Coagulation/flocculation-based removal of algal-bacterial biomass from piggery wastewater treatment

Influence of chitosan properties and operating parameters on the flocculation efficiency and harvesting of microalgae (Scenedesmus sp.)

Physicochemical and structural characteristics of chitosan prepared from Deep-sea shrimp (DCs), including degree of deacetylation (DD), molecular weight (Mw), viscosity, crystallinity index (CrI) and surface morphology were compared with a commercial chitosan (CCs). The DCs had a higher DD of 81.33 ± 0.40 %, whereas the CCs had a lower DD of 74.62 ± 0.64 %. Additionally, the DCs exhibited a lower Mw of 192.47 ± 2.5 kDa and viscosity of 646.00 ± 4.00 cP compared to the CCs, which had a Mw of 202.44 ± 0.28 kDa and viscosity of 689.67 ± 5.91 cP. This study investigated the influence of chitosan properties, particularly DD and Mw on the harvesting of Scenedesmus sp. along with the chitosan dosage, pH of the culture medium, mixing speed and time. Under optimal operating conditions, the microalgae removal efficiency of the DCs reached a significantly higher level (94.71 ± 0.20 %) compared to that of CCs (88.25 ± 0.41 %). Chitosan with a higher DD and low Mw demonstrated superior flocculation efficiency. The results highlight the significance of DD and Mw of chitosan and its influence on the flocculation of microalgae, providing valuable insights for optimizing the harvesting process with the non-toxic and natural flocculent, chitosan.

Perspectives on cultivation and harvesting technologies of microalgae, towards environmental sustainability and life cycle analysis

The development of algae is seen as a potential and ecologically sound approach to address the increasing demands in multiple sectors. However, successful implementation of processes is highly dependent on effective growing and harvesting methods. The present study provides a complete examination of contemporary techniques employed in the production and harvesting of algae, with a particular emphasis on their sustainability. The review begins by examining several culture strategies, encompassing open ponds, closed photobioreactors, and raceway ponds. The analysis of each method is conducted in a systematic manner, with a particular focus on highlighting their advantages, limitations, and potential for expansion. This approach ensures that the conversation is in line with the objectives of sustainability. Moreover, this study explores essential elements of algae harvesting, including the processes of cell separation, dewatering, and biomass extraction. Traditional methods such as centrifugation, filtration, and sedimentation are examined in conjunction with novel, environmentally concerned strategies including flocculation, electro-coagulation, and membrane filtration. It evaluates the impacts on the environment that are caused by the cultivation process, including the usage of water and land, the use of energy, the production of carbon dioxide, and the runoff of nutrients. Furthermore, this study presents a thorough examination of the current body of research pertaining to Life Cycle Analysis (LCA) studies, presenting a perspective that emphasizes sustainability in the context of algae harvesting systems. In conclusion, the analysis ends up with an examination ahead at potential areas for future study in the cultivation and harvesting of algae. This review is an essential guide for scientists, policymakers, and industry experts associated with the advancement and implementation of algae-based technologies.

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.

Microalgal flocculation and sedimentation: spatiotemporal evaluation of the effects of the pH and calcium concentration

The high cost of harvesting microalgae is a major hurdle for the microalgae industry, and an efficient pre-concentration method is required. In this study, the effects of using different pH values (between pH 3 and 11) and calcium (Ca²⁺) concentrations (between 0 and 5 mM) on Chlorella vulgaris sedimentation were investigated by evaluating the spacio-temporal distributions of microalgae cells. Fast and efficient sedimentation occurred (within 10 min) at a high Ca²⁺ concentration (5 mM) at pH 9 and 11. However, the sediment volume was lower at a Ca²⁺ concentration of 3 mM than at a Ca²⁺ concentration of 5 mM. This indicated that the Ca²⁺ concentration strongly affected the sediment volume. Fast sedimentation and a low sediment volume were found at pH 7 and a Ca²⁺ concentration of 5 mM, probably because of the neutral charge in the system (adhesion to calcium precipitates would have occurred at a high pH). The highest Ca²⁺ recovery (82%) was achieved when sediment produced at pH 11 and a Ca²⁺ concentration of 5 mM was acidified to pH 3.

