Stable Flocs Research Articles

Harvesting of marine microalgae using cationic cellulose nanocrystals

Flocculation of marine microalgae is challenging because of the high ionic strength of the culture medium. We investigated cationic cellulose nanocrystals (CNCs) as biobased flocculants for the marine microalgae Nannochloropsis oculata, and compared its performance to chitosan. Cationic CNCs induced flocculation at a low dose of 11 mg L-1, while chitosan required a dose of 35 mg L-1. Our cationic CNCs possess a permanent positive charge, allowing flocculation over a wide pH-range (4 to 10). The CNC maximum flocculation efficiency was 90 %, while chitosan achieved > 95 %, attributed to small flocs remaining in suspension for CNCs. However, centrifugation for 1 min at 180 g or gravity filtration using a 30 μm nylon filter after CNC flocculation resulted in the removal of these small, stable flocs (∼150 μm diam) and > 95% harvesting efficiency. Cationic CNCs can thus serve as a sustainable alternative natural flocculant for harvesting both freshwater and marine microalgae.

Characterization of activated sludge flocs in membrane bioreactor: stable and unstable flocs.

In this study, the properties of unstable and stable flocs were investigated under the steady operation of a membrane bioreactor (MBR). The extracellular polymeric substances (EPS) composition, surface charge, and hydrophobicity of unstable and stable flocs were examined and compared. Interfacial interactions of the membrane with unstable flocs, unstable flocs themselves, and unstable and stable flocs were assessed using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) models. Cake layer resistance was found to contribute more than 80% of total resistance under steady operating conditions. Compared with stable flocs, unstable flocs possessed a higher level of EPS, more diverse protein, more negative charge, weaker hydrophobicity, and higher fouling potential. Thermodynamic analyses showed that unstable flocs had a higher adhesive strength (- 63.4mJ/m2) with the membrane, lower self-cohesive strength (- 18.3mJ/m2), and higher cohesive strength (- 54.3mJ/m2) with stable flocs. Therefore, some unstable flocs remained on the membrane surface to form the cake layer due to their poor cohesion strength.

Cationic polyacrylamide induced flocculation and turbulent dewatering of microalgae on a Britt Dynamic Drainage Jar

Dewatering is one of the major constraints to the large scale production of microalgae biofuels, with many drawbacks in currently deployed technologies. Using an approach similar to papermaking, cationic polyacrylamide (CPAM) based flocculants can stabilize flocs of microalgae under turbulent conditions to achieve effective dewatering of dilute suspensions in high speed filtration systems. The effects of flocculant dosage, stirrer speed (i.e. turbulence) and pH on filtration retention was investigated using a Britt Dynamic Drainage Jar (BDDJ) which simulates a commercial paper machine. The development of a stable floc system that withstood turbulence was achieved through the flocculant’s dual effects of charge neutralization and bridging of the microalgae cells. Retention of microalgae on a 76 µm screen improved from 7% when no flocculant was used to 94% at a flocculant dosage of 10 mg/L of CPAM, stirrer speed of 1200 rpm and pH of 6.5 as optimum conditions. The most significant effects on microalgae retention were that of flocculant dosage; followed by the combined interaction between stirrer speed and flocculant dosage; and that of pH. A key finding is that high retention was obtained under moderately turbulent conditions. This study shows that the paper dewatering technique will be potentially applicable for microalgae preconcentration in the production of biofuels, and further pilot scale studies are recommended.

Enhancement of Electrocoagulation Process for Dye Removal Using Powdered Residuals from Water Purification Plants (PRWPP)

Electrocoagulation (EC) technology is an attractive alternative for industrial wastewater treatment due to its simplicity, low operation cost, less sludge production, less chemical addition, and larger and more stable floc formation. The main objective of this research is to investigate the effect of using powdered residuals from water purification plants (PRWPP) on dye removal efficiency and energy consumption under various operational conditions (pH, initial dye concentration, and applied voltage) in order to enhance the EC performance. A series of laboratory-scale experiments were conducted using an EC bench-scale reactor with iron electrodes. The addition of PRWPP to the EC system increases the dye removal efficiency from 32.9 to 97.2% and decreases the energy consumption from 6.08 to 2.06 kWh/kg dye removed. The highest dye removal efficiency was achieved at a PRWPP dosage of 2 g/l, an applied voltage of 5 V, and an initial pH value ranging from 6 to 9. The higher the dye influent concentrations discharged to EC, the lower the removal efficiency achieved and current efficiency. PRWPP has a beneficial effect on the EC performance in terms of the dye removal efficiency as well as the energy consumption.

