The effect of recycling culture medium after harvesting of Chlorella vulgaris biomass by flocculating bacteria on microalgal growth and the functionary mechanism

A fungal-algal self-flocculation system and its application to treat filter sludge leachate in the sugar industry

The efficient and economical treatment of wastewater using microalgae has attracted much attention. However, harvesting microalgae cells from treated wastewater remains challenging. In the present study, a Chlorella vulgaris suspension containing filamentous fungi Aspergillus niger and Chaetomium gracile was successfully used to construct a self-flocculating system, with a microalgae flocculation efficiency of 99.6% achieved by gravity sedimentation within 4 h. The diameter of fungi played an important role in determining flocculation efficiency, and the optimal particle size was 10 mm. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) results indicated that the sweeping action of fungal mycelia and the interaction between the functional groups of fungi and the C. vulgaris surface contributed to improve flocculation. Co-cultivation of C. vulgaris and fungi could effectively remove 83.53%, 94.45% and 76.88% of total phosphorus, total nitrogen and chemical oxygen demand, respectively, from the sludge leachate from a sugar mill. The fungal-algal biomass reached 5.75 g/L. Herein, the constructed self-flocculation system had coupled efficient flocculation of C. vulgaris with removal of pollutants from wastewater in a short period of time, and providing a green, pollution-free, low-cost method for simultaneous wastewater treatment and the production of high quality biomass.

Microalgae and nano-cellulose composite produced via a co-culturing strategy for ammonia removal from the aqueous phase

This study addresses the pressing need for sustainable bioremediation solutions to combat increasing pollution challenges in alignment with sustainability development goals. The research focuses on developing a co-culture approach involving microalgae and Komagataeibacter europaeus BCRC 14148 bacterium to create a biocomposite for efficient ammonia removal. Nanocellulose, produced by the bacterium, serves as a substrate for microalgae attachment. Optimization using specific growth media ratios resulted in biocomposite yields of 4.05 ± 0.16 g/L and 3.83 ± 0.13 g/L in HS medium with fructose and glucose, respectively. The optimal conditions include a 40:60 ratio of HS-F to TAP medium, 25 ℃ incubation, 6000 Lux light intensity, pH 5.5, and a 48-hour incubation period. When applied to wastewater treatment, the biocomposite demonstrated exceptional ammonium removal efficiency at 91.64 ± 1.27 %. This co-culture-derived biocomposite offers an eco-friendly, recyclable, and effective solution for sustainable environmental bioremediation.

Technologies for harvesting the microalgae for industrial applications: Current trends and perspectives

Microalgae are emerging as a promising source for augmenting the supply of essential products to meet global demands in an environmentally sustainable manner. Despite the potential benefits of microalgae in industry, the high energy consumption for harvesting remains a significant obstacle. This review offers a comprehensive overview of microalgae harvesting technologies and their industrial applications, with particular emphasis on the latest advances in flocculation techniques. These cutting-edge methods have been applied to biodiesel production, food and nutraceutical processing, and wastewater treatment. Large-scale harvesting is still severely impeded by the high cost despite progress has been made in laboratory studies. In the future, cost-effective microalgal harvesting will rely on efficient resource utilization, including the use of waste materials and the reuse of media and flocculants. Additionally, precise regulation of biological metabolism will be necessary to overcome algal species-related limitations through the development of extracellular polymeric substance-induced flocculation technology.

Potential of biogenic and non-biogenic waste materials as flocculant for algal biomass harvesting: Mechanism, parameters, challenges and future prospects

In this review article, waste materials (biogenic/non-biogenic) are focused as the flocculants for harvesting of algal biomass. Chemical flocculants are widely utilized for the effective harvesting of algal biomass at a commercial scale while the high cost is a major drawback. The waste materials-based flocculants (WMBF) are started to utilize as one of the cost-effective performance for dual benefits of waste minimization and reuse for sustainable recovery of biomass. The novelty of the article is articulated with the objective that presents an insight of WMBF, classification of WMBF, preparation methods of WMBF, mechanisms of flocculation, factors affecting flocculation-mechanism, challenges and future recommendations that are required for harvesting of algae. The WMBF are shown similar flocculation mechanisms and flocculation efficiencies as chemical flocculants. Thus, the utilization of waste material for the flocculation process of algal cells minimizes the waste load into the environment and transforms the waste materials into valuable resources.

