*4.3.2 Nutrient recycling from HTL aqueous phase*

Hydrothermal liquification (HTL) is a potential technology to convert wet algal biomass into bio-oil with biochar and aqueous phase (AP) as byproducts. AP is substantial portion because high moisture containing (~10–20% algal slurry) biomass is used as feedstock in HTL [88]. AP is nutritionally rich, containing organic carbon as short chain organic acids, like acetic and propionic acid, nitrogen as NH4 + , nitrate and other nitrogen containing compounds, phosphorous as orthrophosphates and other macro and micro nutrients [89]. This makes AP a potential nutrients source for microalgae when recycled back into cultivation, which are otherwise lost. It is also reported that even harmful algal blooms are also good feedstock for HTL and AP produced is promising nutrient source for microalgae cultivation [90]. AP also has growth inhibitory compounds like phenols, amides, pyrazines, indole, metal ions like Ni etc., which either must be removed or diluted to the extent that they are no more growth inhibitory [89, 91]. Composition of AP is quite variable and depends on algal feedstock used for HTL, processing parameters, biomass loading and use of AP separation method from bio-oil. For instance, high protein content in feedstock leads to higher organic carbon and nitrogen content in AP [92]. Likewise, increasing resident time in HTL process also has shown to result in increased total nitrogen in the AP. Since, the concentration of nutrients and toxic compounds is often high in AP, substantial dilution of AP is needed to bring concentration of nutrients in the usable range and dilute growth inhibitory toxic elements. There are multiple studies reported where AP is used as sole nutrient source for algal cultivation or a supplement with systematic heavy dilutions made either with water or combination of water and standard nutrient medium. Outcome of these studies is quite variable and was dependent on AP composition and strain being used for cultivation. When AP was used as sole nutrient source, growth of the tested algae was relatively compromised. For instance, AP obtained from *Spirulina* HTL was used as sole nutrient source for cultivating *Chlorella minutissima,* where AP consisted ~16,200 mg/L N and 795 mg/L P along with other nutrients. Biomass productivity obtained was 0.035 g/L/d at 0.2% AP (500X dilution), which was significantly less than BG11 control, having 0.07 g/L/d productivity [91]. Likewise, APs obtained from HTL of *Chlorella vulgaris, Scenedesmus dimorphous* or *Spirulina platensis* as feedstocks were also evaluated as sole nutrient source at various dilutions to grow these stains. Growth of *Chlorella* and *Scenedesmus* was less in comparison to standard medium even at 400X dilution, however, *Spirulina* showed comparable growth in AP and standard medium [93]. Alba et al. (2013), presented comparative account of AP diluted with water versus standard medium for cultivation of *Desmodesmus* sp*.* A substantial reduction in growth was observed when AP was diluted with water, however, when mixture of water and AP was enriched with standard medium, growth comparative to standard medium was observed. This study clearly indicates that it is not just N and P content that is important for growth but balancing AP in such a way that other macro and micro nutrients are also not limiting is essential for successful use of AP for cultivation [94]. Similar results were obtained in other studies, where AP diluent was enriched with desired nutrients [95–100]. Interestingly, Lopez Barreiro et al. (2015) observed that growth in AP diluted with standard medium was strain dependent. *Nannochloropsis gaditana* and *Chlorella vulgaris* could grow well in AP diluted with standard medium, however, *Phaeodactylum* 

