Bioactives from microalgae: A review on process intensification using hydrodynamic cavitation

Microalgae are emerging as an important renewable and sustainable source of high-value biomolecules having applications in food, cosmetics, pharmaceutical, agrochemicals and fuel industries. Deriving high-value biomolecules from micro-algae however faces numerous process and technological challenges. It is essential to develop innovative ways of intensifying processes used for valorising microalgae. Hydrodynamic cavitation (HC) offers an attractive platform for process intensification relevant to microalgae because of its scalability, ability to handle dense slurries, intense physicochemical effects, and low cost. Here we briefly review the overall processes involved in deriving high-value biomolecules from micro-algae. Opportunities for intensifying these processes and enhancing productivity of processing microalgae via HC are then identified and critically reviewed. The current state of the art and yet unresolved challenges are highlighted. An attempt is made to identify specific suggestions to help direct future research efforts. The review will be useful for researchers and practitioners aiming to harness HC for deriving high-value products from microalgae.


Introduction
Microalgae are microscopic photosynthetic organisms that account for 40% of global photosynthesis forming the base of food chains. Microalgae are emerging as a renewable and sustainable bio-resource. Bioactive molecules derived from microalgae are finding numerous applications in a variety of industries like food, cosmetics, pharmaceutical, diet supplement, agrochemicals, bio-stimulants, animal feed and fuel (Singh and Kalia 2017;MU et al. 2019;Junior et al. 2020). Residual algal mass after extracting high-value biomolecules can also be further valorised by deriving other relatively low-value biomolecules like carbohydrates and cellulosic material and bioenergy (in the form of biogas) (Chen and Quinn 2021; Goswami et al. 2021;Torres et al. 2021). Microalgae are important photosynthetic organisms due to their high photon-to-biomass conversion efficiency (3%) against 1% in higher plants (Perin et al. 2019). Microalgae grow in various types of habitats and can produce various commercially valuable biomolecules such as proteins, carbohydrates, lipids pigments etc. Microalgae can be cultivated in wastewater on barren waste land, without any significant input in terms of fertilizer or pesticides. Microalgae are also used for wastewater treatment and CO 2 sequestration, adding to the advantages of microalgae cultivation and its bioprocessing for a variety of commercially important bioactive biomolecules (Levasseur et al. 2020).
While significant potential exists for valorising microalgae by deriving high-value bioactives and bioenergy, it is essential to develop cost-effective and scalable solutions for all the relevant steps involved in valorisation of microalgae ranging from cultivation to purification. Due to the microscopic size of microalgae (e.g., the diameter of Arthrospira platensis is about 8 μm), the handling of such a small cell size and collective biomass is a challenge during cultivation and harvesting. For the same reason, conventional pre-treatment methods may not be effective in the breakage of cell wall (cell disruption) for opening up the cell and the release of intramolecular content, thus, is a challenge. The small quantity of biomolecules per cell of microalgae warrants a huge quantity for industrial applications. Using inefficient methods results in high production and operation cost. Methods such as ultrasonication, homogenization, pressurized 1 3 liquid, enzymatic methods, pulse electric method, supercritical fluid extraction, microwave etc., are presently in use for pre-treatment for extraction (Skorupskaite et al. 2019;Lee et al. 2021;Mehariya et al. 2021;Pagels et al. 2021;Vasistha et al. 2021;Timira et al. 2022).
However, the constraints to realise economically feasible commercialization of this nascent bio-industry to realise valorisation of microalgal biomass are summarised in Table 1.
Hydrodynamic cavitation (HC) has the potential to address these limitations and is gaining interest as a pre-treatment method. It is reported to be low cost, energy efficient, minimization of toxic solvent usage, scalable and ability to obtain superior processed products when compared to conventional methods (Ranade and Utikar 2022). Despite its various advantages and potential in microalgal-based industries, it is underutilised and needs the insight to develop an understanding of the HC-based operations to open its scope. Thus, a review is required to show the path for developing the understanding, and new strategies for the efficient processing of microalgae, towards improving the economic feasibility and sustainability. To the best of our knowledge, no review on evaluating the potential of HC in microalgae-based industry is available.
Here, we critically review the potential of harnessing hydrodynamic cavitation for intensifying processes and provide a new perspective for assessing the potential of HC in microalgae valorisation. In the following subsections, we briefly review bioactives derivable from microalgae and various steps involved in the valorisation process. Hydrodynamic cavitation is then discussed. Various HC devices used in practice are briefly reviewed. Later we critically review application of HC for cultivation and harvesting of microalgae, pre-treatment for extraction of bioactives and purification of extracted bioactives.
An attempt is made to critically evaluate the merits of using HC with respect to alternative technologies. Specific comments and suggestions on further research for improving performance are included. Some comments on developing a bio-refinery based on microalgae as feedstock and outlook are included. We believe that it will stimulate further research on harnessing HC for process intensification.

Bioactives from microalgae
The diversity in family and species of microalgae offers a variety of high-value biomolecules, which can be further diversified by modifying the growing conditions. The diversity in terms of strains of microalgae can be classified under different phyla such as Chlorophyta, Rhodophyta, Haptophyta etc. (Kim and Chojnacka 2015;Ruggiero et al. 2015). Microalgae e.g., Aphanizomenon, Arthrospira, Botryococcus, Chlamydomonas, Chlorella, Dunaliella, and Hemiselmis are natural and rich sources of bioactives (Fernandes and Cordeiro 2020;Levasseur et al. 2020). Some of the variety of bioactives (polyunsaturated fatty acids (PUFA), monounsaturated fatty acids (MUFA), docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), pigments (chlorophylls (a & b), phycoerythrin (PE), phycocyanin (PC), allophycocyanin (APC), carotenoids (β-carotene, astaxanthin, lutein), proteins, vitamins, polyphenols, phytosterols, exopolysaccharides) from microalgae are presented in Fig. 1 (Mourelle et al. 2017;Bhalamurugan et al. 2018;Dixon and Wilken 2018;Khanra et al. 2018;Galasso et al. 2019;Scaglioni et al. 2019). These bioactives find applications in different fields of pharmaceuticals, nutraceuticals (single-cell proteins) and cosmetics (Del Mondo et al. 2020;Santhakumaran et al. 2020;Wali et al. 2020;Reynolds et al. 2021), and can be used as food ingredients in functional foods or in the form of tablets, capsules etc., as dietary supplements summarised in Table 2 (Panda and Manickam 2019;Srivastava et al. 2021;Cazarin et al. 2021;Gupta and Gupta 2020;Saha et al. 2015;Molino et al. 2020;Fernandes and Cordeiro 2020;Gharajeh et al. 2020;Gautam and Mannan 2020). For example, lipids from microalgae can be a potential ingredient in feed Table 1 Bottle necks to realise the economically feasible microalgae-based industry • High production cost ○ High installation and operation costs of the cultivation set-up ○ Energy-intensive harvesting ○ Energy-intensive pre-treatment methods • Low product yield ○ Difficulty to disrupt all cell wall types ○ Difficult to recover all the valuable biomolecules ○ Ease of using pre-treatment method ○ Selective extraction ○ Complete recovery of valuable compounds ○ Efficient when choosing a cell lysis method • Variation in their cellular composition due to seasonal and nutritional variation • Scalability of pre-treatment method • Lack of techno-economic analysis • Environment-friendly route • Increase in energy cost and food products and can be a replacement for fish and plantbased polyunsaturated fatty acids (Katiyar and Arora 2020). The global market for phytosterols is expected to reach US$ 935 million by 2022 and plant-based PUFAs may not be able to meet increased demand through 2030 and beyond, whereas microalgae could yield 678-6035 kg ha −1 year −1 of phytosterol which is more than the current productivity of phytosterols from rapeseed plants, highlighting their economic potential (Randhir et al. 2020). The cosmetic industry has recently started the inclusion of algal compounds in innovative formulations. Herein, microalgal pigments such as chlorophyll, β-carotene, astaxanthin, xanthophylls, and phycobiliproteins   (Mittal et al. 2017;Gargouch et al. 2018;Mittal and Raghavarao 2018;Jacob-Lopes et al. 2019;Martínez et al. 2019a;Li et al. 2019b;Mittal et al. 2019;Bueno et al. 2020;Mittal et al. 2020;García et al. 2021;Mittal et al. 2022) Phycocyanin (PC) Anti-hypertensive properties and antioxidant activity A. platensis (Tavanandi et al. 2018b;Bachchhav et al. 2020) Allophycocyanin ( (Gallego et al. 2020b;Khoo et al. 2020b;Lima et al. 2020) (Sá et al. 2019;Almendinger et al. 2020) Astaxanthin Haematococcus pluvialis (Imbimbo et al. 2020;Khoo et al. 2020c;Le-Feuvre et al. 2020;Yang et al. 2020) find applications in innovative cosmetics (Almendinger et al. 2020;Morocho-Jácome et al. 2020;Tabarzad et al. 2020) and lipids find use in skin creams and lotions Vieira et al. 2020;De Luca et al. 2021;Miguel et al. 2021). For example, microalgae (Arthrospira platensis and Dunaliella salina) are being introduced into cookies for their superior nutritional profile (Sahin 2020). Microalgal biomass of Arthrospira and Chlorella have been reported to improve the growth performance of broiler chickens (Sugiharto 2020) and have been added to animal (livestock and fish) feed (Ritu et al. 2023).