Insights into differences between spore-assisted and pellet-assisted microalgae harvesting using a highly efficient fungus: Efficiency, high-value substances, and mechanisms

To achieve efficient and low-cost microalgae harvesting, investigations were conducted on the harvesting efficiency and potential mechanisms of Chlorella sp. HQ by filamentous fungi using two strategies, fungal spore-assisted harvesting (FSH) and fungal pellet-assisted harvesting (FPH). Five of the 19 fungal species isolated from domestic sewage could form pellets, and Aspergillus niger HW8-1 comprised the highest harvesting efficiencies of 99.17 % and 88.70 % for FPH and FSH, respectively. FSH had 2-3 times more lipids and polysaccharides in fungus-alga pellets and caused richer saturated and monounsaturated fatty acids compared with FPH. Moreover, by optimizing the microalgae preculture time, glucose concentration, and microalgae initial density, the contents of high-value substances, such as lipids, polysaccharides, and proteins of fungus-alga pellets after FPH were improved from 5.96 %, 5.67 %, and 7.27 % to 20.18 %, 24.34 % and 10.48 %, respectively. Furthermore, fungal pellets secreted more extracellular polymeric substances (EPS) during FPH than those by FSH, which could chemisorb algal cells by lowering the surface potential of fungal pellets. FPH caused algal cells to cover the outside, which increased the light exposure area of algae, thereby increasing the photosynthesis rate, whereas FSH mainly captured microalgae physically through mycelium entanglement.

Evaluation of Growth and Utilization Potential of Rhodobacter sphaeroides in Reused Medium

Rhodobacter sphaeroides is a metabolically versatile purple non-sulfur bacteria that can produce valuable substances. As the low-cost and high-efficiency production of valuable substances is attracting attention, the reuse of the medium is emerging as a promising strategy. Therefore, in this study, the growth of R. sphaeroides was evaluated by reusing the medium of Escherichia coli and Saccharomyces cerevisiae. As a result, in the reuse of the medium in which S. cerevisiae was cultured, sufficient growth of R. sphaeroides could be confirmed, and especially, the growth of R. sphaeroides was not inhibited under aerobic conditions. Therefore, it is considered that the strategy of reusing the medium of S. cerevisiae is sufficiently feasible. Of the organic compounds investigated, R. sphaeroides grew best in succinic acid, followed by malic acid, citric acid, acetic acid, and glucose. In addition, by comparing photopigment synthesis in the reused medium, we propose the hypothesis that succinic acid may play an important role in photopigment synthesis for the first time.

A low-cost technique for biodiesel production in Ankistrodesmus sp. EHY by using harvested microalgal effluent

The present study aims to use Ankistrodesmus sp. EHY to develop a viable and economic lipid production strategy using recycling of harvested microalgal effluent. In comparison to the control, the highest lipid content (52.4 %) and productivity (250.72 mg L^-1 d^-1) were achieved when 40 % recycled medium was used. Consistent with the trend of lipid accumulation, the six key lipogenetic genes were upregulated, as well as reactive oxygen species (ROS), glutathione (GSH) and genes encoding antioxidant enzymes during cultivation in recycled medium. Moreover, the consumption of dissolved organic carbon (DOC) and the increased humic acid (HA) in the recycled medium might also be associated with lipid biosynthesis. The biodiesel parameters of alga biomass-derived lipids were fitted to the standard of commercial biodiesel. In conclusion, this study offers an economically viable strategy for microalgal biofuel production and wastewater treatment using recycling of harvested microalgal effluent.

Life cycle assessment of microalgae systems for wastewater treatment and bioproducts recovery: Natural pigments, biofertilizer and biogas

The aim of this study was to assess the potential environmental impacts associated with microalgae systems for wastewater treatment and bioproducts recovery. In this sense, a Life Cycle Assessment was carried out evaluating two systems treating i) urban wastewater and ii) industrial wastewater (from a food industry), with the recovery of bioproducts (i.e. natural pigments and biofertilizer) and bioenergy (i.e. biogas). Additionally, both alternatives were compared to iii) a conventional system using a standard growth medium for microalgae cultivation in order to show the potential benefits of using wastewater compared to typical cultivation approaches. The results indicated that the system treating industrial wastewater with unialgal culture had lower environmental impacts than the system treating urban wastewater with mixed cultures. Bioproducts recovery from microalgae wastewater treatment systems can reduce the environmental impacts up to 5 times compared to a conventional system using a standard growth medium. This was mainly due to the lower chemicals consumption for microalgae cultivation. Food-industry effluent showed to be the most promising scenario for bioproducts recovery from microalgae treating wastewater, because of its better quality compared to urban wastewater which also allows the cultivation of a single microalgae species. In conclusion, microalgae wastewater treatment systems are a promising solution not only for wastewater treatment but also to boost the circular bioeconomy in the water sector through microalgae-based product recovery.

