Microalgae-based wastewater treatment for nutrients recovery: A review

Sustainable microalgal biomass production in food industry wastewater for low-cost biorefinery products: a review

Microalgae are recognized as cell factories enriched with biochemicals suitable as feedstock for bio-energy, food, feed, pharmaceuticals, and nutraceuticals applications. The industrial application of microalgae is challenging due to hurdles associated with mass cultivation and biomass recovery. The scale-up production of microalgal biomass in freshwater is not a sustainable solution due to the projected increase of freshwater demands in the coming years. Microalgae cultivation in wastewater is encouraged in recent years for sustainable bioeconomy from biorefinery processes. Wastewater from the food industry is a less-toxic growth medium for microalgal biomass production. Traditional wastewater treatment and management processes are expensive; hence it is highly relevant to use low-cost wastewater treatment processes with revenue generation through different products. Microalgae are accepted as potential biocatalysts for the bioremediation of wastewater. Microalgae based purification of wastewater technology could be a universal alternative solution for the recovery of resources from wastewater for low-cost biomass feedstock for industry. This review highlights the importance of microalgal biomass production in food processing wastewater, their characteristics, and different microalgal cultivation methods, followed by nutrient absorption mechanisms. Towards the end of the review, different microalgae biomass harvesting processes with biorefinery products, and void gaps that tend to hinder the biomass production with future perspectives will be intended. Thus, the review could claim to be valuable for sustainable microalgae biomass production for eco-friendly bioproduct conversions.

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An overview of microalgae biomass as a sustainable aquaculture feed ingredient: food security and circular economy

Sustainable management of natural resources is critical to food security. The shrimp feed and fishery sector is expanding rapidly, necessitating the development of alternative sustainable components. Several factors necessitate the exploration of a new source of environmentally friendly and nutrient-rich fish feed ingredients. Microalgal biomass has the potential to support the growth of fish and shrimp aquaculture for global food security in the bio-economy. Algal biorefineries must valorize the whole crop to develop a viable microalgae-based economy. Microalgae have the potential to replace fish meal and fish oil in aquaculture and ensure sustainability standards. Microalgae biomasses provide essential amino acids, valuable triglycerides such as lipids, vitamins, and pigments, making them suitable as nutritional supplements in livestock feed formulations. Fish and microalgae have similar nutritional profiles, and digestibility is a critical aspect of the aquafeed formulation. A highly digestible feed reduces production costs, feed waste, and the risk of eutrophication. Due to low input costs, low carbon footprint, wastewater treatment benefits, and carbon credits from industrial CO₂ conversion, microalgae-based fish and shrimp feeds have the potential to provide significant economic benefits. However, several challenges must be addressed before microalgal biomass and bioproducts may be used as fish feeds, including heavy metal bioaccumulation, poor algal biomass digestion, and antinutrient effects. Knowledge of biochemical composition is limited and diverse, and information on nutritional value is scattered or contradictory. This review article presents alternative approaches that could be used in aquaculture to make microalgal biomass a viable alternative to fish meal.

Differences in adaptation to light and temperature extremes of Chlorella sorokiniana strains isolated from a wastewater lagoon

Presently, two Chlorella sorokiniana strains sampled during summer (CS-S) and winter (CS-W) from a maturation pond and isolated by dominance were studied on their behavior on temperature and light extremes in batch experiments. Although both strains showed no differences in their tolerance of temperatures up to 45 °C, the growth rates, pigment contents and fatty acid compositions in response to PAR at 700 and 1,500 µmol m^-2sec^-1 differed. CS-W was less affected by photoinhibition and maintained constantly high growth rates. High radiation resulted in both strains in an equivalent decrease of chlorophyll a and accessory pigments indicating that the latter did not function as a light filter. PUFAS (18:3 and 16:3) increased in CS-W at high radiation by > 60% and decreased in CS-S by 8 %. Results indicate that CS-W is highly favorable for mass cultivation particularly in outdoors, in which diurnal variations of solar radiation occur.

Heterotrophic and mixotrophic cultivation of microalgae to simultaneously achieve furfural wastewater treatment and lipid production

Furfural wastewater (FWW) treatment is important in modern chemical production. However, traditional wastewater treatment methods are inappropriate for the treatment of FWW. In this work, Chlorella pyrenoidosa was employed to study the growth and pollutants removal with both heterotrophic and mixotrophic cultures. The results show that the biomass and removal efficiency for COD and TN were the highest under 10-fold dilution. However, TP removal were inconsistent when the algae were cultivated in both mixotrophic and heterotrophic modes. Compared to high nitrogen (0.75 g/L NaNO₃), the algal cells grew faster when adding 0.25 g/L NaNO₃ to the FWW, whether in mixotrophic or heterotrophic conditions. The total lipid content in heterotrophic conditions was 18.53%, which was higher than the values in mixotrophy when the concentration of NaNO₃ was 0.75 g/L. Different carbon assimilation mechanisms of the algal cells result in a discrepancy in cell growth and pollutant removal, under different culture modes.

A comprehensive review on the use of algal-bacterial systems for wastewater treatment with emphasis on nutrient and micropollutant removal

The scarcity of water resources and environmental pollution have highlighted the need for sustainable wastewater treatment. Existing conventional treatment systems are energy-intensive and not always able to meet stringent disposal standards. Recently, algal-bacterial systems have emerged as environmentally friendly sustainable processes for wastewater treatment and resource recovery. The algal-bacterial systems work on the principle of the symbiotic relationship between algae and bacteria. This paper comprehensively discusses the most recent studies on algal-bacterial systems for wastewater treatment, factors affecting the treatment, and aspects of resource recovery from the biomass. The algal-bacterial interaction includes cell-to-cell communication, substrate exchange, and horizontal gene transfer. The quorum sensing (QS) molecules and their effects on algal-bacterial interactions are briefly discussed. The effect of the factors such as pH, temperature, C/N/P ratio, light intensity, and external aeration on the algal-bacterial systems have been discussed. An overview of the modeling aspects of algal-bacterial systems has been provided. The algal-bacterial systems have the potential for removing micropollutants because of the diverse possible interactions between algae-bacteria. The removal mechanisms of micropollutants - sorption, biodegradation, and photodegradation, have been reviewed. The harvesting methods and resource recovery aspects have been presented. The major challenges associated with algal-bacterial systems for real scale implementation and future perspectives have been discussed. Integrating wastewater treatment with the algal biorefinery concept reduces the overall waste component in a wastewater treatment system by converting the biomass into a useful product, resulting in a sustainable system that contributes to the circular bioeconomy.