Steps to extract bioactives
To extract bioactives, which are generally high-value and low-volume biomolecules from microalgae a series of steps needs to be followed. Microalgal cultivation is the starting point followed by harvesting, pre-treatment and extraction, and purification (shown in Fig. 2). Optimised cultivation of microalgae is crucial to achieving desirable productivity of algal biomass and high yield of bioactives. Cultivation modes such as raceway ponds, photobioreactors and their advancements are used at a commercial scale for microalgal Fig. 2 Schematic representation of the steps to valorise microalga-from cultivation to products (the images in this figure are taken from www. commo ns. wikim edia. org; www. pixab ay. com) 1 3 production. The cultivated biomass is then harvested by various methods such as flocculation, filtration, settling for further processing and value addition.
To obtain bioactives, harvesting is followed by pre-treatment and extraction and involves pre-treatment methods (mechanical methods: bead milling, ultrasonication, microwave, high-speed and high-pressure homogenizer, autoclaving, pulsed electric field, and non-mechanical methods: enzymatic or alkali treatment) intending to maximize the yield of target biomolecule. The cell wall provides mechanical and chemical protection to cells, overcoming which is crucial during extraction. The cell wall structure and composition of some of the commercially important microalgae is given below. The cell wall thickness and composition vary with microalgal species for example, D. salina and similar hypersaline strains only have a plasma membrane and lack any rigid polysaccharide in the cell wall (Torzillo and Masojidek 2008). The cell wall of Crypthecodinium cohnii is composed of several membranes surrounded by thecal plates and exopolysaccharides and is 133 nm thick which provides strength to the microalgae. The cell wall of Chlorella vulgaris is of 100-200 nm thickness (Sydney et al. 2014). The cell walls of C. vulgaris and other green microalgae have rigid wall components embedded within a more plastic polymeric matrix containing uronic acids, rhamnose, arabinose, fucose, xylose, mannose, galactose, and glucose. The predominant amino sugar found in the rigid cell wall of Chlorella is a chitin-like glycan, making it rigid (Gerken et al. 2013). Like chitin, sporopollenins is a tough non-hydrolysable polysaccharide, present in the cell wall of H. pluvialis aplanospores making it difficult to extract the antioxidant, astaxanthin from these cells (Ye et al. 2020) and the Arthrospira cell wall gains its strength from peptidoglycan with an absence of cellulosic compounds in the cell wall (Canelli et al. 2021;Machado et al. 2022). The selection of pre-treatment methods is important for efficient extraction owing to different cell wall structure of biomass, which needs to be disrupted (Alhattab et al. 2019).
After extraction, purification is a mandatory step for value addition. Methods such as electrophoresis, membrane separation, ultracentrifugation, aqueous two-phase extraction (ATPE), adsorption, ammonium sulphate precipitation, chromatography, etc., are gaining a lot of attention. After the extraction and purification of high-value biomolecules, valorisation of spent biomass through the biorefinery approach will lead to value addition (Jacob-Lopes et al. 2019). To make these biomolecules commercially viable and feasible, research efforts are being made to develop fast-growing and high-yielding strains, simple cultivation methods, easy and efficient harvesting techniques, advanced cell disruption, extraction and purification methods for enhancing valorisation potential of microalgae (Quinn and Davis 2015;Milano et al. 2016;Waghmare et al. 2019).
Algae processing involves several energy-intensive operations such as aeration, mixing, harvesting, cell disruption and interphase mass transfer. The low biomass concentration in the production media aggravates these limitations further. Thus, it is essential to develop better ways of intensifying these desired transport processes without incurring significant energy penalties.
In this pursuit, cavitation-based methods have the potential to realise process intensification and enhance productivity. Cavitation acts as an energy concentrator and offers an attractive avenue for intensifying processes like particle size reduction, extraction and interphase mass transfer (Subhedar and Gogate 2013;Abiev 2021). The phenomena of cavitation involve the formation, growth and collapse of microbubbles for a very short duration of time (order of nanoseconds) (Lohani et al. 2016). The sudden collapse of the microbubbles results in a rush of liquid to occupy the void, developing a high-velocity jet of stream in the bulk liquid. Cavitation generates very high local temperatures (∼10,000 K) and pressure (∼1000 atm) at millions of locations, resulting in high shear (Ranade and Bhandari 2014;Bhandari et al. 2016;Suryawanshi et al. 2018;Sarvothaman et al. 2019). This jet stream of liquid can disrupt the cell wall and boundary layers, facilitating the rapid migration of biomolecules. Thus, increasing the rate of heat and mass transfer, and increasing the intrinsic rate of chemical reaction at much lower overall energy inputs, in turn, improves the extraction efficiencies. The micron-size vapour/gas bubbles are generated in a low-pressure region which eventually travel to a region of higher pressure where they collapse. The collapse under certain conditions results in intense shear, hammer pressure and localised hot spots. These localised physio-chemical effects can be harnessed for a variety of process intensification applications (Ranade and Utikar 2022). The collapse of cavities, results in the generation of microbubbles and can be useful in microalgal cultivation through media regeneration, aeration, water disinfection, gas mixing and agitation. It acts by removing competitive bacterial cells, rotifers, oxidation etc. the nanobubbles can be useful in harvesting by flocculation.
Cavitation may be realised primarily in two ways: ultrasonic cavitation and hydrodynamic cavitation (Ranade and Utikar 2022). Hydrodynamic cavitation is a method of introducing discrete energy input, where the energy dissipated per unit volume into the system. During HC, the energy is dissipated simultaneously to extremely small zones, but at millions of locations in the system, making it similar to acoustic cavitation. Unlike ultrasonic cavitation, hydrodynamic cavitation is scalable and offers a promising low-cost option for intensifying processes used in valorisation of algae. Hydrodynamic cavitation is the formation, growth and collapse of vapour/gas bubbles due to pressure fluctuations in the flowing liquid. HC is reported to be a possible alternative to ultrasonication for its energy efficiency and scale-up capability (Lohani et al. 2016).
As indicated in Fig. 3 the processes involved in microalgae-based process intensification, need to be optimised to achieve higher efficiency. The ability to generate nanobubbles or microdroplets by HC can be utilised during harvesting and for efficient mass transfer. There are no reports of employing HC for media regeneration, aeration and harvesting. Thus, it was thought prudent to document the gaps and scopes of HC towards valorising microalgae.

