Review ArticleA review on evaluation of applied pretreatment methods of wastewater towards sustainable H2 generation: Energy efficiency analysis
Introduction
Innovation in the field of energy systems becomes a major quest due to the high demand and development of various technologies on environmental emphasis. In the worldwide history of human civilization, energy has furthermost essential status towards progress and prosperity. The present outline of energy demand grows tremendously on a global scale to reduce greenhouse gas (GHG) emissions linked with fossil fuel depletion. Major destructive environmental impacts such as climate change, ozone layer depletion, acidification, and air pollution occur on incinerating fossil fuels. Hydrogen energy generation was a holistic approach to reduce environmental pollution and its related issues [1]. Hydrogen is a very light and more abundant element with no odor and no toxic with the highest energy content (122 kJ/g), besides no harmful emissions during its combustion [2]. Similar to electricity, hydrogen is an energy carrier and used in all economic sectors such as transportation, power, industry, and buildings [3]. In addition, this more intense and vast desirable hydrogen energy was utilized in the refining industry, food processing sector, methanol production, ammonia and fertilizer for agriculture, metal and fabrication industry and electronics sector [4]. Also, by using fuel cells, hydrogen can be converted to electricity and operated continuously under hydrogen and oxygen zone. These fuel cells are available from small to very large sizes liberating heat and water as its by-products [5]. Thus, Hydrogen, an industrial gas, has strong fundamentals on a global scale. As per the report of the 2015 IEA technology roadmap for hydrogen and fuel cells, hydrogen has potential characteristics to reduce CO2 emissions.
The rapid industrialization demands high water and energy resource and overconsumption of rich valuable resources affects the ecosystem. This immense development of the industrial sector leads to drastic environmental changes by disposing of the abundance of wastewater without treatment which causes pollution and health hazards [6]. The treatment of wastewater along with the generation of renewable energy resource is an ideal method to tackle the problems nowadays. Presently, the availability of pilot-scale eco-friendly approach is more for yielding bioenergy such as biomethane, biohydrogen and bioethanol from organic wastewater. In recent decades, hydrogen production from wastewater is gaining attention as an emergent technology.
Hydrogen generation can be achieved by various methods such as dark fermentation, photo fermentation, bio photolysis and microbial electrolysis [7]. The photosynthetic bacteria in the wastewater consume organic acids and light energy to generate hydrogen during photo fermentation process. A major limitation of this fermentation process is excess energy demand, removal of ammonia, photobioreactor size and minimum light conversion efficiency affects the hydrogen yield. Bio- Photolysis process make use of green algae and cyanobacteria which may absorb light energy in direct and indirect way and deplete the water molecules through electron transfer. This in turn enhances the hydrogen generation and oxygen. But, the efficiency of the light energy conversion is very less and restricts the hydrogen generation by inhibiting the enzyme hydrogenease and nitrogenase due to oxygen availability. During microbial electrolysis, the volatile fatty acids (VFA) of wastewater was depleted in low voltage and utilized by the acidophilic microbes which liberates the electrons/protons to produce hydrogen. But, co-generation of VFA and alcohol limits the hydrogen yield during this process. This issues was conquered by integrating the process i.e., two-step process such as photofermentation and microbial electrolysis method. This integrated approach effectively enriches the hydrogen yield but consumes high energy. The dark fermentation process was a highly adopted and efficient method [8] which was carried out by fermentative bacteria. This process consumes various carbon sources in the absence of any external light source to produce hydrogen. The usage of raw industrial wastewater for hydrogen production can be a high energy-intensive and low yield process [9]. Since raw industrial wastewater consists of complex and recalcitrant biodegradable compounds that hinders the hydrogen yield and specific hydrogen production rate. Hence to enrich the process feasibility and sustainability, pretreatment process such as physical, chemical, biological and mechanical was incorporated as an additional step. The pretreatment process accelerates the hydrogen generation capacity of wastewater [10]. So far, no review paper has been particularly focused on this aspect, thus, the scope of the review is to study various pretreatment process involved in wastewater treatment which aids in enhancing the hydrogen generation, overview of wastewater detailing necessity of pretreatment, strategies and reactor utilized for the H2 generation enhancement and evaluation required for commercialization. Fig. 1 details the brief description of the review.
Section snippets
Overview of wastewater
Most of the human and industrial activities necessitate high amount of water for existence and that leads to the way for enormous wastewater generation. The majority of wastewater was disposed to the environment without treatment due to not strict regulations in many developing countries [11]. It causes a detrimental impact on human health, economic productivity, freshwater resource quality and ecosystem [12]. Source of wastewater generated includes domestic residence, commercial properties,
Pretreatment of wastewater for H2 production
Pretreatment of wastewater was highly preferred to enhance microbial-substrate accessibility and reaction rate. The pretreatment method has the advantage of improving the rate-limiting step thereby augmenting the biohydrogen production performances. There are several pretreatments adopted to improve surface area of wastewater substrate such as physical method - thermal and microwave treatment [15,16], chemical method-alkaline, acidification and Ozone treatment [[17], [18], [19]], mechanical
Cost and energy evaluation of pretreatment process for H2 production during commercialization
In cost and energetic analysis, operational costs namely: energy consumption, chemical consumption and other for pretreatment of wastewater was taken into consideration for calculation. Energy consumption cost is the major parameter determining the profitability of the treatment process which accounts to be 62% of the capital cost required for the treating wastewater. Based on the research report [[70], [71], [72], [73], [74], [75], [76], [77]], the energy consumption of the pretreatment
Limitation and challenges in pretreatment techniques
Even though there are many reports on viable approaches intended for enhancing biohydrogen production, an intense rise in commercialization that remains almost unachievable. This is mainly due to the long-lasting gap among the researchers and engineers [82]. Hence, this issue necessitates the researcher to procure the perspective of field application towards an economic output that could readily operable by industry. Researchers adopted different pretreatment approaches for biohydrogen
Key issues and future perspectives
Key issues affecting the hydrogen economy were grouped into four components (1) feedstocks preparation for the smooth and efficient treatment process, (2) suitable selection of pretreatment process for hydrogen enhancement, (3) contaminants exist in hydrogen generation need to be purified by downstream processing, and (4) uniform hydrogen supply and appropriate storage. To achieve clean hydrogen, proper monitoring in the above four components was required. The implementation of suitable
Conclusion
The study details the review of the extensive literature on wastewater pretreatment on the features involved in enhancing biohydrogen generation. The study also revealed the efficiency of various pretreatment agents in enhancing hydrogen generation. However, numerous pretreatment techniques have been extensively studied, but lack of concord in opting universally appropriate method. This necessitates lots of research in the future. Variation in metabolic pathways and microbial distribution
Acknowledgment
The authors acknowledge the National Research Foundation of Korea (NRF) grant funded by the Korea Government (Ministry of Science and ICT, NRF-2019M3E6A1103839).
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