Microalgae based wastewater treatment coupled to the production of high value agricultural products: Current needs and challenges

The Microalgae Chlamydomonas for Bioremediation and Bioproduct Production

The extensive metabolic diversity of microalgae, coupled with their rapid growth rates and cost-effective production, position these organisms as highly promising resources for a wide range of biotechnological applications. These characteristics allow microalgae to address crucial needs in the agricultural, medical, and industrial sectors. Microalgae are proving to be valuable in various fields, including the remediation of diverse wastewater types, the production of biofuels and biofertilizers, and the extraction of various products from their biomass. For decades, the microalga Chlamydomonas has been widely used as a fundamental research model organism in various areas such as photosynthesis, respiration, sulfur and phosphorus metabolism, nitrogen metabolism, and flagella synthesis, among others. However, in recent years, the potential of Chlamydomonas as a biotechnological tool for bioremediation, biofertilization, biomass, and bioproducts production has been increasingly recognized. Bioremediation of wastewater using Chlamydomonas presents significant potential for sustainable reduction in contaminants and facilitates resource recovery and valorization of microalgal biomass, offering important economic benefits. Chlamydomonas has also established itself as a platform for the production of a wide variety of biotechnologically interesting products, such as different types of biofuels, and high-value-added products. The aim of this review is to achieve a comprehensive understanding of the potential of Chlamydomonas in these aspects, and to explore their interrelationship, which would offer significant environmental and biotechnological advantages.

Modelling of anaerobic digestion of microalgae biomass: Effect of overloading perturbation

Anaerobic digestion (AD) of microalgae is an intriguing approach for bioenergy production. The scaling-up of AD presents a significant challenge due to the systematic efficiency losses related to process instabilities. To gain a comprehensive understanding of AD behavior, this study assessed a modified version of the anaerobic digestion model No1 (ADM1) + Contois kinetics to represent microalgae AD impacted by overloading. To this end, two new inhibition functions were implemented: inhibition by acetate for acidogenesis/acetogenesis and total volatile fatty acids for hydrolysis. This proposed ADM1 modification (including Contois kinetics) simulated AD behavior during the stable, disturbed and recovery periods, showing that the inhibition functions described in the original ADM1 cannot explain the AD performance under one of the most common perturbations at industrial scale (overloading). The findings underscore the importance of refining the inhibitions present in original ADM1 to better capture and predict the complexities of microalgae AD against overloading.

Integrating microalgae-based wastewater treatment, biostimulant production, and hydroponic cultivation: a sustainable approach to water management and crop production

A natural appearing microalgae-bacteria consortium was used to process urban wastewater. The process was done in an 80 m2 raceway reactor and the results were compared to an identical reactor operated using freshwater supplemented with commercial fertilisers. The biomass harvesting was done using commercial ultrafiltration membranes to reduce the volume of culture centrifuged. The membrane allowed achieving a biomass concentration of ∼9-10 g L-1. The process proposed avoids the use of centrifuges and the drying of the biomass, two of the most energy consuming steps of conventional processes. The specific growth rate in freshwater and the wastewater-based media was estimated as 0.30 ± 0.05 and 0.24 ± 0.02 days-1, respectively (p < 0.05). The maximum concentration reached at the end of the batch phase was 0.96 ± 0.03 and 0.83 ± 0.07 g L-1 when the biomass was produced using freshwater and wastewater, respectively (p < 0.05). The total nitrogen removal capacity of the system was on average 1.35 g m-2·day-1; nitrogen assimilation into biomass represented 60%-95% of this value. Furthermore, the P-PO4 3- removal capacity of the system varied from 0.15 to 0.68 g m-2·day-1. The outlet effluent of the reactor was used as a nutrient source in the hydroponic production of zucchini seedlings, leading to an increase in the root dry weight and the stem diameter compared to the water alone. The produced biomass showed potential for use as feedstock to produce plant biostimulants with positive effects on root development and chlorophyll retention.

Integrating microalgae growth in biomethane plants: Process design, modelling, and cost evaluation

The integration of microalgae cultivation in anaerobic digestion (AD) plants can take advantage of relevant nutrients (ammonium and ortho-phosphate) and CO2 loads. The proposed scheme of microalgae integration in existing biogas plants aims at producing approximately 250 t·y-1 of microalgal biomass, targeting the biostimulants market that is currently under rapid expansion. A full-scale biorefinery was designed to treat 50 kt·y-1 of raw liquid digestate from AD and 0.45 kt·y-1 of CO2 from biogas upgrading, and 0.40 kt·y-1 of sugar-rich solid by-products from a local confectionery industry. An innovative three-stage cultivation process was designed, modelled, and verified, including: i) microalgae inoculation in tubular PBRs to select the desired algal strains, ii) microalgae cultivation in raceway ponds under greenhouses, and iii) heterotrophic microalgae cultivation in fermenters. A detailed economic assessment of the proposed biorefinery allowed to compute a biomass production cost of 2.8 ± 0.3 €·kg DW-1, that is compatible with current downstream process costs to produce biostimulants, suggesting that the proposed nutrient recovery route is feasible from the technical and economic perspective. Based on the case study analysis, a discussion of process, bioproducts and policy barriers that currently hinder the development of microalgae-based biorefineries is presented.

