Journal of Energy Research and Reviews 7(1): 1-14, 2021; Article no.JENRR.64317 ISSN: 2581-8368 Evaluation of Biogas Production through Anaerobic Digestion of Aquatic Macrophytes in a Brazilian Reservoir R. Nagipe da Silva Paulo1*, A. Viana Gusmão Vieira2 and P. Rodrigues3 1North Fluminense State University, UENF, Brazil. 2Brookfield Energy Renewables, Brazil. 3Jordão Energy, Brazil. Authors’ contributions: This work was carried out in collaboration among all authors. Author RNDSP designed the study, wrote the protocol, performed the experiments, managed the analyses of the study, wrote the first draft of the manuscript and effect all necessary corrections. Authors AVGV and PR managed the literature searches. All authors read and approved the final manuscript. Article Information DOI: 10.9734/JENRR/2021/v7i130180 Editor(s): (1) Dr. Huan-Liang Tsai, Da-Yeh University, Taiwan. Reviewers: (1) C. Vijayanand, Tamil Nadu Agricultural University, India. (2) Otto Corrêa Rotunno Filho, Federal University of Rio de Janeiro, Brazil. Complete Peer review History: http://www.sdiarticle4.com/review-history/64317 Received 25 October 2020 Accepted 30 December 2020 Original Research Article Published 18 January 2021 ABSTRACT Aquatic macrophytes are important components of aquatic habitats. However, the overgrowth of aquatic plants can cause severe problems for the management of bodies of water. As a result, these plants must be removed and disposed of as waste. However, the usage of this biomass as a substrate in biogas plants would appear to be more beneficial. The present work deals with the anaerobic digestion (AD) of macrophytes species that cause inconvenience to power generation at hydroelectric plant in Minas Gerais - Brazil. The study examines the following macrophytes species; Salvinia molest Oxycarium cubense, Eichhornia crassipes, Pistia stratiotes and Brachiaria. The experiments were carried out as stainless steel reactor with temperature, agitation and pressure control. As pre-treatment of macrophytes was used heat treatment at 120°C and pressure of 1.6 atm. The maximum methane content was 60% during 40 days digestion time, for Brachiaria of higher lignin content. The result obtained, mainly with Brachiaria demonstrates the efficiency of pre- treatment for the lignocellulosic samples. _____________________________________________________________________________________________________ *Corresponding author: E-mail: [email protected]; Paulo et al.; JENRR, 7(1): 1-14, 2021; Article no.JENRR.64317 Keywords: Macrophytes; lignocellulosic biomass; renewable energy; biogas; biomass energy. 1. INTRODUCTION To control aquatic macrophytes that are unbalanced in the environment, manual or Bioenergy represents energy from biomass and mechanized withdrawal is preferably applied [3]. plays an important role in promoting renewable In order to take advantage of this biomass, it can alternatives. This way, Brazil is a world reference be used biogas production from anaerobic mono for the use of this type of energy. This is because or co-digestion [4,5]. 43.5% of its energy matrix comes from renewable resources, and the world average is It is also possible to carry out a pre-treatment only 14.2%. If we consider only what is used for (chemical, physical or biological) of the electricity production, the percentage rises to substrates, in order to promote the disruption of 81.7%, since the main source of supply of the the structure of lignocellulose and increase the electricity sector comes from hydroelectric plants surface, improving biodegradability through cell throughout the country. Historically, Brazil invests wall rupture [6]. In this way, access to a lot in this system due to the abundance of microorganisms to the substrate, increasing the available water resources. The burning of non- potential for biogas production [7,8]. fossil organic materials, such as sugarcane In view of this, in order to obtain a more bagasse, alcohol, wood, etc., causes the sustainable development and also to provide a generation of energy through biomass. In this destination for the biomass macrophytes, the case, we are particularly interested in anaerobic biodigestion of these residues is macrophytes as raw material. One of the main shown to be a viable alternative by promote points of the use of this energy generation nutrient recycling, produce bio fertilizer (or system is in the use of agricultural waste that, digestate, a by-product of the process) and also theoretically, would be discarded and in the to generate sustainable energy through biogas possibility of cultivation. Biofuels (biogas, bio [9-11]. alcohol, biodiesel, among others) are obtained in the transformation of these organic wastes. Compared to the mono digestion, anaerobic co- Although