Aerobic granular systems for biological treatment of industrial wastewater: operation and characterization of microbial populations

Abstract

The new regulations and policies, both at a State and European Community level, are the more and more exigent regarding the concentrations of pollutants in the generated effluents at industrial and urban level. This fact, together with the reduction of the available surfaces for their management, is promoting the development of new more effective and compact systems and technologies for wastewater treatment where the simultaneous organic matter and nutrients removal, like nitrogen and phosphorus, can be performed. The use of sequencing batch reactors (SBR) with aerobic granular biomass applied for the biological wastewater treatment in aerobic conditions is presented as an alternative to the conventional activated sludge processes. These reactors are operated in sequential cycles that comprise filling, reaction, settling and withdrawal phases that are characterised by the short length of the filling and settling ones aiming to the development of biomass in the shape of granules in aerobic conditions. These systems allow the treatment of higher loading rates than the activated sludge systems because higher biomass concentrations can be achieved. On the other hand, the excellent settling properties of aerobic granules and their low biomass productivity allow doing without the big and expensive secondary settlers and moreover smaller footprint is required for its implantation. Another advantage of these granular systems is that organic matter and nitrogen removal can be simultaneously carried out in the same unit by means of simultaneous nitrification and denitrification processes (SND). In this way not only the improvement of the quality of the generated effluents but also a reduction of the implementation and sludge management costs are attained. First studies focused on aerobic granulation were developed at laboratory scale reactors using synthetic media in order to better understand all the variables that affected the granulation process. In this way the settling time, the hydrodynamic shear and stress forces, the type of substrate, the organic matter (COD) and nitrogen concentrations, the dissolved oxygen concentration in the bulk liquid or the presence of possible inhibitory substances were identified to be among the key factors affecting the operation of these systems.