*tricornutum* and *Scenedesmus almeriensis* showed poor performance [98]. Apart from deficiency of essential nutrients in AP, other factors which have been reported for inhibited growth are presence of phenolic compounds [91], high Ni concentration [93], NH3 toxicity [92, 101], limitation of carbon availability and generation of toxic metabolites [102]. HTL technology is evolving to address these issues. In direct HTL at temperature 300°C or above, protein converts into pyrazines, pyrroles and amines, whereas, polysaccharides convert into cyclic ketones and phenols [103]. These non-fuel components lower bio-oil quality, in the process polysaccharides are lost and toxic metabolites are generated and accumulated in AP. To improve quality of bio-oil and prevent loss of polysaccharides, sequential HTL (SEQHTL) is developed, where AP is recovered in first stage of HTL operated at lower temperature (~160°C) [104]. Polysaccharides constitute major portion in the AP from SEQHTL in contrary to AP from direct HTL, where N and P dominate. In nutrient reuse experiments using AP from SEQHTL, it was shown that *Chlorella sorokiniana and Chlorella vulgaris* could utilize 77% and 64% of hydrolyzed polysaccharides, respectively, however, *Galdieria sulphuraria* could not use the polysaccharides from AP, suggesting again that the utilization of nutrients from AP of HTL is strain dependent [88]. Apart from altering HTL conditions and dilution of AP, other ways to reduce toxicity of AP is through removal of toxic substance by absorbents like activated charcoal, zeolite and ion exchange resins. In recent study it was shown that AP treated with ion-exchange resin, Dowex 50WX8 supported the growth of *Chlorella vulgaris* at 100X dilution similar to control medium and better than activated charcoal treated AP [105].

Thus, outcome of multiple studies suggests that for successful utilization of HTL-AP for algal cultivation, selection of right strain is crucial, which can grow mixotrophically and can utilize N as NH4 + . Appropriate dilution of AP or treatment with absorbents to reduce toxic metabolites load and supplementation with limiting nutrients are also essential for overcoming growth inhibition in AP.

## **4.4 Pond crashes and mitigation**

Large scale algae cultivation ponds and photobioreactors are usually prone to contamination by unwanted foreign organisms due to nonsterile cultivation conditions. Moreover, suboptimal cultivation conditions (light, temperature, nutrients), poor culture mixing, old and sick cells, allow predators and contaminants overtake and crash the culture [106]. Common contaminants in algae cultivation include, grazers (ciliates, rotifers, flagellates, crustaceans, amoeba), pathogens (bacteria and virus) and parasites (fungi, vampyrellids). Multiple studies have reported culture crash due to these organisms. For instance, chytrid contamination in *Haematococcus pluvialis* [107], *Poterioochromonas* sp. (flagellate) [108] and *Euplotes* sp. (ciliate) [109] contamination in *Chlorella,* pleomorphic bacterial (FD111) contamination in *Nannochloropsis* [110], *Colpoda steinii* (ciliate) contamination in *Synechocystis* sp. [111], *Amoeboaphelidium protococcarum* (amoeba) contamination in *Scenedesmus* sp. [112] etc. are some of the studies where contamination resulted in collapse of the culture at mass scales. Since culture crash results in substantial biomass loss, a scalable, environmentally friendly and economical crop control measures are crucial.

Various chemical and physical methods are available for crop protection; however, selection of a method at large scale depends on its activity against predators, non-toxicity towards algae of interest, scalability and cost effectiveness. In case of chemical methods, availability, stability of the chemical and its environmental toxicity should also be considered [109]. Various chemicals belonging to antimicrobials, fungicides, herbicides, oxidants, pesticides, natural compounds, antiparasitic,

**467**

*Recent Advances in Algal Biomass Production DOI: http://dx.doi.org/10.5772/intechopen.94218*

reported in *Synechocystis* sp. PCC 6803 cultures [121].

tested outdoors in 1 and 20 m2

ciliates without affecting algal cells [125].

Apart from chemical methods, there are multiple physical methods, which have been developed for grazer control in algae cultivation. Hydrodynamic cavitation (HC), ultrasonication, foam flotation, pulse electric field, filtration and electromagnetic stratagem are some of the technologies used for crop protection. HC is considered as simple and economical method to kill zooplanktons in waste-water treatment. Kim et al. (2017) have extended this technology in controlling rotifers in algal cultivation. This method could successfully control 99% rotifers in four passes with little effect on *Nannochloropsis* [122]. Likewise, flagellate *Poterioochromonas* sp., a deleterious contaminant in *Chlorella* mass cultivation was disrupted using ultrasonication. This method was tested at 60 L scale and has potential to be used at mass scale. Ultrasonication was also shown to be effective in controlling fungi, amoeba, and ciliates [108]. Electrocution is another technology, which was successfully

