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"Flocculation"
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Recent Achievements in Polymer Bio-Based Flocculants for Water Treatment
Polymer flocculants are used to promote solid–liquid separation processes in potable water and wastewater treatment. Recently, bio-based flocculants have received a lot of attention due to their superior advantages over conventional synthetic polymers or inorganic agents. Among natural polymers, polysaccharides show many benefits such as biodegradability, non-toxicity, ability to undergo different chemical modifications, and wide accessibility from renewable sources. The following article provides an overview of bio-based flocculants and their potential application in water treatment, which may be an indication to look for safer alternatives compared to synthetic polymers. Based on the recent literature, a new approach in searching for biopolymer flocculants sources, flocculation mechanisms, test methods, and factors affecting this process are presented. Particular attention is paid to flocculants based on starch, cellulose, chitosan, and their derivatives because they are low-cost and ecological materials, accepted in industrial practice. New trends in water treatment technology, including biosynthetic polymers, nanobioflocculants, and stimulant-responsive flocculants are also considered.
Journal Article
Biopolymer-based flocculants: a review of recent technologies
Biopolymer-based flocculants have become a potential substitute for inorganic coagulants and synthetic organic flocculants due to their wide natural reserves, environmental friendliness, easy natural degradation, and high material safety. In recent years, with more and more attention to clean technologies, a lot of researches on the modification and application of biopolymer-based flocculants have been carried out. The present paper reviews the latest important information about the base materials of biopolymer-based flocculants, including chitosan, starch, cellulose, and lignin etc. This review also highlights the various modification methods of these base materials according to reaction types in detail. Via the recent researches, the flocculation mechanisms of biopolymer-based flocculants, such as adsorption, bridging, charge neutralization, net trapping, and sweeping, as well as, some other special mechanisms are comprehensively summarized. This paper also focuses on the water treatment conditions, the removal efficiency, and advantages of biopolymer-based flocculants in applications. Further, this review sheds light on the future perspectives of biopolymer-based flocculants, which may make progress in the sources of base materials, modification processes, multi-function, and deepening application researches. We believe that this review can guide the further researches and developments of biopolymer-based flocculants in the future, to develop them with a higher efficiency, a lower cost, more safety, and multi-function for more diversified applications.
Graphical abstract
Journal Article
Microbial Flocculants as an Alternative to Synthetic Polymers for Wastewater Treatment: A Review
Microorganisms such as bacteria, fungi, and microalgae have been used to produce bioflocculants with various structures. These polymers are active substances that are biodegradable, environmentally harmless, and have flocculation characteristics. Most of the developed microbial bioflocculants displayed significant flocculating activity (FA > 70–90%) depending on the strain used and on the operating parameters. These biopolymers have been investigated and successfully used for wastewater depollution in the laboratory. In various cases, selected efficient microbial flocculants could reduce significantly suspended solids (SS), turbidity, chemical oxygen demand (COD), total nitrogen (Nt), dye, and heavy metals, with removal percentages exceeding 90% depending on the bioflocculating materials and on the wastewater characteristics. Moreover, bioflocculants showed acceptable results for sludge conditioning (accepted levels of dry solids, specific resistance to filtration, moisture, etc.) compared to chemicals. This paper explores various bioflocculants produced by numerous microbial strains. Their production procedures and flocculating performance will be included. Furthermore, their efficiency in the depollution of wastewater will be discussed.
Journal Article
Engineering Zymomonas mobilis for Robust Cellulosic Ethanol Production
Great effort has been devoted to engineering Saccharomyces cerevisiae with pentose metabolism through the oxido-reductase pathway for cellulosic ethanol production, but intrinsic cofactor imbalance is observed, which substantially compromises ethanol yield. Zymomonas mobilis not only can be engineered for pentose metabolism through the isomerase pathway without cofactor imbalance but also metabolizes sugar through the Entner–Doudoroff pathway with less ATP and biomass produced for more sugar to be used for ethanol production. Moreover, the availabilities of genome sequence information for multiple Z. mobilis strains and advanced genetics tools have laid a solid foundation for engineering this species, and the self-flocculation of the bacterial cells also presents significant advantages for bioprocess engineering. Here, we highlight some of recent advances in these aspects.