Understanding the influence of free nitrous acid on microalgal-bacterial consortium in wastewater treatment: A critical review

Microalgal-bacterial consortium (MBC) constitutes a sustainable and efficient alternative to the conventional activated sludge process for wastewater treatment (WWT). Recently, integrating the MBC process with nitritation (i.e., shortcut MBC) has been proposed to achieve added benefits of reduced carbon and aeration requirements. In the shortcut MBC system, nitrite or free nitrous acid (FNA) accumulation exerts antimicrobial influences that disrupt the stable process performance. In this review, the formation and interactions that influence the performance of the MBC were firstly summarized. Then the influence of FNA on microalgal and bacterial monocultures and related mechanisms together with the knowledge gaps of FNA influence on the shortcut MBC were highlighted. Other challenges and future perspectives that impact the scale-up of the shortcut MBC for WWT were illustrated. A potential roadmap is proposed on how to maximize the stable operation of the shortcut MBC system for sustainable WWT and high-value biomass production.

Global market and economic analysis of microalgae technology: Status and perspectives

Microalgae have been a promising alternative source of high-value compounds to replace the non-sustainable fossil fuels resource. The recent research development of algae-based bioproducts has remarkable impact various industries section for its renewability, efficiency, and environmentally friendly crops over those synthetic-made product. However, by utilizing microalgae biomass toward their full potential is still limited due to lack of research funding, social acceptability and challenges in policy implementation. This present review highlights the various microalgae biotechnology with consideration of economical aspect for the global potential of algae market, comparison between the microalgae market in Malaysia and international countries. In addition, the cultivation technologies and feasibility of microalgae biomass production globally, followed by insightful challenges and future development of microalgae industry are mentioned. The current study will contribute to the understanding of upstream and downstream of microalgae processing along with technical economical understandings for the successful commercialisation of microalgae products.

A bacterial strain Citrobacter W4 facilitates the bio-flocculation of wastewater cultured microalgae Chlorella pyrenoidosa

A bacteria strain Citrobacter W4 isolated from the microalgae sewage culture system showed flocculating activity against Chlorella pyrenoidosa. In this work, operation parameters under outdoor conditions were optimized. When the bacterial-algal ratio was 4:1, G value was 26.30 s^-1, and harvesting time was 6 h, the harvesting efficiency achieved 87.37 ± 2.96% without ions addition and pH adjustment. The microbial community structure analysis showed Citrobacter W4 was dominant in the harvesting process. Flocculating active substances were on the surface and metabolites of Citrobacter W4. The main component of bacteria flocculating active substances was protein. Polysaccharides and carboxylic acid also promoted flocculation. The flocculation mechanisms were mainly adsorption bridging, net catching, and sweeping, not electric neutralization. The quality of FAMEs was improved after flocculation. The cost of 1 kg dried microalgae flocculated by Citrobacter W4 was $1.35. The novel flocculating bacteria showed the potential to harvest microalgae cost-effectively and environmentally friendly.

Constructed microalgal-bacterial symbiotic (MBS) system: Classification, performance, partnerships and perspectives

The microalgal-bacterial symbiotic (MBS) system shows great advantages in the synchronous implementation of wastewater treatment and nutrient recovery. To enhance the understanding of different MBS systems, this review summarizes reported MBS systems and proposes three patterns according to the living state of microalgae and bacteria. They are free microalgal-bacterial (FMB) system, attached microalgal-bacterial (AMB) system and bioflocculated microalgal-bacterial (BMB) system. Compared with the other two patterns, BMB system shows the advantages of microalgal biomass harvesting and application. To further understand the microalgal-bacterial partnerships in the bioflocculation of BMB system, this review discusses bioflocs characteristics, extracellular polymeric substances (EPS) properties and production, and the effect of microalgae/bacteria ratio and microalgal strains on the formation of bioflocculation. Microalgal biomass production and application are important for BMB system development in the future. Food processing wastewater characterized by high biodegradability and low toxicity should be conducive for microalgal cultivation. In addition, exogenous addition of functional bacteria for nutrient removal and bioflocculation formation would be a crucial research direction to facilitate the large-scale application of BMB system.

Converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia: A critical review

Conventional wastewater treatment using activated sludge cannot efficiently eliminate nitrogen and phosphorus, thus engendering the risk of water eutrophication and ecosystem disruption. Fortunately, a new wastewater treatment process applying microalgae-bacteria consortia has attracted considerable interests due to its excellent performance of nutrients removal. Moreover, some bacteria facilitate the harvest of microalgal biomass through bio-flocculation. Additionally, while stimulating the functional bacteria, the improved biomass and enriched components also brighten bioenergy production from the perspective of practical applications. Thus, this review first summarizes the current development of nutrients removal and mutualistic interaction using microalgae-bacteria consortia. Then, advancements in bio-flocculation are completely described and the corresponding mechanisms are thoroughly revealed. Eventually, the recent advances of bioenergy production (i.e., biodiesel, biohydrogen, bioethanol, and bioelectricity) using microalgae-bacteria consortia are comprehensively discussed. Together, this review will provide the ongoing challenges and future developmental directions for better converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia.

A fungal immobilization technique for efficient harvesting of oleaginous microalgae: Key parameter optimization, mechanism exploration and spent medium recycling

To confront with energy crisis, microalgae as the promising feedstock have a great potential in exploring renewable energy field, whereas the high costs related to medium preparation and biomass harvesting are the main bottleneck to hinder the development on a large scale. Though cultivation of filamentous fungi for microalgae harvesting is an efficient, sustainable and emerging method, and the studies on specific mechanisms and spent medium recycling for efficiency improvement as well as resource saving through a co-pelletization mode are urgently needed. Hence, in this study, the harvesting process of autotrophic microalgae Chlorella vulgaris by pre-cultured Aspergillus oryzae pellets was investigated systematically. The highest efficiency (99.23%) was obtained within 5 h under the optimized conditions of 30 °C, 130 rpm and fungi:algae ratio of 1:1 on a dry weight basis without demand for pH adjustment (initial value on 9.68). Charge neutralization was not the main mechanisms involved in fungi-algae aggregations, and the functional group changes on cell surfaces as well as secreted metabolites in medium could be mainly responsible for inducing the bioflocculation process. After harvesting, separated water could also effectively support microalgae re-growth. The biomass concentration in medium with 50% recycling was higher than that in fresh medium, while lipid content was increased from 24.37% to 33.97% in fully recycled medium. These results indicated that the pellet-assisted mode for algal harvesting is a promising way to promote biofuel production and resource recycling.

Various potential techniques to reduce the water footprint of microalgal biomass production for biofuel-A review

Due to their rapid growth rates, high lipid productivity, and ability to synthesize value-added products, microalgae are considered as the potential biofuel feedstocks. However, among the several bottlenecks that are hindering the commercialization of microalgal biofuel synthesis, the issue of high water consumption is the least explored. This analysis, therefore, examines the factors that decide water use for the production of microalgae biofuel. Microalgae biodiesel water footprint varies from 3.5 to 3726 kg of water per kg of biodiesel. The study further investigates the cause for large variability in the estimation of the water footprint for microalgae fuel. Various strategies, including the reuse of harvested water, the use of high density cultivation that could be adopted for low water consumption in microalgal biofuel production are discussed. Specifically, the review identified a reciprocal relationship between biomass productivity and water footprint. On the basis of which the review emphasizes the significance of high density cultivation, which can be inexpensive and feasible relative to other water-saving techniques. With the setback of water scarcity due to the rapid industrialization in developing countries, the implementation of the cultivation system with a focus on minimizing the water consumption is inevitable for a successful large scale microalgal biofuel production.

Effects of Co-culture on Improved Productivity and Bioresource for Microalgal Biomass Using the Floc-Forming Bacteria Melaminivora Jejuensis

Bacterial and algal floc formation was induced by inoculating three species of wastewater-derived bacteria (Melaminivora jejuensis, Comamonas flocculans, and Escherichia coli) into algal cultures (Chlorella sorokiniana). Bacterial and algal flocs formed in algal cultures inoculated with M. jejuensis and C. flocculans, and these flocs showed higher sedimentation rates than pure algal culture. The floc formed by M. jejuensis (4988.46 ± 2589.81 μm) was 10-fold larger than the floc formed by C. flocculans (488.60 ± 226.22 μm), with a three-fold higher sedimentation rate (M. jejuensis, 91.08 ± 2.32% and C. flocculans, 32.55 ± 6.33%). Biomass and lipid productivity were improved with M. jejuensis inoculation [biomass, 102.25 ± 0.35 mg/(L·day) and 57.80 ± 0.20 mg/(L·day)] compared with the productivity obtained under pure algal culture conditions [biomass, 78.00 ± 3.89 mg/(L·day) and lipids, 42.26 ± 2.11 mg/(L·day)]. Furthermore, the fatty acid composition of the biomass produced under pure algal culture conditions was mainly composed of C_16:0 (43.67%) and C_18:2 (45.99%), whereas the fatty acid composition of the biomass produced by M. jejuensis was mainly C_16:0 (31.80%), C_16:1 (24.45%), C_18:1 (20.23%), and C_18:2 (16.11%). These results suggest the possibility of developing an efficient method for harvesting microalgae using M. jejuensis and provide information on how to improve biomass productivity using floc-forming bacteria.

Outdoor cultivation of Chlorella sorokiniana in third generation biorefinery: Resource savings through medium recycling

The reduction of resource requirements for the outdoor cultivation of Chlorella sorokiniana using 180 L flat panel photobioreactors through medium recycling was investigated in this study. Without medium recycling, algae grew in 13.6 d from 0.92 to 5.32 gL^-1with a productivity of 0.32 gL^-1d^-1. For the production of 748 g algae dry weight (DW), 152gkg^-1 N, 27 gkg^-1 P and 231 Lkg^-1 water were needed. A realistic cultivation model with the recycling of medium and a productivity of 0.4 gL^-1d^-1 was set up based on experimental data, in which the requirements decreased to 104gkg^-1 N, 24 gkg^-1 P and 141 Lkg^-1 water. Compared to the production of lutein-containing plant Tagetes erecta, water and potassium requirements of up to 91% less and 96% respectively and higher biomass productivity by the factor 3.7 was achieved.

Simultaneous nitriles degradation and bioflocculant production by immobilized K. oxytoca strain in a continuous flow reactor

High cost is one of the limiting factors in the industrial production of bioflocculant. Simultaneous preparation of bioflocculant from the contaminants in wastewater was considered as a potential approach to reduce the production cost. In this study, butyronitrile and succinonitrile were verified as sole nitrogen sources for the growth of strain K. oxytoca GS-4-08 in batch experiments. Moreover, more than 90 % of the mixed nitriles could be degraded in a continuous flow reactor, and the bioflocculant could be prepared simultaneously in the effluent. All the as-prepared bioflocculants exhibited high flocculation efficiencies of over 90 % toward Kaolin solution. FTIR and XPS results further unveiled that, the bioflocculant samples with abundance of carboxyl, amine and hydroxyl groups may play an important role on adsorption of Pd²⁺. The adsorption process could be well simulated by Freundlich model, and the K_f values were as high as 452.8 mg^1-1/n l^1/n g^-1. The results obtained in this study not only confirm the technical feasibility for preparation of bioflocculant from various single nitrile and/or mixed nitriles, but also promise its economic feasibility.

Microalgal bioenergy production under zero-waste biorefinery approach: Recent advances and future perspectives

In view of the globalization and energy consumption, an economic and sustainable biorefinery model is essential to address the energy security and climate change. From this perspective, renewable biofuel production from microalgae along with a wide range of value-added co-products define its potential as a biorefinery feedstock. However, economic viability of microalgal biorefinery at its current state is not considered sustainable. Reduce, recycle, and reuse of waste derived from algal bioenergy conversion process will lead to an energy efficient and sustainable zero-waste microalgal biorefinery. This review focuses on three major aspects of zero-waste microalgal biorefinery approach; (1) recent advances on microalgal bioenergy conversion processes (chemical, biochemical and thermochemical); (2) mitigation and transformation of liquid and solid waste and (3) techno-economic analysis (TEA) and lifecycle assessment (LCA). In addition, the study also focuses on the challenges and future perspectives for an advanced microalgal biorefinery model.