graphite rods for 6 h or more to control ciliates and dinoflagellates, however, algae growth was not affected [123]. Pulse electrophoresis is another technology which has been used to effectively control rotifers in tubular PBR. Technology however, can be used for freshwater algal cultures [124]. Umar et al. (2018) evaluated foam flotation, a physiochemical method to remove ciliates *Tetrahymena pyriformis* from *C. vulgaris* culture grown in PBR. Addition of SDS at 40 mg/L concentration lysed

ponds. Here, 5–10 mA current was applied through

antifeeding categories have been evaluated to control predators in algae cultivation. Majority of chemicals tested at lab scale are not suitable for large scale operation because of environmental toxicity or they are very expensive for use in algae cultivation. However, copper has been successfully used to selectively control rotifer- *Brachionus calyciflorus* at 1.5 ppm concentration in open pond cultivation of *Chlorella kessleri* [113]. Similarly, sodium hypochlorite (NaOCl) at a dosage of 0.45 to 0.6 mg Cl/L with dosing frequency of every two hours also inhibited predation by *B. calyciflorus* while no growth inhibition was observed in *C. kessleri* [114]. Use of NaOCl might be practically more feasible in open ponds as chlorine dissipates rapidly, leaving no long-lasting residual effects. Moreover, it is effective at lower dosage in comparison to commonly used insecticides Fenitrothion (6.7 mg/L) and Chlorpyrifos (12 mg/L) for controlling *Brachionus calyciflorus* [115, 116]. Recently, Karuppasamy et al. (2018), have screened around 100 chemicals and out of these 21 were effective against *Euplotes* sp. and *Oxyrrhis* sp., and did not have noticeable detrimental effect on *Chlorella vulgaris.* Further, considering cost, availability, stability and effectiveness, benzalkonium chloride (a quaternary amine) at a concentration of 2 mg/L was evaluated and recommended for preventing pond crash [109]. Apart from chemical control, temporary alteration of cultivation conditions has also been reported to be effective in pond crash mitigation. For instance, limitation of P in the medium does not affect algae severely but affects growth of zooplanktons. Slowest zooplankton growth was observed under high light/P ratio [117]. Flynn et al. (2017) also reported through predictive modeling that low level of P stress can be strategically applied to create suboptimal conditions to zooplankton growth without causing detrimental conditions for algal growth [118]. Another potential strategy to control certain type of predators is use of high level of CO2 in the culture medium. Ma et al. (2017) demonstrated that CO2 purging temporarily lowered *C. sorokiniana* GT-1 culture pH to 6–6.5 and helped in controlling *Poterioochromonas malhamensis* by lowering its intracellular pH and resulting in cell death. This strategy can be implemented for controlling *P. malhamensis* and other protozoans in large scale cultivation [119]. In addition, CO2 asphyxiation was found to be effective in causing acute mortality of all zooplankton species in t < 10 min [120]. *Poterioochromonas* sp. contamination could also be controlled through cultivation at high pH (>pH 11) as

#### *Recent Advances in Algal Biomass Production DOI: http://dx.doi.org/10.5772/intechopen.94218*

*Biotechnological Applications of Biomass*

activated charcoal treated AP [105].