Effectively Recycling Swine Wastewater by Coagulation–Flocculation of Nonionic Polyacrylamide

Recycling swine wastewater is an environmental and economic issue for promoting the sustainable development of the pig industry worldwide. The application of a flocculant, non-ionic polyacrylamide (NPAM) for treating the contaminants in wastewater was trialed in this study. Firstly, the optimal pH value for the coagulation–flocculation of NPAM was adjusted by hydrochloric acid and sodium hydroxide. The viscosity of the flocculant solution was examined by a rotational viscometer and the morphology of the flocculant on the glass surface was examined by an optical microscope and an atomic force microscope. The result showed that a pH value of 11 or more was best for NPAM coagulation–flocculation. Subsequently, the swine wastewater from the anoxic reactor of a three-stage manure treatment system was adjusted by a pH adjuster, calcium hydroxide, followed by the coagulation–flocculation of NPAM. The quality of the final, treated water was examined by a regular wastewater analysis. The results showed that the removal rates for copper ions, zinc ions, NH4+–N, total phosphate (TP), and total nitrogen (TN) were 96.3%, 97.8%, 99.2%, 94.9%, and 99.1%, respectively. Our study concluded that this water recycling method combining the existing organic fertilizer production and power generation enhanced the recycling strategy for swine wastewater treatment and could further the sustainable development of the pig industry.

Metal-based flocculation to harvest microalgae: a look beyond separation efficiency

Metal-based flocculants are commonly used for biomass harvesting in microalgae-based bio-refineries. Besides the high separation efficiency, additional aspects should be considered, related to the toxicity of metals for the algal biomass. Partitioning tests for commonly used flocculants (i.e., FeCl₃ and Al₂(SO₄)₃) showed that metals were mostly transferred to the solid phase with more than 95% of dosed metal ending up into the biomass, and low metal concentrations in the liquid effluent (lower than 0.4 mg L^-1 for both metals), thus allowing for water reuse. Photosynthesis inhibition was tested on microalgae and microalgae-bacteria cultures, using a standardized photo-respirometry protocol in which typical concentrations used during coagulation-flocculation were assessed. Modelling dose-response curves, concentrations corresponding to 50% inhibition (IC₅₀) were obtained, describing short-term effects. The obtained IC₅₀ ranged from 13.7 to 28.3 mg Al L^-1 for Al, and from 127.9 to 195.8 mg Fe L^-1 for Fe, showing a higher toxicity for the Al-based flocculant. The recovery of photosynthesis inhibition was also quantified, to evaluate the possibility of reusing/recycling the harvested biomass. The results highlighted that the residual photosynthetic activities, evaluated after 1 h and 24 h of exposure to metals were partially recovered, especially for Al, passing from 67.3% to 94.6% activity, respectively, while long-term Fe effects were stronger (passing from 64.9% to 77.6% activity). A non-toxic flocculant (cationic starch) was finally tested, excluding potential effects due to biomass aggregation, as the reduction of photosynthetic activity only reached 3.4%, compared to control. Relevant modifications to the light availability and the optical properties of algal suspensions were assessed, identifying a strong effect of iron which caused an increase of the light absorbance up to approximately 40% at high Fe concentrations. Possible implications of dosing metallic flocculants in MBWWT processes are discussed, and suggestions are given to perform inhibition tests on flocculating chemicals.

Synthesis, characterization, and flocculation performance of cationic starch nanoparticles

A series of cationic starches with different degrees of substitution were synthesized by etherification of potato starch with 3-chloro-2-hydroxypropyl trimethylammonium chloride (CTA). Cationic starch nanoparticles (CTA-StNPs) with different sizes were prepared by precipitation. Flocculation behaviors of the CTA-StNPs in simulated water sample containing kaolin were studied. The results showed that the dosage required to bring the simulated water sample containing kaolin to attain maximum transmittance at pH = 4 was significantly less than that at pH = 7. Both the size and degree of substitution of the CTA-StNPs affected their flocculation performance. The smaller the size and the higher the degree of substitution of CTA-StNPs, the better was the flocculation performance. Charge neutralization played a leading role in the flocculation process. The adsorption process of the CTA-StNPs onto kaolin could be divided into rapid adsorption, stable adsorption and equilibrium adsorption and followed pseudo second-order kinetic equation very well (R² > 0.99).

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.

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.

