Flocculation characteristics of anaerobic sludge driven-extracellular polymeric substance (EPS) extracted by different methods on microalgae harvesting for lipid utilization

Microalgal extracellular polymeric substances (EPS) and their roles in cultivation, biomass harvesting, and bioproducts extraction

Microalgae extracellular polymeric substances (EPS) are complex high-molecular-weight polymers and the physicochemical properties of EPS strongly affect the core features of microalgae cultivation and resource utilization. Revealing the key roles of EPS in microalgae life-cycle processes in an interesting and novelty topic to achieve energy-efficient practical application of microalgae. This review found that EPS showed positive effect in non-gas uptake, extracellular electron transfer, toxicity resistance and heterotrophic symbiosis, but negative impact in gas transfer and light utilization during microalgae cultivation. For biomass harvesting, EPS favored biomass flocculation and large-size cell self-flocculation, but unfavored small size microalgae self-flocculation, membrane filtration, charge neutralization and biomass dewatering. During bioproducts extraction, EPS exhibited positive impact in extractant uptake, but the opposite effect in cellular membrane permeability and cell rupture. Future research on microalgal EPS were also identified, which offer suggestions for comprehensive understanding of microalgal EPS roles in various scenarios.

Microalgae harvesting for wastewater treatment and resources recovery: A review

Microalgae-based wastewater treatment has been conceived to obtain reclaimed water and produce microalgal biomass for bio-based products and biofuels generation. However, microalgal biomass harvesting is challenging and expensive, hence one of the main bottlenecks for full-scale implementation. Finding an integrated approach that covers concepts of engineering, green chemistry and the application of microbial anabolism driven towards the harvesting processes, is mandatory for the widespread establishment of full-scale microalgae wastewater treatment plants. By using nature-based substances and applying concepts of chemical functionalization in already established harvesting methods, the costs of harvesting processes could be reduced while preventing microalgae biomass contamination. Moreover, microalgae produced during wastewater treatment have unique culture characteristics, such as the consortia, which are primarily composed of microalgae and bacteria, that should be accounted for prior to downstream processing. The aim of this review is to examine recent advances in microalgal biomass harvesting and recovery in wastewater treatment systems, considering the impact of consortia variability. The costs of available harvesting technologies, such as coagulation/flocculation, coupled to sedimentation and differential air flotation, are provided. Additionally, promising technologies are discussed, including autoflocculation, bioflocculation, new filtration materials, nanotechnology, microfluidic and magnetic methods.

Sludge biodrying coupled with photocatalysis improves the degradation of extracellular polymeric substances

The efficiency of sludge dewatering is limited by extracellular polymeric substances (EPS) during biodrying. This study investigated the effect of photocatalysis-mediated EPS degradation on sludge dewatering performance during the sludge biodrying process. The photocatalysis of municipal sludge was first carried out to choose a cost-efficient catalyst. Then sludge biodrying tests were performed using TiO2-coated amendment (TCA) and uncoated amendment (TUCA) as the control. Municipal sludge photocatalysis results showed that using TiO2 could efficiently degrade carbohydrates and proteins in the EPS within 60 min. After 20-day biodrying, photocatalysis significantly promoted a reduction in the moisture content and EPS by 17.64% and 6.88%, respectively. The surface-enhanced Raman scattering (SERS) intensities of the C-C-O symmetric stretching vibration peak of D-lactose and the C-S stretching vibration peak of cysteine were significantly decreased by approximately 33.19% and 44.76%, respectively, indicating that photocatalysis indeed promoted the reduction of polysaccharides and cysteine in the EPS, especially after the thermophilic phase. The hydrophilic amino acid content decreased by 23.02%, verifying that photocatalysis could improve EPS hydrophobicity. Consequently, municipal sludge biodrying coupled with photocatalysis promotes sludge EPS degradation and enhances sludge dewaterability, improving the efficiency of sludge biodrying.

Utilization of dark fermentation effluent for algal cultivation in a modified airlift photobioreactor for biomass and biocrude production

Developing an efficient photobioreactor (PBR) and reducing freshwater dependence are among the significant challenges for generating 3rd generation biomass feedstock. Addressing these, the present study focused on developing a modified airlift (MoAL) PBR. Its performance was further evaluated and compared with the traditional airlift PBR by cultivating microalgae in dark fermentation spent wash. Lower mixing time and higher interfacial mass transfer coefficient was observed in the MoAL PBR having a perforated draft tube. Experimentally, the MoAL exhibited the maximum biomass concentration of 3.18 g L^-1, which was 30% higher than that of the conventional airlift PBR. The semi-continuous operation of the MoAL (with water recycling) achieved the maximum biomass productivity of 0.83 g L^-1 d^-1, two folds superior to that of batch culture. The comprehensive biomass characterization (proximate, ultimate, and thermochemical) further confirmed its potential for bioenergy application. Considering that, hydrothermal liquefaction of the biomass resulted in a maximum biocrude yield of 31% w/w with a higher heating value (HHV) of 36.6 MJ kg^-1. In addition, the biocrude comprised 66.6% w/w lighter fraction (<343 °C), including 21.5% w/w of heavy naphtha, 20.5% w/w of kerosene, and 24.6% w/w of diesel. The results can help develop sustainable technology for simultaneous wastewater remediation and biocrude production.

Microalgal Feedstock for Biofuel Production: Recent Advances, Challenges, and Future Perspective

Globally, nations are trying to address environmental issues such as global warming and climate change, along with the burden of declining fossil fuel reserves. Furthermore, countries aim to reach zero carbon emissions within the existing and rising global energy crisis. Therefore, bio-based alternative sustainable feedstocks are being explored for producing bioenergy. One such renewable energy resource is microalgae; these are photosynthetic microorganisms that grow on non-arable land, in extreme climatic conditions, and have the ability to thrive even in sea and wastewater. Microalgae have high photosynthetic efficiencies and biomass productivity compared to other terrestrial plants. Whole microalgae biomass or their extracted metabolites can be converted to various biofuels such as bioethanol, biodiesel, biocrude oil, pyrolytic bio-oil, biomethane, biohydrogen, and bio jet fuel. However, several challenges still exist before faster and broader commercial application of microalgae as a sustainable bioenergy feedstock for biofuel production. Selection of appropriate microalgal strains, development of biomass pre-concentrating techniques, and utilization of wet microalgal biomass for biofuel production, coupled with an integrated biorefinery approach for producing value-added products, could improve the environmental sustainability and economic viability of microalgal biofuel. This article will review the current status of research on microalgal biofuels and their future perspective.

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).

Extracting extracellular polymeric substances from fungi in contrasts: from quantity to quality

Many fungi are able to produce extracellular polymeric substances (EPS) for environmental, food, and industrial applications. This study evaluated the extraction (in vivo) of EPS from Rhodotorula mucilaginosa, a typical yeast with abundant EPS. Three extracting methods were set, i.e., heating, addition of NaCl during heating, and cation exchange resin (CER). The abundance of extracted proteins and polysaccharides showed evident contrasts (elevated to ~ 600 and 1700 mg/L, respectively) after heating at 70 °C in water. Although the higher temperature will increase the extracted abundance of EPS, the leakage of DNA would be enhanced due to cell rupture. The addition of NaCl further promoted the efficiency of extraction, either for proteins (from ~ 550 to ~ 650 mg/L) or polysaccharides (from ~ 1700 to ~ 2010 mg/L). Moreover, the biochemical results showed that the extracted abundance of EPS via heating was dramatically higher than that via CER. Additionally, DNA leakage in the CER treatment (2.0 g/g DW) was significantly higher (up to > 6 mg/L) than that under heating at 70 °C (< 2 mg/L). Furthermore, the three-dimensional excitation-emission matrix spectra showed two characteristic peaks of emission/excitation wavelength at 280/300 and 280/350, suggesting the relative high diversity of organic matters in EPS after heating treatments. Finally, a fluctuation of polysaccharide abundance in EPS at 500-1500 mg/L Pb²⁺ level was elucidated by the extraction based on heating treatment. This study hence confirmed that the heating method might be recommended for extraction of EPS from fungi in vivo KEY POINTS: • 3D-EEM results indicated that heating could extract more EPS compared with CER. • Heating treatments showed lower DNA leakage from fungi than CER treatments. • Addition of NaCl promoted the detachment of EPS from fungal cells in vivo.

The enhanced dewaterability of sludge by a starch-based flocculant combined with attapulgite

Coagulation/flocculation is one of the most widely used and cost-effective pretreatment methods for improving the dewaterability of sludge. In this study, a cationic modified starch-based flocculant (St-CTA) in conjunction with a popular clay, attapulgite (ATP), was used for the conditioning of waste-activated sludge. The dewatering properties, including the filter cake moisture content, filtration specific resistance, capillary suction time, filtration rate and compressibility coefficient, were measured and compared by varying the doses of St-CTA and ATP. By combination of the apparent dewatering performance and the changes in the contents and distributions of the extracellular polymeric substance (EPS) fractions and components, sludge flocs, and microstructures of sludge cakes, the dewatering mechanisms were discussed in detail. St-CTA in conjunction with ATP can exhibit an enhanced dewaterability of sludge and the water content in final sludge cake can be stably reduced below 80% owing to the synergistic effects of St-CTA and ATP. In addition to the efficient charge neutralization of St-CTA, ATP not only acts as a skeleton builder in the sludge dewatering process which makes the sludge flocs more compact and improves the filterability and permeability, but also tightly interacts with the proteins in EPS of the sludge which reduces the protein content and further enhances the dewatering effect. This study provides an economical, green, and effective way to further improve the dewaterability of sludge.

Time-Resolved Kinetic Measurement of Microalgae Agglomeration for Screening of Polysaccharides-Based Coagulants/Flocculants

Time-resolved monitoring of microalgae agglomeration facilitates screening of coagulants/flocculants (CFs) from numerous biopolymer candidates. Herein, a filtering-flowing analysis (FFA) apparatus was developed in which dispersed microalgal cells were separated from coagulates and flocs formed by CFs and pumped into spectrophotometer for real-time quantification. Polysaccharides-based CFs for Microcystis aeruginosa and several other microalgae were tested. Cationic hydroxyethyl cellulose (CHEC), chitosan quaternary ammonium (CQA) and cationic guar gum (CGG) all triggered coagulation obeying a pseudo-second-order model. Maximal coagulation efficiencies were achieved at their respective critical dosages, i.e., 0.086 g/g_M.a. CHEC, 0.022 g/g_M.a. CQA, and 0.216 g/g_M.a. CGG. Although not active independently, bacterial exopolysaccharides (BEPS) aided coagulation of M. aeruginosa and allowed near 100% flocculation efficiency when 0.115 g/g_M.a. CQA and 1.44 g/g_M.a. xanthan were applied simultaneously. The apparatus is applicable to other microalgae species including Spirulina platensis, S. maxima, Chlorella vulgaris and Isochrysis galbana. Bio-based CFs sorted out using this apparatus could help develop cleaner processes for both remediation of harmful cyanobacterial blooms and microalgae-based biorefineries.

