Elsevier

Bioresource Technology

Volume 314, October 2020, 123775
Bioresource Technology

Synergistic effects of rice straw and rice bran on enhanced methane production and process stability of anaerobic digestion of food waste

https://doi.org/10.1016/j.biortech.2020.123775Get rights and content

Highlights

  • Co-digestion of FW and RS with RB had positively synergistic effects.

  • An optimum mixing ratio of FW:RS:RB of 60:10:30 was determined.

  • The synchronous addition of RS and RB in FW AD system favored VFA consumption.

Abstract

This study investigated the synergistic effects of rice straw (RS) and rice bran (RB) addition on methane production and process stability of anaerobic digestion of food waste (FW). Positive synergistic effect (Synergy index (SI) = 1.03–1.24 > 1) was noticed in all the co-digestion reactors. The optimum mixing ratio of FW:RS:RB (volatile solid (VS) basis) was 60:10:30 with the maximum SI (1.24), achieving 27.4% increase in methane yield (235.4 mL/g-VS) and around 5 days shorter of λ (3.7 days) compared to the mono-digestion of FW (184.8 mL/g-VS and 8.2 days). Remarkably high concentration of volatile fatty acids (VFAs) was also accumulated in the mono-digestion of FW, especially propionic acid, which to a great extent caused the methane production to stagnate. Results from this study demonstrate that co-digestion of FW and RS with RB has high potentials for energy recovery from AD of the mixed feedstocks and its stable operation.

Introduction

With the rapid growth of population and economy, the triggered environmental pollution problems are the major challenges at the moment, in which the disposal of ever-increasing food waste (FW) becomes a pivotal global issue. Food and Agriculture Organization (FAO) of the United Nations reported that approximately 1.3 billion tons of food used for human consumption was wasted annually, which is estimated to produce about 2.2 billion tons of FW by 2025 (Mehariya et al., 2018, Algapani et al., 2019). In Japan, ~19 million tons of FW was generated annually, including ~ 11.3 million tons from wholesale, retail, catering, and restaurant activities for food manufacturing and ~ 7.7 million tons from household preparation and cooking (Babalola, 2015). If not handled properly, FW would cause serious problems including environmental contamination, health risks and massive land occupation, etc. According to Liu et al. (2016), in Japan, about half FW produced from food manufacturing industries is used for animal feed, together with 13% recycled as compost, 3% as energy, 12% reduced through dehydration treatment, and 19% being disposed by incineration or landfilling. On the other hand, 94% of FW from households is disposed by incineration or landfilling with only 4% for recycling. As it is known, Japan is short of available land for landfilling, and a large amount of harmful substances (dioxins) and greenhouse gases (CO2) are produced during incineration. As estimated by Yano and Sakai (2016), one third of the incinerators should be replaced by 2020 and the remaining two thirds should be done by 2030. Therefore, more sustainable and environmentally friendly management strategies for FW should be addressed and developed. Compared to landfilling, composting and incineration, anaerobic digestion (AD) is regarded as a more appealing alternative because it can produce biogas (mainly CH4 and CO2) and organic fertilizers from FW simultaneously, largely reducing the risk of FW to human health and the environment. Nevertheless, many anaerobic mono-digesters of FW have been reported to have unstable performance and even process failure due to reactor acidification and ammonia accumulation, especially the more toxic free ammonia to microorganisms (Mahdy et al., 2017, Zhang et al., 2019).

