Sludge bed development in denitrifying reactors using different inocula-performance and microbiological aspects C. Etchebehere, M.I. Errazquin, A. Cabezas, M.J. Pianzzola*, M. Mallo, P. Lombardi, G. Ottonello, L. Borzacconi** and L. Muxí* * Cátedra de Microbiología, Facultad de Química, Udelar, Uruguay ** Departamento de Ingeniería de Reactores, Facultad de Ingeniería, Udelar, Uruguay Abstract Aerobic and methanogenic consortia were evaluated as inocula for laboratory scale denitrifying reactors, fed with a synthetic wastewater with acetate as the main electron donor. The denitrifying microflora of inocula and reactors was evaluated by specific denitrifying activity, enumeration and isolation of denitrifiers, which were screened by amplified ribosomal DNA restriction analysis. Reactor performance was monitored by COD and nitrate removal efficiencies and granule size. The aerobic sludge failed to form granules, probably due to the development of a filamentous, nitrate-reducing organism which was characterised by 16SrDNA sequencing as Bacillus cereus. The methanogenic sludge showed denitrifying activity and adapted very rapidly to denitrifying conditions in the two reactors seeded with granules of different sizes. Denitrifiers grew around the granules, increasing the specific denitrifying activity of the sludge over 10-fold. Exopolymer-forming organisms, belonging to the same species, were isolated from both reactors. Granule size increased during operation, but flotation of the aggregates, related to gas retention was observed. Keywords Denitrification; granulation; inocula; sludge bed anoxic reactor Introduction The need for the removal of nitrogen compounds from effluents before final disposal is being increasingly recognised, and should be considered when designing a biological treatment system. Although a biological denitrification step is widely used to lower nitrate concentrations to acceptable levels, the information about the microbiology and sludge behaviour of denitrifying reactors is still scarce. Recently, it has been shown that the denitrifying organisms isolated from an anoxic reactor treating leachate from a sanitary landfill could not be identified by conventional methods, requiring genetic characterisation based on 16SrDNA sequences (Etchebehere et al., 2001). Anoxic UASB reactors have been successfully used for nitrate removal in an integrated system (anaerobic, aerobic, anoxic) treating effluents with high organic and nitrogen content (Borzacconi et al., 1999; Morgan-Sagastume et al., 1994). However, there are few reports on the parameters that affect granulation of the denitrifying biomass (Cuervo-López et al., 1999). Additionally, buoyancy problems have been reported in UASB anoxic reactors (Cuervo López et al., 2000a; Cuervo López et al., 2000b; Hendriksen and Ahring, 1996). In the present work, we describe the behaviour of laboratory scale UASB denitrifying reactors fed with acetate and nitrate. The aim was to study the effect of the nature of the inoculum on granulation and reactor performance considering both microbiological and operational aspects. Materials and methods Three upflow reactors (4.6 L) were seeded with different inocula: i) aerobic sludge from a RBC reactor treating acetate. ii) intact granules from an anaerobic UASB reactor treating Water Science and Technology Vol 45 No 10 pp 365–370 © IWA Publishing 2002 365 Downloaded from https://iwaponline.com/wst/article-pdf/45/10/365/424904/365.pdf by guest on 04 November 2018