Microalgal-bacterial biofilms for wastewater treatment: Operations, performances, mechanisms, and uncertainties

Microalgal-bacterial biofilms have been increasingly considered of great potential in wastewater treatment due to the advantages of microalgal-bacterial synergistic pollutants removal/recovery, CO2 sequestration, and cost-effective biomass-water separation. However, such advantages may vary widely among different types of microalgal-bacterial biofilms, as the biofilms could be formed on different shapes and structures of attachment substratum, generating "false hope" for certain systems in large-scale wastewater treatment if the operating conditions and pollutants removal properties are evaluated based on the general term "microalgal-bacterial biofilm". This study, therefore, classified microalgal-bacterial biofilms into biofilms formed on 2D substratum, biofilms formed on 3D substratum, and biofilms formed without substratum (i.e. microalgal-bacterial granular sludge, MBGS). Biofilms formed on 2D substratum display higher microalgae fractions and nutrients removal efficiencies, while the adopted long hydraulic retention times were unacceptable for large-scale wastewater treatment. MBGS are featured with much lower microalgae fractions, most efficient pollutants removal, and acceptable retention times for realistic application, yet the feasibility of using natural sunlight should be further explored. 3D substratum systems display wide variations in operating conditions and pollutants removal properties because of diversified substratum shapes and structures. 2D and 3D substratum biofilms share more common in eukaryotic and prokaryotic microbial community structures, while MGBS biofilms are more enriched with microorganisms favoring EPS production, biofilm formation, and denitrification. The specific roles of stratified extracellular polymeric substances (EPS) in nutrients adsorption and condensation still require in-depth exploration. Nutrients removal uncertainties caused by microalgal-bacterial synergy decoupling under insufficient illumination, limited microbial community control, and possible greenhouse gas emission exacerbation arising from microalgal N2O generation were also indicated. This review is helpful for revealing the true potential of applying various microalgal-bacterial biofilms in large-scale wastewater treatment, and will provoke some insights on the challenges to the ideal state of synergistic pollutants reclamation and carbon neutrality via microalgal-bacterial interactions.

A Comprehensive Review of Microalgae and Cyanobacteria-Based Biostimulants for Agriculture Uses

Significant progress has been achieved in the use of biostimulants in sustainable agricultural practices. These new products can improve plant growth, nutrient uptake, crop yield and quality, stress adaptation and soil fertility, while reducing agriculture's environmental footprint. Although it is an emerging market, the biostimulant sector is very promising, hence the increasing attention of the scientific community and agro-industry stakeholders in finding new sources of plant biostimulants. Recently, pro- and eucaryotic microalgae have gained prominence and can be exploited as biostimulants due to their ability to produce high-value-added metabolites. Several works revealed the potential of microalgae- and cyanobacteria-based biostimulants (MCBs) as plant growth promoters and stress alleviators, as well as encouraging results pointing out that their use can address current and future agricultural challenges. In contrast to macroalgae biostimulants, the targeted applications of MBs in agriculture are still in their earlier stages and their commercial implementation is constrained by the lack of research and cost of production. The purpose of this paper is to provide a comprehensive overview on the use of this promising new category of plant biostimulants in agriculture and to highlight the current knowledge on their application prospects. Based on the prevailing state of the art, we aimed to roadmap MCB formulations from microalgae and cyanobacteria strain selection, algal biomass production, extraction techniques and application type to product commercialization and farmer and consumer acceptance. Moreover, we provide examples of successful trials demonstrating the beneficial applications of microalgal biostimulants as well as point out bottlenecks and constraints regarding their successful commercialization and input in sustainable agricultural practices.