Characteristics of Biochemical and Fractal Structure of Activated Sludge with Thermochemical Lysis

In this study, we investigated the structural characteristic and biochemical properties of waste-activated sludge after thermochemical pretreatment. The results show that with the increase dosage of hydrochloric acid or sodium hydroxide, the concentration of suspended solid (SS) and volatile suspended solids (VSS) declined, especially at pH 12 + H (“H” means heating). At the same time, soluble chemical oxygen demand (SCOD) all increased as well, especially at pH 12, the greatest lysis effect appeared. Protein and polysaccharide presented a similar law with SCOD. Furthermore, the specific surface area (SSA), two-dimensional fractal dimension (D 2), and three-dimensional fractal dimension (D 3) all increased to a certain degree with acid/alkali pretreatment whether or not heating. Otherwise, the median particle size (d 0.5) and zeta potential decreased leading to more compact and stable floc structure and reduction effect compared with the original sludge. In Pearson correlation analysis, SSA and SS, SSA and VSS, zeta potential and SCOD, and zeta potential and protein have significant negative correlations; D 3 and SSA have a significant correlation with SS, VSS, SCOD, and protein. Consequently, measuring the structural parameters D 3 and SSA online can reflect the effects of sludge lysis indirectly, which will be helpful to guide the practical application.

Influence of Activated Sludge Surface Properties on Flocculating Settling and Effluent Suspend Solid

In this paper, the influence of the surface properties of the activated sludge on flocculating settling and ESS was studied in seven different sewage treatment plants in Beijing, such as the sludge volume index (SVI), the protein/carbohydrate (P/C) ratio in extracellular polymeric substance, Zeta potential, two-dimensional fractal dimension(D2f). The relationships between the surface properties of the activated sludge and flocculation ability (FA) or effluent suspend solid(ESS) were also analyzed. The results showed that no obvious correlation was obtained between SVI and FA or ESS, but obvious negative correlation between the FA and ESS (R2=0.787) was observed. When the P/C increased from 1.28 to 25.34, the FA could increase from 0.19 to 0.73 correspondingly, but the ESS decreased from 14.89 mg·L-1 to 6.08 mg·L-1. This was because the amino acids in protein with positive charge could neutralize the negative charge on the surface of floc, the increase of protein ratio in EPS could decrease the absolute value of Zeta potential, which made a contribution to form more stable floc structure, and improved the ability of flocculation. Moreover, the Zeta potential had a positive correlation with FA but a negative correlation with ESS. The absolute value of Zeta potential decreased by 1 mV, FA increased by 0.059 and ESS decreased by 0.934 mg·L-1 correspondingly. In addition, there was an obvious index correlation (R2=0.935, P<0.01) between D2f and FA; While D2f increased from 1.10 to 1.45, FA could increase by 4.3 times and ESS decreased linearly (R2=0.868).

