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14Methane production rate (MPR) in waste activated sludge (WAS) digestion processes is typically limited 15 by the initial steps of complex organic matter degradation, leading to a limited MPR due to sludge 16 fermentation speed of solid particles. In this study, a novel microbial electrolysis AD reactor (ME-AD) was 17 used to accelerate methane production for energy recovery from WAS. Carbon bioconversion was 18 accelerated by ME producing H 2 at the cathode. MPR was enhanced to 91.8 gCH 4 /m 3 reactor/d in the 19 microbial electrolysis ME-AD reactor, thus improving the rate by 3 times compared to control conditions 20 (30.6 gCH 4 /m 3 reactor/d in AD). The methane production yield reached 116.2 mg/g VSS in the ME-AD 21 reactor. According to balance calculation on electron transfer and methane yield, the increased methane 22 production was mostly dependent on electron contribution through the ME system. Thus, the use of the 23 novel ME-AD reactor allowed to significantly enhance carbon degradation and methane production from 24 WAS. 25 26 2 29 1. Introduction 30 The large amount of activated sludge generated during wastewater treatment poses a critical threat (when 31 not properly disposed) to ecological systems [1], while proper treatment and disposal of excess sludge is 32 quite expensive (Wei et al. 2003). On the other hand, anaerobic digestion (AD) is widely used for sludge 33 reduction as an energy saving and recovering method [2]. However, AD rate is substantially limited by the 34 first two steps (hydrolysis and acidogenesis) to convert complex organic compounds into suitable substrates 35 for methanogenesis, in raw sludge [3-5]. Commonly, it takes from 20 to 30 days to degrade 30-50% of the 36 total COD or volatile solids (VS) of raw WAS, under mild environmental conditions [6]. The pressure of 37 rapid human population growth and increasing energy demand have thus promoted further research on 38 development and improvement of an rate-accelerating AD process, in order to enhance biogas production 39 and achieve faster degradation rate from WAS [7, 8]. 40Recently, some researchers pointed out that bioelectrochemical systems have the ability to promote carbon 41 oxidation on anode and in-site CO 2 capture and reduction on cathode, thus providing additional CH 4 42 formation in an integrated AD system [9, 10]. Recently a direct interspecies electron transfer for 43 methanogenesis has been proved between Geobacter and Methanosaeta [11]. However, few efforts have 44 been made to better understand bioelectrochemical contributions to organic conversion or methane 45 promotion, which is very important to achieve viable reactor operations in the future. Lately, microbial 46 electrolysis cells (MECs) have been tested for their ability to convert waste organic compounds from 47 3 Therefore, in this study, a coupled system was tested, by putting a microbial electrolysis (ME) system into 59 an AD system, for raw waste activated sludge treatment at mild environmental conditions. The microbial 60 electrolysis system wa...

Understanding the microbial community structure relative to enhancement of methane production from digestion of waste-activated sludge (WAS) coupled with a bioelectrochemical system is a key scientific question for the potential application of bioelectrochemistry in biogas production. Little has been known about the influence of electrode on the structure and function of microbial communities, especially methanogens in a bioelectrochemical anaerobic digestion (AD) reactor. Here, a hybrid reactor, which coupled bioelectrolysis and AD, was developed to enhance methane recovery from WAS. The methane production rate reached up to 0.0564 m 3 methane/(m 3 reactor*d) in the hybrid reactor at room temperature, which was nearly double than that of the control anaerobic reactor (0.0259 m 3 methane/(m 3 reactor*d)) without bioelectrochemical device. Microbial community analysis revealed that hydrogenotrophic methanogen Methanobacterium dominated the cathode biofilm, which was the predominant contributor to accelerate the methane production rate from WAS. While acetoclastic methanogen Methanosaeta was enriched in the sludge phase of all reactors, shifts of the microbial community structure of the biocathode was in significant correlation with the methane production. This study suggested a potential way to utilize a bioelectrochemical system with the regulated microbial community to enhance methane production from WAS.

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BackgroundBioelectrochemical systems have been considered a promising novel technology that shows an enhanced energy recovery, as well as generation of value-added products. A number of recent studies suggested that an enhancement of carbon conversion and biogas production can be achieved in an integrated system of microbial electrolysis cell (MEC) and anaerobic digestion (AD) for waste activated sludge (WAS). Microbial communities in integrated system would build a thorough energetic and metabolic interaction network regarding fermentation communities and electrode respiring communities. The characterization of integrated community structure and community shifts is not well understood, however, it starts to attract interest of scientists and engineers.ResultsIn the present work, energy recovery and WAS conversion are comprehensively affected by typical pretreated biosolid characteristics. We investigated the interaction of fermentation communities and electrode respiring communities in an integrated system of WAS fermentation and MEC for hydrogen recovery. A high energy recovery was achieved in the MECs feeding WAS fermentation liquid through alkaline pretreatment. Some anaerobes belonging to Firmicutes (Acetoanaerobium, Acetobacterium, and Fusibacter) showed synergistic relationship with exoelectrogens in the degradation of complex organic matter or recycling of MEC products (H2). High protein and polysaccharide but low fatty acid content led to the dominance of Proteiniclasticum and Parabacteroides, which showed a delayed contribution to the extracellular electron transport leading to a slow cascade utilization of WAS.ConclusionsEfficient pretreatment could supply more short-chain fatty acids and higher conductivities in the fermentative liquid, which facilitated mass transfer in anodic biofilm. The overall performance of WAS cascade utilization was substantially related to the microbial community structures, which in turn depended on the initial pretreatment to enhance WAS fermentation. It is worth noting that species in AD and MEC communities are able to build complex networks of interaction, which have not been sufficiently studied so far. It is therefore important to understand how choosing operational parameters can influence reactor performances. The current study highlights the interaction of fermentative bacteria and exoelectrogens in the integrated system.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0493-2) contains supplementary material, which is available to authorized users.