Effects of wastewater constituents and operational conditions on the composition and dynamics of anodic microbial communities in bioelectrochemical systems

Highlights

• Anodic microbial communities and the roles of different microbes were examined.

• Syntrophic interaction between fermenting bacteria and exoelectrogens is crucial.

• Wastewater components may lead to enrichment of species competing for current.

• Such species include methanogens and sulphate- and nitrate-reducing bacteria.

Abstract

Over the last decade, there has been an ever-growing interest in bioelectrochemical systems (BES) as a sustainable technology enabling simultaneous wastewater treatment and biological production of, e.g. electricity, hydrogen, and further commodities. A key component of any BES degrading organic matter is the anode where electric current is biologically generated from the oxidation of organic compounds. The performance of BES depends on the interactions of the anodic microbial communities. To optimize the operational parameters and process design of BES a better comprehension of the microbial community dynamics and interactions at the anode is required. This paper reviews the abundance of different microorganisms in anodic biofilms and discusses their roles and possible side reactions with respect to their implications on the performance of BES utilizing wastewaters. The most important operational parameters affecting anodic microbial communities grown with wastewaters are highlighted and guidelines for controlling the composition of microbial communities are given.

Introduction

Bioelectrochemical systems (BES) are devices that enable a transfer of electrons to or from a biocatalyst via a working electrode. In case this electrode is operated as anode (i.e. accepting electrons) it can sustain the biological oxidation of organic or inorganic substrates under anoxic condition. Such a biobased current flow is for instance used in microbial fuel cells (MFC) for the generation of electrical energy from the chemical energy in biomass. For electricity generation in a MFC usually oxygen is reduced at the corresponding cathode. In microbial electrolysis cells (MEC) or microbial electrosynthesis cells (MES), the reductive current is used to produce hydrogen or other compounds, such as acetate, at the cathode with the help of an additional voltage. For further information, see for instance the reviews by Hamelers et al., 2010, Logan and Rabaey, 2012.

At the anodes of BES the organic carbon content of various wastewaters, including domestic, distillery and dairy wastewaters, can be oxidized biologically (Ha et al., 2012, Montpart et al., 2015, Yu et al., 2012). If the wastewaters contain complex organics, such as cellulose, hemicellulose, fats, proteins, sugars, alcohols, or volatile fatty acids (VFAs), the biological degradation often requires syntrophic interaction of mixed species microbial communities or in other words, the capabilities of an interacting microbiome. In nature, the first degradation step is often the hydrolysis of a biopolymer. Thereafter, fermentative bacteria metabolize the monomeric sugar compounds, fatty acids, and amino acids into alcohols, VFAs, H₂, and CO₂ (Fig. 1). Current producing bacteria, called exoelectrogens, can oxidize different sugars, alcohols, and VFAs through anaerobic respiration using the anode of the BES as terminal electron acceptor. The wastewaters can, however, also contain inorganic compounds, such as sulfate or nitrate, which serve as electron acceptors instead of the anode and thus, result in competing metabolic reactions that direct electrons away from current production (Fig. 1).

By optimizing process design and environmental parameters (such as anode potential, pH and presence of O₂, see Section 3), it is possible to direct the growth of microorganisms and improve the performance of BES. However, to be able to select for the right microbial species and to control the oxidation of organic compounds, it is important to understand the microbial community dynamics and interactions at the anode. Different microbial communities at the anode (Logan and Regan, 2006) as well as syntrophic interactions of bacteria utilizing different substrates for electricity generation (Kiely et al., 2011b) have been reviewed earlier. However, the number of publications on microbial community compositions in BES anodes has increased significantly in the past years. Furthermore, the wastewaters used as substrate at the anode may contain inorganic compounds, which may result in competing metabolic reactions (such as nitrate- or sulfate-reduction, see Section 2.1). These metabolic reactions that compete with electricity generation need to be taken into account when designing a bioelectrochemical system, but have not been considered in previous review articles.

The purpose of this review is to describe the roles of different microbial groups at the anodes of bioelectrochemical systems. Furthermore, the abundance of different bacterial phyla and species in the anodic microbial communities are compared and their capabilities to improve or hamper current production are discussed. Based on the literature, the effects of operational parameters on microbial communities are illustrated. In addition, some guidelines are derived regarding the selection of desired microbial communities and inhibition of microbial species hindering current production by changing environmental and technological parameters. At the end, further necessary research topics are highlighted.