Hydrodynamic cavitation -a promising technology
Hydrodynamic cavitation is gaining popularity as a technique with application in various fields such as pre-treatment, food processing, wastewater treatment, cosmetics etc. HC is gaining interest and importance among pre-treatment technologies for the minimization of toxic solvent usage, cost-effectiveness in operation, and ability to obtain superior processed products when compared to conventional methods. Hydrodynamic cavitation was originally a bane for shipping industries and pumping equipment, as it causes erosion of the inner walls. However, on the brighter side, it intensifies various physical and chemical processes to become a novel technique that has extensive applications. The applications ranges from HC-assisted extraction, emulsification, food processing, biofuel synthesis (Iskalieva et al. 2012;Kim et al. 2015a;Hilares et al. 2017;Zieliński et al. 2019;Hilares et al. 2020;Nalawade et al. 2020;Lauberte et al. 2021;Thaker and Ranade 2021b;Bimestre et al. 2022;Nagarajan and Ranade 2022;Thangavelu et al. 2022), biomolecule degradation (Yi et al. 2021) and wastewater remediation (Ranade and Bhandari 2014;Bhandari et al. 2016;Gaikwad 2016;Jain et al. 2019;Burzio et al. 2020;Mane et al. 2020;Abbas-Shiroodi et al. 2021;Gostiša et al. 2021;Mane et al. 2021;Patil et al. 2021a, b;Ranade et al. 2021c) agricultural waste (Garuti et al. 2022), food industry (Asaithambi et al. 2019;Arya et al. 2020;Panda et al. 2020;Vigneshwaran et al. 2022). HC has been reported as a scalable, energy-efficient technique for the cell disruption and extraction of biomolecules (such as phenolics, enzymes, and flavonoids), delignification of wheat straw, biomass pre-treatment etc. for bioresource utilization (Gogate and Pandit 2008;Iskalieva et al. 2012;Lohani et al. 2016;Hilares et al. 2017).
In Fig. 4 the potential steps where HC can be employed in the microalgae industry are represented. Markets are consumer-driven and there is a significant increase in the demand for varieties of consumer products. To meet this demand at lower production costs and to enhance the quality of the product, industries are looking toward greener processing technologies. Here, we present a critical review of potential of HC to realise process intensification for the valorisation of microalgae.
To utilize cavitation for beneficial purposes, several devices are used for generating cavitation and enhancing its effectiveness for cell wall disruption at lower energy requirements (Grillo et al. 2019;Wu et al. 2019b).
HC can be categorized based on design of the reactor, which governs the cavitational phenomena. HC devices can be classified into two main types: devices without moving parts and devices with moving parts (Table 3) Mane et al. 2020;Nagarajan and Ranade 2020;Sarvothaman et al. 2020b;Patil et al. 2021a;Ranade et al. 2021c). The advantages of orifice plates, venturi tubes, and high-pressure nozzles HC devices are that they are simple designs and cheap (Sarvothaman et al. 2020b). However, they require powerful feeding pumps to overcome the high-magnitude pressure drop and to generate cavitation. These pumps face issues of frequent clogging, resulting in profit loss, when treating solid-liquid mixtures. To overcome these issues, novel vortex diodes have been designed and developed (Ranade et al. , 2017Jain et al. 2019;Sarvothaman et al. 2019).
Another type of HC device with moving parts is rotary equipment or dynamic cavitational elements, e.g., rotor-stator devices (RSD) or pinned disc rotating generators. The use of a novel pinned disc rotating hydrodynamic cavitation generator has been reported by Gostiša et al., (2021) for wastewater treatment and various parameters. These were developed to generate HC with less energy input and to overcome issues faced by devices without moving parts (Badve et al. 2015;Cerecedo et al. 2018;Sun et al. 2018). Higher inlet pressure leads to a higher degree of cavitation and is expected to result in better efficiency (Batista et al. 2017), ascribed to higher turbulence (Wu et al. 2012). However, with an increase in inlet pressure, the energy input will increase affecting the efficiency of the process (Batista et al. 2017). The authors also tried to correlate efficiency with the cell concentration cavitation effectiveness and reported mixed conclusions. For instance, Kim et al. (2017) reported that at lower solid loading (cell concentration) the damage to cells is less as the probability of interactions between cells and cavitation bubbles is minimised. On the other hand, Xu et al. (2006) contradicted this observation and reported that a lower initial concentration yielded better results, whereas, Halim et al. (2013) observed an open-down parabolic relationship between cell destruction and initial concentration in the case of high-pressure homogenisation. However, Greenly and Tester (2015) reported that cell concentration does not affect efficiency, however the sample volume may have a role in the efficiency of the process. This is particularly true in the case of extraction by HC, where the processing of higher volume requires more time, thus more time for metabolite release into the extraction medium. Besides this, cell shape and size also have a role to play in deciding cavitation efficiency (Wang et al. 2014a) changing the effectiveness of cavitation with cell type which is governed by the microalgal species. This can be due chemical composition and thickness of the cell wall as mentioned above, making cells more or less susceptible to cavitation.
In any method or technique, process parameters affect the effectiveness and efficiency of operation; thus optimization is an important step. In HC, parameters such as  Hence, several models such as multi-layer and per-pass degradation models have been used to study the relation between cavitation reactor performance, its design and operation (Pathania et al. 2018;Sarvothaman et al. 2019). Further, reports are also available explaining the application of HC in batch operation of cavitation reactor and its modelling (Simpson and Ranade 2018;Cappa et al. 2020). Furthermore, other versions of HC devices, namely orifice with swirl, and venturi with swirl have been modelled (Sarvothaman et al. 2020b).
Despite the advantages of HC, only limited reports are available on employing HC for deriving bioactives from microalgae. The major strategies reported for cell disruption of algae (micro and macro) and extraction of bioactives involve osmotic shock (Ramola et al. 2019), alkali hydrolysis method (Ogretmen and Duyar 2018;Mensi 2019), pressurized liquid extraction (Otero et al. 2018), maceration in the presence of liquid nitrogen (Munier et al. 2014), freeze grinding, freezing and thawing (Senthilkumar et al. 2013), and homogenization (Harnedy and FitzGerald 2013). There are also reports on the application of process intensification methods such as high-voltage pulsed electric field (Prabhu et al. 2019) and ultrasonication (Le Guillard et al. 2016;Mittal et al. 2017;Devi et al. 2020). However, these methods have limitations being energy-intensive, requiring high operating temperatures and have scalability issues, e.g. ultrasonication has the challenge of wave attenuation due to a decrease in sound wave amplitude with distance and the pressure waves generated fade off soon, creating a low penetration through the media, leaving most of the mixture untreated (Kadam et al. 2015), and high-voltage pulsed electric field has issues of high power dissipation and amplitude during the pulse (Reberšek and Miklavčič 2011). By contrast, HC-based extraction methods are reported to have significantly lower energy consumption and enjoy advantages (such as higher efficacy and ease of scale-up) over US (Gogate and Pandit 2008). However, exposure times also have a direct effect cell disruption (Rajasekhar et al. 2012). Optimisation of duration of exposure is important, as overexposure to reactive ionic entities generated during cavitation may affect the stability of sensitive biomolecules by oxidising the bonds in biomolecules.
Accordingly, a HC based pre-treatment method has tremendous potential for deriving metabolites from microalgae. However, only a few reports are available on employing HC for the extraction of biomolecules from microalgae. For the benefit of researchers and algae-based industries, HC and its promising roles in different steps such as cultivation, harvesting etc., to realise valorisation of algae, HC is discussed in subsequent sections.

Cultivation and harvesting of microalgae
Obtaining good quality bioactives in desirable volume requires high productivity of quality microalgal biomass. Recent developments in algal cultivation and harvesting are discussed in a subsequent section and the limitations are summarised in Fig. 5.

