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Thermal/Photocatalytic Conversion of Sewage Sludge and Biomass to Energy

Maria Siddique1, Sumia Akram2, Zainab Liaqat1 and Muhammad Mushtaq1*

1Department of Chemistry, Government College University, Lahore, Pakistan

2Division of Science and Technology, University of Education, Lahore, Pakistan

Abstract

Recently, a great deal of research has been devoted to exploring renewable energy sources like sunlight, geothermal, hydropower, and bioenergy. Many developed countries like the United States of America are getting more than 20% of their energy needs from renewable sources and biomass can work as a fascinating renewable energy source. Biomass and sewage are considered viable alternatives to fossil fuels for sustainable energy production. Thermal and photocatalytic conversion technologies can generate energy from these materials, contributing to cost-effective and sustainable energy systems. Utilizing these technologies could reduce the environmental impact of energy production while creating new opportunities for economic growth and job creation. Additionally, utilizing biomass and sewage for energy production provides benefits such as addressing waste management concerns and mitigating the negative environmental impacts of waste disposal. Challenges and opportunities of biomass energy: The potential of thermal and photocatalytic conversion technologies to generate energy from biomass and sewage has been comprehensively highlighted in this chapter. A thorough examination of the various types of biomass and sewage and their energy content paved the way for scrutinizing the challenges and opportunities associated with using them as energy sources. Further investigation into thermal conversion technologies including pyrolysis, gasification, and combustion, in addition to the photocatalytic conversion process, was carried out. Scrutiny of each technology’s respective advantages and disadvantages was done with great detail and analysis.

Keywords: Biomass, pyrolysis, photocatalysis, biofuel

1.1 Introduction

In recent years, policymakers have been very excited to switch to clean and green energy sources as it not only can help them combat environmental pollution but also provide more renewable energy sources [1]. Meanwhile, rapid industrialization and urbanization have led to a significant increase in global energy demands and a shortfall in fossil fuels. The growing consumption of fossil fuel-based resources for energy has caused several substantial rises in greenhouse gas emissions. These consequences hold immense importance not just for the economy but also for human health [2]. An increase in awareness about future energy and environmental concerns has spurred the utilization of available renewable viable energy sources. Biomass is now regarded as a green renewable energy source owing to its eco-sensitive nature, transformability, and, cost-effectiveness [3–5].

As cities continue to grow and become more heavily populated, per capita energy consumption increases and this may lead to high levels of carbon emissions [6]. The scenario becomes more alarming in countries such as China, Pakistan, India, Indonesia, and other certain parts of the world which totally rely on fossil fuels for their energy demands. With dwindling global oil reserves, treating and recycling copious amounts of wastewater produced by these so-called ‘megacities’ offered new horizons of challenges. Many countries have successfully switched toward solar energy but this technology remained limited to regions of intense and longer sunlight which is not the case with many countries like Russia and other European nations [7, 8]. Consequently, a single energy source may not work fine for all nations and we need a hybrid energy system. For example, countries situated beside oceans like Brazil can exploit algal biomass as a viable source of energy. It has been estimated that algae can produce 30 to 100 times more biomass than that is produced by other photosynthetic sources on Earth. Meanwhile, researchers claim large-scale production of algal biomass is associated with more carbon conversion/capturing present in the air [9, 10].

Although the exact definition and composition of biomass vary from country to country and source to source (Figure 1.1), we, in this monograph, are going to treat the underutilized agricultural residues, seaweeds, organic pollutants, and livestock residues as biomass. In a broader sense, biomass refers to a wide array of living entities that are obtained from the process of photosynthesis [11], including organisms of the plant, animal, microbe, and some byproduct classifications [12]. Various types of biomass waste may comprise dissimilarities, for instance, agricultural waste forms encompass straw, crops, wood chips, and animal byproducts [13, 14]. Furthermore, industrial organic wastes include textile and food processing wastes, leftovers, and organic wastewater [15, 16], whereas municipal solid wastes cover food and kitchen waste, as well as domestic waste.

Figure 1.1 The key biomass/waste residues that can be exploited as alternative energy sources.

In countries undergoing rapid urbanization and an increase in population, the conversion of municipal waste into energy/electricity has been an essential area of research. The global annual municipal solid waste production is anticipated to be reached around 2200 million tons by 2025 as per the World Bank report [17]. The utilization of solid waste as an alternative energy source especially biomass and bio-solid wastes can help us to resolve not only waste management issues but also improve associated social and environmental standards [18, 19]. In another report, the yearly amount of worldwide solid waste production is estimated at 2400 million tons and is projected to reach above 2600 million tons in the next 6 years [20]. Furthermore, wastewater treatment plants produce around 1000 tons of solid waste each day [21]. With the proper technology, loads of energy required for the process can be minimized and also utilized for the production of sustainable energy sources.

1.2 Biomass as Energy Sources

Sewage and biomass are two promising renewable energy sources that have gained the interest of researchers, lately. Sewage or wastewater is an abundant source of organic matter along with other potentials; it can be useful in producing renewable energy such as biogas. Because sewage contains numerous complex components, it cannot be processed directly for energy production, rather it is required to be converted into a suitable form called sewage sludge [22, 23]. Therefore, the residue obtained from wastewater treatment is known as sewage sludge [24]. Sewage sludge can be distinguished both quantitatively and qualitatively, depending on the specific characteristics of the wastewater being processed and the approaches used for treatment. This process is relatively energy-intensive and definitively requires resources. This residue has long been classified as non-useful and often discarded in landfills [25]. A recent perspective regarding wastewater treatment approves of its conversion into energy and other useful products. It has been estimated that sewage sludge can potentially fulfill about 10% of our power demands as an alternative energy source [26, 27]. However, implementation of these perspective technologies is associated with high capital investment and needs considerable land area for sludge disposal. On the other side, sewage sludge production will keep growing with the population, and industrialization and its eco-friendly and sustainable management may cost much more particularly in developing countries [28].

Wastewater treatment plants are responsible for the sanitation of sewage water by separating its components, i.e., solids and minerals. It is also worth knowing that sewage sludge is an unavoidable material residue created at WWTPs, with significant amounts being created on a worldwide basis. According to the statistics, the annual dried sewage production of some countries was as follows: for the United States, it was 6.5 million tons in 2004 [29]; for China, it was almost 6.25 million tons in 2013 [30]; and for the European Union, it was 13 million tons in 2020 [31, 32].

Biomass is a popular phrase that relates to plant-derived and biological waste materials. However, the concept can include any sort of organic material. The total estimated production of carbon is 105 billion metric tons annually, evenly divided between terrestrial sources and oceans (algal biomass). Despite the fact that algal biomass constitutes a major part of world biomass volume, wood remains the primary biomass source used in various applications. Wood sources (cover forests) are often utilized for power generation and biopower, while agricultural remains like sugarcane bagasse, rice straw, cotton stalks, and wheat straw are classified as biomass [33–35].

Biomass can be employed as a powerful energy source to mitigate environmental risks associated with waste disposal and establish a sustainable and renewable energy alternative [36]. Bioenergy is another term used to refer to energy that is derived from biomass resources. It takes a series of stages, which include harvesting, drying, storage, moving, conversion, etc., to convert biomass into an energy source that can be utilized.

The direct...