ORIGINAL PAPER Interpreting the effect of increasing COD loading rates on the performance of a pre-anoxic MBBR system: implications on the attached and suspended biomass dynamics and nitrification–denitrification activity P. S. Lima 1 • M. Dezotti 1 • J. P. Bassin 1 Received: 4 February 2016 / Accepted: 15 February 2016 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract A pre-anoxic MBBR system was subjected to increasing organic loading rates up to 18 gCOD/(m 2 day). At 3 gCOD/(m 2 day), most of the incoming organic matter was removed via denitrification. However, at higher loads, anoxic COD removal became limited by the nitrite/nitrate supply from the aerobic reactor, which assumed an important role in this conversion. Despite the application of low dissolved oxygen (DO) levels (\2 mg/L) in this tank, nitrification was observed to be nearly complete until 8 gCOD/(m 2 day). As the organic input was increased, the maximum specific nitrifying activity gradually declined. Activity tests suggested that an oxygen-limited environ- ment was established in the biofilm. At lower loads [3–8 gCOD/(m 2 day)], the nitrification product obtained was affected by the DO concentration, whereas from 16 to 21 gCOD/(m 2 day), nitrite/nitrate profiles were likely associated with microbial stratification in the biofilm. The results also indicated that the role of the suspended biomass in the overall nitrification and denitrification can be very significant in high loaded MBBRs and should not be neglected, even at low HRTs. Keywords Moving-bed bioreactors Á Biofilms Á Nitrogen removal Á Organic load Á Nitrifying activity Introduction Historically, pollution by organic matter and nutrients (especially nitrogen and phosphorus) in receiving waters has been a big concern worldwide due to their deleterious effect on aquatic life. The removal of oxygen-demanding carbonaceous materials (often expressed in terms of chemical oxygen demand—COD) and eutrophication- causing nutrients from wastewaters is often accomplished by means of bacterial metabolism and subsequent physical solid–liquid separation, in the so-called biological wastewater treatment processes [1]. Traditionally, this treatment is conducted in engineered suspended biomass- based reactors, such as the conventional activated sludge (AS) process. However, given the complexity of some wastewaters, the AS process frequently shows some drawbacks such as poor biomass settling properties due to excessive growth of filamentous bacteria (bulking sludge) [2], high susceptibility of inhibition by the wastewater compounds [3] and large footprint compared to concurrent processes employing other forms of biomass agglomeration rather than flocculent sludge [4]. The operation of sus- pended growth AS-based systems becomes even more complicated when high organic concentrated streams need to be processed [5], a challenge routinely faced by brew- eries, tanneries, dairies, food processing, paper and pulp and textile industries. With the increasing tendency toward the recycle of the treated wastewater driven by water shortage issues, the generation of more concentrated effluent streams will gradually become more common in the industrial sector. In recent years, the need for meeting stringent effluent quality pushed the development of novel biological pro- cesses, with compact design and high treatment perfor- mance. A considerable fraction of the new developments Electronic supplementary material The online version of this article (doi:10.1007/s00449-016-1574-0) contains supplementary material, which is available to authorized users. & J. P. Bassin jbassin@peq.coppe.ufrj.br 1 Chemical Engineering Program/COPPE, Federal University of Rio de Janeiro, P.O. Box 68502, Rio de Janeiro 21941-972, Brazil 123 Bioprocess Biosyst Eng DOI 10.1007/s00449-016-1574-0