Raceway ponds
Raceway ponds are the most common cultivation system for microalgae and are equipped with paddlewheels for mixing nutrients, gases and exposure of the algae to sunlight. The scientific community is working on improving the design of raceway ponds for enhanced yield (Sivakaminathan et al. 2020). Enhanced productivity in the range of 20-59% was achieved by altering the depth of pond  or the light spectra by orange-coloured polyvinyl chloride sheets , light-diffusing systems (red luminescent solar concentrators) (Raeisossadati and Moheimani 2020) for microalgae such as A. platensis and Scenedesmus sp. ). Productivity was enhanced by introducing baffles in a 1000 L multistage raceway pond (MSRP) for the cultivation of C. vulgaris (Matanguihan et al. 2020  (RAB) has reduced electricity and water consumption by up to 83 and 30% per kilogram of biomass (20% DW) of algae meal when comparing open raceways ponds (ORP) for the production of Te. suecica . Enhanced productivity of Arthrospira cells and Nannochloropsis sp. was achieved by microporous diaphragm aerators (MDA) in raceway pond and flue gas, respectively (Bhadra et al. 2020;Cunha et al. 2020;Kumar et al. 2020). Strategies such as mixotrophic cultivation are employed to improve the cultivation of microalgae, leading to utilization of inorganic and organic carbon in the presence of light (Yu et al. 2020).
For economic advantages, raceway ponds are popular and adopted in major commercial plants. However, the fluid dynamics of the raceway operation are quite complex. Thus, to improve the understanding and to gain insight, computational fluid dynamics (CFD) -based software simulation has been employed to optimize the design (Kusmayadi et al. 2020). CFD tools were used to improve the design by modelling, and evaluating the geometry, specifically raceway length, width, flow depth and operational conditions such as operational velocity of raceways ponds (Lima et al. 2021;Romagnoli et al. 2020). The approach of utilising treated wastewater (after disinfection), media recycling and efficient aeration (CO 2 gas exchange) can be used to further improve productivity and reduce the operational cost of raceway ponds. Hydrodynamic cavitation in microalgal cultivation is a novel concept and envisaged the media regeneration, aeration, and harvesting by water disinfection, gas mixing and agitation, and flocculation by nanobubbles, respectively.
Suitable water quality water is a primary requirement for microalgal cultivation, however, fresh water in such huge volumes may not be a feasible approach. Hence, treating wastewater by removing competitive bacterial cells (Kim et al. 2021), rotifers (Kim et al. 2017), and degradation of solvents (Patil et al. 2021b) etc., is required for microalgal cultivation (Fig. 6). Water disinfection employing HC has been achieved (Ranade et al. 2009;Gaikwad 2016;Sarvothaman et al. 2018;Jain et al. 2019;Mule et al. 2021). This degradation of extracellular or metabolic waste produced during cultivation or breaks down complex biomolecules in culture media into monomeric nutrients for media recycling (Kim et al. 2021). HC acts by the formation of different radicals (predominately OH radical and H radical) and consequent oxidation at high local temperatures of 5000 K during collapsing of cavitation bubble (Arrojo et al. 2008), however, oxidation is not a principal method for the eradication of bacteria for their larger sizes. Shock waves, shear flow, supercritical water conditions, and pressure and temperature spikes which accompany the aggressive bubble collapse play an important (sometimes decisive) role in damaging the bacteria (Šarc et al. 2016). Besides disinfection, HC by virtue of the ability to generate nanobubbles can be used to replace spargers and paddles to improve the interfacial area and therefore gas mass transfer (Patel et al. 2021).
Wastewater from slaughterhouses was treated using HC to reduce chemical oxygen demand (COD) for microalgal cultivation (Terán Hilares et al. 2022). Reduction of COD mainly happens due to oxidation happening during HC. Advanced oxidation processes (AOPs) in synergy with HC are reported to significantly lower the wastewater treatment cost (Badmus et al. 2018;Joshi and Gogate 2019;Fedorov et al. 2021). The reduction of ammoniacal nitrogen in wastewaters employing HC was demonstrated by Patil et al. (2021). Advanced oxidation processes (AOPs) in synergy with HC are reported to significantly lower the wastewater treatment cost (Badmus et al. 2018;Joshi and Gogate 2019;Fedorov et al. 2021). HC at standardised pH (Zampeta et al. 2022) and in combination with hydrogen peroxide (Agarkoti et al. 2022) and ozone (Wang et al. 2020b) for their influence on the degradation of pollutants, has resulted in enhanced efficiency of the treatment demonstrating a synergistic effect. Attempts for artificial neural network (ANN) and CFD modelling the HC-based wastewater treatments showed excellent ability to interpolate and reasonable ability to extrapolate Sun et al. 2021;Terán Hilares et al. 2022). Thus, it can be summarised that molecular bond breaking, thermal decomposition, and free radical formation during HC , make it a potential method for wastewater treatment for microalgae cultivation. HC can be used for enhanced gas exchange by nanobubbles generation, resulting in improving the productivity of photobioreactors. This will further advance its edge over raceway ponds and is discussed in the following section.

Photobioreactors
Recent years have witnessed a growth in interest in the cultivation of microalgae. However, industrial production remains a major issue -mainly due to the constraints posed by classical cultivation systems such as raceway ponds. This is due to limitations related to light exposure, shear stress, gas exchange, and biofouling. Productivity of biomass varies with geographical locations and climate factors, including solar irradiance, air temperature, relative humidity, and wind velocity and it was found that the productivity varies from 2000 to 13,000 t km −2 year −1 (Banerjee and Ramaswamy 2019). These issues are addressed by employing photobioreactors and their advanced modifications (Assunção and Malcata 2020).
Mixotrophic cultivation (Nanochloropsis oceanica in association with microbe Halomonas aquamarina) in photobioreactors (Subasankari et al. 2020), airlift photobioreactor comprising a transparent draft tube (Madhubalaji et al. 2020), and presence of a low concentration of Cr (VI) (0.5 and 1.0 mg L −1 ) in MPBR (Lu et al. 2021) also have been found to increase biomass productivity and metabolites.
The growth of microalgae depends on the microenvironment in the media, which is controlled by the mass transfer of nutrients, inhibitors and gases in cultivation media and thus plays an important role to improve the overall productivity of any bioprocess. Mass transfer of gases such as CO 2 and O 2 is an important parameter in microalgae cultivation, as the k L a atm (transfer coefficient) value depends mostly on the surface area between culture and atmosphere (Caia et al. 2018;Oliveira et al. 2018;Li et al. 2019a). Balance of these gases in media is crucial for the growth of microalgae and the maintenance of pH. It has been reported that smaller bubble size during cultivation improves the growth of microalgae (Kim et al. 2014), where 26-44% enhancement in microalgae growth by air nanobubbles due to better mass transfer as demonstrated by Patil et al. (2021a) and enhanced lipid productivity (Yang et al. 2018). The gas-to-liquid interfacial area increases with the decrease in bubble size. Bubble size achieved using baffles, orifice or fine-pore diffusers resulted in air bubble size of equivalent diameter (d e ) in the range of 1.0 to 2.0 mm (Cheng et al. 2016;Yang et al. 2018), whereas bubbles generated by HC can be of the nanoscale levels. Hence, HC will be useful in maintaining the CO 2 and O 2 levels and temperature in the cultivation media. Reports employing devices such as a cavitation tube and a needle valve to generate the size of the bubbles ranging from 100 to 550 nm in all surfactant solutions, Results of specific growth rate (µ, day −1 ), doubling Ɵme (day), and biomass producƟvity (g −1 L −1 day −1 ) verify the reusability of the supernatants (aŌer HC) with bacterial debris.

Biomass
Recycled media

FlocculaƟon
Hydrodynamic cavitaƟon HarvesƟng efficiency of auto-flocculaƟon (%) = 97.9 ± 0.1 CFU (mL -1 ) in supernatant before HC -TYG agar: 1.83 × 10 5 ± 1.95 × 10 4 R2A agar: > 2.00 × 10 5 LB agar: 0 HC device (Orifice plate) with 27 holes and diameter of 1 mm each and a beta opening of 0.067. HC process, fluid velocity was maintained at 19.7 m s −1 at upstream pressure was at 4.0 bar and 75 L/min. CFU (mL -1 ) in supernatant before HC -TYG agar: 0.01 × 10 5 ± 0.14 × 10 4 R2A agar: 0 LB agar: 0 after 30 min of recirculation and pine oil to change the interfacial tension are available (Etchepare et al. 2017;Oliveira et al. 2018). Hence, HC owing to its ability to generate microbubbles can be employed for sparging and generation of microbubbles during microalgae cultivation (Sarvothaman et al. 2020a). Cavitation leads to the generation of nanobubbles via simultaneous generation of microbubbles and nanobubbles and subsequent shrinkage of microbubbles with increasing surface charge density to become nanobubbles. High-shear rotor-stator device was used to generate bulk nanobubbles (BNBs) in pure water (Jadhav et al. 2021). Improved and efficient exchange of gases will reduce the operation cost during microalgal cultivation.

