Il processo cicli alternati - MBR per il trattamento avanzato delle acque reflue ( advanced wastewater treatment by membrane bioreactors operating the alternate cycle process)

Today membrane bioreactors (MBRs) can no longer be considered a novel technology for wastewater treatment. However, the most appropriate practice to design and operate these systems is not consolidated. The major aim of this three-year research activity was to develop a biotechnological system which could be sustainable and ready for the widespread application for advanced municipal wastewater treatment. The basic technological idea was to couple a MBR with the intermittent aeration of the bioreactor and the process control automation. Therefore, the alternate cycles process, which operates the automatic control of the intermittent aeration in the biological reactor on the basis of the on-line signals of dissolved oxygen and redox potential, was applied to a membrane bioreactor in the so-called AC-MBR system. The experimental activity has been carried out, always treating real municipal sewage, on three different scale-levels: the big pilot membrane plant, the demonstration and the full scale MBRs. All the experimental facilities were equipped with submerged modules of hollow fibre ultrafiltration membranes (filtration area respectively of 21,6; 69,9 and 12.130 m2). In particular, the pilot plant was used to assess (a) the impact of biomass bulk parameters on the membrane fouling and (b) the role of fouling/cake layer for metals removal. The demonstration MBR was used to investigate the nutrients and micropollutants removals and reliability of control algorithm. Finally, the full scale validation of the technology was possible by supervising the real AC-MBR system. The demonstration study was carried out over about 500 days. The plant was fed with real wastewater, where salts of nitrogen and phosphorus and external carbon source were dosed in certain runs to increase the influent loadings. Nine steady state experimental runs were carried out increasing the nitrogen loading rate (NLR), run by run, from 0,05 up to 0,25 kgN ⋅m−3 ⋅ d −1 reaction , while the C/N ratio was always ≤ 8÷9. NLRs around 0,16÷0,17 kgN ⋅m−3 ⋅ d −1 reaction were considered the maximal treatment capacity of the ACMBR system in terms of influent nitrogen. Under the maximal treatment capacity, the carbon and nitrogen removal were excellent: the ammonia nitrification was almost complete (nitrification efficiencies >90%), while the denitrification was the rate limiting step for the alternating system. According to the mass balances, the biological process removed 0,1 kgN per kgCODremoved, notwithstanding the over-aeration phenomena were often observed in night-times. The autotrophic nitrification seemed not very sensitive to the sludge age within the range experimented (20÷50 days). On the other hand, the automatic control system played a key role to optimize the nitrogen removal. In fact, it was able to manage the alternation of the anoxic and aerobic phases consistently with the influent loadings. This flexibility was lost for C/N<5÷6, that may be considered a border line for the good efficiency of the dynamic system. The phosphorus was basically removed only for biomass assimilation. However, the P biological removal was enhanced when external acetate was included in the carbon source, and the resulting removal rates were around 60%...