The seasonal variance was only signi®cant with Beggiatoa ssp. This microorganisms was abundant during
spring, summer, and autumn, and decreased to rare in
winter.
The Eikelboom's type 021N presented signi®cant differences in the stage of the RBC, because it was frequently
present in the ef¯uent end. Chung and Strom (1991) only
observed this microorganism in the inlet side of one plant.
The interaction plant-seasons also presented signi®cant
differences (Fig. 2)