Development of a hybrid treatment system for combined sewer overflows using a hydrocyclone and a dissolved air flotation system

Combined sewer overflows (CSOs) contain a mixture of untreated, contaminated sanitary, and storm water. CSOs include both precipitated and suspended matters. In this study, a novel system is proposed to treat CSOs using a combined hydrocyclone and dissolved air flotation (DAF) system for removal of both precipitated and suspended matter. The optimum operating conditions were determined by changing the internal flow rate and the split ratio (Rf) in the hydrocyclone and by changing the angle of the nozzle to determine the floc size, flow rate, and recycle ratio in the DAF system. The types of organic matter contained in prepared synthetic water samples are categorized as soluble CODcr (SCOD) and particulated CODcr (PCOD) in the characterization of the removal rate of the precipitated and suspended matter. The suspended solids (SS) were classified by their specific gravity to determine the removal rate of both precipitated and suspended matter. In a pretreatment process, the system employed a cationic polymer to obtain a low gradient time (G × t) value. The results showed that poly-aluminum chloride (PAC) used with the cationic polymer had a 9.2% higher turbidity removal rate than PAC alone, due to a stable floc being achieved in the hydrocyclone. The optimum operating conditions of the hydrocyclone showed the highest efficiency at Rf = 10% at a flow rate of 2.8 m/s; the DAF-treated solutions showed a higher removal rate for SCOD solutions vs. PCOD solutions, with the highest turbidity removal rate of 65%. In contrast, when the hydrocyclone-treated solutions had a higher PCOD than SCOD, they showed a higher removal efficiency. The removal rates of SS, CODcr, turbidity, T-P, and T-N of the total process were 77, 80, 98, 98, and 45%, respectively.

Effects of irradiation and pH on fluorescence properties and flocculation of extracellular polymeric substances from the cyanobacterium Chroococcus minutus

Microbial extracellular polymeric substances (EPS) may flocculate or be decomposed when environmental factors change, which significantly influences nutrient cycling and transport of heavy metals. However, little information is available on the stability of EPS in natural environments. Fluorescence and flocculation properties of EPS from Chroococcus minutus under different irradiation and pH conditions were studied. Two aromatic protein-like fluorescence peaks and one tyrosine protein-like peak were identified from the excitation–emission-matrix (EEM) fluorescence spectra of EPS. UVB (ultraviolet B) and solar irradiation increased the fluorescence intensity of all the three peaks while UVC (ultraviolet C) irradiation had little effect. EPS formed unstable flocs after exposure to UV (ultraviolet) irradiation and formed stable flocs under solar irradiation. EPS were prone to flocculation under highly acidic conditions and minimal fluorescence of peaks was observed. The fluorophores in EPS were relatively stable under neutral and alkaline conditions. These findings are helpful for understanding the behavior of EPS in aquatic environments and their role in biogeochemical cycles of the elements.

A Research on the Morphology and Composition of Flocs (Part 2: Floc Breakage Included)

The floc breakage plays an important role in the flocculation, the stable floc size or morphology distribution is the result of the equilibrium of formation and breakage. In this paper, the breakage approaches or models for different kind and size of flocs have been discussed. By introducing special breakage functions and comparing their relative values, the numbers of various flocs are verified to be still proportional to the suspension concentration compared with the situation in which the breakage is neglected. However, as illustrated in this paper, the total number of every kind of floc is still a power function of the suspension concentration for dilute suspensions where the number concentration of solid particles is linearly related to the volume concentration. This is similar to that where the breakage is disregarded, although the number for every kind of floc is inevitably changed. In addition, a potential quantitative method to analyze the relative composition of different kinds of flocs is presented by measuring the contributions of the flocs to the suspension viscosity.

Aggregation and breakage kinetics of fresh cement paste

The influence of shear-induced forces on the microstructure of fresh cement pastes was studied. Aggregation and breakage kinetics of the paste matrix are highly influenced by the shear history. It was found that the kinetics of re-aggregation is relatively slow, and time scale for recovery is longer than the time needed for breakdown. When the aggregation kinetics dominates, network interactions among particles develop and the average floc size increases. When the breakage kinetics dominates, network interactions among particles are broken and are accompanied by a decrease in the average floc size. The results suggest that there is a limiting size to floc growth. Minor additions of clays can significantly impact the structural network development and result in a more flocculated structure. The flocs produced by the clays were highly stable flocs with strong interparticle bonds that were able to oppose floc breakage and floc erosion.