Enhancing sustainability through microalgae cultivation in urban wastewater for biostimulant production and nutrient recovery

Microalgae can produce biostimulants in form of phytohormones, which are compounds that, even if applied in low concentrations, can have stimulant effects on plants growth and can enhance their quality and their resistance to stress. Considering that microalgal biomass can grow recovering nutrients from wastewater, this circular approach allows to use residues for the production of high added value compounds (such as phytohormones) at low cost. The interest on biostimulants production from microalgae have recently raised. Scientists are focused on the direct application of these cellular extracts on plants, while the number of studies on the identification of bioactive molecules, such as phytohormones, is very scarce. Two cyanobacteria strains (Synechocystis sp. (SY) and Phormidium sp. (PH)) and a chlorophyte (Scenedesmus sp. (SC)) were cultured in laboratory-scale PBRs with a working volume of 2.5 L in secondary urban wastewater varying N:P ratio in the cultures to obtain the highest productivity. The variation of N:P ratio affects microalgae growth, and SY and PH presented higher productivities (73 and 48 mg L-1 d, respectively) under higher N:P ratio (> 22:1). Microalgal biomass was freeze-dried and phytohormones content was measured with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The three microalgae showed similar phytohormones profiles, being the auxin (indole-3-acetic acid, IAA) the most abundant (72 ng g-1DW in SY). Proteins were major macronutrient for all strains, reaching 48 %DW in PH culture. To optimize the biostimulants production, a balance between the production of such compounds, biomass productivity and nutrients removal should be taken into consideration. In this sense, SC was the most promising strain, showing the highest N and P removal rates (73 % and 59 %, respectively) while producing phytohormones.

Harnessing the potential of microalgae-bacteria interaction for eco-friendly wastewater treatment: A review on new strategies involving machine learning and artificial intelligence

In the pursuit of effective wastewater treatment and biomass generation, the symbiotic relationship between microalgae and bacteria emerges as a promising avenue. This analysis delves into recent advancements concerning the utilization of microalgae-bacteria consortia for wastewater treatment and biomass production. It examines multiple facets of this symbiosis, encompassing the judicious selection of suitable strains, optimal culture conditions, appropriate media, and operational parameters. Moreover, the exploration extends to contrasting closed and open bioreactor systems for fostering microalgae-bacteria consortia, elucidating the inherent merits and constraints of each methodology. Notably, the untapped potential of co-cultivation with diverse microorganisms, including yeast, fungi, and various microalgae species, to augment biomass output. In this context, artificial intelligence (AI) and machine learning (ML) stand out as transformative catalysts. By addressing intricate challenges in wastewater treatment and microalgae-bacteria symbiosis, AI and ML foster innovative technological solutions. These cutting-edge technologies play a pivotal role in optimizing wastewater treatment processes, enhancing biomass yield, and facilitating real-time monitoring. The synergistic integration of AI and ML instills a novel dimension, propelling the fields towards sustainable solutions. As AI and ML become integral tools in wastewater treatment and symbiotic microorganism cultivation, novel strategies emerge that harness their potential to overcome intricate challenges and revolutionize the domain.

Energy balance and life cycle assessments in producing microalgae biodiesel via a continuous microalgal-bacterial photobioreactor loaded with wastewater

Life cycle assessments of microalgal cultivation systems are often conducted to evaluate the sustainability and feasibility factors of the entire production chain. Unlike widely reported conventional microalgal cultivation systems, the present work adopted a microalgal-bacterial cultivation approach which was upscaled into a pilot-scale continuous photobioreactor for microalgal biomass production into biodiesel from wastewater resources. A multiple cradle-to-cradle system ranging from microalgal biomass-to-lipid-to-biodiesel was evaluated to provide insights into the energy demand of each processes making up the microalgae-to-biodiesel value chain system. Energy feasibility studies revealed positive NER values (4.95-8.38) for producing microalgal biomass but deficit values for microalgal-to-biodiesel (0.14-0.23), stemming from the high energy input requirements in the downstream processes for converting biomass into lipid and biodiesel accounting to 88-90% of the cumulative energy demand. Although the energy balance for microalgae-to-biodiesel is in the deficits, it is comparable with other reported biodiesel production case studies (0.12-0.40). Nevertheless, the approach to using microalgal-bacterial cultivation system has improved the overall energy efficiency especially in the upstream processes compared to conventional microalgal cultivation systems. Energy life cycle assessments with other microalgal based biofuel systems also proposed effective measures in increasing the energy feasibility either by utilizing the residual biomass and less energy demanding downstream extraction processes from microalgal biomass. The microalgal-bacterial cultivation system is anticipated to offer both environmental and economic prospects for upscaling by effectively exploiting the low-cost nutrients from wastewaters via bioconversion into valuable microalgal biomass and biodiesel.