Impact of pH on Pollutional Parameters of Textile Industry Wastewater with Use of Chlorella pyrenoidosa at Lab-Scale: A Green Approach

The current study focused on the pollution remediation of textile industry wastewater by using Chlorella pyrenoidosa in two different physical forms: free algal biomass and immobilized algal biomass. The hypothesis behind the present study was to analyze the pollution reduction efficiency of immobilized algal biomass and free algal biomass on comparative scale on the basis of the adsorption process which is directly proportional with the surface area of the adsorbate. So, in this context the immobilized form of algae could enhance the pollution reduction efficiency due to availability of more surface area. So, the textile industry wastewater was treated by both free algal biomass and immobilized algal biomass and the major wastewater contributors like nitrate, phosphate, Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) were assessed before and after the treatment process. To conclude the optimum comparative results, the pH of wastewater was maintained constant, as it can capitalize or moderate the adsorption process (initial pH of was 8.2 ± 0.1, but it was maintained to 8). The contamination remediation was found to be effective with immobilized algal biomass (46.7% of nitrate, 59.4% of phosphate, 83.1% BOD and 83.0% of COD) than free algal biomass (43.2% of nitrate, 56.7% of phosphate, 71.4% of BOD and 78.0% COD).

Wastewater in India: An untapped and under-tapped resource for nutrient recovery towards attaining a sustainable circular economy

Wastewater (WW) contains nitrogen (N) and phosphorus (P), where N oxidizes to nitrate followed by denitrification to release N₂ and P is accumulated in sludge. Higher concentrations of N and P leads to eutrophication and algal blooming, thereby threatening the aquatic life systems. Such nutrients could be potentially recovered avoiding the fertilizer requirements. Distinct nutrient recovery systems have been demonstrated including chemical precipitation, ion-exchange, adsorption, bio-electrochemical systems, and biological assimilation at various scales of volumes. This study focusses on the nutrient recovery possibility from wastewater in India. The resource estimation analysis indicates that at 80% recovery, 1 million liters per day (MLD) of sewage can generate 17.3-kg of struvite using chemical precipitation. When compared with traditional fertilizers, nutrient recovery from sewage has the potential to avoid 0.38-Mt/a in imports. Replacing conventional fertilizer with struvite recovered from WW avoids 663.2 kg CO_2eq/ha in emissions (53%). Prevailing WW treatment looks at maintaining the discharging standards while recovering nutrients is an advanced option for a self-reliant and sustainable circular economy. However, more detailed assessments are necessary from techno-economic and environmental perspective in realizing these technologies at an industrial scale.

Development of an alternative medium via completely replaces the medium components by mixed wastewater and crude glycerol for efficient production of docosahexaenoic acid by Schizochytrium sp

Exorbitant substrates for Schizochytrium culture result in the high cost of docosahexaenoic acid (DHA) production. In order to develop a feasible approach that is expected to reduce DHA production cost, Schizochytrium sp. S31 cultivation with a mixture of saline wastewater (SWW) and tofu whey wastewater (TWW) was investigated in this study. Using glucose as the carbon source, the maximum biomass and DHA yield in cultures using mixed wastewater containing 5% SWW reached 19.08 and 2.66 g/L, respectively, which were 2.29 and 2.66 times higher than those of cultures using control medium. Moreover, a good wastewater treatment performance was achieved as approximately 60% of the COD, TN, and TP were reduced in the cultures using mixed wastewater with a SWW ratio of 5%. The mixed wastewater presented better performance on DHA production than control medium using all tested carbon sources including glucose, fructose, and pure and crude glycerol. The components of control medium can be completely replaced by the mixed wastewater and crude glycerol. It is expected to effectively decrease the medium cost for DHA production and reduce the environmental risk of food processing wastewater.

Current application of algae derivatives for bioplastic production: A review

Improper use of conventional plastics poses challenges for sustainable energy and environmental protection. Algal derivatives have been considered as a potential renewable biomass source for bioplastic production. Algae derivatives include a multitude of valuable substances, especially starch from microalgae, short-chain length polyhydroxyalkanoates (PHAs) from cyanobacteria, polysaccharides from marine and freshwater macroalgae. The algae derivatives have the potential to be used as key ingredients for bioplastic production, such as starch and PHAs or only as an additive such as sulfated polysaccharides. The presence of distinctive functional groups in algae, such as carboxyl, hydroxyl, and sulfate, can be manipulated or tailored to provide desirable bioplastic quality, especially for food, pharmaceutical, and medical packaging. Standardizing strains, growing conditions, harvesting and extracting algae in an environmentally friendly manner would be a promising strategy for pollution control and bioplastic production.

Periodic oxygen supplementation drives efficient metabolism for enhancing valuable bioresource production in photosynthetic bacteria wastewater treatment

Periodic oxygen supplementation (A-O) strategy was proposed to improve pollutant removal and enhance bioresource production of photosynthetic bacteria (PSB). The A-O strategy obtained higher COD (91.4%) and NH₄⁺-N (78.6%) removal compared with the non-oxygen supplementation (N-O) strategy, which was similar to the continuous oxygen supplementation (C-O) strategy. A-O strategy achieved the highest biomass concentration of 1338.5 mg/L. Bacteriochlorophyll and carotenoids concentration in the A-O strategy were 24.9-31.1% and 15.1-23.7% higher than those in the other two strategies; coenzyme Q₁₀ concentration and content were 52.5% and 21.3% higher than that in the N-O strategy. The metabolomic analysis showed that the A-O strategy enhanced the tricarboxylic acid cycle after fumaric acid formation and β-alanine metabolism, then caused higher biomass accumulation. The A-O strategy reduced the inhibition of photophosphorylation by oxidative-phosphorylation and maintained both characteristics. Hence, A-O might be an economic strategy for enhancing pollutant removal and bioresource production in PSB-based wastewater treatment.