Reducing greenhouse gas emissions and stabilizing nutrients from dairy manure using chemical coagulation

Two primary concerns of dairies that store manure wastewater in anaerobic ponds are greenhouse gas (GHG) emissions and unpredictable nutrient availability after applying it to forage crops. Solid-liquid separation of dairy manure wastewater with chemical coagulants significantly reduces the fraction of organic matter stored in anaerobic conditions. However, the effects of coagulants on methane emissions from ponds and nutrient availability following field application are not well understood. In this experiment, several metal salts and organic polymers were used to coagulate dairy manure wastewater for separation into solid (floc) and liquid (effluent) fractions. The coagulants tested were ferric sulfate, ferric chloride, polyaluminum chloride, Superfloc C-569, and chitosan. An anaerobic incubation of manure effluent to simulate liquid manure storage and an aerobic incubation of manure floc-amended soil to simulate field application were conducted with analysis of GHGs and carbon and nitrogen transformations. The treatment of chemically separating organic matter from manure wastewater effectively eliminated methane emissions under anaerobic conditions in the laboratory. In the solid manure fraction, organic carbon was stabilized in the chemically separated flocs, and, apart from flocs produced with ferric iron, nitrogen mineralization was reduced as well. Carbon dioxide emissions were also reduced from the flocs applied to soil compared with untreated manure solids.

Understanding variability in algal solid-liquid separation process outcomes by manipulating extracellular protein-carbohydrate interactions

Coagulation-flocculation followed by sedimentation or dissolved air flotation (DAF) are processes routinely used for separating microalgae from water; however, during algae separation then can exhibit inconsistent separation, high coagulant demand, and high operating cost. To circumvent these problems, previous studies reported the development of a novel DAF process in which bubbles were modified instead of particles. While this process was shown to be sustainable and inexpensive, the problem of inconsistent algal separation across species remained. Recent research has suggested that this could be due to the varying concentration and character of algal-derived proteins and carbohydrates within the extracellular organic matter (EOM) and their associated interactions. This hypothesis is tested in the current study using the novel modified-bubble DAF process, which has been highly susceptible to EOM protein and carbohydrate concentrations and character. Biomolecular additives (commercially available proteins and carbohydrates, and algal-extracted proteins) of widely differing molecular weight (MW) and charge were dosed in varying proportions into samples containing either Chlorella vulgaris CS-42/7, Microcystis aeruginosa CS-564/01, or Microcystis aeruginosa CS-555/1 after removing the intrinsic EOM. These cell-rich suspensions were then subject to flotation using cationic bubbles modified with poly(diallyldimethylammonium chloride) (PDADMAC). When additives were dosed independently, separation increased from <5% to up to 62%. The maximum separation was obtained when the dose was double the respective biopolymer concentration measured in the intrinsic EOM for the equivalent species, and, in the case of protein additives, when MW and charge were >50 kDa, and >0.5 meq·g^-1, respectively, irrespective of the species tested. When evaluating steric- and charge-based protein-carbohydrate interactions on cell separation by simultaneously dosing high MW and high charge protein- and carbohydrate-additives, enhanced separation of up to 79% was achieved. It is suggested that enhanced cell separation is achieved due to proteins and carbohydrates bridging with cells and forming protein-carbohydrate-cell suprastructures in the presence of a flocculant, e.g. PDADMAC, and this only occurs when the intrinsic EOM comprises proteins and carbohydrates that have high MW (>25 kDa) and charge (>0.2 meq·g^-1), and interactions with each other and with the cell surface.

A comprehensive analysis of an effective flocculation method for high quality microalgal biomass harvesting

Flocculation is a low-cost harvesting technique for microalgae biomass production, but flocculation efficiency is species dependent. In this study, we investigated the efficacy of two synthetic (polyacrylamide) and one natural (chitosan) flocculants against three algal species: the cyanobacterium Synechocystis sp., the freshwater Chlorella vulgaris, and the marine Phaeodactylum tricornutum at laboratory and pilot scales to evaluate harvesting efficiency, biomass integrity and media recycling. Growth phase affected the harvesting efficiency of the eukaryotic microalgae. The flocculation was optimal at stationary phase with high flocculation efficiency achieved using polyacrylamides at 24-36 mg/g dry weight. The effect of the flocculants on the harvested biomass was investigated. The flocculated Synechocystis sp. showed a higher proportion of compromised cells compared to C. vulgaris and P. tricornutum likely due to differences in cell walls composition. Compromised cells could lead to the release of valuable products into the surrounding growth media during flocculation. The residual culture media was recycled once with no impact on cell growth for all treatments and algal species. The flocculation technique was demonstrated at pilot-scale using 350 L microalgal suspension, showing an efficiency of 82-90% at a polyacrylamide dosage of 6.5-10 mg/L. This efficiency and the biomass quality are comparable to the laboratory-scale results. Overall, results indicate that polyacrylamide flocculants work on a wide range of species without the need for pre-treatment. The information generated in this study can contribute to making the microalgae industry more competitive.

