Long-term experience with an automatic process control for nitrogen removal in membrane bioreactors

The alternating processes applied in membrane bioreactors for municipal wastewater treatment may be an attractive option to reduce the energy consumptions and optimize carbon and nitrogen removal. However, the knowledge of these systems is often based on empirical results so to discourage the plant operators for its adoption. This paper discusses and compares the empirical evidence coming from two different alternating membrane bioreactors, a demonstration and a full-scale one. The two plants treat two real municipal wastewaters, rather different for both C:N ratio and degree of biodegradability of the influent organics. Nine steady-state runs have been carried out in the demonstration plant, while a one whole year operation has been considered for the full-scale system. Combining the results of the two MBRs, it was found that the alternating process was able to adjust automatically and adequately the aeration of the biological reactor with a nitrogen loading rate in the range 0.05– 0.18 kgN m!3 d!1 and C:N mass ratios greater than 5–6. As a result, the use of the available carbon source, with concern to the total nitrogen removal, was as low as 0.1 kg of total nitrogen removed per kg of total influent COD. Effluent total nitrogen met the standard for reuse with specific energy consumptions in the range 85–109 gTNremoved per kWhconsumed. Considering the usual loading conditions of the municipal wastewater treatment plants in Italy, membrane bioreactors operating alternating processes may be implemented to increase the nitrogen treatment capacity of existing plants and achieve the standards for reuse.

The results from the long-term operation of a demonstration and full-scale MBRs intermittently aerated have been presented. The technology was proposed for the treatment and reuse of municipal wastewater and the key performance indicators proved its reliability. The scale of the experimentations allows to generalize the results, so to outline the “knowhow” for this kind of automatically controlled system applied to membrane bioreactors. The main remarks of the study were the following: 1. The system was able to adjust the alternation and the length of anoxic and aerobic phases, so to optimize the aeration for the biological process for NLRs in the range 0.05– 0.18 kgN m!3 d!1 and C:N mass ratios greater than 5–6. In practice, these values mark the border lines beyond which alternating processes are no more flexible with respect to the influent loading fluctuations. However, considering the actual loading conditions for the major part of the Italian municipal wastewater treatment plants, the AC-MBR can be adopted both for new systems and also to upgrade existing ones, increasing the nitrogen treatment capacity and meeting with the reuse standards. 2. The use of the available carbon source, with concern to the total nitrogen removal, was as low as 0.1 kg of total nitrogen removed per kg of total influent COD. However this value is affected by phenomena of over-aeration of the activated sludge and can reasonably be considered as a minimal performance for such an alternating membrane bioreactor. 3. The system was able to optimize the removal of total nitrogen achieving while at the same time minimizing the power requirements, according to the best energy saving practices. 4. The control strategy was validated at different scales so to consider the technology ready for the widespread, full-scale application.