Towards hydrogen production from waste activated sludge: Principles, challenges and perspectives

https://doi.org/10.1016/j.rser.2020.110283Get rights and content

Highlights

  • H2 is mainly produced from the acidogenesis process of WAS dark fermentation.

  • Microbial electrolysis cell is a prospective technology for H2 production.

  • The poor organics utilization and rapid H2 consumption hinder hydrogen recovery.

  • A promising avenue through integration of the available technologies was proposed.

  • The hybrid process could achieve substantial technical and economic benefits.

Abstract

Hydrogen production from waste activated sludge (WAS) was widely considered and intensively investigated as a promising technology to recover energy from wastewater treatment plants. To date, no efforts have been made on either systematic summarization or critical thinking of the application niche of hydrogen production from WAS treatment. It is therefore time to evaluate whether and how to recover hydrogen in a future paradigm of WAS treatment. In this critical review, the principles and potentials, microorganisms, possible technologies, and process parameters of hydrogen generation were analyzed. Microbial electrolysis cell shows high theoretical hydrogen yield and could utilize a variety of organic compounds as substrates, which is regarded as a prospective technology for hydrogen production. However, the poor organics utilization and rapid consumptions of produced hydrogen hindered hydrogen recovery from WAS. Based on the analysis of the current state of the literatures, the opportunities and challenges of hydrogen production from WAS are rethought, the detailed knowledge gaps and perspective of hydrogen production from WAS were discussed, and the probable solutions of hydrogen recovery from WAS treatment are figured out. To guide the application and development of hydrogen recovery, a more promising avenue through rational integration of the available technologies to form a hybrid process is finally proposed. The integrated operational paradigm of WWTPs could achieve substantial technical, environmental and economic benefits. In addition, how this hybrid process works is illustrated, the challenges of this hybrid process and future efforts to be made in the future are put forward.

Introduction

With the development of society, people need more and more energy, which inevitably leads to the shortage of fossil fuels. In addition, the combustion of fossil fuels makes large amounts of greenhouse gas (i.e., carbon dioxide) release in the atmosphere, causing the climate change and global warming. The issues of energy shortage and global warming drive the efforts to seek clean, recyclable, and renewable energy. Compared with methane, hydrogen possesses higher energy yield (i.e., 142.35 kJ/g which is 2.75 folds than that of other hydrocarbons) and generates water rather than greenhouse gases while it is combusted [1]. Therefore, hydrogen is considered as the green energy and widely accepted to be the most promising alternative to fossil fuels.

Hydrogen can be produced from valuable resources such as water and fossil fuels, and also from wastes such as sewage and food waste via a series of chemical-physical and biological methods. There are many publications that reported hydrogen production by utilizing the chemical-physical methods and biological methods [2]. Currently, most of the hydrogen (>85%) is produced through the pyrolysis of fossil fuels, and the gasification of biomass [3]. Although these chemical-physical methods could obtain high hydrogen yield, they are not sustainable due to high energy consumption [4,5]. By contrast, biological hydrogen production is a more cost-effective and environmental friendly method to produce hydrogen from varieties of organic wastes (e.g., waste activated sludge) due to the simple operational conditions, steady H2 yield and low energy consumption [6,7]. As for different substrates, using wastes to produce hydrogen is an environmentally favorable and economically sustainable way. With the growing energy crisis worldwide, this aspect is becoming more important and pushing forward new attempts employing more wastes.

Waste activated sludge (WAS), which is the main byproduct of municipal wastewater treatment plants (WWTPs), is generated with large amounts annually [8]. For instance, it was documented that 11.2 million metric tons of dry sludge were generated in China while 10 million tons were produced in EU countries [9]. On one hand, treatment and disposal of such massive amount of WAS are costly, accounting for up to 60% of the total operation cost of a WWTP [10]. On the other hand, WAS contains high levels (50–70%) of organic compounds such as protein, carbohydrate, and lipid [11,12], which makes it an ideal renewable resource. For example, Jiang et al. investigate the physicochemical characteristics of WAS and found that when the volatile suspended solid (VSS) of WAS was 10.81 g/L, WAS contained 14.88 g/L total chemical oxygen demand (TCOD), 9.94 g COD/L total protein, 0.86 g COD/L total carbohydrate, and 0.17 g COD/L lipid and oil [13]. Similar WAS characteristics were also reported in other papers [14,15]. In addition, it was reported that ~35% of carbon element, ~3.8% of nitrogen element, ~1.6% of phosphorus element, and other trace elements contained in WAS [16].

Many publications showed that various wastes containing high organic substrates could be utilized to produce hydrogen by anaerobic fermentation process [17,18], indicating that WAS is a potential substrate for hydrogen production. Although WAS is generally treated by the anaerobic digestion to produce methane, several hydrogen producers are found to be present in the digester such as Clostridium pasteurianum [19] and Thermoanaerobacterium [20,21]. In fact, hydrogen is observed as an important intermediate in the anaerobic digestion process [22]. Thus, hydrogen production from WAS attracted much attention in the past decades, by which fossil fuels are saved, greenhouse gas (e.g., CO2) emission is reduced, WAS is reused and reduced, and sustainable clean energy H2 and volatile fatty acids (VFAs) are also obtained. However, practical application of hydrogen production from WAS has not yet been achieved. On the contrary, some doubts and debates have been arisen recently about its technical and economic feasible in full-scale situations due to the low hydrogen yield. There are many challenges in reactor control, system development, and energy recovery. For example, hydrogen is an intermediate product in the anaerobic digestion, thus the produced hydrogen would be quickly consumed by hydrogen-utilizing methanogens to produce methane, homoacetogens to produce acetic acid, or sulfate-reducing bacteria to produce hydrogen sulfide. The highest hydrogen yield from WAS reported so far has been only 20.30 mg per gram volatile suspended solids [23].