Flocculation Kinetics of TiO2-enzyme Microfloccules

TiO2-enzyme microfloccules were prepared by using titanium dioxide nanoparticles as immobilizing carriers, three kinds of polyacrylamide (nonionic, cationic and anionic polyacrylamide) as flocculants and papain as a model of en- zyme. The effects of pH values, dosage and types of polyacrylamide (PAM) on the flocculation and sedimentation behaviour of TiO2-enzyme microfloccules were investigated. The SEM, EDS and particle size analyzer were used to characterize the morphology of TiO2-enzyme microfloccules. The results showed that an effective flocculation was formed among TiO2, pa- pain and different types of PAM flocculant by hydrogen bonding interactions, electrostatic attractions, adsorption bridging action, etc. It was noted that the settling rate of TiO2-enzyme microfloccules, turbidity of the supernatant, floc size and com- pactness of resulting floccules were highly dependent on the PAM dosage. For three kinds of PAM, similar trends of floccu- lation kinetics were observed, a general increase in settling rates were relevant to decrease in turbidity. Attributed to high settling rates, strong flocculation with big size and stable floc occurred with the function of optimum PAM concentrations. But the optimum dosages were different. When the concentration was in the range of 75 mgL -1 to 175 mgL -1 , nonionic PAM (nPAM) displayed the best flocculant performance in all kinds of PAM with a rapid settling rate and large floc size. Moreover, under excessive dosage condition, breakup of floc then occurred. Flocculation kinetics of TiO2-enzyme micro- floccules also could be effectively controlled by changing the pH value of reaction system. Compared nPAM with cationic PAM (cPAM), the microfloccules by using nPAM displayed a high stability and compactness in a wide range of pH values. The settling rate and floc size by using cPAM showed a strong dependence on pH values. It indicated that the immobilized enzyme size could be regulated by the PAM dosage and pH value according to the enzyme structure and properties. Such porous and flexible microstructure was expected to provide the free space as much as possible for the access of substrate molecules to enzyme. Keywords flocculation kinetics; settling rate; particle size distribution; nano TiO2; morphology

Emulsifying Properties of Squid Mantle Actomyosin (Illex argentinus)Stored at 2-4°C. Effect of some Inhibitors

The possible effect of the proteolytic activity on emulsifying properties of squid ( Illex argentinus ) mantle actomyosin stored at 2-4°C and the effects of cocktail of protease inhibitors [phenylmethylsulfonyl fluoride (PMSF), iodo acetic acid (IAA) and ethylene diamine-tetra acetic acid (EDTA)] were investigated. Oil/water emulsion and their stability were studied by optical characterization using a vertical scan analyzer. The particle size distribution of emulsions was obtained with a particle analyzer. O/W emulsions formulated with actomyosin of squid mantle with inhibitors showed certain stability during the first 15-20 min, and then destabilize during the analyzed remaining time, reaching BS (Backscattering) of approximately 20% with no significant changes thereafter. However, in emulsions formulated with actomyosin without inhibitors, the decrease in BS was recorded at 30-40 min, indicating a greater stability, as a function of the storage time, in comparison with those in presence of inhibitors actomyosin. The particle size distribution of both emulsions presented, for actomyosin with and without inhibitors, three droplet populations at zero time. Greater droplet size, De Brouker mean diameter D [4.3], was observed for either emulsions formulated with actomyosin without and with inhibitors, at 24 and 48h of storage. The addition of SDS solution led to a reduction of the population of large particles suggesting the presence of stable flocs under the analyzed conditions. In addition, the P (polidispersity) values corresponding to actomyosin without inhibitors were significantly (p < 0.05) higher than those of actomyosin with inhibitors. Both emulsions exhibited a significant destabilization by creaming and flocculation. The stability of emulsions was enhanced when aggregates appeared in emulsions, mainly for emulsions stored without inhibitors. These results suggest that the proteolytic activity can favour the emulsifying properties of actomyosin obtained from squid mantle stored at 2-4°C. The structure of flocs would affect positively the stability of O/W emulsions.