A Review on the Reliability and the Readiness Level of Microalgae-Based Nutrient Recovery Technologies for Secondary Treated Effluent in Municipal Wastewater Treatment Plants

Algae-based wastewater treatment technologies are promising green technologies with huge economical potential and environmental co-benefits. However, despite the immense research, work, and achievement, no publications were found wherein these technologies have been successfully applied in an operational environment for nitrogen and phosphorus removal of secondary treated effluent in municipal wastewater treatment plants. Based on a literature review and targeted comprehensive analysis, the paper seeks to identify the main reasons for this. The reliability (considering inlet wastewater quality variations, operating conditions and process control, algae harvesting method, and produced biomass) as well as the technology readiness level for five types of reactors are discussed. The review shows that the reactors with a higher level of control over the technological parameters are more reliable but algal post-treatment harvesting and additional costs are barriers for their deployment. The least reliable systems continue to be attractive for research due to the non-complex operation and relieved expenditure costs. The rotating biofilm systems are currently undertaking serious development due to their promising features. Among the remaining research gaps and challenges for all the reactor types are the identification of the optimal algal strains, establishment of technological parameters, overcoming seasonal variations in the effluent’s quality, and biomass harvesting.

Cattle wastewater as a low-cost supplement augmenting microalgal biomass under batch and fed-batch conditions

The utilization of costly chemical fertilizers and large freshwater requirements make the microalgae cultivation process uneconomical and highly unsustainable. To address this challenge, the present study aimed to integrate cattle wastewater (CW) (alternate for fertilizers) with domestic sewage wastewater (DSW) (substitute for freshwater) to cultivate Chlorella thermophile. To maximize the biomass yield, in-depth nutrient consumption patterns in both batch and fed-batch cultivation conditions were analyzed. Out of the eight (1%-4.5%) different CW feed concentrations tested during the batch cultivation, 2.5% CW set gave the highest biomass yield (2.17 g L^-1), which was almost double the yield obtained using Bold Basal Medium (1.24 g L^-1) and DSW without any CW addition (1.22 g L^-1). However, the biomass yield declined with CW> 2.5%, and the ammonium (NH₄⁺) inhibitory effect was observed. To address the (NH₄⁺) toxicity challenge and further enhance the biomass yield, fed-batch experiments were designed with an intermittent CW feeding based on nutrient (NH₄⁺) consumption pattern. The fed-batch cultivation resulted in twofold increased biomass yield (4.52 g L^-1) in comparison to the batch process. The nutrient consumption pattern inferred that the (NH₄⁺) concentration greater than 600 mg L^-1 during the logarithmic phase was inhibitory for Chlorella thermophila cells. On biomass characterization, a significant improvement in protein content with CW addition was observed. The FAME analysis of the derived lipid stated its competitive biofuel quality with up-gradation of C:16 and C:18 groups. Based on the obtained results, projection analysis for an integrated rural model demonstrated the technology's potential for sustainable water management with valuable resource recovery.

Attempts to alleviate inhibitory factors of anaerobic digestate for enhanced microalgae cultivation and nutrients removal: A review

Anaerobic digestion is a well-established process that is applied to treat organic wastes and convert the carbon to valuable methane gas as a source of energy. The digestate that comes out as a by-product is of a great challenge due to its high nutrient content that can be toxic in case of improper disposal to the environment. Several attempts have been done to valorize this digestate. Digestate has been considered as an interesting medium to cultivate microalgae. The nutrients available in the digestate, mainly nitrogen and phosphorus, can be an interesting supplement for microalgae growth requirement. The main obstacles of using digestate as a medium to cultivate microalgae are the dark color and the high ammonium-nitrogen concentration. The focus of this review is to discuss in detail the major attempts in research to overcome inhibition and enhance microalgae cultivation in digestate. This review initially discussed the obstacles of digestate as a medium for microalgae cultivation. Different processes to overcome inhibition were discussed including dilution, supplying additional carbon source, favoring mixotrophic cultivation and pretreatment. More emphasis in this review was given to digestate pretreatment. Among the pretreatment methods, filtration, and centrifugation were of the most applied ones. These strategies were found to be effective for turbidity and chromaticity reduction. For ammonium nitrogen removal, ammonia stripping and biological pretreatment methods were found to play a vital role. Adsorption could work both ways depending on the material used. Combining different pretreatment methods as well as including selected microalgae stains were found interesting strategies to facilitate microalgae cultivation with no dilution. This study recommend that more study should investigate the optimization of microalgae cultivation in anaerobic digestate without the need for dilution.

Removal of Nutrients and Pesticides from Agricultural Runoff Using Microalgae and Cyanobacteria

The use of pesticides in agriculture has ensured the production of different crops. However, pesticides have become an emerging public health problem for Latin American countries due to their excessive use, inadequate application, toxic characteristics, and minimal residue control. The current project evaluates the ability of two strains of algae (Chlorella and Scenedesmus sp.) and one cyanobacteria (Hapalosyphon sp.) to remove excess pesticides and other nutrients present in runoff water from rice production. Different concentrations of wastewater and carbon sources (Na2CO3 and NaHCO3) were evaluated. According to the results, all three strains can be grown in wastewater without dilution (100%), with a biomass concentration comparable to a synthetic medium. All three strains significantly reduced the concentration of NO3 and PO4 (95 and 85%, respectively), with no difference between Na2CO3 or NaHCO3. Finally, Chlorella sp. obtained the highest removal efficiency of the pesticide (Chlorpyrifos), followed by Scenedesmus and Hapalosyphon sp. (100, 75, and 50%, respectively). This work shows that it is possible to use this type of waste as an alternative source of nutrients to obtain biomass and metabolites of interest, such as lipids and carbohydrates, to produce biofuels.

