Simultaneous removal of organic substances and nitrogen using a membrane bioreactor seeded with anaerobic granular sludge under oxygen-limited conditions

An innovative process, the oxygen-limited membrane bioreactor seeded with anaerobic granular sludge, was proposed and its performance investigated for concurrent removal of organic substances and nitrogen from synthetic domestic wastewaters. An air diffuser was installed just above the granular sludge bed to supply air to the reactor at an intermittent mode. The internal recycle from the upper part of the reactor to the bottom was introduced to provide the granular sludge bed under the oxygen-limited conditions. The oxygen addition rates were controlled at 3–4 g O2 l!1d!1. The total COD removal efficiency of more than 94% was achieved throughout the whole operation period. N was removed through the simultaneous nitrification and denitrification process that took place in the granular sludge bed. TN levels decreased with the decrease of ammonium levels, indicating that nitrification was the rate-limiting step. The TN removal efficiency reached 80–91% at an hydraulic retention time of 15 h. Nitrate was scarcely detected and nitrite was the main NOx-N species in the effluent, indicating that nitrite oxidizers were inhibited in the system.

This investigation showed that simultaneous removal of organic substances and nitrogen from synthetic domestic wastewater was successful in a membrane bioreactor seeded with anaerobic granular sludge under oxygen-limited conditions. Preliminary batch experiments have been carried out to acclimatize the anaerobic granular sludge to the microaerophilic environment. The loss of COD and N occurred in the batch systems. From the data of DO profiles, the membrane bioreactor could be classified into two zones: microaerophilic (inside the granular sludge bed) and aerobic (above the granular sludge bed). Throughout the entire operational period, the total COD removal efficiency was above 94%, unaffected by the change in HRT. Organic substance was oxidized both by microaerophilic and aerobic processes. TN was successfully nitrified, and 80– 91% of TN was removed at 15 h HRT. Throughout the study, nitrite instead of nitrate was the major nitrification product. This led to the conclusion that nitrite oxidizers were inhibited. Based on the observed results, the use of an oxygen-limited membrane bioreactor seeded with granular sludge seems to be a potential technology for concurrent removal of COD and N. If the process is to be used for technical applications, the operation parameters such as COD/N ratio and supplied oxygen rate need to be further studied. Moreover, the biochemical and microbiological mechanisms have to be investigated.