Several review papers were published on hydrogen production using the various wastes such as biodegradable municipal wastes [24], food waste [25], agriculture waste, wastewater [26], and lignocellulosic materials [27,28]. These works were mainly to review the progress of hydrogen production in one aspect or several aspects. For example, Yang et al. provided a review on fermentative hydrogen production from sewage sludge but only focus on pretreatment methods and co-fermentation with other substrates [5]. Systematic summarization and critical thinking of the application niche of hydrogen production from WAS is still lacking. In addition, many endeavors were dedicated recently to improve hydrogen yield from WAS through enhanced the disintegration of WAS and suppressed or killed the competitive microorganisms (e.g., methanogens), which have made great progress [7,29]. Hence, this review article aimed to comprehensively sum up the knowledge obtained in this field and critically think the prospect of biohydrogen production from WAS. To find out whether and how to recover hydrogen in a future paradigm of WAS treatment, the principles and advances of hydrogen production from WAS were needed to systematically review, its opportunities and challenges were required to critically re-examine, and the possible solutions were essential to carefully think about.

Based on reviewing more than 200 publications and critically analyzing the opportunities and challenges of hydrogen production from WAS, this review aims to offer useful information to remove key barriers that hinder hydrogen production from WAS to be applied in full-scale situations, and to stimulate more thinking and discussion of a probable application niche for hydrogen production from WAS. To guide the application and development of hydrogen recovery, a more promising hybrid process through rational integration of the available technologies is proposed as an example, and how this hybrid process works is illustrate d and future efforts to be made in the future is discussed.

Section snippets

The pathways of hydrogen production from waste activated sludge

Several methods such as direct-biophotolysis, indirect-biophotolysis, photo-fermentation, dark-fermentation, and microbial electrolysis cell can be theoretically used for hydrogen production [30]. To date, however, only three approaches, i.e., dark-fermentation, photo-fermentation, and microbial eletrolysis cell, have been documented to produce hydrogen from WAS (Fig. 1). Dark-fermentative hydrogen production is a process that uses organic matters in either soluble or solid state as electron

The potential of hydrogen production from WAS

WAS shows huge theoretical potential in hydrogen production. The theoretical methane yield of 354 mg-CH4 can be produced from WAS anaerobic digestion if 1 g sludge cells (expressed as C5H7NO2) are completely digested [64]. It is reported that 28% methane is produced from the hydrogenotrophic methonogenesis pathway [65], this indicates ~50 mg-H2 would be consumed theoretically in this process. Besides, two other processes, i.e., homoacetogenesis and sulfate-reducing processes, are known to

Recent progress in hydrogen production from WAS dark fermentation

In the past years, many endeavors were dedicated to improving hydrogen yield from WAS (Table 1). In order to enhance the disintegration of WAS and to suppress or kill the competitive microorganisms (e.g., methanogens) in dark fermentative systems, many WAS pretreatment methods such as heating [87], acid [88], alkaline, and ultrasonic [89,90] pretreatments were tested. Xiao et al. [91] found that compared with the control, thermal pretreatment at 121 °C for 30 min enhanced soluble chemical

Research gap and perspective

Although numerous efforts have been performed, hydrogen production from WAS is still far from achieving at practical scales due to low hydrogen yield, high energy (chemical) input in WAS pretreatment and fermenter control, or cost-prohibitive materials in photo-bioreactors (or anodes). In view of the state of the art relate to hydrogen bioproduction from WAS, further research on the following perspectives is urgently needed:

  • (1)

    Many pretreatments were developed to promote sludge disintegration, but

The hybrid process for the operation of WWTPs

It is widely acceptable that WWTPs are not only the places to purify sewage but also the facilities to recovery energy and resource. The yield of each useful product (e.g., hydrogen) from WWTPs should be maximized, and meanwhile the operation input should be minimized as far as possible. In addition, the investment of WWTPs to recovery resource should be controlled at a reasonable level. Based on these principles, the combining dark fermentation technology with other available technologies

Conclusions

In this paper, the principles and potentials, microorganisms, possible technologies, and process parameters of hydrogen production were reviewed. Besides, both the opportunities and challenges of hydrogen production were analyzed. Microbial electrolysis cells were regarded as the prospective technology for hydrogen production due to high theoretical hydrogen yield and the ability of using varieties of organics as substrate. Nonetheless, the low organics utilization from WAS and the quick

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

This study was financially supported by the project of National Natural Science Foundation of China (51779089), the Natural Science Funds of Hunan Province for Distinguished Young Scholar (2018JJ1002), and Huxiang High Level Talent Gathering Project (2019RS1029).

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