Dynamics of protozoan and metazoan communities in a full scale wastewater treatment plant by rotating biological contactors

Performance of a full-scale wastewater treatment plant by rotating biologic al contactors (RBC) system was monitored during a year by physico-chemical and microbial characterisation. Six points along wastewater treatment were selected in the plant: three points along the water line (influent, sedimen tation tank and effluent) and three points along RBC system (RBC1, RBC2 and RBC3). Although a large seasonal change in the values of physico-chemical parameters was observed, operation of the plant was optimal during all year (90% of removal in BOD5 and SS influent content). Microbial characterisati on was approached by determining the structure and dynamics of protozoan an d metazoan communities. Protozoa were the most abundant in all stages in th e plant, heterotrophic flagellates being the most representative group in t he water line and ciliates in the RBC system. The same seasonal preference was only observed for heterotrophic flagellates in the water line and green flagellates in the RBC system, both groups having highest abundances in su mmer and spring, respectively. Identification of ciliated protozoa populations rendered 58 species of cili ates in the plant. Most of these species are typical of aerobic wastewater treatment systems except three of them, which are cited for the first time in this type of ecosystems: Chaenea stricta, Holosticha mancoidea and Oxytr icha lanceolata. Along the water line 34 species were identified, and half of them only appeared occasionally (once in all the study), while along the RBC system biofilms 55 species were observed, and the majority appeared pe rmanently in this system. Our results indicate that the type of habitat, rather than the physico-chem ical water parameters, was the primary factor in determining the different distribution of protozoan and metazoan communities in the plant. In RBC bio films, the structure of ciliate protozoa community was found to be quite se nsitive to changes in physico-chemical parameters; mainly to organic loadin g (BODS) variations.