Innovative microalgae biomass harvesting methods: Technical feasibility and life cycle analysis

In order to ease one of the main challenges of biomass production in wastewater, the harvest stage, this study proposes as main innovations: the comparison of technical and environmental performance of different methods of harvesting biomass which have not been addressed in the literature and the projection of an optimal environmental scenario for biomass harvesting. For this, three harvesting methods were evaluated and compared, namely the gravitational sedimentation (GS) via settling tank, coagulation with tannin followed by gravitational sedimentation (TC/GS), and a biofilm reactor operated in parallel with a settling tank (BR/GS). TC/GS required less time to concentrate the biomass (121.13 g/day); however, the biomass had a higher moisture content (99.02%), which may compromise its direct application for production of most bioproducts and bioenergy, only a dewatering step is recommended. The harvesting methods interfered in biomass characterisation, mainly in carbohydrate content, which was higher in biomass concentrated over time (28-37%) than biomass concentrated in a single day by coagulation (13.8%). The results of the life cycle assessment revealed that in scenarios which included TC/GS methods and the BR/GS presented less environmental impact in relation to only GS. Additionally, the combination of these two methods comprises the best scenario and promises to optimise the harvest of biomass grown in wastewater.

The biological performance of a novel microalgal-bacterial membrane photobioreactor: Effects of HRT and N/P ratio

A microalgal-bacterial membrane photobioreactor (MB-MPBR) was developed for simultaneous COD and nutrients (N and P) removals from synthetic municipal wastewater in a single stage for a long-term operation over 350 days. The effects of hydraulic retention time (HRT) and N/P ratio on the biological performance were systematically evaluated for the first time. The results showed that a lower N/P ratio (3.9:1) and shorter HRT (2 d) promoted more biomass production, as compared to a high HRT (3 d) and a high N/P ratio (9.7:1). The highest biomass concentration (2.55 ± 0.14 g L^-1) and productivity (127.5 mg L^-1·d^-1) were achieved at N/P ratio of 3.9:1 and HRT of 2 d due to the highest nitrogen and phosphorus loadings under such conditions. A COD and ammonia-N removal efficiency of over 96% and 99%, respectively, were achieved regardless of HRTs and N/P ratios. In the absence of nitrogen or phosphorus deficiency, shorter HRT (2 d) yielded a higher nitrogen and phosphorus uptake but lower removal efficiency. In addition, the imbalance N/P ratio (9.7:1) would decrease nitrogen or phosphorus removal. Overall, the results suggested that it was feasible to simultaneously achieve complete or high removal of COD, nitrogen, and phosphorous in MB-MPBR under the appropriate conditions. This study demonstrated for the first time that MB-MPBR is a promising technology that could achieve a high-quality effluent meeting the discharge standards of COD and nutrients in one single step.

Nutrient and pathogen removal from anaerobically treated black water by microalgae

The demand for systems that efficiently and sustainably recover value-added compounds and materials from waste streams is a major challenge. The use of wastewater as a source for recovery of carbon and nutrients is an attractive and sustainable alternative. In this study, anaerobically treated black water was treated in photobioreactors (PBRs) inoculated with Chlorella sorokiniana, and the process was investigated in terms of phosphorus and nitrogen removal, biomass growth, and the removal of pathogens. The consumption of bicarbonate (alkalinity) and acetate (volatile fatty acids) as carbon sources by microalgae was investigated. The average nutrient removal achieved was 66% for N and 74% for P. A high consumption of alkalinity (83%) and volatile organic acids (76%) was observed, which suggests that these compounds were used as a source of carbon. The biomass production was 73 mg L^-1 day^-1, with a mean biomass of 0.7 g L^-1 at the end of the batch treatment. At the end of the experiments, a log removal/inactivation of 0.51 log for total coliforms and 2.73 log for Escherichia coli (E. coli) was observed. The configuration used, a flat-panel PBR operated in batch mode without CO₂ supplementation, is a cost-effective and environmentally sustainable method for recovering of nutrients and production of algal biomass.

