ORIGINAL ARTICLE Co-digestion of orange peels and marine seaweed with cattle manure to suppress inhibition from toxicants Viviana Negro 1 & Merlin Alvarado-Morales 2 & Panagiotis Tsapekos 2 & Debora Fino 1 & Bernardo Ruggeri 1 & Irini Angelidaki 2 Received: 16 April 2020 /Revised: 13 July 2020 /Accepted: 23 July 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Traditionally, anaerobic digestion (AD) is applied for the simultaneous generation of energy and treatment of various organic wastes. For example, despite orange peels (OP) and seaweed (SW) are highly available for AD, their physicochemical compo- sition can inhibit the microbiome and collapse the process. In the present work, we examined the combination of OP and SW with livestock manure (MN) at a co-digestion process. At first, batch toxicity tests revealed that D-limonene and gallic acid at 0.2% v/v and 2.28 g L -1 respectively solely inhibit methane generation. At batch assays and based on added volatile solids (VS), the mono- digestion of OP and SW had methane yields of 398.4 ± 9.4 and 348.6 ± 10.2 m 3 t VS -1 , respectively. Furthermore, continuous mode experiments were conducted in order to examined process performance and bioenergy outcome at the co-digestion of OP, SW, and MN at various organic loading rates and substrate contributions. This work revealed two successful operation strategies when OP and SW were co-digested with manure resulting in methane yields of 264.5 ± 5.2 and 220.5 ± 5.2 m 3 t VS -1 , respec- tively. The results of this work potentially could be applied when dealing with other types of algal biomass. Keywords Anaerobic co-digestion . Livestock manure . Orange peels . Seaweed . Inhibition Abbreviations AD Anaerobic digestion (-) BMP Practical biomethane potential (m 3 t VS -1 ) HRT Hydraulic retention time (day) OP Orange peels (-) ORL Organic loading rate (kg VS m -3 day -1 ) SW Seaweed (-) MN Manure (-) ThBMP Theoretical biomethane potential (m 3 t VS -1 ) TKN Total Kjeldahl nitrogen (g kg -1 ) TS Total solids (kg kg -1 ) VFA Volatile fatty acids (kg m -3 ) VS Volatile solids (kg kg -1 ) 1 Introduction European Union regulations aim to boost the application of environmentally friendly methods (i.e., collection, transporta- tion, and management) to treating citrus waste and seaweed and avoid local nuances. However, these actions add extra costs and in the hunting of finding sustainable waste manage- ment solutions, the anaerobic digestion (AD) is exploited as a biological process to produce renewable energy. Furthermore, the depletion of fossil fuels and the adverse environmental impacts originated from overexploitation pave the way to find alternative engineering approaches capable of meeting the growing demands of humanity in a sustainable way. In this frame, the AD of livestock slurries is a well- established method for energy recovery. For instance, the Danish biogas plant is widely co-digesting livestock slurries with various wastes and wastewaters in Denmark motivated by their high availability (> 40 million tons are annually gen- erated) [1]. Co-digestion can be a solution for substrates that are associated with low bioenergy outcome, as orange peels (OP) and seaweed (SW), due to the existence of inhibitory molecules. Indeed, co-digestion appears as a solution to simul- taneously face the disposal issue of voluminous industrial * Panagiotis Tsapekos ptsa@env.dtu.dk * Irini Angelidaki iria@env.dtu.dk 1 Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy 2 Department of Environmental Engineering, Technical University of Denmark, Bld 115, 2800 Lyngby, Denmark Biomass Conversion and Biorefinery https://doi.org/10.1007/s13399-020-00912-w