**4.4 Pond crashes and mitigation**

mixotrophically and can utilize N as NH4

*tricornutum* and *Scenedesmus almeriensis* showed poor performance [98]. Apart from deficiency of essential nutrients in AP, other factors which have been reported for inhibited growth are presence of phenolic compounds [91], high Ni concentration [93], NH3 toxicity [92, 101], limitation of carbon availability and generation of toxic metabolites [102]. HTL technology is evolving to address these issues. In direct HTL at temperature 300°C or above, protein converts into pyrazines, pyrroles and amines, whereas, polysaccharides convert into cyclic ketones and phenols [103]. These non-fuel components lower bio-oil quality, in the process polysaccharides are lost and toxic metabolites are generated and accumulated in AP. To improve quality of bio-oil and prevent loss of polysaccharides, sequential HTL (SEQHTL) is developed, where AP is recovered in first stage of HTL operated at lower temperature (~160°C) [104]. Polysaccharides constitute major portion in the AP from SEQHTL in contrary to AP from direct HTL, where N and P dominate. In nutrient reuse experiments using AP from SEQHTL, it was shown that *Chlorella sorokiniana and Chlorella vulgaris* could utilize 77% and 64% of hydrolyzed polysaccharides, respectively, however, *Galdieria sulphuraria* could not use the polysaccharides from AP, suggesting again that the utilization of nutrients from AP of HTL is strain dependent [88]. Apart from altering HTL conditions and dilution of AP, other ways to reduce toxicity of AP is through removal of toxic substance by absorbents like activated charcoal, zeolite and ion exchange resins. In recent study it was shown that AP treated with ion-exchange resin, Dowex 50WX8 supported the growth of *Chlorella vulgaris* at 100X dilution similar to control medium and better than

Thus, outcome of multiple studies suggests that for successful utilization of HTL-AP for algal cultivation, selection of right strain is crucial, which can grow

+

nutrients are also essential for overcoming growth inhibition in AP.

with absorbents to reduce toxic metabolites load and supplementation with limiting

Large scale algae cultivation ponds and photobioreactors are usually prone to contamination by unwanted foreign organisms due to nonsterile cultivation conditions. Moreover, suboptimal cultivation conditions (light, temperature, nutrients), poor culture mixing, old and sick cells, allow predators and contaminants overtake and crash the culture [106]. Common contaminants in algae cultivation include, grazers (ciliates, rotifers, flagellates, crustaceans, amoeba), pathogens (bacteria and virus) and parasites (fungi, vampyrellids). Multiple studies have reported culture crash due to these organisms. For instance, chytrid contamination in *Haematococcus pluvialis* [107], *Poterioochromonas* sp. (flagellate) [108] and *Euplotes* sp. (ciliate) [109] contamination in *Chlorella,* pleomorphic bacterial (FD111) contamination in *Nannochloropsis* [110], *Colpoda steinii* (ciliate) contamination in *Synechocystis* sp. [111], *Amoeboaphelidium protococcarum* (amoeba) contamination in *Scenedesmus* sp. [112] etc. are some of the studies where contamination resulted in collapse of the culture at mass scales. Since culture crash results in substantial biomass loss, a scalable, environmentally friendly and economical crop control

Various chemical and physical methods are available for crop protection; however, selection of a method at large scale depends on its activity against predators, non-toxicity towards algae of interest, scalability and cost effectiveness. In case of chemical methods, availability, stability of the chemical and its environmental toxicity should also be considered [109]. Various chemicals belonging to antimicrobials, fungicides, herbicides, oxidants, pesticides, natural compounds, antiparasitic,

. Appropriate dilution of AP or treatment

**466**

measures are crucial.

