A Preliminary Study of the Effect of Bioavailable Fe and Co on the Anaerobic Digestion of Rice Straw

Gabriele Mancini,Stefano Papirio,Giovanni Esposito,Piet Lens

doi:10.3390/en12040577

Abstract

Rice straw is an abundant and sustainable substrate for anaerobic digestion (AD), but it is often deficient in essential trace elements (TEs) for proper microbial growth and metabolism. A lack of TEs leads to AD imbalances and suboptimal biogas yields. However, the total TE concentration is not a sufficient indicator of the amount of TEs available to the microorganisms. Therefore, this study investigated the degree of bioavailability of iron (Fe) and cobalt (Co) during the AD of rice straw, and correlated it to the biomethane yields and volatile fatty acids (VFAs) produced. When the two TEs were dosed at 205 µg Fe/g TS and 18 µg Co/g TS of rice straw, the biomethane production was approximately 260 mL CH4/g VS, i.e., similar to that obtained when Fe and Co were not added. Despite an increased bioavailable fraction of 23 and 48% for Fe and Co, respectively, after TEs addition, the AD performance was not enhanced. Moreover, VFAs did not exceed 250 mg HAc/L both in the presence and absence of added TEs, confirming no enhancement of the methanogenesis step. Therefore, the bioavailability of Fe and Co was not a limiting factor for the biomethane production at low total VFAs concentration.

Highlights

In the last two decades, pressing issues—such as world population increase, global warming, growing energy demand, and the need for national energy security and rural economic development—have been the driving forces urging for a switch from traditional fossil fuels towards sustainable energy sources [1]
Biomethane can be produced from a wide range of lignocellulosic materials (LMs), with the term LM referring to any plant dry matter, which is mainly composed of carbohydrate and aromatic polymers [3]
More than 3 trillion kilos of LMs are available from agricultural sources every year and methane yields from the anaerobic digestion (AD) of crop residues are in the range of 3200 to 4500 cubic meters per hectare per year [5]

Summary

Introduction

In the last two decades, pressing issues—such as world population increase, global warming, growing energy demand, and the need for national energy security and rural economic development—have been the driving forces urging for a switch from traditional fossil fuels towards sustainable energy sources [1]. Biomethane, produced through the anaerobic digestion (AD) process, has the potential to yield more energy than any other available biofuel, such as bioethanol or biodiesel [2], and is normally used in combined heat and power (CHP) units for the production of heat and electricity or directly injected in the gas grids. Biomethane can be produced from a wide range of lignocellulosic materials (LMs), with the term LM referring to any plant dry matter, which is mainly composed of carbohydrate (i.e., cellulose and hemicellulose) and aromatic (i.e., lignin) polymers [3]. More than 3 trillion kilos of LMs are available from agricultural sources every year and methane yields from the AD of crop residues are in the range of 3200 to 4500 cubic meters per hectare per year [5]. Rice straw is one of the most common agricultural wastes and its biogas production

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