A bibliometric analysis of published literature on membrane photobioreactors for wastewater treatment from 2000 to 2022

With the focus on limiting greenhouse gas emissions, microalgae-based technology is a promising approach for wastewater treatment, combining cost-effective operation, nutrient recovery, and assimilation of CO2. In addition, membrane technology supports process intensification and wastewater reclamation. Based on a bibliometric analysis, this paper evaluated the literature on membrane photobioreactors to highlight promising areas for future research. Specifically, efforts should be made on advancing knowledge of interactions between algae and bacteria, analysing different strategies for membrane fouling control and determining the conditions for the most cost-effective operation. The Scopus® database was used to select documents from 2000 to 2022. A set of 126 documents were found. China is the country with the highest number of publications, whereas the most productive researchers belong to the Universitat Politècnica de València (Spain). The analysis of 50 selected articles provides a summary of the main parameters investigated, that focus in increasing the biomass productivity and nutrient removal. In addition, microalgal-bacterial membrane photobioreactor seems to have the greatest commercialisation potential. S-curve fitting confirms that this technology is still in its growth stage.

Potential application of a newly isolated microalga Desmodesmus sp. GXU-A4 for recycling Molasses vinasse

The development of cost-effective and energy-efficient technologies for the stabilization of organic wastewater by microalgae has been essential and sought after. In the current study, GXU-A4 was isolated from an aerobic tank treating molasses vinasse (MV) and identified as Desmodesmus sp. based on its morphology, rbcL, and ITS sequences. It exhibited good growth with a high lipid content and chemical oxygen demand (COD) when grown using MV and the anaerobic digestate of MV (ADMV) as the growth medium. Three distinct COD concentrations for wastewater were established. Accordingly, GXU-A4 removed more than 90% of the COD from molasses vinasse (MV1, MV2, and MV3) with initial COD concentrations of 1193 mgL^-1, 2100 mgL^-1, and 3180 mgL^-1, respectively. MV1 attained the highest COD and color removal rates of 92.48% and 64.63%, respectively, and accumulated 47.32% DW (dry weight) of lipids and 32.62% DW of carbohydrates, respectively. Moreover, GXU-A4 grew rapidly in anaerobic digestate of MV (ADMV1, ADMV2, and ADMV3) with initial COD concentrations of 1433 mgL^-1, 2567 mgL^-1, and 3293 mgL^-1, respectively. Under ADMV3 conditions, the highest biomass reached 13.81 g L^-1 and accumulated 27.43% DW of lipids and 38.70% DW of carbohydrates, respectively. Meanwhile, the removal rates of NH₄-N and chroma in ADMV3 reached 91.10% and 47.89%, respectively, significantly reducing the concentration of ammonia nitrogen and color in ADMV. Thus, the results demonstrate that GXU-A4 has a high fouling tolerance, a rapid growth rate in MV and ADMV, the ability to achieve biomass accumulation and nutrient removal from wastewater, and a high potential for MV recycling.

The potential of wastewater grown microalgae for agricultural purposes: Contaminants of emerging concern, heavy metals and pathogens assessment

In the coming years, the use of microalgal biomass as agricultural biofertilizers has shown promising results. The use of wastewater as culture medium has resulted in the reduction of production costs, making microalgae-based fertilizers highly attractive for farmers. However, the occurrence of specific pollutants in wastewater, like pathogens, heavy metals and contaminants of emerging concern (CECs), such as pharmaceuticals and personal care products may pose a risk on human health. This study presents an holistic assessment of the production and use of microalgal biomass grown in municipal wastewater as biofertilizer in agriculture. Results showed that pathogens and heavy metals concentrations in the microalgal biomass were below the threshold established by the European regulation for fertilizing products, except for cadmium. Regarding CECs, 25 out of 29 compounds were found in wastewater. However, only three of them (hydrocinnamic acid, caffeine, and bisphenol A) were found in the microalgae biomass used as biofertilizer. Agronomic tests were performed for lettuce growth in greenhouse. Four treatments were studied, comparing the use of microalgae biofertilizer with a conventional mineral fertilizer, and also a combination of both of them. Results suggested that microalgae can help reducing the mineral nitrogen dose, since similar fresh shoot weights were obtained in the plants grown with the different assessed fertilizers. Lettuce samples revealed the presence of cadmium and CECs in all the treatments including both negative and positive controls, which suggests that their presence was not linked to the microalgae biomass. On the whole, this study revealed that wastewater grown microalgae can be used for agricultural purposes reducing mineral N need and guaranteeing health safety of the crops.