Microalgae Biomass as a New Potential Source of Sustainable Green Lubricants

Lubricants are materials able to reduce friction and/or wear of any type of moving surfaces facilitating smooth operations, maintaining reliable machine functions, and reducing risks of failures while contributing to energy savings. At present, most worldwide used lubricants are derived from crude oil. However, production, usage and disposal of these lubricants have significant impact on environment and health. Hence, there is a growing pressure to reduce demand of this sort of lubricants, which has fostered development and use of green lubricants, as vegetable oil-based lubricants (biolubricants). Despite the ecological benefits of producing/using biolubricants, availability of the required raw materials and agricultural land to create a reliable chain supply is still far from being established. Recently, biomass from some microalgae species has attracted attention due to their capacity to produce high-value lipids/oils for potential lubricants production. Thus, this multidisciplinary work reviews the main chemical-physical characteristics of lubricants and the main attempts and progress on microalgae biomass production for developing oils with pertinent lubricating properties. In addition, potential microalgae strains and chemical modifications to their oils to produce lubricants for different industrial applications are identified. Finally, a guide for microalgae oil selection based on its chemical composition for specific lubricant applications is provided.

Microalgae-based swine wastewater treatment: Strain screening, conditions optimization, physiological activity and biomass potential

Using microalgae to treat swine wastewater (SW) can achieve wastewater purification and biomass recovery at the same time. The algae species suitable for growth in SW were screened in this study, and the response surface combined with the desirability function method was used for multi-objective optimization to obtain high algal biomass and pollutant removal. Chlorophyll fluorescence parameters and biomass composition were analyzed to evaluate the cell physiological activity and its application potential. Chlorella sp. HL was selected as the most suitable species for growth in SW, and after 9 d of cultivation, the maximum specific growth rate and highest algal density were achieved 0.51 d^-1 and 2.43 × 10⁷ cells/mL, respectively. In addition, the removal of total phosphate and chemical oxygen demand were reached 69.13% and 72.95%, respectively. The optimum conditions for maximum algal density and highest pollutant removal were determined as the light intensity of 58.73 μmol/m²/s, inoculation density of 5.0 × 10⁶ cells/mL, and a light/dark ratio of 3 using response surface model, and the predicted overall desirability value was 0.96. The potential maximum quantum yield of PSII (Fv/Fm) of Chlorella sp. HL in the early stage of cultivation was 0.60-0.70, while under high light and long photoperiod, the value of Fv/Fm and performance index of Chlorella decreased, trapped and dissipated energy flux per reaction center increased. The higher heating value of 18.25 MJ/kg indicated that the Chlorella cultivated in SW could be a good feedstock for biofuel production.

Effects of Different pH Control Strategies on Microalgae Cultivation and Nutrient Removal from Anaerobic Digestion Effluent

This study investigated nutrient removal from anaerobic digestion effluent by cultivating mixed-culture microalgae enriched from anaerobic sludge under different pH conditions: R_UC (uncontrolled), R_7–8 (maintained at 7–8), and R_<8 (maintained below 8). Significant amounts of NH₄⁺-N were lost by volatilization in R_UC cultures due to increased pH values (≤8.6) during the early period of cultivation. The pH control strategies significantly affected the biological NH₄⁺-N removal (highest in R_7–8), microalgal growth (highest in R_7–8), biomass settleability (highest in R_<8), and microalgal growth relative to bacteria (highest in R_<8) in the cultures. Parachlorella completely dominated the microalgal communities in the inoculum and all of the cultures, and grew well at highly acidic pH (<3) induced by culture acidification with microalgal growth. Microalgae-associated bacterial community structure developed very differently among the cultures. The findings call for more attention to the influence and control of pH changes during cultivation in microalgal treatment of anaerobic digestion effluent.

Growth kinetics and quantification of carbohydrate, protein, lipids, and chlorophyll of Spirulina platensis under aqueous conditions using different carbon and nitrogen sources

This study evaluated different carbon and nitrogen sources on the growth and production of carbohydrates, protein, lipids, and chlorophyll of Spirulina platensis LEB-52 through an easy successive methodology under aqueous conditions. Spirulina platensis was cultivated at 120 rpm and light intensity of 156 µmol m^-2 s^-1 in a 500 mL Erlenmeyer flask with a working volume of 250 mL, using Zarrouk's medium. The biomass, carbohydrate, and protein production together with the specific growth rate did not show a significant difference between NaHCO₃ and Na₂CO₃. The salts of urea and ammonium are not an alternative nitrogen sources of low cost for Spirulina platensis cultivation. From the experimental results obtained in this study, a successful estimate of carbohydrate, protein, lipids, and chlorophyll content inside Spirulina platensis was achieved without use advanced analytical techniques, allowing saves resources and time. This method can be extrapolated to other microorganisms and cultivation regimens.

Cultivable microalgae diversity from a freshwater aquaculture filtering system and its potential for polishing aquaculture-derived water streams

AIMS

Microalgae are ubiquitous in aquatic environments, including aquaculture farms, but few studies have delved into their phytoplankton diversity and bioremediation potential. In this study, the cultivable phytoplankton of a rainbow trout freshwater aquaculture farm was isolated, phylogenetically analysed and used to assemble a consortium to polish an aquaculture-derived effluent, with low concentrations of ammonium, nitrite and nitrate.

METHODS AND RESULTS

Through standard plating in different selective media, a total of 15 microalgae strains were isolated from sludge from a rotary drum filtering system which removes suspended solids from the water exiting the facility. Based on 18S rRNA gene sequences, isolates were assigned to nine different genera of the Chlorophyta phylum: Asterarcys, Chlorella, Chloroccocum, Chlorosarcinopsis, Coelastrella, Desmodesmus, Micractinium, Parachlorella and Scenedesmus. Species from most of these genera are known to inhabit freshwater systems in Galicia and continental Spain, but the Coelastrella, Asterarcys or Parachlorella genera are not usually present in freshwater streams. In an onsite integrative approach, the capacity of a consortium of native microalgae isolates to grow on aquaculture-derived effluents and its nutrient removal capacity were assessed using a raceway pond. After 7 days, removal efficiencies of approximately 99%, 92% and 49% for ammonium, nitrite and nitrate, respectively, were achieved concomitantly with a microalgae biomass increase of ca. 17%.

CONCLUSIONS

Sludge from the aquaculture filtering system presents a high diversity of microalgae species from the Chlorophyta phylum, whose application in a consortial approach revealed to be efficient to polish aquaculture-derived effluents with low nutrient content.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of native microalgae consortia from aquaculture systems can contribute to the development of efficient treatment systems for low-nutrient wastewater, avoiding nutrients release to the environment and promoting water recirculation. This may further strengthen the use of phycoremediation at the industrial scale, as an environment-friendly strategy.

