Performance evaluation of a novel anaerobic–anoxic sludge blanket reactor for biological nutrient removal treating municipal wastewater Rubén Díez-Montero ⇑ , Loredana De Florio, Marta González-Viar, María Herrero, Iñaki Tejero Department of Water and Environmental Sciences and Technologies, University of Cantabria, Avda. Los Castros s/n, 39005 Santander, Spain highlights  A novel upflow anaerobic–anoxic sludge blanket reactor, AnoxAn, is presented.  Separate anoxic and anaerobic conditions were achieved inside the AnoxAn reactor.  The single reactor successfully performed denitrification and phosphate release.  Excellent nitrogen and phosphorus removal of AnoxAn coupled with an aerobic reactor.  AnoxAn promotes biomass retention and particulate organic matter hydrolysis. article info Article history: Received 18 November 2015 Received in revised form 15 February 2016 Accepted 18 February 2016 Available online 27 February 2016 Keywords: Anaerobic–anoxic reactor Biological nutrient removal Denitrification Enhanced biological phosphorus removal Sludge blanket abstract A novel anaerobic–anoxic sludge blanket reactor, AnoxAn, unifies the non-aerated zones of the biological nutrient removal treatment train in a single upflow reactor, aimed at achieving high compactness and efficiency. The environmental conditions are vertically divided up inside the reactor with the anaerobic zone at the bottom and the anoxic zone above. This contribution presents the performance evaluation of the novel reactor in the removal of organic matter and nutrients from municipal wastewater, coupled with an aerobic hybrid MBR. The overall system achieved total nitrogen and phosphorus removal with average efficiencies of 75% and 89%, respectively. Separate anoxic and anaerobic conditions were main- tained in AnoxAn, allowing anaerobic phosphate release and nearly complete anoxic denitrification in the single reactor operating with an HRT of 4.2 h. Biomass was retained in the reactor achieving TSS concen- tration up to 10 g L 1 and partial hydrolysis of influent particulate organic matter. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Nitrogen and phosphorus are the main nutrient elements dis- charged with wastewaters whose presence in the receiving water bodies is the major cause of eutrophication. Biological nutrient removal (BNR) processes avoid the use of chemicals and chemical sludge disposal but conventional configurations require complex and large treatment systems providing anaerobic, anoxic and aer- obic compartments. An aerobic reactor sufficiently large to estab- lish nitrification is required, which should be coupled with additional non-aerated (anoxic and anaerobic) reactors, resulting in a significant volume increase compared to the one needed for organic matter removal only. In the anoxic reactor, denitrification takes place where nitrate serves as an electron acceptor allowing organic matter consumption. In the anaerobic one, phosphate is released through the phosphate accumulating organisms (PAO) metabolism, while the subsequent accumulation of phosphate by PAO takes place in excess of metabolic requirements, under aerobic conditions. Phosphate uptake is also feasible under anoxic condi- tions using nitrate as sole electron acceptor, instead of oxygen (Vlekke et al., 1988), through the denitrifying phosphate accumu- lating organisms (DPAO) metabolism, which can lead to savings in plant operational costs due to energy savings for aeration, less sludge production and maximal influent organic substrate exploitation (Kuba et al., 1993; Oehmen et al., 2007). In order to reduce the BNR system complexity and volume requirements, compact and efficient aerobic reactors have been proposed, as well as the inclusion of the anaerobic and/or anoxic zones into the same aerobic reactor. In a different approach, the anaerobic and anoxic zones are unified in a single non-aerated reactor. This approach takes advantage of the complete separation from the aerobic reactor, making it easier to prevent the undesired intrusion of oxygen into the anoxic and anaerobic zones. For instance, Ahn et al. (2003) and Song et al. (2009, 2010) proposed http://dx.doi.org/10.1016/j.biortech.2016.02.084 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +34 942202286. E-mail address: ruben.diezmontero@unican.es (R. Díez-Montero). Bioresource Technology 209 (2016) 195–204 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech