Comparative evaluation of inorganic and organic amendments for their flocculation efficiency of selected microalgae

Cost-efficient harvesting of microalgae is a major challenge due to their small size and often low concentration in the culture medium. The flocculation efficacy of different inorganic and organic amendments was evaluated on various microalgae genera—one strain each belonging to Chlamydomonas, Chlorococcum, two of Botryococcus, and of Chlorella. An improvised medium comprising of commercial grade urea, single super phosphate, and muriate of potash was used to grow the microalgae for flocculation experiments. High pH induced increased flocculation efficiency (72–76 %) in selected microalgal strains. Ferric chloride was found to be the most efficient for most of the microalgal strains, while maize starch and rice starch proved superior for Chlorella sp. MCC6 and Botryococcus sp. MCC32. Although the highest flocculation efficiency was obtained with inorganic flocculant, i.e., ferric chloride (87.3 %) with Botryococcus MCC31, this was comparable with rice starch (86.8 %) for Botryococcus MCC32. This study showed that widely available cheaper biopolymers such as rice starch, maize, and potato starch can be promising flocculants due to their better harvesting efficiency (>80 %) and low price, thereby contributing to economical production of biodiesel from algae.     

Flocculation of microalgae in brackish and sea waters

Flocculation is an essential step in the concentration and harvesting of microalgae from aquatic media. Salinity of brackish water and sea water requires high flocculant dosages and renders flocculation less effective than in freshwater algal media. Experiments with the marine microalgae Isochrysis galbana and Chlorella stigmatophora showed that effective alum or ferric chloride flocculation was obtained only with dosages which are 5 to 10 times higher than the dosages required for the flocculation of freshwater microalgae. The flocculant dosages required for removing over 90% of the algae from suspensions were found to increase linearly with salinity as expressed in ionic strength. High salinity was found to inhibit flocculation with polyelectrolytes which are quite effective in freshwater algae flocculation. This inhibition was diminished at reduced salinity levels and effective flocculation was attained at salinity levels of 5 g/liter and below, which is typical of desert brackish water. Two methods were found to induce flocculation in sea water: (a) combining polyelectrolytes with inorganic flocculants such as ferric chloride or alum, and (b) ozone oxidation pretreatment followed by flocculation with inorganic flocculants.     

Effective biomass harvesting of marine diatom Chaetoceros muelleri by chitosan-induced flocculation,preservation of biomass,and recycling of culture medium for aquaculture feed application

Microalgae are a promising new source of biomass; however, large-scale economical harvesting of microalgal biomass is a major technological and economic challenge, limiting the commercial production of microalgal biomass for high-value compounds. In this study, the cationic polymer chitosan was used for the harvesting of the marine diatom Chaetoceros muelleri. Natural flocculation, and pH and chitosan-induced flocculation were studied in detail. The effects of flocculant dosage, culture pH, initial biomass concentration, and sedimentation time were investigated on biomass recovery. The results showed that flocculation efficiency can reach > 99% with an optimum dosage of chitosan (80 mg L^?1) at pH 9.6 and settling time of 40 minutes for biomass concentration from 0.2 to 1.2 g L^?1. The reusability of the recycled water, preservation of biomass after harvesting, and cost of the harvesting process were evaluated. The results showed that the chitosan-induced flocculation offers an efficient, cost-effective, rapid, and sustainable harvesting method for C. muelleri biomass for food and feed applications in aquaculture.

     

Selection of optimal flocculant for effective harvesting of the fucoxanthin-rich marine microalga <Emphasis Type="Italic">Isochrysis galbana</Emphasis>

Flocculation harvesting of the fucoxanthin-rich marine microalga Isochrysis galbana has received little attention. Therefore, we attempted to screen for an optimal chemical flocculant and optimize flocculation conditions from five chemical flocculants—ferric chloride (FC), aluminum sulfate (AS), polyaluminum chloride (PAC), aluminum potassium sulfate (APS), and zinc sulfate (ZS)—for effective flocculation of I. galbana. The growth rate, photosynthetic performance, and fucoxanthin content were determined in re-suspended flocculated algal cells and in the flocculation supernatant cultured algal cells. The results showed that high growth rate and fucoxanthin accumulation were observed when FC was used as the flocculant in I. galbana cultures, which indicated that FC may cause less harm to I. galbana than the other aluminum-based flocculants. Furthermore, satisfactory flocculation efficiency was also observed when FC was used to flocculate I. galbana, and the FC dosage was less than that required for flocculation of I. galbana using PAC, APS, and AS. Thus, we selected FC as the optimal flocculant for harvesting I. galbana based on its flocculation efficiency together with algal physiological performance, growth rate, and fucoxanthin content.     

Flocculation of Chlorella vulgaris by Lactobacillus casei

Summary The flocculation of Chlorella vulgaris by Lactobacillus casei was studied to determine whether the latter could act as a suitable flocculant for the removal of Chlorella from algal ponds. The flocculating activity of the Lactobacilli was caused by the bacterial cells themselves, and not by diffusible products of bacterial metabolism. Diffusible products of algal metabolism inhibited flocculation. For algae resuspended in water, the best flocculation occurred at pH values less than 3.5 where the charges on the bacterial and algal cells were opposite. For flocculation at least one bacterium was required for every algal cell; in terms of cell concentrations,10 mg/l of bacteria were required to flocculate an algal suspension of 1,000 mg/l. The mechanism of flocculation implied by the results is that positively charged cells of L. casei adsorb to the surface of negatively charged cells of C. vulgaris neutralizing the charge and thus destabilizing the algal suspension. Because of the low pH required and because diffusible products of algal metabolism inhibit the flocculation, it is unlikely that L. casei could be usefully employed as a flocculant of Chlorella from algal ponds.     

Evaluation of electro‐coagulation–flocculation for harvesting marine and freshwater microalgae

Although microalgae are considered as a promising feedstock for biofuels, the energy efficiency of the production process needs to be significantly improved. Due to their small size and low concentration in the culture medium, cost‐efficient harvesting of microalgae is a major challenge. In this study, the use of electro‐coagulation–flocculation (ECF) as a method for harvesting a freshwater (Chlorella vulgaris) and a marine (Phaeodactylum tricornutum) microalgal species is evaluated. ECF was shown to be more efficient using an aluminum anode than using an iron anode. Furthermore, it could be concluded that the efficiency of the ECF process can be substantially improved by reducing the initial pH and by increasing the turbulence in the microalgal suspension. Although higher current densities resulted in a more rapid flocculation of the microalgal suspension, power consumption, expressed per kg of microalgae harvested, and release of aluminum were lower when a lower current density was used. The aluminum content of the harvested microalgal biomass was less than 1% while the aluminum concentration in the process water was below 2 mg L⁻¹. Under optimal conditions, power consumption of the ECF process was around 2 kWh kg⁻¹ of microalgal biomass harvested for Chlorella vulgaris and ca. 0.3 kWh kg⁻¹ for Phaeodactylum tricornutum. Compared to centrifugation, ECF is thus more energy efficient. Because of the lower power consumption of ECF in seawater, ECF is a particularly attractive method for harvesting marine microalgae. Biotechnol. Bioeng. 2011;108: 2320–2329. © 2011 Wiley Periodicals, Inc.     