Reuse of sea water reverse osmosis brine to produce Dunaliella salina based β-carotene as a valuable bioproduct: A circular bioeconomy perspective

Due to population growth and global warming, the use of the sea water reverse osmosis process to obtain freshwater is increasing rapidly. A sustainable method with low environmental impact is limited for the management of brine with a high salt content, which is released as a result of the process. Some microalgae species can grow in salty environments and produce β-carotene. This study aims to evaluate the commercial potential of β-carotene production from microalgae grown in sea water reverse osmosis brine from a bioeconomy perspective. Synthetic media are often used for the production of β-carotene from algae, the use of sea water reverse osmosis brine is not common and the commercial potential of this application has not been evaluated before. In terms of the development of the β-carotene market, the strengths and weaknesses of the process, opportunities, and threats are thoroughly examined in this report. Also, with the use of sea water reverse osmosis, a daily 750 tons of algal β-carotene can be produced. The biotechnological production of microalgal β-carotene and the reuse of salt water within the scope of circular bioeconomy are seen as a sustainable solution due to the fact that the strengths of the process are dominant, and the market value of natural β-carotene is increasing day by day.

Assessment of Nutrients Recovery Capacity and Biomass Growth of Four Microalgae Species in Anaerobic Digestion Effluent

Four microalgae species were evaluated for their bioremediation capacity of anaerobic digestion effluent (ADE) rich in ammonium nitrogen, derived from a biogas plant. Chlorella vulgaris, Chlorella sorokiniana, Desmodesmus communis and Stichococcus sp. were examined for their nutrient assimilation efficiency, biomass production and composition through their cultivation in 3.7% v/v ADE; their performance was compared with standard cultivation media which consisted in different nitrogen sources, i.e., BG-11NO3 and BG-11ΝH4 where N-NO3 was replaced by N-NH4. The results justified ammonium as the most preferable source of nitrogen for microalgae growth. Although Stichococcus sp. outperformed the other 3 species in N-NH4 removal efficiency both in BG-11NH4 and in 3.7% ADE (reaching up to 90.79% and 69.69% respectively), it exhibited a moderate biomass production when it was cultivated in diluted ADE corresponding to 0.59 g/L, compared to 0.89 g/L recorded by C. vulgaris and 0.7 g/L by C. sorokiniana and D. communis. Phosphorus contained in the effluent and in the control media was successfully consumed by all of the species, although its removal rate was found to be affected by the type of nitrogen source used and the particular microalgae species. The use of ADE as cultivation medium resulted in a significant increase in carbohydrates content in all investigated species.

Microalgal bioremediation of heavy metal pollution in water: Recent advances, challenges, and prospects

With the rapid economy development and population surge, the water resources available for direct use on the earth have been in shortage. Therefore, water pollution remediation inevitably becomes the focus of global attention. Aside from their capacity to fix and effectively control the emission of carbon dioxide thus achieve negative carbon emission, microalgae and its products modified by genetic engineering and other technologies also have a broad prospect in sewage treatment such as efficiently removing all kinds of pollutants in water and producing high-quality biofuels after use. Therefore, research on these organisms has gradually deepened in recent years. This paper summarizes the bioremediation mechanism of heavy metal ions in water by using microalgae and their modified products. The relevant research progresses since 2015 are critically reviewed and discussed. Challenges and prospects are also put forward for their industrial implementation.

Hybrid support vector regression and crow search algorithm for modeling and multiobjective optimization of microalgae-based wastewater treatment

Microalgae-based wastewater treatment (and biomass production) is an environmentally benign and energetically efficient technique as compared to traditional practices. The present study is focused on optimization of the major treatment variables such as temperature, light-dark cycle (LD), and nitrogen (N)-to-phosphate (P) ratio (N/P) for the elimination of N and P from tertiary municipal wastewater utilizing Chlorella kessleri microalgae species. In this regard, a hybrid support vector regression (SVR) technique integrated with the crow search algorithm has been applied as a novel modeling/optimization tool. The SVR models were formulated using the experimental data, which were furnished according to the response surface methodology with Box-Behnken Design. Various statistical indicators, including mean absolute percentage error, Taylor diagram, and fractional bias, confirmed the superior performance of SVR models as compared to the response surface methodology (RSM) and generalized linear model (GLM). Finally, the best SVR model was hybridized with the crow search algorithm for single/multi-objective optimizations to acquire the global optimal treatment conditions for maximum N and P removal efficiencies. The best-operating conditions were found to be 29.3°C, 24/0 h/h of LD, and 6:1 of N/P, with N and P elimination efficiencies of 99.97 and 93.48%, respectively. The optimized values were further confirmed by new experimental data.

Advanced biokinetic and hydrodynamic modelling to support and optimize the design of full-scale high rate algal ponds

High rate algal ponds (HRAP) are known for their suitability to treat wastewater and to produce microalgal biomass, which can be converted into bioproducts. However, full-scale application of HRAP is still limited to few cases, and design procedures are not consolidated or standardized. In this study, a demonstrative-scale HRAP system for secondary wastewater treatment to be implemented in India (treatment capacity of 50 m3·d-1) has been designed combining conventional dimensioning techniques and advanced modelling tools. The objective of the study was to assist, verify and optimize the conventional dimensioning of the secondary HRAP by means of simulations predicting the behaviour of the system in the specific local conditions under different configurations and operational strategies. Biokinetic modelling and hydrodynamic analysis using Computational Fluid Dynamics (CFD) were carried out. The simulations performed with the biokinetic model showed that the optimal hydraulic retention time to enhance nutrient removal and biomass production is 4 days. For the hydrodynamic modelling, a 3D model of the HRAP was built to simulate the hydrodynamic behaviour of 36 different designs. Simulations allowed quantifying the presence of low velocity zones as well as the land use efficiency of the different designs in terms of the useful area vs. the total occupied area. Two baffles and tear-shapes with a diameter equal to ¼ of the channel width was the most efficient configuration. Moreover, a technical-economic assessment of the system was carried out, resulting in an investment cost of 483 € per population equivalent and an operational cost of 0.19 € per m3 of treated wastewater.

