistic loading rates (90-100 g N/(m(3)·d)). The operational strategy, which included increased stress on the sludge settling velocity, showed development of a semi-granular sludge, with average particle size of 280 μm (ø(4,3)), resulting in increased suppression of nitrite-oxidizing bacteria. The process was able to remove part of the nitrogen (51 ± 23%) over nitrite, with COD/N removal ratios of 3.2 ± 0.9. The latter are lower than the current operation of the full-scale B-stage in Breda (6.8-9.4), showing promising results for carbon-efficient N-removal, while producing a well settling sludge (SVI(30) < 100 mL/g).