Optimization of Chlorella biomass harvesting by flocculation and its potential for biofuel production

Optimization of microalgal biomass harvesting is essential to produce effective and optimum outcomes that can contribute towards a feasible and economical harvesting technique. Two Chlorella species were used, namely, C. vulgaris and C. sorokiniana UKM3. Two essential factors affecting microalgal biomass harvesting via flocculation, namely, the initial pH of the microalgal broth and flocculant (chitosan) concentration were studied. The optimization process was conducted by using a response surface methodology (RSM) based on the model of face-centered-central composite design (FC-CCD). The potential for biofuel application of the harvested biomass was evaluated based on the production of fatty acid methyl esters (FAMEs) by transesterification. The quadratic models obtained from the RSM significantly fitted the experiment data as the p-values were less than 0.05. The initial pH of the microalgal suspension was found to have a more significant effect on the flocculation process than flocculant concentration. For C. vulgaris, the highest flocculant efficiency of 98.7% was obtained at a chitosan concentration of 0.2 g L^?1 and an initial pH of 12.0, whereas for C. sorokiniana UKM3, at 0.15 g L^?1 of chitosan and initial pH of 12.0 produced the highest efficiency of 97.1%. The harvested biomass of both species exhibited a high content of palmitic acid (C16:0) with 29.74 wt% and 11.81 wt% of dry biomass for C. vulgaris and C. sorokiniana UKM3, respectively. This study showed that Chlorella species can be harvested efficiently using the flocculation process and manifested an excellent potential for biodiesel production where palmitic acid (C16:0) is one of the main compounds for high-acid oil-biodiesel.

     

Phycoremediation of Tannery Wastewater Using Microalgae Scenedesmus Species

A number of microalgae species are efficient in removing toxicants from wastewater. Many of these potential species are a promising, eco-friendly, and sustainable option for tertiary wastewater treatment with a possible advantage of improving the economics of microalgae cultivation for biofuel production. The present study deals with the phycoremediation of tannery wastewater (TWW) using Scenedesmus sp. isolated from a local habitat. The test species was grown in TWW under laboratory conditions and harvested on the 12th day. The results revealed that the algal biomass during the growth period not only reduced the pollution load of heavy metals (Cr-81.2–96%, Cu-73.2–98%, Pb-75–98% and Zn-65–98%) but also the nutrients (NO₃ >44.3% and PO₄ >95%). Fourier Transform Infrared (FTIR) spectrums of Scenedesmus sp. biomass revealed the involvement of hydroxyl amino, carboxylic and carbonyl groups. The scanning electron micrograph (SEM) and Energy Dispersive X-ray Spectroscopic analysis (EDS) revealed the surface texture, morphology and element distribution of the biosorbent. Furthermore, the wastewater generated during wet-blue tanning process can support dense population of Scenedesmus sp., making it a potential growth medium for biomass production of the test alga for phycoremediation of toxicants in tannery wastewaters.     

Recent progress in the use of processed microalgae in aquaculture

Mass-cultured algal biomass has been tested as a food source for a number of aquaculture animals because of its low cost and convenience. This paper reviews the results of nutritional studies on processed microalgae with respect to mollusc, crustacean, rotifer and fish culture. Research using species of Spirulina, Chlorella, Scenedesmus and other mass-produced algae indicates that microalgae can be an effective dietary component provided that processing, diet formulation and presentation requirements are met. Processed microalgae can be used to correct specific dietary deficiencies in artificial diets. Our research found that the growth and pigmentation of marron, Cherax tenuimanus (Decapoda, Crustacea), can be significantly enhanced by the incorporation of Dunaliella salina in its artificial diet. Likewise, rainbow trout, Oncorhynchus mykiss, were pigmented by Haematococcus pluvialis.     

Optimization of pH induced flocculation of marine and freshwater microalgae via central composite design

Microalgae harvesting via pH induced flocculation along with utilization of recovered medium after flocculation is one of the most economical methods for separating the microalgal biomass in order to reduce the dewatering cost. In this study, optimization of marine and freshwater microalgae flocculation by pH adjustment was investigated via central composite design methodology. One molar of KOH and NaOH solutions were used to increase the pH level of the microalgal culture. Increasing pH value of the medium provided the highest flocculation efficiency up to 92.63 and 86.18% with pH adjusted to 10.5 with KOH and NaOH solutions for marine microalgae Nannochloropsis oculata and freshwater microalgae Chlorella minutissima, respectively. Also, it was revealed that microalgae cells were still alive after flocculation process and their biochemical composition was not changed, and flocculated medium can be used again for the next microalgal production. According to the results, it can be said that this method is cheap and effective, simple to operate and provides the utilization of flocculated medium again.     