Nutrients removal from piggery wastewater coupled to lipid production by a newly isolated self-flocculating microalga Desmodesmus sp. PW1

A newly isolated microalgal strain, Desmodesmus sp. PW1, possessing not only high potential for removing nitrogen and phosphorous from piggery wastewater but excellent self-flocculating ability, was provided here. Strain PW1 grew well in diluted and undiluted piggery wastewater, and could effectively remove total nitrogen and total phosphorus with removal rates up to 90% and 70%, respectively. In the laboratory scale by 30-L photobioreactor, microalga also performed well in TN (65.3%) and TP (83.5%) removal. Strain PW1 cultivated in the stationary phase achieved high self-flocculating efficiency (>90%) in 2.5 h of settling; meanwhile, temperature and pH slightly influenced on the flocculation. The potential mechanism on self-flocculation was considered related to hydrophobic extracellular polymeric substances. Furthermore, the fatty acid compositions of PW1 were mainly hexadecanoic acid, oleic acid and linoleic acid. Taken together, Desmodesmus sp. PW1 was the promising candidate to overcome the microalgae harvesting problem in piggery wastewater treatment.

Flocculating properties and potential of Halobacillus sp. strain H9 for the mitigation of Microcystis aeruginosa blooms

Microcystis aeruginosa can cause harmful algal blooms in freshwaters worldwide. It has already seriously affected human lives and prevented the use of water resources. Therefore, there is an urgent need to develop ecofriendly and effective methods to control and eliminate M. aeruginosa in aquatic environments. In this study, Halobacillus sp. strain H9, a bacterium that showed high M. aeruginosa flocculation activity, was isolated and selected to assess its potential for the removal of M. aeruginosa. The analyses of flocculation activity and mode indicated that the strain H9 induced M. aeruginosa flocculation by secreting active flocculating substance rather than by directly contacting algal cells. A 5% concentration of the H9 supernatant could efficiently flocculate M. aeruginosa cells with a density of up to 5 × 10⁷ cells/mL. Dramatic increases in the zeta potential indicated that charge neutralization could be the mechanism of the flocculation process. The strain H9 flocculated M. aeruginosa with no damage to the algal cell membrane, and did not result in microcystin being released into the surrounding environment. The flocculated algal culture was less toxic to zebrafish larvae, suggesting an environmentally friendly benefit of the H9 supernatant. In addition to M. aeruginosa, the H9 strain was also able to flocculate two other species causing harmful algal blooms, Phaeocystis globose and Heterosigma akashiwo. Furthermore, the flocculation activity of the H9 supernatant was stable at different temperatures and over a wide pH range. These characteristics give the H9 strain great potential for mitigating the influences of harmful algal blooms.

Urban wastewater photobiotreatment with microalgae in a continuously operated photobioreactor: growth, nutrient removal kinetics and biomass coagulation-flocculation

The aim of this study was to investigate the growth, nutrient removal and harvesting of a natural microalgae bloom cultivated in urban wastewater in a bubble column photobioreactor. Batch and continuous mode experiments were carried out with and without pH control by means of CO₂ dosage. Four coagulants (aluminium sulphate, ferric sulphate, ferric chloride and polyaluminium chloride (PAC)) and five flocculants (Chemifloc CM/25, FO 4498SH, cationic polymers Zetag (Z8165, Z7550 and Z8160)) were tested to determine the optimal dosage to reach 90% of biomass recovery. The maximum volumetric productivity obtained was 0.11 g SS L^-1d^-1 during the continuous mode. Results indicated that the removal of total dissolved nitrogen and total dissolved phosphorous under continuous operation were greater than 99%. PAC, Fe₂(SO₄)₃ and Al₂(SO₄)₃ were the best options from an economical point of view for microalgae harvesting.

