Contents lists available at ScienceDirect Algal Research journal homepage: www.elsevier.com/locate/algal Combining medium recirculation with alternating the microalga production strain: a laboratory and pilot scale cultivation test J. Fret a, ⁎ , L. Roef a , L. Diels b,c , S. Tavernier d , W. Vyverman e , M. Michiels a a Proviron NV, G. Gilliotstraat 60, 2620 Hemiksem, Belgium b Department of Biology, Faculty of science, Antwerp University, Groenenborgerlaan 171, 2020 Antwerp, Belgium c Flemish Institute for Technological Research, VITO, Boeretang 200, 2400 Mol, Belgium d Faculty of Applied Engineering Sciences, Antwerp University, Salesianenlaan 90, 2660 Hoboken, Belgium e Department of Biology, Ghent University, Krijgslaan 281, S8, 9000 Ghent, Belgium ABSTRACT Reuse of growth medium after biomass harvesting is a cost-saving approach to improve the economic feasibility of algae mass cultivation. Algal exudates, cell debris and varying amounts of residual nutrients, impose challenges to the recycling of spent medium. In this study, the potential of combining reused medium from different algae species for growing monocultures of other algal strains was evaluated by making use of three successive cultivation setups with increasing volume; 400 mL in turbidostat mode, 2.6 L and 220 L in semi-continuous mode. Cultivation on replenished medium derived from Nannochloropsis sp. and Tisochrysis lutea, had no adverse effect on the productivity of either of the strains, regardless of whether they were grown in their own recycled medium or that of the other alga. Microfiltration of the reused medium proved to be sufficient to avoid cross-contamination. Moreover, a substantial average reduction in water footprint (77%) and nutrient cost (68% or 9 €·kg −1 dry biomass) was achieved. Extension and validation of the medium recycling approach to other economically interesting algae species can contribute to improving the economic feasibility of large scale microalgae production systems. 1. Introduction The number of algal species has been estimated to be anything from 30,000 to > 1 million [1]. However, only a few species (Chlorella, Ar- throspira, Dunaliella and Haematococcus) are commercially cultivated on a scale of hundreds to thousands of tonnes per year today, whereas about 10 additional species are produced on a smaller scale, aimed mainly at high-value products such as nutritional supplements, natural pigments, or aquaculture feed [2]. Nannochloropsis sp. CCAP211/78 (Eustigmatophyceae) and Tisochrysis lutea CCAP927/14 (Coccolithophyceae, formerly known as Isochrysis affinis galbana (Tahiti strain) [3]) are commercially cultivated for aquaculture in view of their nutritional value [4]. The Belgian based company Proviron NV cultivates these algae species using their pa- tented flat panel photobioreactor, dubbed ProviAPT (Proviron Ad- vanced Photobioreactor Technology) [5]. To improve the economic feasibility of mass cultivation, reducing material and energy costs associated with the production process is needed. Closing the water loop is a prerequisite [6,7] which permits not only to reduce water demand and to avoid or reduce waste water treatment costs, but also allows re-using remaining nutrients. Using waste effluents from for example aquaculture as growth medium can reduce production cost of microalgal cultivation and contributes to wastewater remediation [8–11]. Another approach is used in this study whereby the feasibility to reuse effluents from the microalgae cultiva- tion itself multiple times to cultivate the same or a different alga is investigated. Multiple studies have assessed the feasibility of recycling culture medium for several microalgae species. However, divergent results were obtained, including decreased [12–14], enhanced [15–17] or unaffected [18–23] growth. Most studies, however, suggest that re- circulation of growth medium is possible and beneficial, depending on the choice of harvesting method and rejuvenation technique. Still, large-scale recycling experiments under commercial production condi- tions are limited. Furthermore, alternating the cultivated species can challenge the medium reusability. When multiple monocultures are cultivated on large-scale installa- tions, mixing of filtered harvest broths sourced from these cultures will occur due to the use of a centralized harvest unit. While reusing this medium, allelopathic effects may occur and influence growth [24]. For example, Yingying et al. [25] isolated a component, 1-[hydroxyl-die- thyl malonate]-isopropyl dodecenoic acid, C 22 H 38 O 7 , from an Isochrysis galbana culture which inhibited growth of I. galbana itself and of eight selected algae species. Richmond et al. and Zou et al. [26,27] observed that the production of auto-inhibitors is a common phenomenon in high cell density cultures of Nannochloropsis. The species-specificeffect of https://doi.org/10.1016/j.algal.2019.101763 Received 23 August 2019; Received in revised form 13 November 2019; Accepted 14 December 2019 ⁎ Corresponding author. E-mail address: jorien.fret@proviron.com (J. Fret). Algal Research 46 (2020) 101763 2211-9264/ © 2019 Elsevier B.V. All rights reserved. T