Introduction

Phenolic compounds have been shown to be toxic and potentially carcinogenic chemicals, which could be found in various types of industrial wastewaters including pharmaceutical plants, coal gasification and petroleum refineries [1]. Therefore, potentially environmental problems would be induced when the phenolic compounds containing wastewater was discharged into environments without proper treatment. Various treatment processes, including adsorption [2], photocatalysis [3] and electrochemistry [4] have been investigated for the phenolic compounds removal, while high organic matters increased the operation costs and energy investment which impeded their application in practical application. Anaerobic digestion is considered to be the promising method for organics containing wastewater treatment due to higher energy recovery and lower costs [5,6]. However, as to phenolic compounds containing wastewater, the recalcitrant and toxic organics would limited their biological removal efficiency [7]. Previous study indicated that during the anaerobic digestion of hard-degradable compounds, the deficiency of bioavailable organic matters would reduce the microbial growth rate [8] and increase the washout amount of biomass [9], which finally limited the biomass in the continuous operation reactor. To provide better growing environment and prevent biomass washing out, supporting materials such as polyurethane foam, polyvinyl chloride (PVC), polystyrene particles and plastic materials are placed into the digesters to serve as the biofilm carriers for promoting biofilm layer growth [9,10]. However, the low bio-affinity and hydrophilicity of these materials limit the extensive use as biofilm carriers in anaerobic digestion [10]. Nowadays, natural materials, owing to the low cost and own features, are used more and more widely in environmental wastes treatment [11,12]. Volcanic rocks (VR) as the easily available minerals were reported to be successfully applied in adsorption field due to their great advantages in cost, porosity and hydrophilicity [13]. Previous study indicated that VR as the adsorbent could obtained satisfactory results during nitrogen [14], phosphorus [15], heavy metals ion [16] and organics [17] removal. Furthermore, some researchers found that VR possesses high bio-affinity, which is benefit for biofilm growth during wastewater treatment process [18,19]. For example, in previous study, VR was added into aerated submerged biofilm reactor serving as the microbial carriers to treat municipal wastewater for nitrogen and carbon removal [20]. Considering the adsorption and the bio-affinity, it can be speculated that when VR was added into the anaerobic wastewater treatment reactors, it could promote the removal efficiency through two aspects: (1) organics in wastewater could be removed through the adsorption of VR; (2) VR could promote the biofilm growth and reduce the amount of biomass washout. For some industrial wastewater, organic compounds generally have the characteristics of stubbornness and inhibition of biological activity [6]. Therefore, the adsorption property and activity of the biofilm would be inhibited if the adsorbed organics not be degraded immediately. It meant that improving the adsorbed organics degradation efficiency is important for the continuous organics removal. It was found that adding iron oxides into an anaerobic digester could effectively promote organics removal efficiency [6,21]. The potential mechanisms were considered to be that Fe(III)-reducing microorganisms were enriched with the existence of iron oxides (i.e. Fe3O4), which played an important role in complex organics decomposition with Fe(III) serving as the electron acceptor (i.e. dissimilatory iron reduction process) [22]. In addition, further study indicated that during anaerobic digestion Fe(III)-reducing microorganisms could accept electrons through direct interspecies electron transfer (DIET) [23]. Previous study found that DIET is more efficient compared with traditional interspecies H2 transfer (IHT) during the syntrophic metabolism electron transfer process [23]. However, iron oxides would aggregate at the bottom of the reactor when simple adding into reactor as the powder or particle, which would reduce the promotion effects on microorganisms owing to the insufficient contact. Considered the porous structure of VR, it can be believed that aggregation phenomenon would be effectively improved when the iron oxides loading on the VR surface. Therefore, it can be speculated that adding the volcanic rock loaded with iron oxides into the wastewater anaerobic digester could not only take advantages of volcanic rock to promote the biofilm growth and adsorb organics, but also enhance the bioactivity through enriching the Fe(III)-reducing microorganisms. To the best of our knowledge, few researchers conducted experiments on the effects of volcanic rock loaded with iron oxides during wastewater anaerobic digestion. The overall objective of this study was as following: (1) the effects of VR on organics removal efficiency during continuous anaerobic digestion; (2) metabolism of VR promoting anaerobic organics removal; (3) exploration of the microbial community structure and functional gene changeset.