Comparison of organic matter removals in single-component and bi-component systems using enhanced coagulation and magnetic ion exchange (MIEX) adsorption

Natural organic matter (NOM) in aquatic environments have a significant impact on NOM-organic compound interactions, which could strongly affect the distribution and transformation of organic compounds during water treatment. This study focused on the removals of NOM (humic acid, HA) and synthetic organic matter (ibuprofen, IBP) through enhanced coagulation and magnetic ion exchange (MIEX) resin adsorption in single and bi-component systems. Two coagulants, traditional aluminum sulfate (AS) and lab-prepared polyaluminum chloride (PACl), were employed. The charge properties, particle size distribution, and fractal dimension (D_f) during organic matter removal were studied in both the single and bi-component systems to explore the purification behaviors and mechanistic effects of interactions between coagulants, MIEX, and organic matters. The experimental results indicated that the Al-based coagulants could remove over 80% of HA in both the single and IBP-HA combined systems, while the presence of HA could considerably improve the IBP removal rate. The aggregates formed during single-component coagulation were larger, but weaker and more loosely structured than those formed in the bi-component system under the same coagulation conditions. In the single-component system, the maximum removal efficiencies of IBP and HA by MIEX adsorption were 65% and 72%, respectively, at a resin dosage of 20.0 mL/L and mixing time of 60 min. Under the same conditions, the removals of these components in the bi-component system were improved to 68% and 98%, respectively. The reaction rate between IBP and MIEX resin was found faster than that between HA and MIEX resin.

Fabrication of Bifunctional Chitosan-Based Flocculants: Characterization, Assessment of Flocculation, and Sterilization Performance

In this study, a series of chitosan-based quaternary ammonium graft flocculants, namely chitosan-graft-poly(acrylamide and methacryloyl ethyl trimethyl ammonium chloride) [CTS-g-P(AM-DMC)], was successfully synthesized by plasma initiation, and the as-prepared [CTS-g-P(AM-DMC)] had both flocculation and sterilization functions. Various characterization techniques were used to study the structure and physicochemical properties of the chitosan-based flocculants. ¹H nuclear magnetic resonance spectroscopy (¹H NMR), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction spectroscopy (XRD), and thermogravimetric analysis/differential scanning calorimetry (TG/DSC) confirmed the successful synthesis of CTS-g-P(AM-DMC). Scanning electron microscopy (SEM) analysis exhibited that CTS-g-P(AM-DMC) contained a smooth convex and porous structure with an enormous surface area. CTS-g-P(AM-DMC) was then used to flocculate the simulated wastewater of the kaolin suspension and the Salmonella suspension. Besides external factors, such as the dosage of flocculant and pH, the effect of the internal factor graft ratio was also evaluated. The experimental results showed that CTS-g-P(AM-DMC) also revealed a strong sterilization effect, aside from the excellent flocculation effect. Moreover, the sterilization mechanism was investigated through a series of conductivity measurements and the analysis of fluorescence-based cell live/dead tests. The results indicated that CTS-g-P(AM-DMC) destroyed the cell membrane of Salmonella through its grafted quaternary ammonium salt, thereby exhibiting sterilization property.

Variations of floc morphology and extracellular organic matters (EOM) in relation to floc filterability under algae flocculation harvesting using polymeric titanium coagulants (PTCs)

The work evaluated the algae cells removal efficiency using titanium salt coagulants with different degree of polymerization (PTCs), and the algae cells aggregates and extracellular organic matter (EOM) under chemical flocculation were investigated. The results indicated that PTCs performed well in algae cells flocculation and separation. The main mechanism using PTCs of low alkalisation degree for algae flocculation was associated with charge neutralization, while adsorption bridging and sweep flocculation was mainly responsible for algae removal by PTCs of high alkalisation degree treatment. In addition, the flocs formed by PTC_1.0 showed the best filtration property, and EOM reached the minimum at this time, indicating the flocs formed by PTC_1.0 were more compact than other PTCs, which can be confirmed by SEM analysis. Three-dimensional excitation emission matrix fluorescence (3D-EEM) and high performance size exclusion chromatography (HPSEC) revealed that the EOMs were removed under PTCs flocculation, which improved floc filterability.