Harvesting
Harvesting is crucial but difficult step, due to the very small cell size and buoyancy of the cells. High volume of growing media in raceway ponds and bioreactors and low biomass concentration ( ≈ 3 g L −1 ) adds to the cost due to the energy-intensive dewatering and drying step, thus warranting a favourable method to bypass the dewatering and drying steps (Waghmare et al. 2019). This section aims to summarise both conventional and recently developed techniques of harvesting. The reported methods are employed alone or in combinations to concentrate algal suspensions to process them into the final products. At the time of harvesting algal suspension consists of 2-7% w/v biomass. Harvested biomass is dewatered to reach final biomass concentration of 15-25%, which contributes to 3-15% of algae biomass production costs (Fasaei et al. 2018). Harvesting is reported to be carried out employing methods such as centrifugation (Pahl et al. 2013;Barros et al. 2015;Kim et al. 2015b), chemical flocculation (Vandamme et al. 2013;Lama et al. 2016;Roselet et al. 2017), nanoparticle flocculation (Wang et al. 2014b;Zhao et al. 2018;Liu et al. 2019;Liu et al. 2020a;Ma et al. 2020;Wang et al. 2020a), bio-flocculation (Liu et al. 2013;Ummalyma et al. 2016;Úbeda et al. 2017;Pandey et al. 2019), electrocoagulation (Shuman et al. 2016;Fayad et al. 2017), filtration (Monte et al. 2018;Singh and Patidar 2018;Leam et al. 2020), and dewatering-independent methods (Gross et al. 2013;Prajapati et al. 2014;Prochazkova et al. 2015;Peng et al. 2020). Among these, methods such as microwave-assisted flocculation and magnetic nanoparticles-assisted flocculation are recent developments (Liu et al. 2020b). The advantage of these methods is reported to be their higher flocculation efficiency and non-destructive nature on cell walls (Liu et al. 2020b;Vashist et al. 2020).
Bio-flocculant (alum (Al 2 (SO 4 ) 3 ), eggshells and calcium carbonate have been used for harvesting C. pyrenoidosa and alum resulted in maximum harvesting efficiency (99%) with a few deformities in algal cell surfaces. (Pandey et al. 2019). Membranes (mainly ultrafiltration) are also used as separating tools to harvest the biomass e.g., D. salina (Monte et al. 2018) (Hafiz et al. 2020). Harvesting efficiency of Isochrysis zhanjiangensis was improved by increasing the cross-flow velocity (in turn permeate flux)  or by reducing the membrane fouling by reversing the feed flow direction (Yang et al. 2019). Kim et al. (2017) reported that HC was the lowest energy-consuming method among other physical methods. Kim et al. (2021) have reported the application of HC for bacterial disinfection and media recycling for the cultivation of Ettlia sp. HC also resulted in a slight increase (1.6-fold) in lipid yield (Kim et al. 2021). Work on the application of HC mainly for cell disruption employing HC and US is reported (Zupanc et al. 2019;Liu et al. 2022). However, it has also been reported that nanobubbles improve the flotation of particles (Oliveira et al. 2018;Nazari et al. 2019;Pourkarimi et al. 2021) and the same is demonstrated by Itoh et al. (2019) in their work where HC was employed for the generation of microbubbles for dispersed air flotation of microalgae using venturi tube type microbubble generator to achieve harvesting. Hence, due to the ability of HC to generate nanobubbles Nazari et al. 2022), it can be used to harvest microalgal biomass by flocculation. Followed by harvesting, pre-treatment of harvested biomass is the next step. Thus, in the subsequent section various pretreatment methods are discussed and potential role of HC in the intensification of the process.

Pre-treatment methods for extraction of bioactives
Bioactives extraction from the biomass is facilitated by cell disruption, making pre-treatment an important downstream processing step. Pre-treatment becomes crucial in case of high-value and low-volume target biomolecules. The selection of pre-treatment methods for efficient extraction is important due to different cell wall composition, making cell disruption a critical step (Praveenkumar et al. 2015;Mittal et al. 2017;Mittal and Raghavarao 2018;Tavanandi and Raghavarao 2020).
Various methods such as ultrasonication or ultrasoundassisted extraction, high-pressure homogenization, pressurized liquid extraction, enzyme-assisted extraction, pulse electric field assisted extraction, solvent and supercritical fluid extraction, Ionic liquid assisted extraction, ozonation, microwave-assisted extraction and HC are explored for cell breakage and release of biomolecules of interest (Fig. S1). These methods and their advantages are discussed in the following section and are summarised in Table 4.
In recent times, the application of HC for the processing and valorisation of biomass has been of interest. The advancements in the exploitation of cavitation for various applications such as pre-treatment of biomass (cell disruption), droplet size reduction, mixing, hydrolysis etc., are available in the literature. However, reports on application of HC for the pre-treatment of microalgae are very limited (summarized in Table 5).
Lipids extracted from microalgae can be coverted to biofuels, however, the main problem in the production of biodiesel from microalgae is the high cost of extraction which escalates the production cost. Thus, to improve the economic feasibility of biodiesel production, improved cell disruption is important (Setyawan et al. 2018a(Setyawan et al. , 2020. A few process optimization studies of HC for cell disruption to extract lipid and chlorophyll demonstrate that HC has higher Lipids (Setyawan et al. 2018b) product yield HC has resulted in 26-99% lipid yield when compared to autoclave and ultrasonication, which resulted in 16-65 and 5 to 27% yield from N. salina, respectively. In another study, HC was able to process 40 L of T. suecica slurry in 4 min whereas it took 600 min with US (Lee and Han 2015; . It was demonstrated that HC is a more energy-efficient method for microalgal cell disruption Waghmare et al. 2019) and the mechanism of action is represented in Fig. 7. HC was employed for cell disruption and simultaneous lipid extraction from N. salina. It resulted in a high lipid yield (25.9-99.0%) when compared with an autoclave (16.2-66.5%) and ultrasonication (5.4-26.9%) in terms of the specific energy input (500-10,000 kJ kg −1 ). Setyawan et al. (2018aSetyawan et al. ( , 2020 using Nannochloropsis sp., demonstrated that maximum lipid yield was obtained at 5 bar driving pressure and the energy extraction requirement of 17.79 kJ g −1 lipids is less than the biodiesel heating value, with value of volumetric mass transfer coefficient almost 20-fold higher than the conventional extraction method.  in a study found that for lipid extraction HC has a disruption energy requirement of 3 MJ kg −1 , making it 10 times more energy-efficient than sonication, However, it still accounted for approximately 13% of the total energy in the biomass, which is too high for biofuel production and thus, needs further improvement in terms of design and development and process optimisation. A venturi-hydrodynamic cavitation reactor has been used to treat a mixture of Nannochloropsis sp. within methanol/ hexane (20.5 mL/47.5 mL) at 34 °C and 6.8 bar of sample chamber pressure. Hydrodynamic cavitation extraction (HCE) is more rapid than the agitation method at 260 and 1000 rpm. A lipid yield of 8.9% was achieved by HCE in 10 min, while 5.44-7.3% yields were achieved in 60 min by stirring, respectively. This translates into a volumetric mass transfer coefficient of 7.37 s −1 and 0.53 to 0.12 s −1 with HCE and stirring, respectively. According to the thermodynamic parameters, ΔH 0 (20.72 kJ mol −1 ), ΔS 0 (58.05 J mol −1 K −1 ) and DG (1.97-3.01 kJ mol −1 at 34-50 °C), the process of lipid extraction was endothermic, non-spontaneous and irreversible, indicating that more energy was consumed compared to the extraction from other oil plant sources (Setyawan et al. 2018b).
