Elsevier

Bioresource Technology

Volume 369, February 2023, 128395
Bioresource Technology

Emerging trends in the pretreatment of microalgal biomass and recovery of value-added products: A review

https://doi.org/10.1016/j.biortech.2022.128395Get rights and content

Highlights

  • Microalgae are a promising source for wastewater treatment and resource recovery.

  • Advanced microalgal pretreatment methods are critically evaluated and discussed.

  • Pretreatment cost and complexity are major drawbacks for large-scale application.

  • Hybrid/combination of pretreatment methods can lead to high-value product recovery.

  • Scope, challenges, and perspectives of pretreatment methods are discussed.

Abstract

Microalgae are a promising source of raw material (i.e., proteins, carbohydrates, lipids, pigments, and micronutrients) for various value-added products and act as a carbon sink for atmospheric CO2. The rigidity of the microalgal cell wall makes it difficult to extract different cellular components for its applications, including biofuel production, food and feed supplements, and pharmaceuticals. To improve the recovery of products from microalgae, pretreatment strategies such as biological, physical, chemical, and combined methods have been explored to improve whole-cell disruption and product recovery efficiency. However, the diversity and uniqueness of the microalgal cell wall make the pretreatment process more species-specific and limit its large-scale application. Therefore, advancing the currently available technologies is required from an economic, technological, and environmental perspective. Thus, this paper provides a state-of-art review of the current trends, challenges, and prospects of sustainable microalgal pretreatment technologies from a microalgae-based biorefinery concept.

Introduction

With rapid urbanization and industrialization, the rate of wastewater generation did not match the available wastewater treatment infrastructure and technology, thus leading to the discharge of poorly treated effluent (Mahari et al., 2022). According to Qadir et al. (2020), worldwide 380 trillion L/y of wastewater is generated, with a major contribution from Asian countries (nearly 159 trillion L/y), Europe (68 trillion L/y), and North America (67 trillion L/y). Of this huge amount of wastewater, only 52 % is treated globally and has different treatment rates ranging from low-income (4.3 %) to high-income (74 %) countries (Jones et al., 2021).

Biological wastewater treatments have recently gained significant interest and made much progress due to their sustainable and low-cost approach to treating and recovering resources from various waste streams (Ummalyma and Singh, 2022). Due to its multifaceted benefits, microalgae-based wastewater treatment has been widely explored to treat liquid waste streams and recover nutrients simultaneously. High growth rate, better photosynthesis efficiency, comparative calorific value to fossil fuel, the feasibility of large-scale production, and value-added product development are the key features of microalgae (Goswami et al., 2021, Guldhe et al., 2017). Despite all these desired characteristics, downstream processing of microalgae biomass is energy-intensive, costly, and time-consuming due to the cell’s complex tiny structure and thick cell wall (Phwan et al., 2018). Integrated approaches concerned with microalgae biorefinery targets the fractionation of microalgal biomass into fuel and commercially value-added products through enhanced downstream processing.

Wastewater sourced from industrial, agricultural, dairy, pharmaceutical, and swine contended with high nutrients, is considered a potential medium for microalgal biomass cultivation (Goswami et al., 2021). Microalgae-assisted integrated strategies toward pollutant remediation and value-added product formation by utilizing wastewater are highly promising (Goswami et al., 2021). But selecting appropriate species, optimizing cultivation conditions, and deploying efficient pretreatment methods for cell wall disruption to recover products are key governing factors (Zabed et al., 2020). Costly harvesting and extraction methods with poor product quality are still a challenging issue when applying a biorefinery concept for algal-based wastewater treatment systems (Zhou et al., 2022).

Pretreatment is, thus, recognized as a crucial step in the destruction of complex microalgal cell-wall and the recovery of intracellular products. Mechanical pretreatments such as ultrasonication, high-pressure homogenization, and thermal heating are prominently used for microalgae cell wall destruction (Phwan et al., 2018). In thermochemical pretreatment, the hydrothermal method is considered the most efficient compared to traditional approaches, including acid, microwave, and enzymatic methods (Passos et al., 2016). Microwave (Sirohi et al., 2021), enzymatic (Zhao et al., 2014), fungal (Zhao et al., 2014), and using cationic surfactants like cetylpyridinium chloride, cetylpyridinium bromide, and cetrimonium bromide are also explored as effective pretreatment methods.