antifeeding categories have been evaluated to control predators in algae cultivation. Majority of chemicals tested at lab scale are not suitable for large scale operation because of environmental toxicity or they are very expensive for use in algae cultivation. However, copper has been successfully used to selectively control rotifer- *Brachionus calyciflorus* at 1.5 ppm concentration in open pond cultivation of *Chlorella kessleri* [113]. Similarly, sodium hypochlorite (NaOCl) at a dosage of 0.45 to 0.6 mg Cl/L with dosing frequency of every two hours also inhibited predation by *B. calyciflorus* while no growth inhibition was observed in *C. kessleri* [114]. Use of NaOCl might be practically more feasible in open ponds as chlorine dissipates rapidly, leaving no long-lasting residual effects. Moreover, it is effective at lower dosage in comparison to commonly used insecticides Fenitrothion (6.7 mg/L) and Chlorpyrifos (12 mg/L) for controlling *Brachionus calyciflorus* [115, 116]. Recently, Karuppasamy et al. (2018), have screened around 100 chemicals and out of these 21 were effective against *Euplotes* sp. and *Oxyrrhis* sp., and did not have noticeable detrimental effect on *Chlorella vulgaris.* Further, considering cost, availability, stability and effectiveness, benzalkonium chloride (a quaternary amine) at a concentration of 2 mg/L was evaluated and recommended for preventing pond crash [109]. Apart from chemical control, temporary alteration of cultivation conditions has also been reported to be effective in pond crash mitigation. For instance, limitation of P in the medium does not affect algae severely but affects growth of zooplanktons. Slowest zooplankton growth was observed under high light/P ratio [117]. Flynn et al. (2017) also reported through predictive modeling that low level of P stress can be strategically applied to create suboptimal conditions to zooplankton growth without causing detrimental conditions for algal growth [118]. Another potential strategy to control certain type of predators is use of high level of CO2 in the culture medium. Ma et al. (2017) demonstrated that CO2 purging temporarily lowered *C. sorokiniana* GT-1 culture pH to 6–6.5 and helped in controlling *Poterioochromonas malhamensis* by lowering its intracellular pH and resulting in cell death. This strategy can be implemented for controlling *P. malhamensis* and other protozoans in large scale cultivation [119]. In addition, CO2 asphyxiation was found to be effective in causing acute mortality of all zooplankton species in t < 10 min [120]. *Poterioochromonas* sp. contamination could also be controlled through cultivation at high pH (>pH 11) as reported in *Synechocystis* sp. PCC 6803 cultures [121].

Apart from chemical methods, there are multiple physical methods, which have been developed for grazer control in algae cultivation. Hydrodynamic cavitation (HC), ultrasonication, foam flotation, pulse electric field, filtration and electromagnetic stratagem are some of the technologies used for crop protection. HC is considered as simple and economical method to kill zooplanktons in waste-water treatment. Kim et al. (2017) have extended this technology in controlling rotifers in algal cultivation. This method could successfully control 99% rotifers in four passes with little effect on *Nannochloropsis* [122]. Likewise, flagellate *Poterioochromonas* sp., a deleterious contaminant in *Chlorella* mass cultivation was disrupted using ultrasonication. This method was tested at 60 L scale and has potential to be used at mass scale. Ultrasonication was also shown to be effective in controlling fungi, amoeba, and ciliates [108]. Electrocution is another technology, which was successfully tested outdoors in 1 and 20 m2 ponds. Here, 5–10 mA current was applied through graphite rods for 6 h or more to control ciliates and dinoflagellates, however, algae growth was not affected [123]. Pulse electrophoresis is another technology which has been used to effectively control rotifers in tubular PBR. Technology however, can be used for freshwater algal cultures [124]. Umar et al. (2018) evaluated foam flotation, a physiochemical method to remove ciliates *Tetrahymena pyriformis* from *C. vulgaris* culture grown in PBR. Addition of SDS at 40 mg/L concentration lysed ciliates without affecting algal cells [125].

It is clear from the above description that there are multiple methods available to control the crop loss. However, not all methods are equally effective in controlling all types of predators. Therefore, careful selection of a chemical or physical method based on algae and its intended use is needed to prevent the pond crashes or to control the predators without affecting the algal growth.