Zymomonas mobilis metabolizes glucose through the Entner–Doudoroff (ED) pathway, with less ATP generated and biomass accumulated for more ethanol production.
The large specific cell surface of Z. mobilis together with the ED pathway facilitates glucose uptake and ethanol fermentation.
Its metabolic characteristics and narrow substrate spectrum make Z. mobilis unsuitable for fuel ethanol production from sugar- and starch-based feedstocks, but it would be a good host to be engineered for cellulosic ethanol production.
When self-flocculated, Z. mobilis can be immobilized within fermenters for high cell density to improve ethanol productivity. Meanwhile, its tolerance to environmental stresses may be enhanced by this morphological change.
Both Z. mobilis ZM4 and its self-flocculating mutant ZM401 can tolerate more than 100g/L ethanol, which is sufficient for cellulosic ethanol production.
Journal Article
Cationic Pullulan Derivatives Based Flocculants for Removal of Some Metal Oxides from Simulated Wastewater
Modified polysaccharides have been increasingly used as flocculants in wastewater treatment due to their non-toxicity, low price, biodegradability, etc. However, the pullulan derivatives are less used in wastewater purification processes. Therefore, this article presents some data regarding FeO and TiO2 particle removal from model suspensions by some pullulan derivatives with pendant quaternary ammonium salt groups, trimethylammonium propyl carbamate chloride (TMAPx–P). The influence of the polymer ionic content, dose, and initial solution concentration as well as of the dispersion pH and composition (metal oxide content, salts, and kaolin) on the separation efficacy were considered. UV-Vis spectroscopy measurements have shown a very good removal efficacy of TMAPx–P for the FeO particles (around 95% and more), irrespective of the polymer and suspension characteristics; a lower clarification of the TiO2 particles suspension (removal efficiency between 68% and 75%) was noticed. Both the zeta potential and the particle aggregates size measurements revealed the charge patch as the main mechanism which governs the metal oxide removal process. The surface morphology analysis/EDX data provided supplementary evidence regarding the separation process. A good removal efficiency (90%) of the pullulan derivatives/FeO flocs for the Bordeaux mixture particles from simulated wastewater was found.
Journal Article
Molecular self-assembly mediates the flocculation activity of benzimidazole derivatives against E. coli
Bacterial flocculation is a process in which bacteria aggregate to form cloudy, flake-like clusters known as flocs. While this phenomenon is commonly associated with water treatment, it also has interesting industrial applications, particularly as a method for cell immobilisation.
Escherichia coli
, extensively employed in industrial processes, typically does not possess inherent flocculation ability. In this study, we found that certain bisbenzimidazole derivatives can rapidly induce flocculation in
E. coli
(K-12 MG1655) in a structure-dependent manner. Among others, high-resolution microscopy (SEM, fluid AFM) revealed a dense fibrillar network within the flocs, initially suggestive of an extracellular matrix. Mechanistic investigations demonstrated that this phenomenon cannot be linked to the secretion of extracellular polymeric substances (EPS). Our findings suggest that flocculation arises from the self-assembly of bisbenzimidazole derivatives into supramolecular fibres that anchor to bacterial membranes. These results uncover an atypical flocculation process distinct from charge neutralisation or EPS-mediated pathways, broadening the potential applications of bisbenzimidazole derivatives in bacterial immobilisation.
