Nitrogen and carbon balance in a novel near-zero water exchange saline recirculating aquaculture system Uri Yogev a , Kevin R. Sowers b , Noam Mozes c , Amit Gross a, ⁎ a Zuckerberg Institute for Water Research, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Israel b Department of Marine Biotechnology, Institute of Marine and Environmental Technology, University of Maryland Baltimore County, Baltimore, MD 21202, United States c Israeli Ministry of Agriculture and Rural Development, Department of Fishery and Aquaculture, Beit Dagan 50250, Israel abstract article info Article history: Received 31 January 2016 Received in revised form 11 April 2016 Accepted 26 April 2016 Available online xxxx In response to increasing demand for aquaculture products and strict new regulations on organic matter and nitrogen discharge, inland closed recirculating aquaculture systems (RASs) are being developed as a viable eco-sustainable alternative to traditional aquaculture (e.g. ponds, raceways and cages) because of their minimal environmental impact and controlled operation. Fish feed is virtually the only source of carbon and nitrogen to the system. It is estimated that 20 to 30% of the feed nitrogen and 50% of the feed carbon are assimilated or uti- lized by the fish, while the rest is released to the water. Understanding the fate and utilization of these elements can help optimize RAS efficiency and economics. The fate of carbon and nitrogen was studied by mass balance in a novel near-zero discharge (b 1% water exchange of system's volume per day) saline research scale RAS. The sys- tem included a fish tank attached to three treatment loops: (a) a solid filter followed by an aerated nitrification fixed-film reactor, (b) a single-stage anoxic denitrification activated sludge bioreactor which utilizes fish sludge as a carbon source, and (c) an anaerobic bio-digester (upflow anaerobic sludge blanket [UASB]) for treatment of excess denitrification biomass for the production of biogas. About 50% of the introduced carbon (from feed) was removed by fish assimilation and respiration, and another 10% by aerobic biodegradation in the nitrification bioreactor. In the denitrification reactor, 10% carbon was removed and 25% carbon was introduced into the UASB reactor, of which 12.5% was converted to methane, 7.5% to CO 2 and the rest (5%) remained as nondegradable carbon in the UASB. Using the UASB can save up to 12% of the system's energy demands, both directly as energy (methane) input and indirectly by reducing the system's oxygen demand. Of the feed nitrogen, 29% was assimilated by the fish and bacteria in the nitrification reactor and 40–50% was removed in the denitrification reactor, of which 10–20% was removed by anammox. Lastly, ~20% of the nitrogen was removed in the UASB reactor, likely by precipitation. It was demonstrated that the system was operating at high stocking density, with almost complete nitrogen and carbon removal and energy recovery. Statement of relevance: The fate of carbon and nitrogen was studied by mass balance in a novel near-zero discharge (b 1%) saline RAS. A novel approach which may significantly reduce pollution, save water and energy and improve intensive aquaculture operations was demonstrated. It was postulated that the system operated at high efficiency, with almost complete nitrogen and carbon removal and energy recovery. © 2016 Elsevier B.V. All rights reserved. Keywords: RAS Carbon balance Nitrogen balance Anaerobic digestion Denitrification N 2 O emission 1. Introduction Aquaculture is among the fastest growing animal food-producing sectors, accounting for almost half of the total food fish supply (FAO, 2014). If not adequately addressed, current aquaculture practices can have negative environmental impacts as a result of eutrophication of water bodies, landscape modification, and changes in biodiversity (Tovar et al., 2000). For this reason, regulations have been introduced related to water use (quantity and quality) and waste discharge (Jokumesen and Svendsen, 2010). These and other concerns (e.g. disease control and weather effects) have motivated the industry to explore land-based recirculating aquaculture systems (RASs) as an alternative to the traditional open ponds and cage culture systems (Avenue and Kong, 1995; Timmons and Ebeling, 2007). In RASs, water from the fish tank is circulated through bioreactors that treat it for return and reuse in the growth tank. This concept provides enhanced control over water quality, fish performance, biosecurity and energy use (Ebeling, 2000; Timmons and Ebeling, 2007; Tal et al., 2009). In the RAS, fish feed is virtually the only source of carbon and nitrogen solids, which are the major sources of pollution. It is estimated that by weight, the amount of solids produced in a RAS accounts for about 30 to Aquaculture xxx (2016) xxx–xxx ⁎ Corresponding author at: Zuckerberg Institute for Water Research, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus 84990, Israel. E-mail address: amgross@bgu.ac.il (A. Gross). AQUA-632121; No of Pages 9 http://dx.doi.org/10.1016/j.aquaculture.2016.04.029 0044-8486/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aquaculture Please cite this article as: Yogev, U., et al., Nitrogen and carbon balance in a novel near-zero water exchange saline recirculating aquaculture system, Aquaculture (2016), http://dx.doi.org/10.1016/j.aquaculture.2016.04.029