Methods coagulation/flocculation and flocculation with ballast agent for effective harvesting of microalgae

Use of biochar and a post-coagulation effluent as an adsorbent of malachite green, beneficial bacteria carrier, and seedling substrate for plants belonging to the poaceae family

Wastewater treatment plants produce solid and semi-solid sludge, which treatment minimises secondary environmental pollution because of wastewater treatment and obtaining new bioproducts. For this reason, in this paper, the co-pyrolysis of biogenic biomasses recovered from a biological reactor with immobilised fungal and bacterial biomass and a tertiary reactor with Chlorella sp. used for dye-contaminated wastewater treatment was carried out. Biogenic biomasses mixed with pine bark allowed the production and characterisation of two types of biochar. The raw material and biochar were on the "in vitro" germination of Lolium sp. seeds, followed by adsorption studies for malachite green (MG) dye using the raw material and the biochar. Results showed that using 60 mg L-1 of a cationic coagulant at pH 6.5 allowed for the recovery of more than 90% of the microalgae after 50 min of processing. Two biochar resulted: BC300, at pH 5.08 ± 0.08 and BC500, at pH 6.78 ± 0.01. The raw material and both biochars were co-inoculated with growth-promoting bacteria; their viabilities ranged from 1.7 × 106 ± 1.0 × 101 to 7.5 × 108 ± 6.0 × 102 CFU g-1 for total heterotrophic, nitrogen-fixing and phosphate-solubilising bacteria. Re-use tests on Lolium sp. seed germination showed that with the post-coagulation effluent, the germination was 100%, while with the biochar, with and without beneficial bacteria, the germination was 98 and 99%, respectively. Finally, BC500 adsorbed the highest percentage of malachite green at pH 4.0, obtaining qecal values of 0.5249 mg g-1 (R2: 0.9875) with the pseudo-second-order model.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03766-x.

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.

Sequential modelling for carbohydrate and bioethanol production from Chlorella saccharophila CCALA 258: a complementary experimental and theoretical approach for microalgal bioethanol production

Bioethanol production from microalgal biomass is an attractive concept, and theoretical methods by which bioenergy can be produced indicate saving in both time and efficiency. The aim of the present study was to investigate the efficiencies of carbohydrate and bioethanol production by Chlorella saccharophila CCALA 258 using experimental, semiempirical, and theoretical methods, such as response surface methods (RSMs) and an artificial neural network (ANN) through sequential modeling. In addition, the interactive response surface modeling for determining the optimum conditions for the variables was assessed. The results indicated that the maximum bioethanol concentration was 11.20 g/L using the RSM model and 11.17 g/L using the ANN model under optimum conditions of 6% (v/v %) substrate and 4% (v/v %) inoculum at 96-h fermentation, pH 6, and 40 °C. In addition, the value of the experimental data for carbohydrate concentration was 0.2510 g/g biomass at ANN with the maximums of 50% (v/v) wastewater concentration, 4% (m/m) hydrogen peroxide concentration, and 6000 U/mL enzyme activity. Finally, although the RSM model was more effective than the ANN model for predicting bioethanol concentration, the ANN model yielded more precise values than the RSM model for carbohydrate concentration.

A critical review on different harvesting techniques for algal based biodiesel production

The fuels retrieved from renewable sources which are usually employed as both carbon and energy sources are termed as neutral based biofuels. The most promising feedstock from renewable sources with great potentiality in contributing to the inclining energy demand is microalgae. These microalgae can be harnessed readily in terms of obtaining qualitative biodiesel with greater energy consumption under limited operational cost. The process of harvesting or dewatering microalgae could be carried under single or sequential combinations of operations. The major drawback of harvesting such as huge operational cost could be lowered by increasing the level of automation than cost of investments. The present review concentrates and explores on the techno-economic analysis of the microalgal harvesting and dewatering processes on a large scale. Along with these advanced techniques enclosing the utilization of nanoparticles for harvesting has also been explored. And it also adds with the impacts of concerning facts on energy consumption, processing cost and recovery of resources during harvesting.