Journal Article
Enhancing oil production and harvest by combining the marine alga Nannochloropsis oceanica and the oleaginous fungus Mortierella elongata
Background: Although microalgal biofuels have potential advantages over conventional fossil fuels, high production costs limit their application in the market. We developed bio‑flocculation and incubation methods for the marine alga, Nannochloropsis oceanica CCMP1779, and the oleaginous fungus, Mortierella elongata AG77, resulting in increased oil productivity.Results: By growing separately and then combining the cells, the M. elongata mycelium could efficiently capture N. oceanica due to an intricate cellular interaction between the two species leading to bio‑flocculation. Use of a high‑salt culture medium induced accumulation of triacylglycerol (TAG) and enhanced the contents of polyunsaturated fatty acids (PUFAs) including arachidonic acid and docosahexaenoic acid in M. elongata. To increase TAG productivity in the alga, we developed an effective, reduced nitrogen‑supply regime based on ammonium in environmental photobioreactors. Under optimized conditions, N. oceanica produced high levels of TAG that could be indirectly monitored by following chlorophyll content. Combining N. oceanica and M. elongata to initiate bio‑flocculation yielded high levels of TAG and total fatty acids, with ~ 15 and 22% of total dry weight (DW ), respectively, as well as high levels of PUFAs. Genetic engineering of N. oceanica for higher TAG content in nutrient‑replete medium was accomplished by overexpressing DGT T5, a gene encoding the type II acyl‑CoA:diacylglycerol acyltransferase 5. Combined with bio‑flocculation, this approach led to increased production of TAG under nutrient‑replete conditions (~ 10% of DW ) compared to the wild type (~ 6% of DW ).Conclusions: The combined use of M. elongata and N. oceanica with available genomes and genetic engineering tools for both species opens up new avenues to improve biofuel productivity and allows for the engineering of poly‑unsaturated fatty acids.
Journal Article
Microalgal bio-flocculation: present scenario and prospects for commercialization
The need for sustainable production of renewable biofuel has been a global concern in the recent times. Overcoming the tailbacks of the first- and second-generation biofuels, third-generation biofuel using microalgae as feedstock has emerged as a plausible alternative. It has an added advantage of preventing any greenhouse gas (GHG) emissions with simultaneous carbon dioxide sequestration. Dewatering of microalgal culture is one of the many concerns regarding industrial-scale biofuel production. The small size of microalgae and dilute nature of its growth cultures creates huge operational cost during biomass separation, limiting economic feasibility of algae-based fuels. Considering the recovery efficiency, operation economics, technological feasibility and cost-effectiveness, bio-flocculation is a promising method of harvesting. Moreover, advantage of bio-flocculation over other conventional methods is that it does not incur the addition of any external chemical flocculants. This article reviews the current status of bio-flocculation technique for harvesting microalgae at industrial scale. The various microbial strains that can be prospective bioflocculants have been reviewed along with its application and advantages over chemical flocculants. Also, this article proposes that the primary focus of an appropriate harvesting technique should depend on the final utilization of the harvested biomass. This review article attempts to bring forth the beneficial aspects of microbial aided microalgal harvesting with a special attention on genetically modified self-flocculation microalgae.
Journal Article
Characterisation, Flocculation Efficiencies and Mechanisms of Bioflocculants Derived from Klebsiella pneumoniae and Meyerozyma guilliermondii
Evaluation of characteristics and flocculation mechanisms of microbial flocculants facilitates the identification of potential applications and informs the fine-tuning of operational conditions for maximum activity. Therefore, this study aimed to characterise and optimise the operational conditions of bioflocculants produced from Klebsiella pneumoniae and Meyerozyma guilliermondii for potent wastewater treatment. Scanning electron microscopy, X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA) and Fourier Transform Infrared Spectroscopy (FTIR) were employed to assess the surface morphology, crystalline structure, thermal stability, and functional group composition of the bioflocculants. Their cytotoxicity was assessed using the tetrazolium bromide-based assay against human colorectal adenocarcinoma (CaCO-2) cell lines. Flocculation efficiencies and mechanisms were determined using Jar and zeta potential assays, respectively. The bioflocculant from K. pneumoniae (Kp1) revealed a fibrous morphology, whereas that from M. guilliermondii (Mg1) displayed a granular structure. FTIR spectra revealed hydroxyl, amine, and alkene groups as key functional groups, while TGA analysis indicated that Kp1 was thermally unstable, contrary to Mg1, which exhibited good thermal stability. Bioflocculants Kp1 and Mg1 exhibited COD removal of 90.86% and 93.12% and turbidity reductions of 92.65% and 92.74%, respectively. Zeta potential analysis revealed that bioflocculant Kp1 primarily flocculated through charge neutralisation, while Mg1 employed a bridging mechanism. These bioflocculants illustrated strong potential to treat wastewater. However, the observed cytotoxic effect at increased concentrations warrants cautious handling and application in lower doses.
Journal Article