Reducing Phosphorus Concentration in Animal Feed Coproducts from the Corn Distilling Industry

High phosphorus (P) content of animal feed coproducts of dry grind corn ethanol plants can lead to dietary P levels that are incompatible with P-based nutrient management plans. Therefore, treatment of thin stillage, the dominant P contributor of coproducts, was investigated using centrifugation followed by chemical treatment with coagulants (lime, alum, ACH) and flocculants (anionic and cationic polymers). The majority of the P and solids in thin stillage are in the dissolved form (<0.45 m). Centrifugation resulted in a solids fraction with high crude protein and low P content, which could be appropriate for use in animal feed rations. Removal of both inorganic (dissolved reactive P) and organic (phytase hydrolyzable P) P was achieved with chemical treatment. Lime treatment resulted in significantly (p < 0.05) higher P removal (up to 85%) than Al-based coagulants (up to 70%). Anionic polymers resulted in better flocculation and produced bigger and more stable flocs compared to those obtained with cationic polymers. The low total solids removal achieved could be attributed to the inability of chemicals used to separate the dissolved solids component. Our results suggest that while both Ca-based and Al-based coagulants are effective in reducing the P content of thin stillage, extremely low alkalinity of thin stillage limits the effectiveness of acidic coagulants.

Symbiotic algal bacterial wastewater treatment: effect of food to microorganism ratio and hydraulic retention time on the process performance

Algal incorporation into the biomass is important in an innovative wastewater treatment that exploits the symbiosis between bacterial activated sludge and microalgae (Chlorella vulgaris sp. Hamburg). It allows a good and easy algae separation by means of clarification. The effect of process parameters food to microorganisms ratio (F/M) and hydraulic retention time (HRT) on the process performance, evaluated by settleability, microalgae incorporation to biomass and nutrient removal, was studied. HRT hinted at a significant influence in the growth rate of algae, while F/M turned out to be important for stability when algae are incorporated into the biomass. This parameter also affects the total nitrogen removal of the treatment. Stable flocs with incorporated algae and supernatants with low free swimming algae concentrations were obtained at high HRT and low F/M values.

Evaluation of an Alternative Flocculation System for Manufacture of Fiber−Cement Composites

Flocculation has been a key issue for fiber cement manufacturing since cellulose or poly(vinyl alcohol) (PVA) fibers are used to replace asbestos. Due to the complexity of flocculation, many fiber cement companies face difficulties in optimizing it, which leads to unpredicted production problems and lower process efficiency. This paper studies the behavior of a dual system (PFR/PEO) and compares it with two anionic polyacrylamides commonly used in this industry. The work was carried out using a focused beam reflectance measurement as a sensor to monitor the flocculation process in real time. Results show that the proposed alternative dual system induces the formation of larger but less stable flocs than the ones obtained with the anionic polyacrylamides. A flocculation mechanism was proposed for the dual system consisting of the initial adsorption of the phenol−formaldehyde resin (PFR) onto the particles, which provides junction points for bridge formation by poly(ethylene oxide) (PEO). The flocculation k...

Characterization of floc size, strength and structure under various coagulation mechanisms

Flocs generated by various coagulation mechanisms exhibit different size, strength and structure. The properties and fractal dimensions of flocs formed under three common coagulation mechanisms, i.e. charge neutralization, sweep and bridging, were investigated at various hydraulic conditions. The results showed that the floc size decreased with the increasing average velocity gradient G and the stable floc size exponent γ was of the following hierarchy: charge neutralization (0.6107) > sweep (0.5618) > bridging (0.3674). Furthermore, fractal dimensions of flocs were the highest when formed by sweep and the lowest when generated by bridging flocculation. The mass fractal dimensions measured by light scattering were between 2.0 and 3.0 and the floc strength was between 0.01 and 0.58 N m − 2 . An intrinsic unity of the relationship among floc size, fractal dimensions, floc strength under the three coagulation mechanisms was demonstrated.