#### **4.5 Harvesting efficiencies and energy targets**

Harvesting and dewatering of microalgae is a very challenging process due to their small cell size (<20 μm), low biomass concentration (0.2–1 g/L in ponds and 2–9 g/L in PBRs) [126], density comparable to water (1.08–1.13 g/mL) and negative charge on algal cells, keeping cells in suspension due to repulsive forces [127]. Common harvesting technologies of microalgae include flocculation, centrifugation, sedimentation, filtration and flotation. These methods can be used individually or in combination to improve the effectiveness and economics of harvesting. For example, flocculation can be combined with sedimentation or dissolved air flotation (DAF), DAF can be combined with filtration or centrifugation. First stage of algae harvesting is generally called primary harvesting process, which concentrates cells up to 2–7% and the second stage is called secondary harvesting or dewatering. It uses primary harvested biomass as feed and further concentrates it up to 15–25% [128]. Fasaei et al. (2018) have discussed 28 combinations of primary and secondary harvesting and recommended filtration followed by centrifugation or flocculation followed by membrane filtration and a finishing step with spiral plate technology or centrifugation as economically attractive solutions. Further, when initial biomass concentration and separation techniques are considered, the estimated operational costs and energy consumption for various harvesting methods were estimated to be in the range of 0.1–2 €/kg and 0.1–5 kWh/kg, respectively. Based on these estimates, harvesting cost was projected to be between 3 and 15% of the production cost, which is significantly lower than the earlier estimate of 20–30%, reported in other studies [129, 130].

Flocculation is most common primary harvesting technique, where cell aggregation is achieved through charge neutralization by cationic flocculants, polymers and metal salts like ferric chloride, alum, aluminum sulfate and ferric sulfate [128]. The flocks formed in association with chemicals are either allowed to settle under gravity in a settling tank or floated by attaching micro-bubbles to their surface using a DAF. Energy consumption range for this process as reviewed by Mo et al. (2015) is 0.1–14.8 kWh/m3 [131]. Chemical flocculation has resulted in variable outcome as harvesting efficiency of flocculation is dependent on the flocculent dosage, pH of the culture medium, surface charge and salinity. Under optimal conditions, greater than 90% harvesting efficiency was achieved in many studies, for instance, flocculation of *Chlorella sorokiniana, Chaetoceros muelleri, Chlorella vulgaris* and *Scenedesmus costatum* with chitosan [132–134]. Likewise, *Chlorococcum* sp. and *Dunaliella tertiolecta* were harvested with more than 90% harvesting efficiency using Al2(SO4)3 or Fe2(SO4)3 as flocculants [135]. However, chemical flocculation in large scale algae production may not be economically viable because of high cost of chemical and high dosage requirement. Also, accumulation of residual flocculant in the harvested water and with microalgae might affect the downstream process and may pose environmental concerns [136].

Filtration is another promising harvesting method, which can give 100% biomass recovery and clean biomass, as the process is devoid of chemical input. However, low flux, frequent membrane fouling and high cost of filtration process are key bottlenecks in the large-scale operations. To improve filtration performance and reduce membrane fouling, filtration process has been clubbed with accessory

**469**

*Recent Advances in Algal Biomass Production DOI: http://dx.doi.org/10.5772/intechopen.94218*

negative surface charges [126].

energy consumption was 55 kWh/m3

tion ranged between 1.3–8 kWh/m3

efficiency of 97% [144].

**5. Commercial scale up**

with reported energy consumption ~1 kWh/m3

kWh/m3

*nutum* and reported energy consumption of 0.27 kWh/m3

coagulation flocculation process is reported to be 1.3–9.5 kWh/m3

technologies, like aeration [137], vibration [138], use of electro membrane [139] and rotating disk [140]. Bilad et al. (2012) used submerged microfiltration equipped with vibrator for harvesting *Chlorella vulgaris* and *Phaeodactylum tricor-*

species, which was substantially high [141]. Recently, pilot scale ultra-filtration membrane trial clubbed with air assisted backwashing technology has been successfully used to harvest *Scenedesmus acuminatus*. The culture was concentrated from 0.5 g/L to 136 g/L with 93% biomass recovery. The energy consumption reported was 0.59 kWh/kg dry biomass [142]. Though filtration is less energy intensive [130], further improvements in filtration technology is required and can also be achieved by using membranes with advanced hydrophilic material and introducing

