Granular sludge aggregates are particular types of biofilms that display significantly different metrics and physical-chemical characteristics than activated sludge flocs. The efficiency of intensified processes using granular sludge relies on the application of selection pressures. Operational conditions are engineered to force microorganisms to form specific intrinsic physiological, phenotypic, and metabolic traits for granulation and high-rate biological removal of nutrients and/or recalcitrant organic matter. Granular sludge and conventional activated sludge share a core microbiome, while the distribution of the underlying populations can significantly differ in relative abundance and localization in the architecture of granules and flocs. Analogous ecological principles of microbial selection apply from activated sludge to granular sludge ecosystems with the essential difference that granules are governed by diffusion limitations through which different redox potentials are created on micrometre scale. Integrating the microbiology dimension together with the physical–chemical features of granules in engineering practice will make a difference at process level, besides offering new opportunities for bioaugmentation of granules in existing infrastructure. With this review article we critically examine the macro-scale factors impacting granulation, the physical-chemical characteristics of granular sludge, and fundamental and applied questions driven by the microbial ecology of granular sludge, to generate useful concepts for process design and evaluation in engineering practice.

Anammox bacteria can perform dissimilatory nitrate reduction to ammonium (DNRA) with nitrite as intermediate coupled to the oxidation of volatile fatty acids (VFA). Batch tests with enriched anammox and a co-culture of anammox and heterotrophic bacteria showed the capacity of Candidatus ‘Brocadia fulgida’ to perform the DNRA coupled to the anammox reaction (DNRA-anammox) at a high rate although the culture was not previously adapted to VFA. From thermodynamic calculations it could be stated that low COD/N influent ratios favour the DNRA-anammox transformation over heterotrophic conversions since more free energy is gained. A process scheme is proposed for an innovative nitrogen removal system in which the nitrate produced by nitrite oxidizing bacteria and/or anammox bacteria is converted during DNRA-anammox pathway, resulting in a sustainable nitrogen removal from municipal wastewater while circumventing the troublesome out-selection of nitrite oxidizing bacteria encountered in mainstream applications.