Air-liquid interface cultivation of Navicula incerta using hollow fiber membranes

Microalgae cultivation in open ponds requires a large footprint, while most photobioreactors need improvement in the ratio of surface to volume and energy consumption. In this study, polyethersulfone (PES) and poly(vinylidene fluoride) (PVDF) hollow fiber membranes with a large surface area were rearranged into open-ended and dead-ended configurations to improve the air-liquid interface cultivation of Navicula incerta. N. incerta were successfully grown on the porous membrane surface with the nutrients circulating inside the lumen. Fourier-transform infrared spectra showed the accumulation of polysaccharides, proteins and humic acids. Hydrophilic polysaccharides reduced water contact angles on PES and PVDF membranes to 37.2 ± 2.6° and 55.7 ± 3.3°, respectively. However, the porosity of PES (80.1 ± 1.1%) and PVDF (61.3 ± 4.5%) membranes were not significantly affected even after cultivation and harvesting of N. incerta. Scanning electron images further confirmed that N. incerta, cell debris and extracellular organic matter accumulated on the membrane. With large pores and a hydrophobic surface, PVDF hollow fiber membranes offered a greater improvement in N. incerta cell growth rate compared to PES hollow fiber membranes despite using different configurations. In the dead-ended configuration, they even attained the greatest improvement in N. incerta growth rate, up to 54.0%. However, PES hollow fiber membranes only achieved improvement in harvesting efficiency within the range of 18.7-38.0% due to weak cell adhesion. PVDF hollow fiber membranes significantly promoted the growth of microalgae N. incerta through the air-liquid interface system, leading to potential applications in wastewater treatment.

The Potential of Marine Microalgae for the Production of Food, Feed, and Fuel (3F)

Whole-cell microalgae biomass and their specific metabolites are excellent sources of renewable and alternative feedstock for various products. In most cases, the content and quality of whole-cell biomass or specific microalgal metabolites could be produced by both fresh and marine microalgae strains. However, a large water footprint for freshwater microalgae strain is a big concern, especially if the biomass is intended for non-food applications. Therefore, if any marine microalgae could produce biomass of desired quality, it would have a competitive edge over freshwater microalgae. Apart from biofuels, recently, microalgal biomass has gained considerable attention as food ingredients for both humans and animals and feedstock for different bulk chemicals. In this regard, several technologies are being developed to utilize marine microalgae in the production of food, feed, and biofuels. Nevertheless, the production of suitable and cheap biomass feedstock using marine microalgae has faced several challenges associated with cultivation and downstream processing. This review will explore the potential pathways, associated challenges, and future directions of developing marine microalgae biomass-based food, feed, and fuels (3F).

Insights into the microalgae-bacteria consortia treating swine wastewater: Symbiotic mechanism and resistance genes analysis

This study investigated the effects of microalgae-bacteria consortia (MBC) (Chlorella pyrenoidosa-activated sludge (AS)) treating swine wastewater with low C/N ratios. After co-culture, the removal rates of NH₄⁺-N and PO₄^3--P increased by 53.84% and 43.52%. Furthermore, the sulfamethoxazole (SMX) degradation rates in MBC were slightly higher than in the activated sludge process. Interestingly, the absolute abundance of antibiotic resistance genes (ARGs) in effluent from MBC is relatively less than in the AS process. C. pyrenoidosa has a negative zeta potential that allows bacteria to adhere to its surface. The concentrations of carbohydrates and proteins in extracellular polymeric substance (EPS) of MBC dramatically increased compared with the AS process. At the phylum level, Proteobacteria, Bacteroidota, and Cyanobacteria were the main bacteria, while Ascomycota and Basidiomycota were the primary fungi in MBC. Overall, those findings lead to a better understanding of the swine wastewater containing antibiotic treatment by MBC.

Responses of simultaneous anammox and denitrification (SAD) process to nitrogen loading variation: Start-up, performance, sludge morphology and microbial community dynamics

The effects of loading variation on the efficiency, EPS, sludge morphology and microbial population of simultaneous anammox and denitrification (SAD) were thoroughly investigated with the low-abundance SAD sludge. Results indicated that the first stage lasted the longest (33d), and the average removal rate of TN can be maintained above 95%. The specific anammox activity (SAA), specific denitrification activity and PN/PS continued to increase, but the excessive loading caused the effluent to deteriorate rapidly, and SAA and PN/PS also decreased slightly, but it could be recovered quickly. The contribution rate of anammox and denitrification to N removal reached 87.6% and 12.4% eventually, respectively. The abundance of AnAOB was 10.68%-18.01%, 9.01%-15.54%, 5.74%-12.88% in the upper, middle and lower layers, respectively. Candidatus Kuenenia was always the dominant AnAOB, especially after high loading inhibition. The abundance of denitrifying bacteria (mainly Bacillus, Comamonas and Denitratisoma) gradually became the highest.