Base type determines the effects of nucleoside monophosphates on microalgae-bacteria symbiotic systems

Microalgae are promising sources of clean energy. Bioflocculation by cocultured bacteria is an effective way to harvest microalgae. As a key foundation for microorganisms, phosphorus is theoretically effective in shaping microalgae production and flocculation. In this study, the impacts of 23 nucleoside monophosphates on Auxenochlorella pyrenoidosa growth, lipid synthesis, and self-settlement and on the symbiotic bacterial system were investigated. Adenosine monophosphate was the most effective in enhancing microalgae development (2.14-3.16 × 10⁸ cells/mL) and lipid production (average 10.48%) and resulted in a low settling velocity. Samples were divided into two groups, purine and pyrimidine feeding, according to a random forest analysis (OOB = 0%, p < 0.001). Purine feeding resulted in the highest soluble extracellular protein and polysaccharide secretion (p < 0.01). KEGG ortholog count prediction of functional genes related to biofilm formation was conducted using PICRUSt2, and significant upregulation (FC ≥ 1.77, p < 0.05) of the extracellular polymeric substance formation functional group was observed in the adenosine and guanosine treatments. The symbiotic bacterial community structure differed substantially between purine- and pyrimidine-feeding systems. In summary, these results indicated that the effect of nucleoside monophosphates on the microalgae-bacteria system is determined by the base type (purine or pyrimidine) rather than the molecular structure (cyclic or noncyclic).

Nitrogen removal from wastewater by an immobilized consortium of microalgae-bacteria in hybrid hydrogels

The high content of nitrogen in wastewater brings some operational, technical, and economical issues in conventional technologies. The aim of this study was to evaluate the nitrogen removal by hybrid hydrogels containing consortium microalgae-nitrifying bacteria in the presence of activated carbon (AC) used as an adsorbent of inhibitory substances. Hybrid hydrogels were synthesized from polyvinyl alcohol (PVA), sodium alginate (SA), biomass (microalgae-nitrifying bacteria), and AC. The hybrid hydrogels were evaluated based on the change in ammonium (NH₄), nitrate (NO₃), and chemical demand of oxygen (COD) concentrations, nitrification rate, and other parameters during 72 h. Results indicated that NH₄ removal was more effective for hydrogels without AC than with AC, without significant differences regarding consortium biomass concentration (5 or 16%), presenting final concentrations of 3.13 and 3.75 mg NH₄/L for hydrogels with 5 and 16% of the biomass, respectively. Regarding NO₃ production, hydrogels without AC reached concentrations of 25.9 and 39.77 mg NO₃/L for 5 and 16% of the biomass, respectively, while treatments with AC ended with 2.17 and 1.37 mg NO₃/L. This confirms that hydrogels can carry out the nitrification process and do not need AC to remove potential inhibitors. The best performance was observed for the hydrogel with 5% of biomass without AC with a nitrification rate of 0.43 mg N/g TSS·h.

Sustainability and carbon neutralization trends in microalgae bioenergy production from wastewater treatment: A review

Reducing CO2 emissions using biomass is gaining popularity as an environmentally friendly strategy. Due to high growth rates, low production costs, and ability to withstand harsh conditions, microalgae have become quite popular. Microalgae may also undertake photosynthesis, converting CO2 and solar energy into sugar before becoming biomass, making them an excellent source of renewable and promising biofuels. CO2 sequestration and biofixation was utilized to compare the synthesis of biodiesel as a third-generation biofuel from various types of wastewater was also used as a source for the algal cultivation. This review article focuses on recent developments, research discoveries in the field of microalgal CO2 capture modification and the optimization of conversion efficiency. This review is intended to serve as a helpful and reference for the use of wastewater treatment with microalgae to collect CO2. The overarching objective of this study is to assist wastewater treatment systems in achieving carbon neutrality.

Enhanced wastewater treatment performance by understanding the interaction between algae and bacteria based on quorum sensing

In order to further obtain sustainable wastewater treatment technology, in-depth analysis based on algal-bacterial symbiosis, quorum sensing signal molecules and algal-bacterial relationship will lay the foundation for the synergistic algal-bacterial wastewater treatment process. The methods of enhancing algae and bacteria wastewater treatment technology were systematically explored, including promoting symbiosis, reducing algicidal behavior, eliminating the interference of quorum sensing inhibitor, and developing algae and bacteria granular sludge. These findings can provide guidance for sustainable economic and environmental development, and facilitate carbon emissions reduction by using algae and bacteria synergistic wastewater treatment technology in further attempts. The future work should be carried out in the following four aspects: (1) Screening of dominant microalgae and bacteria; (2) Coordination of stable (emerging) contaminants removal; (3) Utilization of algae to produce fertilizers and feed (additives), and (4) Constructing recombinant algae and bacteria for reducing carbon emissions and obtaining high value-added products.