Efficient Bioflocculation of Chlorella vulgaris with a Chitosan and Walnut Protein Extract

Simple Summary

With the increase in population size, global climate changes, and the improvement of living standards, the fossil fuel resources may run out in the future. Microalgae have been considered the next generation of sustainable and renewable feedstock to produce biofuel and a large spectrum of high-value products, such as healthy oils, carotenoids, and proteins. Unlike terrestrial plants, the production of added-value chemicals from microalgal species is not seasonal; they can be grown under climate-independent conditions in bioreactors; can use wastewater as a source of nutrients, contributing to wastewater treatment; and can convert CO₂ into organic compounds more efficiently. However, the utilization of microalgal biomass is heavily dependent on microalgal biomass harvesting and concentration technology. Flocculation represents a relatively low-cost and efficient approach for the harvesting of microalgal biomass at a large scale. However, in traditional flocculation, most of the chemical flocculants covalently bind to the microalgal surfaces, contaminating the final product, which significantly limits their application. This study aims to develop an efficient and convenient bioflocculation technique to harvest microalgae.

Abstract

Bioflocculation represents an attractive technology for harvesting microalgae with the potential additive effect of flocculants on the production of added-value chemicals. Chitosan, as a cationic polyelectrolyte, is widely used as a non-toxic, biodegradable bioflocculant for many algal species. The high cost of chitosan makes its large-scale application economically challenging, which triggered research on reducing its amount using co-flocculation with other components. In our study, chitosan alone at a concentration 10 mg/L showed up to an 89% flocculation efficiency for Chlorella vulgaris. Walnut protein extract (WPE) alone showed a modest level (up to 40%) of flocculation efficiency. The presence of WPE increased chitosan’s flocculation efficiency up to 98% at a reduced concentration of chitosan (6 mg/L). Assessment of co-flocculation efficiency at a broad region of pH showed the maximum harvesting efficiency at a neutral pH. Fourier transform infrared spectroscopy, floc size analysis, and microscopy suggested that the dual flocculation with chitosan and walnut protein is a result of the chemical interaction between the components that form a web-like structure, enhancing the bridging and sweeping ability of chitosan. Co-flocculation of chitosan with walnut protein extract, a low-value leftover from walnut oil production, represents an efficient and relatively cheap system for microalgal harvesting.

Application progress of enhanced coagulation in water treatment

Water industries worldwide consider coagulation/flocculation to be one of the major treatment methods for improving the overall efficiency and cost effectiveness of water and wastewater treatment. Enhancing the coagulation process is currently a popular research topic. In this review article, the latest developments in enhanced coagulation are summarized. In addition, the mechanisms of enhanced coagulation and the effect of process parameters on processing efficiency are discussed from the perspective of ballast-enhanced coagulation, preoxidation, ultrasound, and composite coagulants. Finally, improvements and new directions for enhanced coagulation are proposed.

Genetical Surface Display of Silicatein on Yarrowia lipolytica Confers Living and Renewable Biosilica-Yeast Hybrid Materials

In this work, a biological engineering-based biosilica-yeast hybrid material was developed. It was obtained by the aggregation of Yarrowia lipolytica through biosilicification catalyzed using genetically displayed silicatein on its cell surface. With orthosilicate or seawater as the substrate, the silicatein-displayed yeast could aggregate into flocs with a flocculation efficiency of nearly 100%. The resulting floc was found to be a sheetlike biosilica-yeast hybrid material formed by the biosilica-mediated immobilization of yeast cells via cross-linking and embedding, turning the original hydrophilicity of yeast cells into hydrophobicity. In addition, this material was characterized to be porous with an average pore diameter of approximately 10 μm and porosity of over 70%. Because of these properties, this hybrid material could achieve enhanced removal efficiencies for chromium ions and n-hexadecane, which were both above 99%, as compared to the free cells of Y. lipolytica in aqueous environments. Importantly, this hybrid material could be recultivated to generate new batches of yeast cells that maintain parallel properties to the first generation so that the same hybrid material could be reproduced with unchanged highly efficient removal of chromium and n-hexadecane to those of the first generation, demonstrating that this biosilica-yeast hybrid material was living and renewable. This work presented a novel way of harnessing silicatein and Y. lipolytica to achieve biological synthesis of a living inorganic-organic hybrid material that has potential to be applied in water treatment.

Natural plant extracts as an economical and ecofriendly alternative for harvesting microalgae

The study investigated the ability of plant based natural coagulants from Azadirachta indica; Ficus indica; Moringa oleifera; Citrus sinensis; Punica granatum and Musa acuminata to harvest the microalgal biomass. Influence of eluent type (water and NaCl) and concentration (1-5 N) on coagulant extraction; coagulant dosage (1-5 g) and volume (20-100 ml); pH (6-12) and algal concentration (0.1-1 g l^-1) on harvesting were analyzed. The results obtained were compared with alum and chitosan. FTIR and biochemical analysis confirmed the presence of bioactive compounds to aid coagulation. Biomass removal efficiency of 75.50% was obtained with M. oleifera extracts (8 mg ml^-1) at pH 7.5-7.8, within 100 min. The harvesting efficiency increased to 95.76% when 4 mg ml^-1M. oleifera extracts was combined with 0.75 mg ml^-1 chitosan. The life cycle and cost analysis acknowledged the eco-friendly coagulants as strong alternative for conventional coagulants used in microalgal harvesting, thereby improvising the overall bioprocess.