Scale-inhibition and flocculation dual-functionality of poly(acrylic acid) grafted starch

Natural-polymer based water treatment agents have recently received much more attention due to their environmental friendliness, widespread availability, and prominent structural features. Starch-graft-poly(acrylic acid) (St-g-PAA) is a simple natural-polymer based material that can be obtained easily by a one-step graft copolymerization. When used as a water treatment agent, St-g-PAA exhibits both effective scale-inhibition performance and high turbidity removal efficiency. The scale-inhibition efficiency of St-g-PAA against calcium carbonate (CaCO₃) is approximately 94% at the optimal dose in a static test of approximately 40 mg/L. Dispersion, crystal lattice distortion, and chelating effects all play important roles in the scale inhibition. When St-g-PAA is used as a coagulant aid for polyaluminum chloride (PAC) in the flocculation of a real hairwork wastewater, the highest reduction of the optimal PAC dose is more than 30% while the turbidity reduction is about 97% at the same time, both floc size and compactness increase, and the final settling efficiency also improves evidently. The efficient bridging flocculation effects account for the effective turbidity removal. The prominent scale-inhibition and flocculation dual-functionality of St-g-PAA is intrinsically ascribed to its distinct anionic linear branched-chain structure.

The effects of low levels of trivalent ions on a standard strain of Escherichia coli (ATCC 11775) in aqueous solutions

Considering the ever‐growing usage of trivalent salts in water treatment, for example, lanthanum salts in rare earth, AlCl₃ and FeCl₃, the effects of different trivalent cations on the bacterium Escherichia coli (E. coli) ATCC 11775 strain have been studied in aqueous solutions. From colony incubation studies, the colony‐forming unit (CFU) densities were found to decrease significantly in the presence of even low levels (10⁻⁵ mol/L) of lanthanum chloride. This level of reduction in CFU number is comparable to the results obtained using the known bacteriocidal cationic surfactant, C₁₄TAB. By comparison, exposure of the cells to low levels of trivalent ion, aluminum and chromium ion solutions produced only modest reductions in CFU density. The results from the incubation studies suggest that the bacteriostatic mechanism of La³⁺ ions has similarities to that of the cationic surfactant, and different to that of the other trivalent ions. Size distribution and zeta potential measurements of E. coli cells and phospholipid vesicles in the presence of trivalent cations solutions suggested significant cell shrinkage probably caused by membrane disruption.

Microalgae Chlorella vulgaris biomass harvesting by natural flocculant: effects on biomass sedimentation, spent medium recycling and lipid extraction

BACKGROUND

Microalgal biomass harvesting using traditional chemicals is costly for the production of biofuels, hindering the scale-up process of the technology. Thus, the search for a cost-effective microalgal harvesting method is extremely important. Using chitosan as a natural flocculant to harvest microalgal biomass seems to be an efficient and convenient solution. Although microalgal biomass flocculation by chitosan has been reported in some previous studies, literature on the harvesting of microalgae C. vulgaris biomass using such polymer is scanty. In addition, there is limited information available on whether the usage of chitosan during the harvesting will affect downstream lipid extraction. Still, whether microalgae can be re-grown with the spent medium after chitosan flocculation is still unknown.

RESULTS

In this study, microalgal biomass harvesting using chitosan as a natural flocculant and aluminum sulfate as a traditional flocculant was compared and evaluated. Optimal doses and effects on biomass sedimentation, spent medium recycling and lipid extraction were investigated. The results showed that the optimal doses for chitosan and aluminum sulfate to achieve more than 90% biomass recovery were 0.25 and 2.5 g/L, respectively. The sedimentation time of 10 min was found to be the most appropriate to remove over 90% biomass from culture. The spent medium after chitosan flocculation could be potentially recycled for the re-cultivation of microalgae, which demonstrated robust growth in comparison with those grown in the recycled medium from aluminum sulfate flocculation. The lipid content of microalgae harvested by chitosan reached 32.9, 4.6% higher than that of those harvested by aluminum sulfate, indicating that the application of the natural flocculant would not impact the downstream extraction of microalgal lipids.

CONCLUSION

The results herein presented, demonstrated that chitosan is applicable for microalgal harvesting during the upscaling process. Flocculation method developed by using chitosan as a natural flocculant is a worthy microalgal harvesting method for microalgae-based biofuel production. There is hope that chitosan can be reasonably and technically realistically applied in a full-scale process for the harvesting of microalgal biomass.

Bioflocculation: An alternative strategy for harvesting of microalgae - An overview

Microalgae based research has been extensively progressed for the production of value added products and biofuels. Potential application of microalgae for biofuel is recently gained more attention for possibilities of biodiesel and other high value metabolites. However, high cost of production of biomass associated with harvesting technologies is one of the major bottleneck for commercialization of algae based industrial product. Based on the operation economics, harvesting efficiency, technological possibilities, flocculation of algal biomass is a superior method for harvesting microalgae from the growth medium. In this article, latest trends of microalgal cell harvesting through flocculation are reviewed with emphasis on current progress and prospect in environmental friendly bio-based flocculation approach. Bio-flocculation based microalgae harvesting technologies is a promising strategy for low cost microalgal biomass production for various applications.

