Bioflocculation management through high-rate contact-stabilization: A promising technology to recover organic carbon from low-strength wastewater Arifur Rahman a, c, * , Francis A. Meerburg b , Shravani Ravadagundhi c , Bernhard Wett d , Jose Jimenez e , Charles Bott f , Ahmed Al-Omari c , Rumana Riffat a , Sudhir Murthy c , Hayd  ee De Clippeleir c a Department of Civil & Environmental Engineering, The George Washington University, 800 22nd Street, NW, Washington, DC 20052, USA b Center for Microbial Ecology and Technology (CMET), Ghent University, Coupure Links 653, 9000 Ghent, Belgium c DC Water, 5000 Overlook Ave., SW., Washington, DC 20032, USA d ARA Consult GmbH, Unterbergerstrasse 1, 6020 Innsbruck, Austria e Brown and Caldwell, 2301 Lucien Way, Suite 250, Maitland, FL 32751, USA f Hampton Roads Sanitation District, 1436 Air Rail Ave., Virginia Beach, VA 23455, USA article info Article history: Received 16 June 2016 Received in revised form 11 August 2016 Accepted 21 August 2016 Available online 23 August 2016 Keywords: Adsorption Carbon capture Energy neutrality Extracellular polymeric substances Observed yield Municipal wastewater abstract A series of pilot-scale studies were performed to compare conventional high-rate activated sludge sys- tems (HRAS) (continuous stirred tank reactor (CSTR) and plug flow (PF) reactor configurations) with high-rate contact-stabilization (CS) technology in terms of carbon recovery potential from chemically enhanced primary treatment effluent at a municipal wastewater treatment plant. This study showed that carbon redirection and recovery could be achieved at short solids retention time (SRT). However, bio- flocculation became a limiting factor in the conventional HRAS configurations (total SRT  1.2 days). At a total SRT 1.1 day, the high-rate CS configuration allowed better carbon removal (52e59%), carbon redirection to sludge (0.46e0.55 g COD/g COD added ) and carbon recovery potential (0.33e0.34 gCOD/ gCOD added ) than the CSTR and PF configurations (28e37% COD removal, carbon redirection of 0.32 e0.45 g COD/g COD added and no carbon harvesting). The presence of a stabilization phase (famine), achieved by aerating the return activated sludge (RAS), followed by low dissolved oxygen contact with the influent (feast) was identified as the main reason for improved biosorption capacity, bioflocculation and settleability in the CS configuration. This study showed that high-rate CS is a promising technology for carbon and energy recovery from low-strength wastewaters. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The conundrum of aerobic wastewater treatment is that a considerable amount of electrical energy is used for aeration, to remove and oxidize chemical energy contained in the influent that could otherwise be harvested to produce energy (Reardon, 1995). It has been shown that the potential chemical energy available in the raw municipal wastewater influent or primary effluent exceeds the electrical energy requirement of the treatment process by a factor of 1.2e6.0 (Svardal and Kroiss, 2011). Energy-neutral wastewater treatment should therefore be possible, especially when using technologies that minimize energy consumption and maximize recovery, such as high-rate activated sludge (HRAS) treatment (Wett et al., 2007). HRAS systems can be one of the most successful carbon redirection and carbon harvesting technologies in temperate and colder climates and can be retrofitted into existing infrastructure (Jimenez et al., 2015; Rahman et al., 2016). Carbon redirection denotes the transformation of organic carbon (partic- ulates, colloids and soluble) from wastewater into the sludge matrix through biosorption (i.e., extracellular adsorption or enmeshment and intracellular storage) and microbial growth phenomena (Rahman et al., 2015). Subsequently, carbon harvesting denotes the recovery of sludge carbon through settling and wasting of the * Corresponding author. Department of Civil & Environmental Engineering, The George Washington University, 800 22nd Street, NW, Washington, DC 20052, USA. E-mail address: arifur@gwmail.gwu.edu (A. Rahman). Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres http://dx.doi.org/10.1016/j.watres.2016.08.047 0043-1354/© 2016 Elsevier Ltd. All rights reserved. Water Research 104 (2016) 485e496