Algal Research 52 (2020) 102120 Available online 7 November 2020 2211-9264/© 2020 Elsevier B.V. All rights reserved. An integrated, two-step biofltration system with Ulva fasciata for sequenced removal of ammonia and nitrate in mariculture effuents Ben Shahar a, b , Lior Guttman c, * a Department of Life Sciences, Ben-Gurion University of the Negev Eilat Campus, Beer-Sheva 84105, Israel b Shamir Research Institute, University of Haifa, P.O. Box 97, Qatzrin, Israel c National Center for Mariculture, Israel Oceanographic and Limnological Research, Eilat 8811201, Israel A R T I C L E INFO Keywords: Phosphorus Ulva Aquaculture effuent Bioflter Nitrogen ABSTRACT Secondary treatment of fshpond effuent is imperative for the removal of excess nitrate and phosphorus. Bio- flters comprised of the green macroalgae, Ulva, exhibit several advantages for this process, including usability of the produced edible biomass. However, even low levels of ammonia in fshpond effuent inhibit the uptake of nitrate by Ulva. In order to overcome this drawback we examined Ulva faciata in a two-step biofltration system. The biofltration system comprised of two sequenced tanks where the upstream Ulva bioflter was fed directly with mariculture effuent containing both ammonia (as total ammonium nitrogen - TAN) and nitrate (as NO 3 -N) and the TAN-depleted but NO 3 -N-rich effuent from this bioflter were transferred to the downstream Ulva bioflter. The performance of each separate Ulva bioflter as well as that of the whole system was measured on a weekly regime over a culture period of three weeks. Between 81 and 95% of the dissolved inorganic nitrogen (TAN + NO 3 -N) in mariculture effuent was removed by the two-step bioflter, with specifc removal of 89–100% of TAN and 57–78% of NO 3 -N in mariculture effuent by the upstream and downstream Ulva, respectively. Exposure of Ulva in upstream bioflters to higher N input with both TAN and NO 3 -N did not affect algal pro- duction rate, which was relatively similar to that in downstream bioflters fed with TAN-depleted, NO 3 -N-rich effuent, with mean specifc growth rate of 17.85% d 1 (±3.1) in the different bioflters. Removal rates of TAN or NO 3 -N by upstream and downstream Ulva, respectively, were relatively similar, between 90 and 135 mmol N m 2 d 1 , while PO 4 -P removal was much faster in downstream Ulva bioflters having only NO 3 -N as a N source. While TAN removal induced production of Ulva biomass with higher N, P and protein content, culture in NO 3 -N- rich effuent resulted in Ulva biomass with higher lipid content. 1. Introduction Excess nitrogen and phosphorus in fshpond effuent results from the relatively poor assimilation, only between 20 and 30%, of these feed components by fsh [1]. The rest is excreted into their culture water as dissolved ammonia, i.e., NH 3 -N + NH 4 -N, here referred as total ammonia nitrogen (TAN), which may harm fsh even at relatively low concen- trations of the unionized form, NH 3 -N [2]. While effcient removal of toxic ammonia can be achieved via the combination of bacterial nitri- fcation and denitrifcation, the energy requirement for these processes makes them expensive [3] while not preventing accumulation of nitrate NO 3 -N [4]. Moreover, since bacterial nitrifcation and denitrifcation are both dissimilatory metabolisms, the result is emission of the expensive nitrogen from aquafeeds into the atmosphere rather than its assimilation by extractive organisms. Algal bioflters present several advantages, as they balance the pH and recycle nitrogen and phosphorus waste into nutrient-rich biomass via assimilation. The green algae, Ulva sp. (Chlorophyta), reveals great potential due to its rapid growth and weight gain, as high as 18% d 1 and 55 g wet weight (WW) m 2 d 1 , respectively [5]. When integrated in multitrophic aquaculture systems (IMTA), Ulva sp. removes ammonia effciently from effuents of fsh culture ponds, while its assimilation of this nutrient results in protein- rich, edible algal biomass containing up to 37% protein of dry weight (DW) [6,7]. Fresh Ulva can be further used for feeding of sea urchins [8–10], mollusks [11–13], and other algivores, or for incorporation after drying into pelleted aquafeeds for shrimps [14] and carnivorous fsh (e. g., sea bream [15,16]) to replace expensive protein-rich ingredients such as fshmeal. However, NO 3 -N in fshpond effuent is also of major * Corresponding author. E-mail address: lior.guttman@ocean.org.il (L. Guttman). Contents lists available at ScienceDirect Algal Research journal homepage: www.elsevier.com/locate/algal https://doi.org/10.1016/j.algal.2020.102120 Received 16 June 2020; Received in revised form 27 October 2020; Accepted 27 October 2020