Co-digestion has been demonstrated as a feasible strategy to ameliorate the limitations of mono-digestion, including balance of nutrients and trace elements, toxicity dilution, improved buffering capacity and control of acidogenesis (Kim et al., 2017, Kainthola et al., 2019). Crop straw is frequently added as a co-substrate to the FW AD reactors because it is the most readily available organic solid waste with a high C/N ratio (Zhang et al., 2018, Zhao et al., 2018). Rice straw (RS), occupying a relatively large proportion (25.1%) of crop residues (Dai et al., 2019), is a common substrate for biogas production via AD process. For instance, some previous studies have proven that the mixing of RS with high organic content matters, such as FW, feces of humans and animals, and waste activated sludge, can adjust the feedstocks’ C/N ratio, resulting in improved stability of AD process (Ye et al., 2013, Sukhesh and Rao, 2018, Xin et al., 2018, Xu et al., 2019). However, direct addition of straw materials into co-digestion system generally can’t achieve increased methane production because the major organic components of straw materials, i.e. cellulose, hemicellulose, and lignin, are difficult to biodegrade (Zhang et al., 2018). Therefore, a suitable pretreatment is necessary before AD of RS. For example, use of ultrasonic pretreatment can change the physical and chemical properties of maize straw and dairy manure, thus enhancing the co-digestion process (Zou et al., 2016). In addition, ammonia pretreatment is also regarded as a technically feasible method. As Zhang et al. (2018) reported that, after being urea-ammoniated, the co-digestion of RS with FW can increase methane yield by 8.83% in comparison to the sole substrate at the optimal mixing ratio of 1:3. Yuan et al. (2020) reported that the highest methane yield of RS was 250.34 mL/g-VS at a 4% ammonia concentration together with a moisture content of 70%, about 28.55% increase in methane yield compared to the control. However, these pretreatment methods also increase the operation cost.

Rice bran (RB), as the major by-product in polished rice production, is generated in the large parts of Asia, Latin America, Caribbean and Africa, > 70 million metric tons annually (Uraipong and Zhao, 2016, Zou et al., 2019). Despite the high biological values of RB, limited amount of RB is utilized for economic benefits and human diet purposes due to the fact that RB is easy to become rancid within a few hours (Irakli et al., 2018, Ertürk and Meral, 2019). Thus it is necessary to quickly stabilize RB before its being utilized as food ingredient, which to a great extent increases the processing cost. Therefore, a great majority of the defatted RB is remained to use as feed to poultry and livestock at present and an important amount of RB is underused every year (Wongwaiwech et al., 2019, Benito-Román et al., 2019). The energy (like methane) obtained from the unprocessed RB via AD process is easy to use, making the most value of RB. As reported, RB contains interesting amounts of carbohydrates (34.0%–62.0%, mainly starch) and nutrients, which could be used as carbon source in AD process (Ning et al., 2012). Additionally, RB contains vitamin B group (mainly B1 and B12) and some microelements which can support microbial growth and improve microbial resistance (Hou et al., 2019). Some researchers found that addition of trace elements to FW AD systems enhanced the degradation rates, which was helpful for the recovery of acidified reactors (Qiang et al., 2013, Capson-Tojo et al., 2018). Therefore, RB is considered as another commonly and abundantly supplementary feedstock to improve methanogenesis in the co-digestion of FW and RS.

The objective of this study was to investigate the synergistic effect of anaerobic co-digestion of FW and RS with RB on operation stability and methane production. Both optimum mixing ratio and synergistic effect of FW-RS-RB mixtures on methane yield were determined according to biochemical methane potential (BMP) from batch AD tests using the co-substrates at different mixing ratios. In this study, volatile fatty acids (VFA) accumulation and consumption was paid more attention in order to examine the stability of co-digesters. The energy conversion efficiency and economic assessment were also conducted.

Section snippets

Substrates and inoculum

FW was collected from the dormitory kitchen in University of Tsukuba, which was mainly composed of peels of vegetables and fruits with smaller amounts of meat and leftovers. The indigestible components such as bones, chopsticks and plastics were manually removed before the FW was smashed and homogenized into slurry using a food grinder. The processed FW was stored at 4 ℃ before use. RS and RB were obtained from the rice processing in the Food Department of University of Tsukuba, which were then

Daily methane production and cumulative methane yield

The daily methane production and cumulative methane yield in the BMP tests at different FW/RS/RB ratios are shown in Fig. 1. All the reactors showed some similar trend in daily methane production, which increased to the peak values and then declined until almost no biogas production (Fig. 1a). During the first 10 days, the daily methane productions from the RS added reactors were higher than other reactors (with no RS addition), and the peak values were noted earlier than the mono-digestion of