Aggregation dynamics in systems of coalescing non-deformable droplets

Emulsion stability simulations (ESS) are used to study the aggregation behavior of oil-in-water (O/W) emulsions composed of non-deformable drops. In the absence of a strong repulsive force (low surfactant concentration), the number of aggregates decreases along with the number of drops following the dynamics predicted by Smoluchowski for irreversible flocculation. During this initial stage, the drops coalesce as soon as they make contact and no stable flocs are formed. The polydispersity of the system increases while its interfacial area decreases. This behavior disfavors the formation of aggregates with the characteristic fractal dimensions exhibit by suspensions under diffusion limited cluster aggregation (DLCA) and reaction limited cluster aggregation (RLCA) regimes. Moreover, the increase of the surfactant surface excess due to the redistribution of surfactant molecules among the available interfaces progressively augments the repulsive force between the remaining drops. In the absence of surface deformation, coalescence stops once the total interfacial area of the emulsion can be stabilized with the available surfactant concentration. At this point, aggregation occurs. In this terminal stage the aggregation behavior is likely to be determined by the characteristics of the secondary minimum of the interaction potential. A high surfactant concentration in the initial system leads to very different results. It prevents coalescence making the drops behave as solid particles. In this case clusters with the fractal dimensions corresponding to the DLCA and RLCA regimes are observed.

Reversible shear gelation of polymer–clay dispersions

Reversible, shear-induced gelation of semi-dilute aqueous colloidal dispersions consisting of monodisperse discoid particles (Laponite) and weakly adsorbing polymer (polyethylene oxide) is studied through a combination of small angle neutron scattering and oscillatory shear. When shaken the samples undergo a dramatic transition from a low viscosity fluid to a self-supporting, turbid gel. This complex non-linear behavior is found to occur over a narrow composition regime near a composition commensurate with saturation of the clay surface with polymer. Through a combination of SANS and rheology, shear gelation is found to occur through the deformation of large stable flocs that expose fresh surface area for the formation of new polymer bridges. At rest, the temporary shear-induced flocs slowly fractionate with time as the polymer desorbs from the clay surface. The shear-induced gelation is time reversible and strongly temperature-dependent suggesting that relaxation is an activated process. Samples showing shear induced gelation are also able to form stiff stable gels which are characteristically similar to pure clay dispersions.

Possible occurrence of a Crabtree effect in the production of lactic and butyric acids by a floc-forming bacterial consortium.

Lactic and butyric acid production by bacterial flocs in a continuous culture obeyed different physiological constraints. The butyric acid rate of production was constant and independent of the growth rate [0.012 +/- 0.001 gBUT/(L.h)], whereas lactic fermentation occurred only beyond a critical growth rate (0.25 +/- 0.05 h(-1)) and was apparently associated with an abrupt drop in biomass. Principles of modeling used to describe a Crabtree effect in Saccharomyces cerevisiae were found to apply to lactic acid production by flocs. A rank of "physiological unit" (or "metabolic unit") can be attributed to the bacterial floc. From a practical point of view, the production of fermentation products by stable flocs, naturally resistant to contamination, opens the possibility of industrial production by continuous cultivation by using flocs-forming consortia.

Extracellular polymer production and potential for aggregate formation by classical propionibacteria

Nineteen Propionibacterium strains were screened for extracellular polymer (ECP) production. The best producer, P. jensenii S1, was introduced into two different media, Yeast Extract Lactate (YEL)-medium and Apricot Effluent (AE)-medium. The YEL medium samples were incubated in different mixing systems (a roller-table and a linear shaking platform) for 24 days at 35 °C. According to the volatile fatty acids and pH profiles, no real differences could be detected between the two mixing systems. Bacterial aggregates were, however, only observed in the roller-table samples. The process was repeated with AE-medium on the roller-table. Larger and more stable flocs were observed in the AE-medium samples. Scanning electron microscopy and PCR analysis confirmed the presence of propionibacteria in these flocs even after 5 months of storage at 4 °C. It was concluded that ECP-producing Propionibacterium strains could be manipulated to form bacterial flocs under certain environmental conditions, which might be enhanced in the presence of fibrous material occurring naturally in food industry effluents.