Investigating hydrothermal pretreatment of food waste for two-stage fermentative hydrogen and methane co-production

Highlights

• Hydrothermal pretreatment (HTP) was used to facilitate food waste solubilization.

• Soluble carbohydrates initially rose and then fell when HTP temperature increased.

• Solubilization of proteins was dramatically promoted with increasing HTP temperature.

• Effect of HTP retention time on solubilization and fermentation was moderate.

• Two-stage H₂ and CH₄ co-production effected an energy conversion efficiency of 78.6%.

Abstract

The growing amount of food waste (FW) in China poses great pressure on the environment. Complex solid organics limit the hydrolysis of FW, hence impairing anaerobic digestion. This study employed hydrothermal pretreatment (HTP) to facilitate the solubilization of FW. When HTP temperature increased from 100 to 200 °C, soluble carbohydrate content first increased to a peak at 140 °C and then decreased, whereas total carbohydrate content was negatively correlated with increasing temperature due to the enhanced degradation and Maillard reactions. Protein solubilization was dramatically promoted after HTP, whereas protein degradation was negligibly enhanced. The hydrogen and methane yields from hydrothermally pretreated FW under the optimum condition (140 °C, 20 min) through two-stage fermentation were 43.0 and 511.6 mL/g volatile solids, respectively, resulting in an energy conversion efficiency (ECE) of 78.6%. The ECE of pretreated FW was higher than that of untreated FW by 31.7%.

Introduction

With the rapid development of economy and improvement of living standard in China, increasing amounts of municipal solid wastes (MSWs) are being generated. In 2015, the MSWs generated in 246 large and medium-sized cities reached up to 186 million tons (MEP of China, 2016a), which was higher than the decontamination and processing quantity (180 million tons) of MSWs for the entire country (MEP of China, 2016b). The accumulation of MSWs exerts considerable pressure on the environment.

In 2015, 63.9% and 33.9% of the processed and decontaminated MSWs were disposed through sanitary landfill and incineration, respectively (MEP of China, 2016b). However, food waste (FW), which accounts for more than half of the generated MSWs (Dou, 2015), is unsuitable for either landfill disposal or incineration. Although the landfill disposal of FW presents some advantages, such as low investment and facile operation, this approach is associated with numerous problems, such as large land occupation and environmental pollution caused by the underground leaching of hazardous substances and the excessive emission of landfill gas when improperly managed (Dou, 2015). The high moisture content of FW renders the use of incineration energy inefficient. Alternatively, anaerobic digestion, which combines organic waste reduction and energy recovery through methane production (Ariunbaatar et al., 2014a), can serve as an ideal strategy for FW disposal.

Anaerobic digestion of FW has been extensively investigated because of its environmental and energetic benefits, (Browne and Murphy, 2013, Zhang et al., 2014). However, complex organics in FW, such as lipids from animal fats and vegetable oils, insoluble proteins, and large-molecular-weight carbohydrates, always limit the hydrolysis step, hence leading to the inhibition and even failure of anaerobic digestion (Ariunbaatar et al., 2014a, Xia et al., 2016). Moreover, the dietary customs of Chinese contribute to the high lipid contents of FW (Li et al., 2017). In addition to hindering hydrolysis, high lipid content can inhibit the metabolism of microbial cells because of the absorption of lipid derivatives (i.e., long-chain fatty acids) on cell surfaces (Hanaki et al., 1981). To address these issues and promote methane production, researchers have subjected different substrates to various mechanical, chemical, thermal, and biological methods prior to anaerobic digestion (Ariunbaatar et al., 2014a). Hydrothermal pretreatment (HTP), a thermal treatment method with a history of successful industrial applications because of its high efficiency and environmental friendliness, has been widely explored as a pretreatment method for methane production (Ariunbaatar et al., 2014a, Tekin et al., 2014). By disintegrating cell membranes, HTP facilitates the dissolution of recalcitrant organic compounds and further promotes the hydrolysis of dissolved macromolecular organics under suitable temperatures and retention times (Ariunbaatar et al., 2014a, Yin et al., 2014). Most studies have focused on the HTP of sewage sludge and lignocellulosic substrates, whereas reports on the HTP of FW are limited. Yin et al. (2014) and Yu et al. (2016) performed the HTP of FW for fermentative volatile fatty acid (VFA) production. They reported that HTP simultaneously enhances organic compound dissolution and VFA production. Similarly, Jin et al. (2016) and Li et al. (2017) applied HTP on kitchen waste and found that solubilization of organics and the corresponding biodegradability in subsequent anaerobic digestion increased.

As an alternative to pretreatments, a two-stage process involving the dark hydrogen fermentation of FW and the anaerobic digestion of fermentation effluent has been suggested to facilitate the hydrolysis of FW substrates and stabilize anaerobic digestion (Kobayashi et al., 2012, Lee et al., 2010, Voelklein et al., 2016). Methanogenesis is blocked during dark hydrogen fermentation. Hydrolysis and acidogenesis can be remarkably strengthened through the optimization of process parameters (e.g., pH value and retention time) of dark hydrogen fermentation (Liu et al., 2013, Voelklein et al., 2016). Voelklein et al. (2016) observed that compared with one-stage anaerobic digestion, dark hydrogen fermentation facilitates the hydrolysis of FW, allows higher loading rates, and increases the subsequent methane production. In addition to VFAs, hydrogen is produced through the dark hydrogen fermentation of FW. Hydrogen is considered as an extremely clean energy carrier and an important chemical feedstock. Kobayashi et al. (2012) established a two-stage system for FW disposal and obtained hydrogen and methane yields of 147.3 and 383.0 mL/g volatile solids (VS), respectively.

Rafieenia et al. (2017) combined aerobic pretreatment and two-stage fermentation to energetically valorize synthetic protein-rich FW. However, to our best knowledge, a study on two-stage fermentative hydrogen and methane co-production using FW subjected to HTP has yet to be reported in literature to date. Thus, to fill in this knowledge gap, this study specifically aims to:

• Evaluate the effects of temperature and time of HTP on the solubilization and acidification of FW.

• Investigate HTP conditions (temperature and time) on FW for two-stage fermentative hydrogen and methane production.

• Assess the energy conversion efficiencies (ECEs) from FW to hydrogen and methane following the two-stage process.