Conclusions

Co-digestion of FW, RS, and RB is an economical approach for sustainable management of FW, enhancing their potential as energy resources and solving the long start-up time of mono-FW digestion. In this study, the optimum mixing ratio of FW:RS:RB was 60:10:30 (VS basis) with the strongest positive synergistic effect (SI = 1.24), achieving 27.4% and 35.23% increase in methane yield and energy recovery in addition to 5 days shorter λ compared to the mono-FW digestion. The simultaneous

CRediT authorship contribution statement

Tingting Hou: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing - original draft, Writing - review & editing. Jiamin Zhao: Formal analysis, Investigation, Methodology. Zhongfang Lei: Conceptualization, Formal analysis, Funding acquisition, Project administration, Supervision, Writing - review & editing. Kazuya Shimizu: Formal analysis, Methodology, Supervision. Zhenya Zhang: Conceptualization, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The first author appreciated the financial support from the China Scholarship Council (CSC.201806400011) for her study in University of Tsukuba, Japan.

References (48)

  • J. Kainthola et al.

    Optimization of methane production during anaerobic co-digestion of rice straw and hydrilla verticillata using response surface methodology

    Fuel

    (2019)
  • M. Kamali et al.

    Anaerobic digestion of pulp and paper mill wastes-An overview of the developments and improvement opportunities

    Chem. Eng. J.

    (2016)
  • J. Kim et al.

    Energy production from different organic wastes by anaerobic co-digestion: Maximizing methane yield versus maximizing synergistic effect

    Renew. Energy

    (2019)
  • J. Kim et al.

    Anaerobic co-digestion of spent coffee grounds with different waste feedstocks for biogas production

    Waste Manage.

    (2017)
  • J. Kim et al.

    Anaerobic co-digestion of food waste, human feces, and toilet paper: Methane potential and synergistic effect

    Fuel

    (2019)
  • J. Lin et al.

    Effects of mixture ratio on anaerobic co-digestion with fruit and vegetable waste and food waste of China

    J. Environ. Sci.

    (2011)
  • C. Liu et al.

    Food waste in Japan: Trends, current practices and key challenges

    J. Clean. Prod.

    (2016)
  • A. Mahdy et al.

    Ammonia tolerant inocula provide a good base for anaerobic digestion of microalgae in third generation biogas process

    Bioresour. Technol.

    (2017)
  • S. Mehariya et al.

    Co-digestion of food waste and sewage sludge for methane production: Current status and perspective

    Bioresour. Technol.

    (2018)
  • Y. Miron et al.

    The role of sludge retention time in the hydrolysis and acidification of lipids, carbohydrates and proteins during digestion of primary sludge in CSTR systems

    Water Res.

    (2000)
  • H. Qiang et al.

    Trace metals requirements for continuous thermophilic methane fermentation of high-solid food waste

    Chem. Eng. J.

    (2013)
  • F.M.S. Silva et al.

    Hydrogen and methane production in a two-stage anaerobic digestion system by co-digestion of food waste, sewage sludge and glycerol

    Waste Manage.

    (2018)
  • Y. Wang et al.

    Effects of co-digestion of cucumber residues to corn stover and pig manure ratio on methane production in solid state anaerobic digestion

    Bioresour. Technol.

    (2018)
  • D. Wongwaiwech et al.

    Comparative study on amount of nutraceuticals in by-products from solvent and cold pressing methods of rice bran oil processing

    J. Food Drug Anal.

    (2019)
  • Cited by (30)

    • Valorization of food waste by anaerobic digestion: A bibliometric and systematic review focusing on optimization

      2022, Journal of Environmental Management
      Citation Excerpt :

      They related that the FW and co-substrate proportion are both significant on the optimization of hydrogen production should take the combination of these two parameters into account to control accumulation of VFAs and excessive nitrogen source for microbial cell growth (Zhang et al., 2019). Hou et al. (2020) investigated the synergistic effects of rice straw and rice bran addition on methane production and process stability of anaerobic digestion of food waste. Positive synergistic effect was observed in all co-digestion reactors.

    View all citing articles on Scopus
    View full text