Enhanced removal of bio-refractory dissolved organic matter from cassava distillery wastewater by powdered activated carbon-ballasted coagulation: Detailed study of separation characteristics and mechanisms

Efficient removal of bio-refractory dissolved organic matter (DOM) and colorants is essential for discharging or reusing the distillery wastewater. An important part of recalcitrant DOM still exists in the effluent of regular coagulation though the ferric coagulant has been found to be effective in decoloration. The present work adopted powdered activated carbon (PAC) as ballasting agent to achieve robust separation effect and efficiency of bio-refractory DOM from the bio-chemically treated cassava distillery wastewater (BTDWW). More than 90% of DOC could be removed at the PAC and Fe(III)-coagulant dosage of 1.40 g/L and 0.84 g/L as Fe when the BTDWW was neutral. PAC should be dosed before coagulant in order to mix well with the DOM in the BTDWW. The analyses of DOM in effluent reveal that PAC facilitated the removal of lignin breakdown products which could not be well eliminated by regular coagulation; the removal of DOM with MW < 5 kDa was mostly enhanced. The characteristics of flocs demonstrate that PAC reinforced the interaction between Fe(III) species and DOM by providing more reaction sites. The sedimentation could be completed within the initial 5 min, and the highest settling velocity was almost 8 times higher than that of the only Fe(III)-involved flocs. The large size and favorable robustness of PAC-involved flocs enabled decent sedimentation even though their stretched structure might not be desirable in regular coagulation. The PAC-ballasted coagulation is recommended as tertiary treatment of BTDWW considering its high efficiency and sound economic feasibility.

Using photocatalyst powder to enhance the coagulation and sedimentation of cyanobacterial cells and enable the sludge to be self-purified under visible light

Removing harmful cyanobacteria intact by coagulation can prevent cell lysis and toxin release, which provides many benefits for drinking water production, including reduction of the burden on subsequent processes and guaranteeing the water quality. But the electronegativity and buoyancy of cyanobacterial cells make them settle slowly and their accumulation and concentration in flocs would still have severe adverse effects. In this study, we introduced a photocatalyst powder to act as a ballasting agent in the coagulation process and to degrade the cells and cyanotoxins during sludge storage. Results showed that adding N-TiO₂ would decrease the coagulant dose to half of the conventional value, and also allow Microcystis aeruginosa cells to completely settle within 10 min. During sludge storage, the algal cells, cyanotoxins and other organics in sludge would be degraded to safe levels after 32 h' visible-light irradiation. Meanwhile, the N-TiO₂, water and some of the coagulant in purified sludge will be directly and safely reused. Thus, this is an environmentally friendly and cost-effective technology which incorporates photocatalyst in algal flocs to enhance coagulation and sedimentation and to enable the sludge produced to be self-purified under visible-light.

Biodiesels from microbial oils: Opportunity and challenges

Although biodiesel has been extensively explored as an important renewable energy source, the raw materials-associated cost poses a serious challenge on its large-scale commercial production. The first and second generations of biodiesel are mainly produced from usable raw materials, e.g. edible oils, crops etc. Such a situation inevitably imposes higher demands on land and water usage, which in turn compromise future food and water supply. Obviously, there is an urgent need to explore alternative feedstock, e.g. microbial oils which can be produced by many types of microorganisms including microalgae, fungi and bacteria with the advantages of small footprint, high lipid content and efficient uptake of carbon dioxide. Therefore, this review offers a comprehensive picture of microbial oil-based technology for biodiesel production. The perspectives and directions forward are also outlined for future biodiesel production and commercialization.

