A rotating biological contactor (RBC) system operating in a full-scale wastewater treatment plant has been described by several approaches accounting for performance, composition and structure of biofilms in three stages through biological wastewater treatment (RBC1, RBC 2, RBC 3). RBC biofilms were effective in removing the BOD loading from 13 g BOD5 d−1 m−2 in RBC 1 to 6 g BOD5 d−1 m−2 in RBC 3. Analysis of biofilm composition showed: i) the volatile solids were similar in the three RBCs (0.6 g m−2 VS per g m−2 of TS); ii) the protozoan and metazoan biocenosis was mainly made up of ciliated protozoa, which were most abundant in RBC 2 (1.84 × 106 ciliates g−1 VS). Relationship between ciliate species and physical-chemical profile of the system by cluster analysis indicated that the species Acineria uncinata, Amphileptus punctatus, Cinetochilum margaritaceum and Holosticha mancoidea were associated with the best RBC performance; iii) the exopolymeric matrix of the three RBC biofilms was mainly constituted by proteins, although humic substances, polysaccharides, uronic acids and DNA were also found. Analysis of biofilm structure by confocal microscopy indicated changes in biofilm organisation with depth. Results have been brought together and a graphic representation of the composition and architecture of RBC biofilms is presented.

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