Nitrogen removal from sludge reject water with a membrane-assisted bioreactor

Abstract The start-up of a membrane-assisted bioreactor for nitrogen removal from sludge reject water was examined. The aim was to rapidly achieve a high nitrogen loading rate while maintaining complete nitrification and to maximize denitrification with addition of methanol or acetic acid. The use of an ultrafiltration unit with a positive displacement pump for sludge recirculation resulted in complete nitrification at an aerobic sludge loading rate of 0.16 kg N (kg SS) −1  d −1 within two weeks. The use of a microfiltration unit with a centrifugal pump for sludge recirculation, resulted in complete nitrification at a maximal aerobic sludge loading rate of 0.08 kg N (kg SS) −1  d −1 . With the centrifugal pump, a sudden collapse of nitrification was observed. The rapid decrease of the specific sludge activity was attributed to shear stress resulting from the high recirculation rate of biomass through the centrifugal pump. NH 3 was found to inhibit Nitrobacter at a concentration higher than 0.1 mg N l −1 . Denitrification with methanol required an adaptation period of three weeks after which more than 80% denitrification was achieved. Denitrification of nitrite with methanol required a COD:N ratio of 2.3 g g −1 , denitrification of nitrate required a COD:N ratio of 3.8 g g −1 . With acetate no adaptation was needed and 90% denitrification was achieved. A low biomass growth of 0.092 g SS (g COD removed ) −1 or 0.398 g SS (g N removed ) −1 was obtained. Polymer ultrafiltration of activated sludge at a concentration of 4 g SS l −1 , a transmembrane pressure (TMP) of 220 kPa and a linear flow velocity (V) of 1.5 m s −1 resulted in a long term flux of 8 l m −2  h −1 . For the ceramic microfiltration membrane, a long term flux of 160 l m −2  h −1 was achieved (5.3 to 18.1 g SS l −1 , TMP=200 kPa, V =3.0 m s −1 ).