Despite several reported microalgae-based wastewater treatment studies, research is still lagging in the direction of comparative assessment of microalgae valorization in different types of wastewater, cost-effective pretreatment strategies, extensive techno-economic analysis (TEA)-life cycle assessment (LCA) of value-added product recovery to find out economic feasibility and environmental sustainability of applied technology. In this direction, the present review provides an enhanced investigation of microalgae-based process integrations for the treatment of different types of wastewater with simultaneous production of commercially valuable products like proteins, lipids, carbohydrates, anti-cancer compounds, and animal feed (Fig. 1). This review critically analyses various conventional and novel pretreatment strategies applied for value-added product recovery from wastewater grown microalgae. Further, critical challenges concerned with pretreatment technologies are discussed based on large-scale applicability, input energy, cost, and recovery efficiency. Techno-economic feasibility analysis for microalgae cultivation in wastewater with value-added product formation is discussed. Thus, the current review highlights the advancements in pretreatment technology associated with microalgae-based biorefinery with in-depth analysis of different kinds of wastewater treatment and value-added product recovery.

Section snippets

Wastewater for microalgal cultivation

Microalgae are explored as a potential feedstock for biofuel production and high-value products with a reduced greenhouse gas footprint. Microalgae culture can be cultivated in three types of modes: photoautotrophic, heterotrophic, and mixotrophic. However, large-scale cultivation requires a large quantity of water and nutrients; thus, wastewater may be considered an inexpensive source to cultivate lipid-rich biomass (see Supplementary material). As a phycoremediation agent, microalgae exhibit

Pretreatment methods for microalgae

Marine or freshwater microalgal species can be grown using different wastewater, and harvested biomass can be used as a source or supplement in foods, feed additives, and raw materials in pharmaceutical and cosmetic industries (Hu et al., 2018, Liaqat et al., 2022). But, the extraction and purification of different valued compounds (i.e., protein, fat, polysaccharides, carotenoid, lutein) from algae are complex and depend on its cell wall composition and structure, which further varies

Polysaccharides

Polysaccharides are one of the major components obtained from microalgae (Table 2), and they are also exploited for their gelation properties in different industrial sectors. Both homo- and heteropolysaccharides can exist as structural, storage, and extracellular polysaccharide (Mendez et al., 2013). However, the occurrence of heteropolysaccharides is predominantly observed with microalgae. The carbohydrate profile generally varies among the species and growth stages of microalgae. Glucose,

Techno-economic analysis of microalgae-based value-added products

Algae-based products are used in different sectors, starting with pharmaceuticals, pigments, chemicals, biofuel, food, and feed additives (Bhatia et al., 2022, Zhang et al., 2021). Interestingly, the United States Foods and Drug Administration considers the variety of algal-based products that are especially used as food and feed additive is generally considered safe. This created a niche market demand for various algal-based products; however, the supply is limited due to various

Challenges and future perspective

Currently, available pretreatment processes have certain drawbacks and can markedly affect the overall production cost. For example, the physical methods currently available are expensive due to high energy input, while the chemicals treatment are either corrosive (due to the use of acids) or result in poor digestibility in the case of alkaline hydrolysis. Small-scale studies on the application of sonication and microwave disruption have been done extensively. But energy input needs to be

Conclusions

Commercialization of algal biorefinery relies on the successful extraction and purification of value-added intracellular products. Approximately 50 % of the cost is used for the pretreatment of algal biomass. The physical pretreatment method is simple but energy-intensive; chemical pretreatment is efficient but generates toxic byproducts. Similarly, biological methods are environmentally friendly but time-consuming; hence ionic liquids or physicochemical-based are becoming more promising but

CRediT authorship contribution statement

Nirakar Pradhan: Writing – original draft, Writing – review & editing, Formal analysis, Resources. Sanjay Kumar: Writing – original draft. Rangabhashiyam Selvasembian: Writing – original draft, Writing – review & editing. Shweta Rawat: Writing – original draft. Agendra Gangwar: Writing – original draft. R. Senthamizh: Writing – original draft. Yuk Kit Yuen: Writing – original draft. Lijun Luo: Writing – original draft. Seenivasan Ayothiraman: Writing – original draft. Ganesh Dattatraya Saratale:

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.

Acknowledgment

This work was supported by the Hong Kong Baptist University with grant numbers: RC-OFSGT2/20-21/SCI/010.

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