energies Article Co-Digestion of Rice Straw with Cow Manure in an Innovative Temperature Phased Anaerobic Digestion Technology: Performance Evaluation and Trace Elements Furqan Muhayodin 1,2, *, Albrecht Fritze 1 , Oliver Christopher Larsen 1 , Marcel Spahr 3 and Vera Susanne Rotter 1   Citation: Muhayodin, F.; Fritze, A.; Larsen, O.C.; Spahr, M.; Rotter, V.S. Co-Digestion of Rice Straw with Cow Manure in an Innovative Temperature Phased Anaerobic Digestion Technology: Performance Evaluation and Trace Elements. Energies 2021, 14, 2561. https:// doi.org/10.3390/en14092561 Academic Editor: Daehwan Kim Received: 30 March 2021 Accepted: 26 April 2021 Published: 29 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Environmental Technology, Chair of Circular Economy and Recycling Technology, Technische Universität Berlin, 10623 Berlin, Germany; a.fritze@tu-berlin.de (A.F.); oliver.larsen@tu-berlin.de (O.C.L.); vera.rotter@tu-berlin.de (V.S.R.) 2 Department of Farm Machinery and Power, University of Agriculture, Faisalabad 38000, Pakistan 3 Herbst Umwelttechnik GmbH, Goerzallee 305E, 14167 Berlin, Germany; marcel.spahr@herbstumwelt.de * Correspondence: furqan.muhayodin@campus.tu-berlin.de Abstract: Rice straw is an agricultural residue produced in abundant quantities. Open burning and plowing back the straw to the fields are common practices for its disposal. In-situ incorporation and burning cause emissions of greenhouse gas and particulate matter. Additionally, the energy potential of rice straw is lost. Anaerobic digestion is a technology that can be potentially used to utilize the surplus rice straw, provide renewable energy, circulate nutrients available in the digestate, and reduce greenhouse gas emissions from rice paddies. An innovative temperature phased anaerobic digestion technology was developed and carried out in a continuous circulating mode of mesophilic and hyperthermophilic conditions in a loop digester (F1). The performance of the newly developed digester was compared with the reference digester (F2) working at mesophilic conditions. Co- digestion of rice straw was carried out with cow manure to optimize the carbon to nitrogen ratio and to provide the essential trace elements required by microorganisms in the biochemistry of methane formation. F1 produced a higher specific methane yield (189 ± 37 L/kg volatile solids) from rice straw compared to F2 (148 ± 36 L/kg volatile solids). Anaerobic digestion efficiency was about 90 ± 20% in F1 and 70 ± 20% in F2. Mass fractions of Fe, Ni, Co, Mo, Cu, and Zn were analyzed over time. The mass fractions of Co, Mo, Cu, and Zn were stable in both digesters. While mass fractions of Fe and Ni were reduced at the end of the digestion period. However, no direct relationship between specific methane yield and reduced mass fraction of Fe and Ni was found. Co-digestion of rice straw with cow manure seems to be a good approach to provide trace elements except for Se. Keywords: rice straw; cow manure; anaerobic digestion; trace elements; temperature; methane; nutrients; renewable energy 1. Introduction Rice straw (RS) is an agricultural residue that is available in abundant supply. Ap- proximately 846 million tons were produced in 2017 globally [1]. Present uses are limited to cooking, feeding animals, constructing buildings, and making paper [2]. However, the largest share of produced RS is left in the fields [2]. Open burning and plowing the straw back to the fields are common practices for its disposal, contributing to greenhouse gas emissions to the atmosphere [3,4], and its energy potential is also lost [5]. Anaerobic digestion (AD) may offer a promising approach to convert RS into biogas. It can play a dual role in producing renewable energy and treating waste [6]. RS also has good theoretical methane potential (TMP) and it can be calculated as proposed by Baserga [7], resulting in 207 to 211 L/kg VS [8]. Moreover, the specific methane yield (SMY) under different experimental conditions were reported by various authors in their studies, such as 231 L/kg VS [9], 120 L/kg VS [10], 226 L/kg VS [11], and 100 L/kg TS [12]. Energies 2021, 14, 2561. https://doi.org/10.3390/en14092561 https://www.mdpi.com/journal/energies