Microalgal Systems for Wastewater Treatment: Technological Trends and Challenges towards Waste Recovery

Wastewater (WW) treatment using microalgae has become a growing trend due the economic and environmental benefits of the process. As microalgae need CO2, nitrogen, and phosphorus to grow, they remove these potential pollutants from wastewaters, making them able to replace energetically expensive treatment steps in conventional WW treatment. Unlike traditional sludge, biomass can be used to produce biofuels, biofertilizers, high value chemicals, and even next-generation growth media for “organically” grown microalgal biomass targeting zero-waste policies and contributing to a more sustainable circular bioeconomy. The main challenge in this technology is the techno-economic feasibility of the system. Alternatives such as the isolation of novel strains, the use of native consortia, and the design of new bioreactors have been studied to overcome this and aid the scale-up of microalgal systems. This review focuses on the treatment of urban, industrial, and agricultural wastewaters by microalgae and their ability to not only remove, but also promote the reuse, of those pollutants. Opportunities and future prospects are discussed, including the upgrading of the produced biomass into valuable compounds, mainly biofuels.

Diatom microalgae as smart nanocontainers for biosensing wastewater pollutants: recent trends and innovations

Microalgae have been recognized as one of the most efficient microorganisms to remediate industrial effluents. Among microalgae diatoms are silica shelled unicellular eukaryotes, found in all types of water bodies and flourish very well even in wastewater. They have their silica cell wall made up of nano arrayed pores arranged in a uniform fashion. Therefore, they act as smart nanocontainers to adsorb various trace metals, dyes, polymers, and drugs which are hazardous to human as well to aquatic life. The beautiful nanoarchitecture in diatoms allows them to easily bind to ligands of choice to form a nanocomposite structure with the pollutants which can be a chemical or biological component. Such naturally available diatom nanomaterials are economical and highly sensitive compared to manmade artificial silica nanomaterials to help in facile removal of the toxic pollutants from wastewater. This review is thus focused on employing diatoms to remediate various pollutants such as heavy metals, dyes, hydrocarbons detected in the wastewater. It also includes different microalgae as biosensors for determination of pollutants in effluents and the perspectives for nanotechnological applications in the field of remediating pollutants through microalgae. The review also discusses in length the hurdles and perspectives of employing microalgae in wastewater remediation.

Exploration of Microalgal Species for Nutrient Removal from Anaerobically Digested Swine Wastewater and Potential Lipids Production

Bio-treatment of anaerobically treated swine wastewater (ADSW) mediated by microalgae has been deemed as a promising strategy. In the present study, six microalgal strains were used to conduct batch experiments in 0~100% ADSW in order to evaluate their potentials for nutrient removal and biodiesel production. Two strains, Chlorella vulgaris FACHB-8 and Chlorella sp. FACHB-31, were selected based on their better growth performances, higher tolerance to wastewater (up to 100%), and better nutrient removal abilities. The capacity of each strain to remove TN, TP, NH₄⁺-N, as well as lipid production and biomass composition in 100% ADSW were further examined. After 15 days of culture, 87.68~89.85%, 92.61~93.68%, and 97.02~97.86% of the nitrogen, phosphorus, and ammonia nitrogen were removed by Chlorella sp. FACHB-31 and C. vulgaris FACHB-8. Their lipid content and lipid productivities were 29.63~33.33% and 18.91~23.10 mg L^-1 d^-1, respectively. Proteins were both the major biomass fraction followed by lipids and then carbohydrates. Their fatty acid profiles both mainly consisted of C-16:0, C-18:1, C-18:0, and C-18:2. Taken together, our results suggest that C. vulgaris FACHB-8 and Chlorella sp. FACHB-31 are potential candidates for biodiesel production by using ADSW as a good feedstock.

Potential of microalgae as a sustainable feed ingredient for aquaculture

An increase in fish consumption, combined with a decrease in wild fish harvest, is driving the aquaculture industry at rapid pace. Today, farmed seafood accounts for about half of all global seafood demand for human consumption. As the aquaculture industry continues to grow, so does the market for aquafeed. Currently, some of the feed ingredients are coming from low-value forage fishes (fish meal) and terrestrial plants. The production of fish meal can't be increased as it would affect the sustainability and ecosystem of the ocean. Similarly, increasing the production of terrestrial plant-based feed leads to deforestation and increased freshwater use. Hence, alternative and environmentally sustainable sources of feed ingredients need to be developed. Microalgae biomasses represent potential feed source ingredients as the cell metabolites of these microorganisms contain a blend of essential amino acids, healthy triglycerides as fat, vitamins, and pigments. In addition to serving as bulk ingredient in aquafeed, their unique array of bioactive compounds can increase the survivability of farmed species, improve coloration and quality of fillet. Microalgae has the highest areal biomass productivities among photosynthetic organisms, including fodder crops, and thus has a high commercial potential. Also, microalgal production has a low water and arable-land footprint, making microalgal-based feed environmentally sustainable. This review paper will explore the potential of producing microalgae biomass as an ingredient of aquaculture feed.

Microalgae Harvesting by Self-Driven 3D Microfiltration with Rationally Designed Porous Superabsorbent Polymer (PSAP) Beads

Microalgae are emerging as next-generation renewable resources for production of sustainable biofuels and high-value bioproducts. Conventional microalgae harvesting methods including centrifugation, filtration, flocculation, and flotation are limited by intensive energy consumption, high capital cost, long treatment time, or the requirement of chemical addition. In this study, we design and fabricate porous superabsorbent polymer (PSAP) beads for self-driven 3D microfiltration of microalgal cultures. The PSAP beads can swell fast in a microalgal suspension with high water absorption capacity. During this process, microalgal cells are excluded outside the beads and successfully concentrated in the residual medium. After treatment, the beads can be easily separated from the microalgal concentrate and reused after dewatering. In one PSAP treatment, a high concentration factor for microalgal cultures up to 13 times can be achieved in 30 min with a harvesting efficiency higher than 90%. Furthermore, microalgal cultures could be concentrated from 0.2 g L^-1 to higher than 120 g L^-1 with minimal biomass loss through multistage PSAP treatments. Therefore, the use of PSAP beads for microalgae harvesting is fast, effective, and scalable. It does not require any complex instrument or chemical addition. This technique potentially provides an efficient and feasible alternative to obtain high concentrations of functional biomass at a very low cost.