As design and process parameters such as geometric configuration, number of passes etc., affect the cavitation and in turn cell disruption, optimization of process parameters is important. Different types of orifice plates with different diameters were used to find the best conditions for lipid extraction from microalgae (46.0 ± 3.7%). It is reported that lower orifice hole areas resulted in high vapour pressure and resulted in a higher degree of cell disruption ( Lee et al. 2019). In a study with closed-type HCR with an orifice plate (0.5 mm × 13 holes in radial pattern) for the lipid extraction from wet microalgae N. salina with hexane (1:0.8 v/v). A lipid yield reached 45.5% by using 0.89% sulfuric acid at 0.4 MPa inlet pressure in 25 min by using HCE compared to autoclave and ultrasonication (Lee and Han 2015). Similarly, lipids in Nannochloropsis sp. were extracted with a co-solvent of hexane and methanol within an HCR. Under the optimum conditions, the specific energy consumption reached 16.743 MJ kg −1 lipid (Setyawan et al. 2018a).
Pre-treatment efficiency of any method can be improved by employing combination of two methods, such as HC in combination with alkaline, enzyme or thermal treatment etc., however, literature on HC in such combination is scant. Such an approach must be used with caution for not to degrade sensitive biomolecules. However, a combination of HC with harsh treatments may be useful for biofuels.
Like HC, ultrasound-assisted extraction (UAE) is used as an eco-friendly method for cell disruption. Ultrasonication (US) creates cavitation in the medium that disrupts the cell wall and facilitates the release of biomolecules. The effect of cavitation depends on the frequency of ultrasonication (Yamamoto et al. 2015;Kurokawa et al. 2016; Zupanc et al. Ultrasound-assisted extraction (UAE) of lipids and carotenoids from Heterochlorella luteoviridis (Jaeschke et al. 2017) and C-phycocyanin (C-PC) and allophycocyanin (A-PC) from the dried biomass of A. maxima (Devi et al. 2020). To exploit synergy, ultrasound in combination with ionic liquids (ILs) of phycobiliproteins (Rodrigues et al. 2018), and the presence of surfactant (Triton X-100, Tween-20 and Tween-80) and lysozyme resulted in C-PC extraction efficiency of 77.92% and purity of 1.09 from A. platensis (Tavanandi and Raghavarao 2020). US in combination with freezing and thawing for the C-PC extraction from A. maxima resulted in extraction efficiency of 92% (Tavanandi et al. 2018b). The spent biomass was valorised for the extraction of chlorophyll employing ultrasonication and an increase of 125% in yield and a reduction of 83.3% in extraction time is reported . Ultrasonication integrated with ATPE resulted in a higher degree of extraction and purification of astaxanthin from H. pluvialis when compared to ATPE alone (Khoo et al. 2020a).
Pulsed electric field (PEF) is a promising technology that allows the selective extraction of high-added value compounds by electroporation. The electric current is drawn into parallel plates to develop an electrical field that breaks the cell wall to release intracellular content (Nafis et al. 2015). It enhanced carotenoid (73%) and lipid (13%) extraction from Nannochloropsis sp., at a moderate electric field (1 Hz frequency) (Nafis et al. 2015). PEF is reported to be used for energy-efficient isolation of valuable microalgal bioactive substances (i.e., pigments and polyphenols) . PEF pre-treatment increases esterase activity resulting in higher yield on pre-treatment followed by incubation in ethanol for 6 h. It resulted in extraction of 96% (18.3 mg car g −1 dw) of the total carotenoid from H. pluvialis, compared to 80% (15.3 mg car g −1 dw) (Martínez et al. 2019b).
Lutein (2.2 ± 0.1-fold) and chlorophyll yields (5.2 ± 3.4fold) from C. vulgaris also increased when compared to non-treated cells using a single-stage ethanol extraction process (Soto-Sierra et al. 2020a). Extraction of several other biomolecules such as pigments (phycoerythrin) and polyphenols, from fresh biomass of P. cruentum (Martínez et al. 2019a), T. chuii and P. tricornutum (Kokkali et al. 2020) was also enhanced employing PEF at lower energy input. A microwave-assisted pre-treatment method is also reported for effective microalgal lipid extraction (Min et al. 2018).
Like PEF, a photoelectrochemical method for cell disruption is reported, where N-doped TiO2-coated photoanode captures UV-Visible light and generates electrons (e − ). The e − is captured by palladium phosphorus-based cathode to generate hydroxyl radicals (•OH). The •OH interacts with cell wall components of microalgal cell walls and membranes causing cell disruption and release of intracellular metabolites .
High-pressure homogenization (HPH) is another method reported to employ for the extraction of biomolecules microalgae. Navarro-López et al., (2020) reported that HPH pretreatment (at 600 bar pressure) of Scenedesmus sp., resulted in 72.9% of amino acids in concentrate. High and ultra-high pressure-based extraction (UHPE) processes using 'Generally Recognized As Safe' (GRAS) solvents are reported for simultaneous cell disruption and extraction of carotenoids from H. pluvialis, B-phycoerythrin (B-PE), carotenoids, and PUFAs from P. cruentum, Chlorococum and Chlorella thermophila (Gallego et al. 2019;Babadi et al. 2020;Bueno et al. 2020;Sarkar et al. 2020). Similarly, high hydrostatic pressure (HHP) treatment for extraction is in trend as a "green" extraction method and is explored for B-PE extraction from P. cruentum without damaging the structural integrity of B-PE, however, it is an energy-intensive method (Bueno et al. 2020;Navarro-López et al. 2020). Besides this, HHP, UHPE technique did not improve carotenoid extraction from N. oceanica compared to pressurized liquid extraction (PLE) (Gallego et al. 2020a). Similarly, pressurized liquid extraction in combination with enzymes did not show a significant increase in lipid yield (Castejón and Señoráns 2019).
Enzyme-assisted extraction acts by weakening the cell wall in turn increasing the permeability. The selection of enzymes is substrate dependent and requires optimization of process parameters. Enzyme-assisted extraction resulted in the release of 73 ± 7% of lipids from C. reinhardtii (Soto-Sierra et al. 2020b). The synergistic effect of the sequential alkaline-enzymatic approach by Callejo-López et al. 2020 resulted in high extraction yields of protein-and/or amino acid peptide-enriched extract from fresh biomass of a consortium of microalgae such as C. vulgaris, N. gaditana and S. obliquus (Callejo-López et al. 2020) and enhanced lipid extraction from Nannochloropsis sp. biomass. Similarly, a combination of an enzyme-assisted method with ultrasonication resulted in the A-PC yield of 44.08 mg g −1 (db) with a purity of 1.09 . Enzyme-assisted three-phase partitioning (EA-TPP) process by cellulase enhanced extraction yield of lipids from wet Nannochloropsis sp. biomass by (90.40%) at standardised conditions (Qiu et al. 2019). Similar enhancement in extraction in lipid yield is reported employing consortia of enzymes namely cellulase, protease, lysozyme, and pectinase (Wu et al. 2017). However, enzyme (viscozyme and celluclast) in combination with PLE and UAE for extraction of lipids from microalga Isochrysis galbana has no significant effect (Señoráns et al. 2020).
Chemical methods such as solvent extraction, ionic liquid-based methods and their combinations are also reported. Solvent extraction method was employed for the extraction of astaxanthin, fatty acids and lutein from H. pluvialis Sanzo et al. 2018). Supercritical CO 2 extraction and its combination with deep eutectic solvents (DESs) and microwaves (MWs) resulted in extraction of neutral lipids, chlorophylls, carotenoids lutein and β-carotene from Nannochloropsis and Pseudokirchneriella subcapitata (Chronopoulou et al. 2019;Iovine et al. 2019;Leone et al. 2019;Di Caprio et al. 2020;Mouahid et al. 2020;Saliu et al. 2020) and to improve triglyceride in an extract from P. tricornutum (Tommasi et al. 2017).
Besides conventional solvents, ionic liquids have emerged as an alternative solvent for the extraction of phycobiliproteins (allophycocyanin, phycocyanin, phycoerythrin), lipids, PUFA, lutein etc. from Chlorella sp, Chlorococum sp., A. platensis, Thraustochytrium sp. etc., (Rodrigues et al. 2018(Rodrigues et al. , 2019Zhang et al. 2018;Khoo et al. 2020bKhoo et al. , c, 2021Morandeira et al. 2020). However, these methods have limitations of scalability and high energy consumption to be applied at an industrial scale when compared to the orifice or venturi devices driven cavitation (Waghmare et al. 2019). These limitations thus can be overcome by employing HC and in particular by a vortex-based diode. The extracted high-value bioactives need to be processed and purified for value addition and end-use.