Centrifugation is another physical method of harvesting, but the harvesting efficiency is less than filtration and highly depends on the gravitational forced applied. Centrifugation is also energy intensive, difficult to scaleup, requires high maintenance and considered expensive for low value products like oil. Using centrifugation as sole harvesting method is not recommended as energy consumption and cost of harvesting is significantly higher compared to a process, where centrifugation is used as secondary harvesting method. In a study where Evodos spiral plate centrifuge was solely used to harvest 10,000 L of *Chlorella* culture,

tion was used as secondary harvesting step [128]. Other common centrifuge types are disc stack and decanter. Disc stack is the most common industrial centrifuge

was further reduced to 50% by design changes, like modifying flow paths of rotor, reduction of aerodynamic losses by air removal outside rotor and use of direct drive instead of belt or gear drive [143]. In case of decanter centrifuge, energy consump-

Energy Laboratory (NREL), energy consumption in concentrating microalgae

In conclusion, it is clear from above description that significant developments are made in harvesting technology but none of the techniques seems to be economical and efficient enough. Combination of two to three technologies have been proposed to give economically viable solution but still significant optimization and innovation is necessary in current technologies and there is substantial scope for development of new, cheaper and more efficient harvesting technologies.

High cost of biomass production and subsequent extraction processes have limited the progress of upscaling of microalgae for commercial fuel and other value-added products. The technoeconomic analyses reported thus far have a wide variation in the cost estimates, primarily due to non-existence of standardized cost assumptions across different geographic locations. For example, in a study conducted in the US, production of microalgal biomass is estimated at \$4.92/ kg with current technology status [145]. In another study conducted in Europe, production cost was estimated to be €4.95, 4.16, and 5.96/kg of biomass from open ponds, horizontal tubular and flat panel photobioreactors, respectively [146]. Even the biomass production cost drops down to \$0.5/ kg, still scaling-up of microalgae

from 13–20% using centrifuge was estimated to be 1.3 kWh/m3

, as opposed to 5.5 kWh/m3

(0.98 kWh/kg), respectively [138]. Corresponding energy for electro-

(0.64 kWh/kg) and 0.25

for the same

, when centrifuga-

, with a dewatering

. However, energy consumption

. In another study by National Renewable

#### *Recent Advances in Algal Biomass Production DOI: http://dx.doi.org/10.5772/intechopen.94218*

*Biotechnological Applications of Biomass*

other studies [129, 130].

is 0.1–14.8 kWh/m3

may pose environmental concerns [136].

control the predators without affecting the algal growth.

**4.5 Harvesting efficiencies and energy targets**

It is clear from the above description that there are multiple methods available to control the crop loss. However, not all methods are equally effective in controlling all types of predators. Therefore, careful selection of a chemical or physical method based on algae and its intended use is needed to prevent the pond crashes or to

Harvesting and dewatering of microalgae is a very challenging process due to their small cell size (<20 μm), low biomass concentration (0.2–1 g/L in ponds and 2–9 g/L in PBRs) [126], density comparable to water (1.08–1.13 g/mL) and negative charge on algal cells, keeping cells in suspension due to repulsive forces [127]. Common harvesting technologies of microalgae include flocculation, centrifugation, sedimentation, filtration and flotation. These methods can be used individually or in combination to improve the effectiveness and economics of harvesting. For example, flocculation can be combined with sedimentation or dissolved air flotation (DAF), DAF can be combined with filtration or centrifugation. First stage of algae harvesting is generally called primary harvesting process, which concentrates cells up to 2–7% and the second stage is called secondary harvesting or dewatering. It uses primary harvested biomass as feed and further concentrates it up to 15–25% [128]. Fasaei et al. (2018) have discussed 28 combinations of primary and secondary harvesting and recommended filtration followed by centrifugation or flocculation followed by membrane filtration and a finishing step with spiral plate technology or centrifugation as economically attractive solutions. Further, when initial biomass concentration and separation techniques are considered, the estimated operational costs and energy consumption for various harvesting methods were estimated to be in the range of 0.1–2 €/kg and 0.1–5 kWh/kg, respectively. Based on these estimates, harvesting cost was projected to be between 3 and 15% of the production cost, which is significantly lower than the earlier estimate of 20–30%, reported in

