The application of an innovative continuous multiple tube reactor as a strategy to control the specific organic loading rate for biohydrogen production by dark fermentation

Highlights

• An innovative reactor configuration was assessed for biohydrogen production.

• The multiple tubes reactor prevents the accumulation of solids in the long-term.

• Better performances were observed by placing support material at the outlet chamber.

• Solid retention within the reactor reached up to 23% of the total biomass generated.

Abstract

Biohydrogen production in fixed-bed reactors often leads to unstable and decreasing patterns because the excessive accumulation of biomass in the bed negatively affects the specific organic loading rate (SOLR) applied to the reactor. In this context, an innovative reactor configuration, i.e., the continuous multiple tube reactor (CMTR), was assessed in an attempt to better control the SOLR for biohydrogen production. The CMTR provides a continuous discharge of biomass, preventing the accumulation of solids in the long-term. Sucrose was used as the carbon source and mesophilic temperature conditions (25 °C) were applied in three continuous assays. The reactor showed better performance when support material was placed in the outlet chamber to enhance biomass retention within the reactor. Although the SOLR could not be effectively controlled, reaching values usually higher than 10 g sucrose g⁻¹ VSS d⁻¹, the volumetric hydrogen production and molar hydrogen production rates peaked, respectively, at 1470 mL H₂ L⁻¹ d⁻¹ and 45 mmol H₂ d⁻¹, indicating that the CMTR was a suitable configuration for biohydrogen production.

Introduction

Hydrogen production can be achieved by employing different technologies, such as water electrolysis, thermochemical and biological processes. Considering biological processes in particular, anaerobic fermentation comprises the most attractive pathway because wastewater streams and organic wastes may be used as substrates (Hafez et al., 2010, Sreethawong et al., 2010). Most studies on fermentative biohydrogen (BioH₂) production have been conducted in batch mode, based on the simple operational and controlling features of such reactors. Although such studies are important to understand the behavior of the systems by simulating different operational conditions, large-scale plants would require continuous production processes for practical engineering reasons (Arimi et al., 2015).

Several factors may influence continuous hydrogen production in biological reactors including the pH (Hwang et al., 2009, Antonopoulou et al., 2011), hydraulic retention time (HRT) (Kumar et al., 2014, Rosa et al., 2014), organic loading rate (OLR) (Hafez et al., 2010, Perna et al., 2013, Ferraz et al., 2014), temperature conditions (Gadow et al., 2013, Sivagurunathan et al., 2014), and type and concentration of the carbon source (Fontes Lima et al., 2013, Kumar et al., 2014, Lucas et al., 2015). The reactor configuration may also play a key-role in obtaining high BioH₂ production rates in continuous systems because the concentration of active biomass directly depends on the type of configuration employed. Most studies on continuous fermentative BioH₂ production are based on the use of continuously stirred tank reactors (CSTR) (Gadow et al., 2013, Kumar et al., 2014, Sivagurunathan et al., 2014), based on the operational simplicity of such systems. However, in suspended cell systems the solids retention time (SRT) and HRT are equivalent, which may limit the establishment of a high cell density for obtaining high hydrogen production rates due to a reduced contact-time between the biomass and the substrate (Han et al., 2012).

Immobilized cell systems, especially the anaerobic packed-bed reactor (APBR), may be considered as promising technologies for BioH₂ production because high biomass concentrations are achieved simultaneously with the application of high organic loading rates to the reactor (Fernandes et al., 2013). The literature for BioH₂ production indicates several studies based on the use of APBR in acidogenic systems, focusing on different aspects that may influence the hydrogen production, i.e., the carbon/nitrogen (C/N) ratio and specific organic loading rate (SOLR) (Anzola-Rojas et al., 2015), back-mixing degree (Fontes Lima and Zaiat, 2012), support material and bed porosity (Fernandes et al., 2013), nutritional supplementation (Peixoto et al., 2011), organic loading rate (Perna et al., 2013, Ferraz et al., 2014, Andreani et al., 2015) and type of inoculum and seed sludge pretreatment methods (Penteado et al., 2013). Although these studies have contributed to the enhancement of the performance of the systems, difficulties have been observed in maintaining high and stable hydrogen production for long-term operations.

Higher biomass concentrations in the reactors improve the hydrogen yield (van Ginkel and Logan, 2005, Hafez et al., 2009). However, low hydrogen yields are usually observed after some days of operation, especially for mesophilic conditions, when the concentration of the biomass is excessive in the reactor. Some studies have indicated that unstable hydrogen production can be associated with the food-to-microorganism ratio (F/M) or specific organic loading rate. This parameter may reach unfavorable values for hydrogen production at both excessively low and high concentrations of biomass in the reactor (Hafez et al., 2010, Anzola-Rojas et al., 2015). Hafez et al. (2010) reported hydrogen production rates for SOLR values ranging from 4.4 g COD g⁻¹ VSS d⁻¹ to 6.4 g COD g⁻¹ VSS d⁻¹, using glucose as the carbon source. The aforementioned authors also observed decreasing system performances when a SOLR of 8.5 g COD g⁻¹ VSS d⁻¹ and 12.1 g COD g⁻¹ VSS d⁻¹ were applied. Similar results were observed by Anzola-Rojas et al. (2015) and better results regarding hydrogen production have been obtained with a SOLR of 6.0 g sucrose g⁻¹ VS d⁻¹ (6.72 g COD g⁻¹ VS d⁻¹). In this case, the tested conditions included SOLR values of 3.9, 4.7, 6.0, 8.5 and 12.4 g sucrose g⁻¹ VS d⁻¹.

Considering the difficulties in maintaining SOLR values in the recommended range when using APBR, innovative configurations of bioreactors should be proposed to simultaneously provide a controlled biomass washout and maintain an adequate concentration of active microorganisms inside the reactor. In this context, the concept of a continuous multiple tube reactor (CMTR) is presented, in which the reaction volume is formed by a group of parallel small diameter tubes, providing high length-to-diameter ratios (L/D) and improving control over the superficial flow velocity in the bed. Theoretically, this configuration could provide a larger contact surface for biomass attachment than a conventional tube reactor without support material for biomass attachment. It also provides continuous discharge of biomass due to the high superficial velocity, which helps to prevent biomass accumulation and maintain the appropriate biomass concentration for hydrogen production. Additionally, the application of CMTR to acidogenic systems should be more attractive than using fluidized-bed reactors because less energy would be required to achieve high upflow velocities. Thus, this study assessed BioH₂ production in a bench-scale CMTR, using sucrose as the main carbon source under mesophilic conditions. Different operational conditions were applied to the CMTR in a preliminary attempt to understand the behavior of this reactor, with emphasis on the control of the SOLR.