Low-cost biochar adsorbents prepared from date and delonix regia seeds for heavy metal sorption

In this study, low-cost biochar as bio-adsorbents derived from locally accessible delonix regia seed and date seeds were explored for heavy metal environmental cleaning. These prepared biochars were characterized by proximate and elemental analyses, CHNS/O analysis, Fourier-transformed infrared spectroscopy and thermo-gravitational methods. Bio-sorbent's ability to adsorb arsenic ions in synthetic wastewater was studied and optimized at varying solution pH, adsorbent dose, and starting metal concentrations. Experimentation and optimization studies were also carried out with the help of Design-software 6.0.8. The trials were designed by using response-surface methods, which includes three components and stages of Box-Behnken design. Date seeds derived-biochars eliminated 95% of arsenic from synthetic wastewater, whereas Delonix regia seeds removed 93.8%. The kinetics, isotherms and mechanism of As adsorption were also postulated. This study proposes that these seed's biochars might be employed as an effective, low-cost, and environmentally friendly adsorbent to remove heavy metals from the environment.

Integrated microalgal biorefinery for the production and application of biostimulants in circular bioeconomy

Adverse detrimental impacts of environmental pollution over the health regimen of people has driven a shift in lifestyle towards cleaner and natural resources, especially in the aspects of food production and consumption. Microalgae are considered a rich source of high value metabolites to be utilized as plant growth biostimulants. These organisms however, are underrated compared to other microbial counterparts, due to inappropriate knowledge on the technical, enviro-economical constrains leading to low market credibility. Thus, to avert these issues, the present review comprehensively discusses the biostimulatory potential of microalgae interactively combined with circular bio-economy perspectives. The biochemical content and intracellular action mechanism of microalgal biostimulants were described. Furthermore, detailed country-wise market trends along with the description of the existing regulatory policies are included. Enviro-techno-economic challenges are discussed, and the consensus need for shift to biorefinery and circular bio-economy concept are emphasized to achieve sustainable impacts during the commercialization of microalgal biostimulants.

Mitigation of self-shading effect in embedded optical fiber in Chlorella sorokiniana immobilized polyvinyl alcohol gel beads

The addition of optical fibers to improve light penetration into microalgae-immobilized poly (vinyl) alcohol (PVA) gel beads is proposed herein. Live/dead staining in combination with confocal laser scanning microscopy analysis revealed greater light penetration in Chlorella sorokiniana-immobilized PVA gel beads with optical fibers (PVA-OF) than in C. sorokiniana-immobilized PVA gel beads (PVA-NOF). The improved light penetration had positive effects on the initial nutrient removal efficiency, which resulted in a 1.2-fold higher initial nutrient removal efficiency in PVA-OF than in PVA-NOF. The use of polymeric PVA materials is effective for maintaining structural integrity. The findings demonstrate a methodology for mitigating self-shading effects in microalgae immobilized on opaque polymeric materials.

Using green alga Haematococcus pluvialis for astaxanthin and lipid co-production: Advances and outlook

Astaxanthin is one of the secondary carotenoids involved in mediating abiotic stress of microalgae. As an important antioxidant and nutraceutical compound, astaxanthin is widely applied in dietary supplements and cosmetic ingredients. However, most astaxanthin in the market is chemically synthesized, which are structurally heterogeneous and inefficient for biological uptake. Astaxanthin refinery from Haematococcus pluvialis is now a growing industrial sector. H. pluvialis can accumulate astaxanthin to ∼5% of dry weight. As productivity is a key metric to evaluate the production feasibility, understanding the biological mechanisms of astaxanthin accumulation is beneficial for further production optimization. In this review, the biosynthesis mechanism of astaxanthin and production strategies are summarized. The current research on enhancing astaxanthin accumulation and the potential joint-production of astaxanthin with lipids was also discussed. It is conceivable that with further improvement on the productivity of astaxanthin and by-products, the algal-derived astaxanthin would be more accessible to low-profit applications.

Effect of nano-TiO2 on humic acid utilization from piggery biogas slurry by microalgae

Resource recovery from piggery biogas slurry has become an inevitable demand for sustainable development of pig industry. Microalgae show great potential in recovering nitrogen and phosphorus from piggery slurry, but struggle to utilize organic pollutants, as most of them are inert components (e.g., humic acids, HAs). In this study, nano-TiO₂ was used to enhance the utilization of HAs by microalgae from piggery biogas slurry. Results showed that the optimal conditions for microalgal growth and HAs removal by the microalgae-TiO₂ coupling system were TiO₂ dosage of 0.30 g/L, microalgal inoculation concentration of 0.40 g/L, light intensity of 360 µmol photon/(m²·s) and temperature of 30 °C, with the microalgal chlorophyll concentration of 6.51 mg/L and HAs removal efficiency of 50.14%. Analysis of HAs composition variations in the piggery biogas slurry indicated that the decrease of HAs was caused by their decomposition into small molecules under the photocatalytic reactions of TiO₂.

Resource recovery from industrial effluents through the cultivation of microalgae: A review

Industrial effluents such as pharmaceutical residues, pesticides, dyes, and metal processes holds abundant value-added products (VAPs), where its recovery has become essential. The purpose of such recovery is for sustainable treatment, which is an approach that considers the economic, social, and environmental aspects. Microalgae with its potential in the recovery process from effluents, can reduce energy usage of waste management strategies and regenerate nutrients such as carbon, phosphorus, and nitrogen. Microalgae cultures offer the use of inorganic materials by microalgae for their growth and the help of bacteria to produce biomass, thus, resulting in the absence of secondary emissions due to its ability to eliminate volatile organic compounds. Moreover, recovered bioactive compounds are transformed into bioethanol, bio-fertilizers, biopolymer, health supplements and animal feed. Therefore, it is significant to focus on an economical and efficient utilization of microalgae in recovering nutrients that can be further used in various commercial applications.