the burning of these materials release- digestion has great potential to maximize biogas polluting gases, it is still considered a source of production and therefore has grown interest in clean energy, because all CO2 generated is the search for new waste to act as co substrates sequestered in the cultivation of organic [12-14]. materials [1,2]. Lignocellulosic materials are the best sources Aquatic macrophytes are, for the most used for biofuel production, such as biogas, and part, terrestrial vegetables that, throughout include residues from agriculture and forests, their evolutionary process, have adapted energy crops, and municipal and food waste. In to the aquatic environment, so they have Europe, almost 72% of the feedstock used in the some characteristics of terrestrial vegetables anaerobic digestion (AD) process for biogas and a great capacity to adapt to different production come from the agricultural sector, types of environments. In many bodies of such as energy crops, manure, and other water, undesirable macrophytes exhibit agricultural residues. The main issue of using excessive growth, generating adverse Lignocellulosic (LC) biomass for the biogas effects on the aquatic environment, which production is biomass recalcitrance, which accelerates the eutrophication process. represents biomass resistance to chemical and Eutrophication is a natural process resulting biological breakdown. from the accumulation of organic matter resulting from the excessive accumulation of organic The external and internal structures of the plant, matter from sewage and the development of its lignin content and many other factors algae. In hydroelectric reservoirs, the determine how well the anaerobic bacteria in the uncontrolled growth of macrophytes causes biogas process can digest the plant. Lignin problems in the turbines, forcing frequent content and the available surface area are discharges by the spillway to the exit of the considered the two most important factors [15, plants, generating water waste and decreased 16]. A pretreatment used to break the structures electrical production. These plants need to be and increase the surface area. Many controlled or removed from many aquatic pretreatments have previously been evaluated; environments. mechanical, thermal, acid, alkaline, biological 2 Paulo et al.; JENRR, 7(1): 1-14, 2021; Article no.JENRR.64317 and others [17,18]. Acidic and alkali split into amino acids. The hydrolysis is often pretreatments have shown of great promise in known to be the rate-limiting step in the biomass solubilization because of their multiple anaerobic digestion (AD) process [20]. In the advantages, for example, a simple device, ease second step, Acidogenesis, the smaller units of operation, high methane conversion efficiency from the hydrolysis can be taken up through the and low cost. Acidic hydrolysis is performed bacterial cell wall and are digested in the second using acids such as HCl, H2SO4, H3PO4, and step by acidogenic fermentative bacteria. Some HNO3 while alkali pretreatment usually employs of the products from this fermentation (mainly several alkaline solutions, inducing NaOH, KOH, acetate, CO2 and hydrogen) can be utilized Ca(OH)2 , Mg(OH)2, CaO, and ammonia. [19]. directly in the fourth step; methanogenesis. Products like alcohols and volatile fatty acids on In the biogas process, bacteria digest organic the other hand must be oxidized in the third step matter under anaerobic conditions and produce by acetogenic bacteria [20,21]. Acetogenesis. methane and carbon dioxide as final products. It The acetogenic step is where alcohols and is a multi-step reaction where at least three volatile fatty acids produced during the groups of different bacteria are involved. The fermentation are converted to acetic acid, carbon process is often divided into four main steps (Fig. dioxide and hydrogen, which is used in the last 1). Hydrolysis. The first step is called the step (methanogenesis). These acetogenic hydrolysis, where hydrolytic bacteria excrete bacteria live in symbiosis with the methanogenic enzymes that break insoluble polymers to soluble organisms [22]. Methanogenesis. This is the final oligomers and monomers. In this step step where methanogens convert acetic acid, carbohydrates are converted into sugars, lipids hydrogen and CO2 to methane, CO2 and some are broken down to fatty acids and proteins are trace gases [22]. Fig. 1. Stages of anaerobic digestion 3 Paulo et al.; JENRR, 7(1): 1-14, 2021; Article no.JENRR.64317 Biogas obtained in this way is, therefore, a constantly degraded by the release of industrial renewable form of energy that may contribute to effluents. Its use for dilution of waste from the mitigate environmental pollution [22], and industries has caused strong