Flocculation is most common primary harvesting technique, where cell aggregation is achieved through charge neutralization by cationic flocculants, polymers and metal salts like ferric chloride, alum, aluminum sulfate and ferric sulfate [128]. The flocks formed in association with chemicals are either allowed to settle under gravity in a settling tank or floated by attaching micro-bubbles to their surface using a DAF. Energy consumption range for this process as reviewed by Mo et al. (2015)

as harvesting efficiency of flocculation is dependent on the flocculent dosage, pH of the culture medium, surface charge and salinity. Under optimal conditions, greater than 90% harvesting efficiency was achieved in many studies, for instance, flocculation of *Chlorella sorokiniana, Chaetoceros muelleri, Chlorella vulgaris* and *Scenedesmus costatum* with chitosan [132–134]. Likewise, *Chlorococcum* sp. and *Dunaliella tertiolecta* were harvested with more than 90% harvesting efficiency using Al2(SO4)3 or Fe2(SO4)3 as flocculants [135]. However, chemical flocculation in large scale algae production may not be economically viable because of high cost of chemical and high dosage requirement. Also, accumulation of residual flocculant in the harvested water and with microalgae might affect the downstream process and

Filtration is another promising harvesting method, which can give 100% biomass recovery and clean biomass, as the process is devoid of chemical input. However, low flux, frequent membrane fouling and high cost of filtration process are key bottlenecks in the large-scale operations. To improve filtration performance and reduce membrane fouling, filtration process has been clubbed with accessory

[131]. Chemical flocculation has resulted in variable outcome

**468**

technologies, like aeration [137], vibration [138], use of electro membrane [139] and rotating disk [140]. Bilad et al. (2012) used submerged microfiltration equipped with vibrator for harvesting *Chlorella vulgaris* and *Phaeodactylum tricornutum* and reported energy consumption of 0.27 kWh/m3 (0.64 kWh/kg) and 0.25 kWh/m3 (0.98 kWh/kg), respectively [138]. Corresponding energy for electrocoagulation flocculation process is reported to be 1.3–9.5 kWh/m3 for the same species, which was substantially high [141]. Recently, pilot scale ultra-filtration membrane trial clubbed with air assisted backwashing technology has been successfully used to harvest *Scenedesmus acuminatus*. The culture was concentrated from 0.5 g/L to 136 g/L with 93% biomass recovery. The energy consumption reported was 0.59 kWh/kg dry biomass [142]. Though filtration is less energy intensive [130], further improvements in filtration technology is required and can also be achieved by using membranes with advanced hydrophilic material and introducing negative surface charges [126].

Centrifugation is another physical method of harvesting, but the harvesting efficiency is less than filtration and highly depends on the gravitational forced applied. Centrifugation is also energy intensive, difficult to scaleup, requires high maintenance and considered expensive for low value products like oil. Using centrifugation as sole harvesting method is not recommended as energy consumption and cost of harvesting is significantly higher compared to a process, where centrifugation is used as secondary harvesting method. In a study where Evodos spiral plate centrifuge was solely used to harvest 10,000 L of *Chlorella* culture, energy consumption was 55 kWh/m3 , as opposed to 5.5 kWh/m3 , when centrifugation was used as secondary harvesting step [128]. Other common centrifuge types are disc stack and decanter. Disc stack is the most common industrial centrifuge with reported energy consumption ~1 kWh/m3 . However, energy consumption was further reduced to 50% by design changes, like modifying flow paths of rotor, reduction of aerodynamic losses by air removal outside rotor and use of direct drive instead of belt or gear drive [143]. In case of decanter centrifuge, energy consumption ranged between 1.3–8 kWh/m3 . In another study by National Renewable Energy Laboratory (NREL), energy consumption in concentrating microalgae from 13–20% using centrifuge was estimated to be 1.3 kWh/m3 , with a dewatering efficiency of 97% [144].

In conclusion, it is clear from above description that significant developments are made in harvesting technology but none of the techniques seems to be economical and efficient enough. Combination of two to three technologies have been proposed to give economically viable solution but still significant optimization and innovation is necessary in current technologies and there is substantial scope for development of new, cheaper and more efficient harvesting technologies.