Valorization of swine wastewater in a circular economy approach: Effects of hydraulic retention time on microalgae cultivation

To optimize the swine wastewater (SWW) treatment, this study investigated different hydraulic retention times (HRTs) for microalgae cultivation. For this purpose, five pilot-scale reactors operated in semi-continuous flow, with HRTs equal to 9, 12, 15, 18, 21 days were evaluated in terms of SWW polishing and biomass production. The effluent treatment was discussed accompanied by principal component analysis, which allowed identification of causes of variance in the data set, ideal for studies with real effluent and influenced by environmental conditions. All reactors show satisfactory removals of N-NH₄⁺ (91.6-95.3%), COD (15.8-39.9%), DO increment (in average 7.5 mg O₂/L) and, only the longest HRT (21 days) was able to remove Ps (21%). The results obtained indicated that a consortium of microalgae and bacteria was developed for all the tested HRTs. On the other hand, HRT = 12 days provided a healthier culture of photosynthesizing organisms (chl-a/VSS = 3.04%). Carbohydrates (20.8-31.3%) and proteins (2.7-16.2%) were the compounds of commercial interest in the highest proportion in the biomass of all reactors, with contents comparable to that of terrestrial crops. Thus, it was suggested a valorization route of these compounds of high added value to return to pig farming, where the nutrients were intended to supplement the swine feed and clarified water for cleaning the pig stalls. Thus, in the circular economy context, this research contributes to water footprint reduction and the sustainability of the pig farming production chain. The economic and environmental analysis of the route is suggested to enable its implementation on a large scale, as well as further technical feasibility research (reactor types, exposure to external environment, evaluation of pathogen removal and animal feed supplementation from SWW microalgae biomass).

Integrating Microalgae Cultivation with Wastewater Treatment: a Peek into Economics

Microalgae cultivation on wastewater is one of the most promising processes in perspective of green and circular economy. This study investigated the economics of integrating the microalgae cultivation with the wastewater treatment in perspective of biomass production and wastewater treatment. The cost of integrated process was evaluated for six cases: three cases for domestic wastewater at different stages of treatment including sewage, anaerobically digested domestic effluent, and centrate and three cases for industrial wastewater including agro-industrial wastewater, anaerobically digested piggery effluent, and anaerobically digested abattoir effluent. The cost of biomass production was found ranging from $ 0.39 to $ 0.92/kg with minimum for the anaerobically digested domestic effluent and centrate. The cost of wastewater treatment was found ranging from 0.18 to 1.69/m3 with minimum for the sewage. These costs did not include any credits generated from the biomass or the treated wastewater. The concentration of limiting nutrient, flowrate of wastewater, and the extent of nutrient removal are the major cost-influencing parameters for the integrated process.

Treatment of Synthetic Ammonium Sulfate Wastewater by Mixed Culture of Chlorella pyrenoidosa and Enriched Nitrobacteria

Ammonium sulfate wastewater can cause eutrophication and black odor of water body. Although ammonia nitrogen can be used as nutrient of microalgae, high ammonia nitrogen levels could inhibit the growth of microalgae. Nitrobacteria can transform ammonia nitrogen into nitrate nitrogen. In this study, mono Chlorella pyrenoidosa culture (mono-C.py), synchronous mixed culture (mixed-a), and asynchronous mixed culture (mixed-b) systems were examined for their ability to treat ammonium sulfate wastewater. Nitrogen removal rate of mixed-b at the end of culture (52.96%) was higher than that of the mono-C.py (46.37%) and the mixed-a (39.11%). Higher total suspended solid concentration (2.40 g/L), crude protein yield (0.76 g/L), and heating value yield (35.73 kJ/L) were obtained in mixed-b, meanwhile with excellent settlement performance (91.43 ± 0.51%). Mechanism analysis of settlement showed that the relative abundance of floc-forming-related bacteria Sphingopyxis and Acidovorax were increased generally, while nitrification/denitrifying members were decreased in mixed-b along with the culture proceeding.

Plastic wastes biodegradation: Mechanisms, challenges and future prospects

The growing accumulation of plastic wastes is one of the main environmental challenges currently faced by modern societies. These wastes are considered a serious global problem because of their effects on all forms of life. There is thus an urgent need to demonstrate effective eco-environmental techniques to overcome the hazardous environmental impacts of traditional disposal paths. However, our current knowledge on the prevailing mechanisms and the efficacy of synthetic plastics' biodegradation still appears limited. Under this scope, our review aims to comprehensively highlight the role of microbes, with special emphasis on algae, on the entire plastic biodegradation process focusing on the depolarization of various synthetic plastic types. Moreover, our review emphasizes on the ability of insects' gut microbial consortium to degrade synthetic plastic wastes. In this view, we discuss the schematic pathway of the biodegradation process of six types of synthetic plastics. These findings may contribute to establishing bio-upcycling processes of plastic wastes towards biosynthesis of valuable metabolic products. Finally, we discuss the challenges and opportunities for microbial valorization of degraded plastic wastes.

Valorization of microalgae biomass into bioproducts promoting circular bioeconomy: a holistic approach of bioremediation and biorefinery

The need for alternative source of fuel has demanded the cultivation of 3rd generation feedstock which includes microalgae, seaweed and cyanobacteria. These phototrophic organisms are unique in a sense that they utilise natural sources like sunlight, water and CO₂ for their growth and metabolism thereby producing diverse products that can be processed to produce biofuel, biochemical, nutraceuticals, feed, biofertilizer and other value added products. But due to low biomass productivity and high harvesting cost, microalgae-based production have not received much attention. Therefore, this review provides the state of the art of the microalgae based biorefinery approach to define an economical and sustainable process. The three major segments that need to be considered for economic microalgae biorefinery is low cost nutrient source, efficient harvesting methods and production of by-products with high market value. This review has outlined the use of various wastewater as nutrient source for simultaneous biomass production and bioremediation. Further, it has highlighted the common harvesting methods used for microalgae and also described various products from both raw biomass and delipidified microalgae residues in order to establish a sustainable, economical microalgae biorefinery with a touch of circular bioeconomy. This review has also discussed various challenges to be considered followed by a techno-economic analysis of the microalgae based biorefinery model.