Methods for purification of bioactives
During extraction more than one biomolecule gets extracted depending on solubility, making purification a mandatory step to separate compounds of interest for desired end applications. Purification of the biomolecule adds to its value and fetches higher value in the market and thus gained a lot of attention from researchers evident by several reports. Purification methods such as electrophoresis, membrane separation, ultracentrifugation, ATPE, adsorption, ammonium sulphate precipitation, chromatography, etc., are summarised in Fig. S2. The purification methods along with HC as a potential purification method are discussed in the following section.
Centrifugation is employed for purification mainly by separating molecular components. It works on the principle of differential sedimentation of molecules based on their densities and under the influence of gravity or g-force.
Centrifugation can remove impurities (e.g., chlorophyll, cell debris) in an extract of C-phycocyanin (Mittal et al. 2019;. It is used to separate precipitates during protein purification. However, centrifugation is an energy-intensive procedure and is not suitable for the separation of biomolecules. Electrophoresis is another wellknown technique for purification. However, this technique is mostly used for analytical and characterization studies by separating proteins, DNA and other macromolecules (Santos and Brodbelt 2020).
Adsorption is the surface phenomenon where adhesion of atoms/molecules takes place on the surface of adsorbents and has grabbed attention, attributed to its low cost and minimal effects on yield. The adsorption of biomolecule, such as phycoerythrin, depends on various conditions mainly pH and ionic strength (IS) (Mittal et al. 2022). Electrostatic attraction is responsible for adsorption at pH below pI, whereas hydrophobic interaction at pI and pH above pI. Adsorption is the combination of hydrophobic attraction, reaction of water with adsorbent and electrostatic interaction (Biesheuvel et al. 2004;Mittal et al. 2022). Adsorbents like chitosan and activated charcoal have been reported to produce effective results for the purification of phycobiliproteins. Activated carbon was used for the purification of astaxanthin from H. pluvialis (Casella et al. 2020) and fucoxanthin (carotenoid pigment) from Tisochrysis lutea by selective adsorption of chlorophyll (Gallego et al. 2020b). Purity index of 5.26 for C-PC employing chitosan, activated charcoal, ammonium sulfate precipitation and ion-exchange chromatography (Safaei et al. 2019). EBA (expanded bed adsorption) column and streamline DEAE (Diethylaminoethyl) resin in a new vortex flow reactor are used for the purification of B-phycoerythrin from P. cruentum and resulted in 71-78% (Bermejo et al. 2013) and 86.6% of yield (Ibáñez-González et al. 2016). Marennine is a blue-green pigment from the diatom Haslea ostrearia and is purified using a graphitic stationary phase to achieve a yield of 66% (higher compared to 57% from ultrafiltration) (Bélanger et al. 2020).
Cavitation has been reported to create a homogeneous solution to facilitate adsorption on the adsorbent surface. HC in integration with the adsorption process is reported for the removal of recalcitrant organic pollutants like dyes and phenols. HC in combination with polymer hydro gel packed bed adsorption for the removal organic pollutants from wastewater is reported (Bethi et al. 2017). However, HC may not have any direct role during adsorption, however, can be used during desorption. The shear forces generated during HC can detach adsorbed compound or particle from the adsorbent, helping in regeneration. Besides shear forces, oxidation of adsorbent may occur at longer processing duration.
Aqueous two-phase extraction (ATPE) is an environmentally benign method for purification with the ability to concentrate the purified biomolecules, such as natural colours of commercial importance from a variety of sources (Chang et al. 2018;Santos et al. 2018;Chandralekha et al. 2020). Aqueous two-phases are formed by the combination of polymer/polymer, polymer/salt or alcohol/salt as phaseforming components. In ATPE, the target solute selectively partitions to one phase and contaminant biomolecules to the other phase, achieving both purification and concentration of the target biomolecule at a suitable phase volume ratio (Chang et al. 2018;Mittal et al. 2019). Polymer-salt phase systems have the additional advantages of being economical and having lower phase viscosity, thus requiring a shorter time duration for phase separation when compared to the polymer-polymer phase systems. It provides advantages over other methods in terms of low processing time, ease of scale-up, simultaneous enrichment and concentration of the product, and the scope for continuous operation on complex mixtures consisting of undesired biomolecules (Diamond and Hsu 1992;Zaslavsky 1994;Raghavarao et al. 1995;Walter 2012) and thus can be employed in integrated bio-refinery approaches for purifying high-value low-volume biomolecules. Biomolecules such as C-phycocyanin, allophycocyanin, phycoerythrin, chlorophyll, lutein, proteins and carbohydrates were extracted and purified from several microalgae using polymer-polymer and polymer-ionic liquid phases (Chethana et al. 2015;Kaewdam et al. 2020;Porav et al. 2020;Ruiz et al. 2020). Aqueous two-phase extraction is integrated with various cholinium-based ionic liquids for the fractionation of microalgal components such as proteins, pigments, lipids, and carbohydrates (Chang et al. 2018;Suarez Ruiz et al. 2018;Suarez Ruiz et al. 2020). In another very interesting application of ATPE the valuable compounds of Botryococcus braunii were simultaneously extracted into organic solvent phase (limonene, n-decanol and n-decane) without significant negative effects on cell growth (Concha et al. 2018). The ability of aqueous twophase systems (ATPSs) to fractionate different biomolecules simultaneously, makes ATPE an important method towards establishing the microalgae industry as a multiproduct biorefinery.
The ability of ATPE is improved by employing advanced fluidic devices. Devices namely, straight-tube extractor, helical-coiled extractor and coiled flow inverter extractor were used and coiled flow inverter at a flow rate from 10 to 30 mL min −1 resulted in an increase in partition coefficient by 1.74-fold of C-phycocyanin extraction from A. maxima (Ruiz-Ruiz et al. 2019). The same pigment-algae combination was subjected to vortex fluidic device intensified ATPE (PEG and potassium phosphate) resulted in a 9.3-fold increase in phase demixing efficiency, increasing C-PC purity and yield 1.18-fold and 22%, respectively compared to conventional ATPE (Luo et al. 2016). From this preview, HC can be integrated with ATPE for its ability to generate microdroplets. Stirring during ATPE for achieving equilibrium (for mixing two phases forming components) can be replaced by HC. As mentioned in Sect. 3 and shown in Fig. 8.
Membrane processing is gaining attention for its advantage of being functional at ambient temperature, requiring low energy for operation, and easy to scale-up methods Mittal et al. 2020). Separation of biomolecules with membrane processing occurs mainly based on molecular weight or size of the biomolecule. Membranes act as a barrier for the passage of molecules higher than the molecular weight cut-off (MWCO) of the membrane. Different types of membrane processing namely microfiltration, ultrafiltration (UF), reverse osmosis (RO), and nanofiltration (NF) are available, out of these, ultrafiltration is mainly employed for the separation of biomolecules. UF being a scalable, cost-effective, and energy-efficient method can be used for purification of biomolecules from microalgae, meeting the global demand (Aron et al. 2020). Ultrafiltration resulted in extraction of 48% of polysaccharide-free B-phycoerythrin with a purity index of 2.3 from Porphyridium (Marcati et al. 2014). Recovery of lipids from Parachlorella kessleri by cross-flow filtration employing hydrophilic polyacrylonitrile (PAN) 500 kDa membrane (Rivera et al. 2020) and peptides from Synechococcus sp. by ultrafiltration has been reported (Suttisuwan et al. 2019). UF in different configurations such as conventional flow, tangential flow, and hollow fibre was employed for the recovery of proteins from A. platensis (Menegotto et al. 2020). However, irreversible/reversible fouling can be a factor limiting the performance during membrane processing (Zaouk et al. 2020). Thus, represents a potential method for the downstream processing of natural anti-inflammation food-grade ingredients, drugs, and cosmetic products.
HC pre-treatment of contaminated water stream at low H 2 O 2 concentrations resulted in effective removal of cells from the water column. It also resulted in reduction in contamination by the release of organic compounds in the cells (especially cyanotoxins) without disrupting cells and the cells remain alive. HC in combination with ozonation was employed for the purification of eutrophic water (Zezulka et al. 2020).
As discussed earlier, the ability of HC for generating nanobubbles will enhance mixing and facilitate mass transfer between the two phases of the system. Higher interfacial area provided by nanobubbles will enhance the movement of biomolecules across the interface, resulting in process intensification (Thaker and Ranade 2022;Mittal and Ranade 2023). Hence, HC has the potential to be used indirectly for the purification of biomolecules (Fig. 8). Purification by selective extraction of biomolecules can be achieved by manipulating the operating variables involved, and a wide range of intensity of cavitation can be achieved resulting in a varying extent of disruption. Such observations on the selective release of α-glucosidase (periplasmic) (Mevada et al. 2019), invertase (cell wall-bound) (Balasundaram and Pandit 2001), alcohol dehydrogenase (cytoplasmic) and glucose-6-phosphate dehydrogenase (cytoplasmic) (Balasundaram and Harrison 2006) employing HC are available. HC is reported to assist in the purification of phycobiliproteins, where vortex flow devices for the intensified purification of B-phycoerythrin from P. cruentum by adsorption on Streamline DEAE resin (Ibáñez-González et al. 2016) and C-Phycocyanin from A. maxima by aqueous two-phase extraction (Luo et al. 2016). It is demonstrated that hydrodynamic cavitation can be employed indirectly for the separation and purification of biodiesel and water (Kolhe et al. 2017;Zezulka et al. 2020).
HC is being used for micromixing, formation of microdroplets and emulsions (Thaker and Ranade 2021b, a) and formation of radical ions or high localised temperature had no adverse effect on the stability or properties of biomolecules was reported (Lee and Han 2015;Lee et al. 2019;Setyawan et al. 2020;Thaker and Ranade 2021a, b). The stability of thermolabile biomolecules was not jeopardised during ultrasound-assisted extraction of biomolecules from algae (Mittal et al. 2017;Lamdande et al. 2020;Chandralekha et al. 2021). However, exposure to the radical ions generated at high intensities of cavitation and localised temperatures for a longer duration can affect the stability of metabolites. Hence, the pre-treatment should be performed at standardised conditions, to minimise the generation of reactive ions and exposure duration, without compromising on the performance of HC (Zupanc et al. 2019;Liu et al. 2022). HC as a method has not been directly used for the purification of any biomolecule, except for purification of water by disinfection. However, it can be useful for purification mainly by virtue of selective extraction of biomolecules and by promoting mass transfer for the ability to generate micro and nanodroplets and enabling macro and micromixing. This formation of nanodroplets and micromixing facilitates purification and will help to realise the process intensification of purification, this is particularly true in case of liquid-liquid extraction such as ATPE, three-phase partitioning (Narayan et al. 2008) and reverse-micellar extraction (also known as nanoreactors) (Saran Chaurasiya et al. 2015).
Hydrodynamic cavitation is mainly used for wastewater treatment and transesterification. For achieving these objectives, mainly orifice or venturi-based cavitation devices were used. Modelling studies are reported to investigate the influence of venturi configuration on the inception and extent of cavitation (Simpson and Ranade 2019c) and the influence of different orifice designs (Simpson and Ranade 2018).
Besides reports on orifice and venture-based devices, vortex-based cavitation device is gaining popularity for their higher efficiency. With the vortex flow-based diode coming into existence, advancements are reported in terms of its modelling and flow characterisation employing CFD (Sarvothaman et al. 2019;Simpson and Ranade 2019b;Thaker and Ranade 2021a, b). Artificial neural networkbased modelling for biomass pre-treatment and wastewater treatment is the most recent advancement in the development and understanding of HC . However, modelling and simulation of the HC-based processing of microalgae are required to understand the functioning of HC at microlevels, as no report on modelling and simulation of HC concerning is available.