Industrial potential of carotenoid pigments from microalgae: Current trends and future prospects

Microalgae are rich source of various bioactive molecules such as carotenoids, lipids, fatty acids, hydrocarbons, proteins, carbohydrates, amino acids, etc. and in recent Years carotenoids from algae gained commercial recognition in the global market for food and cosmeceutical applications. However, the production of carotenoids from algae is not yet fully cost effective to compete with synthetic ones. In this context the present review examines the technologies/methods in relation to mass production of algae, cell harvesting for extraction of carotenoids, optimizing extraction methods etc. Research studies from different microalgal species such as Spirulina platensis, Haematococcus pluvialis, Dunaliella salina, Chlorella sps., Nannochloropsis sps., Scenedesmus sps., Chlorococcum sps., Botryococcus braunii and Diatoms in relation to carotenoid content, chemical structure, extraction and processing of carotenoids are discussed. Further these carotenoid pigments, are useful in various health applications and their use in food, feed, nutraceutical, pharmaceutical and cosmeceutical industries was briefly touched upon. The commercial value of algal carotenoids has also been discussed in this review. Possible recommendations for future research studies are proposed.

UV-initiated synthesis of a novel chitosan-based flocculant with high flocculation efficiency for algal removal

In this study, maleyl chitosan-graft-polyacrylamide (MHCS-g-PAM), a novel chitosan-based flocculant, was prepared through UV irradiation, and maleyl chitosan (MHCS) was designed and prepared with maleic anhydride and acrylamide (AM) through maleyl acylation reaction. The effects of monomer concentration, MHCS-to-AM ratio, illumination time, initiator concentration, pH on viscosity, and grafting efficiency were investigated to optimize the synthesis of these substances. MHCS-g-PAM was characterized through Fourier transform infrared spectroscopy, nuclear magnetic resonance hydrogen spectroscopy, scanning electron microscopy, and thermal gravimetric analysis. Flocculation mechanisms in alga-containing wastewater at various pH levels and dosages were examined in detail on the basis of zeta potential measurements. Zeta potential experiments indicated that the adsorption-bridging and charge neutralization mechanisms played an important role in algal removal. Flocculation tests on algal removal demonstrated that the flocculation performance of MHCS-g-PAM was more effective than that of cationic polyacrylamide, polyferric sulfate, and polymeric aluminium. The optimal Chl-a and COD removal rate obtained by MHCS-g-PAM was 98.6% and 94.9% at pH7 and 4mg·L^-1, respectively.

Using quartz sand to enhance the removal efficiency of M. aeruginosa by inorganic coagulant and achieve satisfactory settling efficiency

In this study, low-cost and non-polluting quartz sand was respectively mixed with AlCl₃, FeCl₃ and PAFC to synergistically remove Microcystis aeruginosa. Results showed that quartz sand could markedly increase the algae removal efficiency and decrease the coagulant doses. The increase of removal efficiency with AlCl₃ and FeCl₃ was only due to the enhancement of floc density by the quartz sand. However, the removal efficiency with PAFC was increased not only by the enhanced floc density, but also by the enlarged floc size. Flocs from 50 mg/L sand addition were larger than that with other sand doses, which was on account of the appropriate enhancement of collision efficiency at this dose. After coagulation, the extracellular organic matter (EOM) and microcystins (MCs) in system with quartz sand was remarkably reduced. That’s because quartz sand can enhance the coagulation so as to improve capping the EOM and MCs in flocs during coagulation process. Owing to 200 mg/L quartz sand could damage the cell’s membrane during coagulation proces, algal cells in the system lysed two days earlier than with 50 mg/L sand during flocs storage. In addition, cells with PAFC incurred relatively moderate cellular oxidative damage and could remain intact for longer time.

Waste activated sludge (WAS) dewatering properties of an original hydrophobically modified polyacrylamide containing a cationic microblock structure

Chemical conditioning, as one of the core technologies used for the dewatering pretreatment of sludge, can efficiently improve the dewaterability of WAS and hence reduce the expense of the transportation and disposal of WAS.

Flocculation kinetics mechanism and floc formation prepared by poly aluminum chloride coupled with polyacrylamide for ship ballast water

The performance of flocculants prepared by poly aluminum chloride (PAC) and polyacrylamide (PAM) on treating ballast water collected at the Dalian new port area, the evaluation depending on the values of reaction parameters, and kinetics mechanism of flocculation were investigated in this study. Accordingly, the flocculants of 0.1 g·L(-1), prepared by mixing PAC of 10% with PAM of 2.0‰, enabled the removal rate of zooplankton and phytoplankton to reach 91% in ballast water at 20 °C. Based on flocculation kinetics mechanism analysis, the efficient vortex size during stirring should be larger than the floc particles, and gradient of fluctuating velocity provide the impetus for turbulence flocculation. The results of this study could be relevant to understanding particle-floc interactions during developmental flocculation, and during application of ballast water treatment.