Development of a novel partial nitrification, fermentation-based double denitrification bioprocess (PN-F-Double/DN) to simultaneous treatment of mature landfill leachate and waste activated sludge

Introducing fermentation technology into sewage treatment is a sustainable development concept, but future application still faces many challenges. A novel partial nitrification, fermentation-based double denitrification bioprocess (PN-F-Double/DN) was achieved in three separated SBR type reactors, simultaneously treating high ammonia (1766.6 mg/L) mature landfill leachate and external waste activated sludge (WAS, MLSS = 20.6 g/L). Firstly, NH₄⁺-N was oxidized to NO₂^--N in partial nitrification reactor (PN-SBR), with nitrite accumulation ratio (NAR) of 96.5%. Next, the PN-SBR effluent (NO₂^--N = 1529.8 mg/L) coupled with the WAS were introduced to an anoxic reactor for integrated fermentation-denitrification (IFD-SBR). The occurrence of fermentation was mainly attributed to free nitrous acid (FNA, nitrite protonate form) promoting the splitting decomposition of sludge spatial configuration and interfacial forces. The released volatile fatty acids (VFAs) were utilized in situ during the denitrification process (NO₂^--N→N₂), obtaining 0.6 kg/m³•d nitrogen removal rate and 3.3 kg/m³•d sludge reduction rate. Finally, undesirable fermentation by-products from IFD-SBR (NH₄⁺-N = 119.2 mg/L) were further removed in the endogenous post-denitrification reactor (EPD-SBR) through operational strategy of anaerobic/aerobic/anoxic by residual VFAs as the carbon source. In the EPD-SBR, Defluviicoccus (0.9%) and Candidatus Competibacter (5.8%) dominated carbon source storage and nitrogen removal, acting as a typical denitrifying glycogen-accumulating organism (DGAO), with an intracellular carbon storage efficiency of 83.1% and nitrogen removal contribution of 93.7%. After 200 days of operation, the PN-F-Double/DN process provided effluent containing, on average, 1.86 mg/L NH₄⁺-N and 5.5 mg/L NO_x^--N, with 98.5% TN removal. Compared with traditional bioprocesses, PN-F-Double/DN allowed up to 25% saving in aeration energy consumption, 100% decrease in carbon source demand, and achieve 46.1% external WAS reduction.

Insights into bioflocculation of filamentous cyanobacteria, microalgae and their mixture for a low-cost biomass harvesting system

Cyanobacteria and microalgae are considered as interesting feedstocks for either the production of high value bio-based compounds and biofuels or wastewater treatment. Nevertheless, the high costs of production, mainly due to the harvesting process, hamper a wide commercialization of industrial cyanobacteria and microalgae based products. Recent studies have found in autoflocculation and bioflocculation promising spontaneous processes for a low-cost and environmentally sustainable cyanobacteria and microalgae biomass harvesting process. In the present work, bioflocculation process has been studied for three different inocula: filamentous cyanobacteria, microalgae and their mixture. Their cultivation has been conducted in batch mode using two different cultivation media: synthetic aqueous solution and urban wastewater. The removal of nutrients and flocculation process performance were monitored during the entire cultivation time. Results have proved that bioflocculation and sedimentation processes occur efficiently for filamentous cyanobacteria cultivated in synthetic aqueous solution, whereas such processes are less efficient in urban wastewater due to the specific characteristics of this medium that prevent bioflocculation to occur. Besides different efficiencies associated to cultivation media, this work highlighted that bioflocculation of sole microalgae is not as effective as when they are cultivated together with filamentous cyanobacteria.