Hydrodynamic cavitation for valorisation and biorefinery
Hydrodynamic cavitation, for its advantages and characteristics, can be employed in various processes (Fig. 9). Its application as a pre-treatment method in almost all processes ranging from conversion of lignocellulosic biomass to extraction of metabolites from plant sources makes it a suitable method for the valorisation of biomass and to realise the concept of biorefinery. Though several reports on employing HC as a pre-treatment method are available, most are on the valorisation of lignocellulosic biomass (Nagarajan and Ranade 2019;Hilares et al. 2020;Konde et al. 2021).
In order to understand the hurdles in realising the biorefinery of microalgae, it can be divided into different stages of the process. The concept of biorefinery has been developed to improve the economic feasibility of the process. However, there are various challenges in making process profitable and realising the concept in practice, it is necessary to address the issues and overcome the challenges.
As discussed, algae have gained attention as a potential source of feedstock for commercial applications. Life cycle assessment (LCA) is to evaluate and ensure environmentally sustainable, production/consumption of various goods and services. It is a systematic, rigorous, and standardized approach aimed at quantifying resources consumed/depleted, pollutants released, and the related environmental and health impacts through the course of consumption and production of goods/services (Hosseinzadeh-Bandbafha et al. 2019) and thus has been applied widely to algal biorefineries, and results vary across studies (Sills et al. 2020). It has been noticed that LCA of mainly biofuel-related processes have been performed without giving much importance to other biomolecules (Gnansounou and Raman 2016;Roux et al. 2017;Sharma et al. 2018). Production of biomolecules (be it bioactive molecules or biofuels) from microalgae deals with two major steps that involve upstream processes (cultivation aspects) and downstream processes (aspects related to harvesting, cell disruption, and recovery purification). Assessment of process on these steps indicates the feasibility and acceptability of the process and is vital for the identification and mitigation of limiting steps.
Algal production for biomolecules commonly contains stages of algal growth, harvest and extraction. Which are decided based on different algal species, their biomolecule yield and characteristic growth parameters. The life cycle of biomolecules influences the choice of algae as each algal is rich range of biomolecules. For example, C. protothecoides, C. zofingiensis, Ochromonas danica etc., are high lipid-yielding species (Lin et al. 2019;Rebello et al. 2020). Microalgae are mainly used as lipid sources and the lipid is converted into biodiesel employing a wide array of physical, chemical, mechanical, enzymatic, and hydrothermal methods. LCA analysis indicates that the cultivation of microalgal biomass on wastewater is a promising option, as it will reduce the expenses on photoreactors and fresh water and major portion of the cost for algal by-products incurs towards downstream processing methods (or extraction techniques), however, it is suitable for biofuel application. The results of LCA can be different for the production of biomolecules for pharmaceutical and nutraceutical applications. This can be understood with an example of LCA analysis of algal biofuel production employing USLCI recommended conventional Fig. 9 Schematic representation of process steps to realise Biorefining of microalgae biomass method for dewatering, consuming 3556 kg kg −1 of fossil energy. Alternative methods (such as centrifugation and pressing or solar drying) to thermal dewatering can be used at optimized conditions for dewatering. Besides this, other novel methods such as impinging jet mixers (generate turbulence) for mixing lipids with solvents permitting faster lipid recovery of algae to bypass dewatering step (Tseng et al. 2019), and electroporation of C. reinhardtii algae to facilitate easy recovery of by-products (Bodenes et al. 2019). LCA is focused on identification of different strategies helpful in reducing environmental hazards such as the emission of greenhouse gases against base methods (Shi et al. 2019;Wu et al. 2019a). LCA for HC-based operations in microalgae algae-based industry is identified as a knowledge gap, as such reports are not available. Improvement in process efficiency can also be achieved by properly understanding the micro-processes happening at micro-scale levels during cavitation. Modelling and simulation tools can help to visualize and get insight (Ranade 2022;. The understanding can be used to modify the conditions to achieve the desired outcome. Models and equations can be developed and explored to gain insight into various dimensions of chemical reactions or physical changes occurring during processing (Ranade and Utikar 2022). The outcome can be used for a better understanding of process and knowledge generation.
Besides using the tools for understanding, management aspects of the processing by developing an onsite establishment (establishing the HC set-up at the site of harvesting and subjecting the wet biomass to cavitation), with an online approach (sequential approach) can be improved. Onsite establishment will be helpful by minimizing the logistics cost, cost of land, biomass availability etc. Onsite set-up will open the possibility of continuous process from both wet, as well as dry bulk biomass to derive high-value low-volume biomolecules. Processing of wet biomass is desirable, as it omits drying and soaking steps, besides easy disruption of cell wall. It is easy to disrupt fresh (wet biomass) as the biomass undergoes several physiological changes such as hardening during drying, possibly adversely affecting the extraction efficiency. Wet biomass leads to better biomolecules both qualitatively and quantitively. The extraction efficiency may also vary with the change in species of microalgae as cellular strength is the function of microalgae species. The semi-processed or final product then can be supplied for further processing or consumption. Process efficiency can also be improved by integrating two or more techniques to maximize the effect by utilizing a synergistic effect. Integration generally results in the improvement of energy efficiency of the process. However, integration warrants more resources and expertise to perform different unit operations in the same process. On a similar platform, HC can be integrated with other methods such as grinding, to achieve higher energy efficiency. During HC the circulation of feed aids in the dissipation of heat generated during cavitation, thus minimising the requirement of a cooling system. HC can overcompensate for the need for multiple US systems, working in series or parallel mode (Das et al. 2022). Thaker and Ranade (2022) reported the energy efficiency of vortex-based devices to be significantly higher than conventional devices. The processing capacity of the vortex-based HC system can be modified by changing the device scale, for example, vortex-based HC device with a larger throat diameter (d t = 3, 6, 12, 48 mm) can be operated for larger volumes for the same processing duration (Sarvothaman et al. 2018;Ranade et al. 2021c;Thaker and Ranade 2022). However, parameters need to be optimised to achieve the same efficiency. Also, rotor-statorbased cavitation systems can be used for pre-treatment as cavitation in a rotor-stator is a function of RPM (rotation per min) and larger volumes can be processed at various flow rates through the device. The use of soundproof systems and earmuffs is also not necessary in the case of HC for its low decibel operating conditions, resulting in no health hazards to the operator (Wu et al. 2018). Making HC a suitable pre-treatment method for larger scale and scale-up.
Hydrodynamic cavitation has huge potential for the processing of microalgae, due to the small cell size of microalgae and the earlier-mentioned advantages of HC. The scope of employing different HC generators for the extraction process, its modelling and simulation are of huge scope in research and development and commercial applications.

Summary and conclusions
Microalgae are a promising source of high-value biomolecules, for various industries and applications with added advantages of reducing carbon footprint to realise the net negative CO 2 goal and ability to grow on wastelands and wastewater. For realising the economics, complete utilization of any resource to its full capacity is mandatory, making it necessary to develop an efficient downstream process including pre-treatment for cell disruption and extraction. In this review various aspects of upstream and downstream processing are discussed with an emphasis on HC, it's design and scale-up. It is realised that very limited reports on microalgal pre-treatment by HC are available, opening the doors for HC in the microalgae industry for various commercially important microalgae. As it is not a muchexplored field, standardization of the process parameters and attempting various strategies will be required. We hope that this review provides a useful basis and starting point for harnessing HC for the valorisation of microalgae. Data Availability Data sharing not applicable to this article as no datasets were generated during the current study.

Conflict of interest The authors declare no competing interests.
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