Influence of the gas-liquid flow configuration in the absorption column on photosynthetic biogas upgrading in algal-bacterial photobioreactors

The potential of an algal-bacterial system consisting of a high rate algal pond (HRAP) interconnected to an absorption column (AC) via recirculation of the cultivation broth for the upgrading of biogas and digestate was investigated. The influence of the gas-liquid flow configuration in the AC on the photosynthetic biogas upgrading process was assessed. AC operation in a co-current configuration enabled to maintain a biomass productivity of 15gm^-2d^-1, while during counter-current operation biomass productivity decreased to 8.7±0.5gm^-2d^-1 as a result of trace metal limitation. A bio-methane composition complying with most international regulatory limits for injection into natural gas grids was obtained regardless of the gas-liquid flow configuration. Furthermore, the influence of the recycling liquid to biogas flowrate (L/G) ratio on bio-methane quality was assessed under both operational configurations obtaining the best composition at an L/G ratio of 0.5 and co-current flow operation.

Optimization of ferric chloride concentration and pH to improve both cell growth and flocculation in Chlorella vulgaris cultures. Application to medium reuse in an integrated continuous culture bioprocess

Combined effect of ferric chloride and pH on Chlorella vulgaris growth and flocculation were optimized using DoE. Afterwards, an integrated bioprocess for microalgae cultivation and harvesting conceived as a sole step was run in continuous operation mode. Microalgae concentration in a 2L-photobioreactor was about 0.5gL(-1) and the efficiency of flocculation in the coupled sedimentation tank was about 95%. Dewatered microalgae reached a biomass concentrations increase about 50-fold, whereas it was only about 0.02gL(-1) in the clarified medium. Then, the reuse of the clarified medium recovered was further evaluated. The clarified medium was reused without any further nutrient supplementation, whereas a second round of medium reuse was performed after supplementation of main nutrients (phosphate-sulfate-nitrate), micronutrients and ferric chloride. The medium reuse strategy did not affect cell growth and flocculation. Consequently, the reuse of medium reduces the nutrients requirements and the demand for water, and therefore the production costs should be reduced accordingly.

Influences of iron and calcium carbonate on wastewater treatment performances of algae based reactors

The influences of iron and calcium carbonate (CaCO3) addition in wastewater treatments reactors performance were investigated. Adding different concentrations of Fe(3+) (5, 10, 30 and 50mmol/m(3)), iron and CaCO3 powder led to changes in algal characteristics and physico-chemical and microbiological properties. According to the investigation results, nutrient removal efficiency in algae based reactors was obviously increased by the addition of 10mmol/m(3) Fe(3+), iron (5mmol/m(3)) and CaCO3 powder (0.2gm(-3)) and the removal efficiencies of BOD5, TN, and TP in Stage 2 were respectively increased by 28%, 8.9%, and 22%. The improvements in physico-chemical performances were verified by microbial community tests (bacteria quantity, activity and community measured in most probable number, extracellular enzymes activity, and Biolog Eco Plates). Microbial variations indicated the coexistence of Fe ions and carbonate-bicarbonate, which triggered the synergistic effect of physico-chemical action and microbial factors in algae based reactors.

A review on chitosan-based flocculants and their applications in water treatment

In recent years, the use of chitosan and its derivatives as flocculants in water treatment has received considerable attention due to their many advantages, including their widespread availability, environmental friendliness, biodegradability, and prominent structural features. However, it is a significant strategy for selection and design of the high-performance materials on the basis of their structure-activity relationships. Here we describe several of the chemical modification methods commonly used to prepare chitosan-based flocculants. These methods allow convenient control and adjustment of the structures of the obtained materials to meet the different practical requirements. The influence of structural elements of the chitosan-based flocculants on their flocculation properties are emphasized in this review by examining different flocculation mechanisms and their applications in the treatment of various wastewaters containing different pollutants (insoluble suspended colloids but also dissolved matters). Above all, the chitosan-based flocculants with proper structures by precise structure control bear great application potentials in water treatment.