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

The start-up of a membrane-assisted bioreactor for nitrogen removal from sl udge reject water was examined. The aim was to rapidly achieve a high nitro gen loading rate while maintaining complete nitrification and to maximize d enitrification with addition of methanol or acetic acid. The use of an ultr afiltration 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 un it with a centrifugal pump for sludge recirculation, resulted in complete n itrification 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 wa s observed. The rapid decrease of the specific sludge activity was attribut ed to shear stress resulting from the high recirculation rate of biomass th rough the centrifugal pump. NH3 was found to inhibit Nitrobacter at a conce ntration higher than 0.1 mg N l(-1). Denitrification with methanol required an adaptation period of three weeks after which more than 80% denitrificat ion 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 CODremoved)(-1) or 0.3 98 g SS (g N-removed)(-1) was obtained. Polymer ultrafiltration of activate d sludge at a concentration of 4 g SS l(-1), a transmembrane pressure (TMP) of 220 kPa and a linear how velocity (V) of 1.5 m s(-1) resulted ina long term flux of 81 m(-2) h(-1). For the ceramic microfiltration membrane, a lo ng 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)). (C) 1998 Elsevier Science Ltd. All rights res erved.