利用絮凝进行微藻采收的研究进展

微藻可生产不饱和脂肪酸及色素等多种高附加值产品,同时也可用来生产可再生清洁能源如生物柴油等,具有良好的应用前景。但是,目前微藻细胞的采收成本高居不下,已成为限制微藻生物技术大规模应用的重要因素之一。与其他方法相比,絮凝采收成本低、操作简便,是很有应用前景的采收方法。本文综述了国内外利用化学絮凝、物理絮凝及生物絮凝等方法对不同微藻细胞进行采收的研究,重点对生物絮凝方法进行了总结。利用微生物絮凝剂及微藻细胞的自絮凝进行微藻生物量的回收,是微藻采收技术中环境友好、低成本和行之有效的新方法之一。     

Harvesting of microalgae by bio-flocculation

The high-energy input for harvesting biomass makes current commercial microalgal biodiesel production economically unfeasible. A novel harvesting method is presented as a cost and energy efficient alternative: the bio-flocculation by using one flocculating microalga to concentrate the non-flocculating microalga of interest. Three flocculating microalgae, tested for harvesting of microalgae from different habitats, improved the sedimentation rate of the accompanying microalga and increased the recovery of biomass. The advantages of this method are that no addition of chemical flocculants is required and that similar cultivation conditions can be used for the flocculating microalgae as for the microalgae of interest that accumulate lipids. This method is as easy and effective as chemical flocculation which is applied at industrial scale, however in contrast it is sustainable and cost-effective as no costs are involved for pre-treatment of the biomass for oil extraction and for pre-treatment of the medium before it can be re-used.     

pH-induced flocculation,indirect electrocoagulation,and hollow fiber filtration techniques for harvesting the saltwater microalga Dunaliella

This research assessed the efficacy of three harvesting methods on a strain of Dunaliella viridis. While there is strong potential to use lipids from microalgae as a feedstock for biofuels to replace petroleum-based fuel, at present microalgal harvesting for biofuel production is not yet economically feasible or energy efficient. pH-induced flocculation (by adjusting the pH of exponentially growing cells), indirect electrocoagulation (applying aluminum hydroxide coagulant to culture), and hollow fiber filtration (separating biomass from medium using tangential flow) were compared as potential harvesting mechanisms for small-scale (3–10 L) and large-scale (30–150 L) volumes of D. viridis. Both pH-induced flocculation and electrocoagulation yielded significant biomass recovery (>95 %), but both methods required removal of added chemicals and/or coagulant before the medium could be reused. In contrast, hollow fiber filtration did not require added chemicals or coagulant, and as another advantage, the filtrate was successfully reused as culture medium without apparent detrimental effects on cell size, cell shape, or cell production. When high salinity stress was imposed on the concentrate produced from the filtration method, total fatty acids (FAs) did not increase. However, total FAs did significantly increase after hollow fiber filtration (49 %) in comparison to FA content before filtration (36 %). This research indicates that hollow fiber filtration as a commercial harvesting mechanism offers attractive advantages as a harvesting mechanism for microalgae such as Dunaliella, relative to pH-induced flocculation and indirect electrocoagulation.     

Comparative life cycle assessment of autotrophic cultivation of Scenedesmus dimorphus in raceway pond coupled to biodiesel and biogas production

Life cycle assessment (LCA) of indigenous freshwater microalgae, Scenedesmus dimorphus, cultivation in open raceway pond and its conversion to biodiesel and biogas were carried out. The LCA inventory inputs for the biogas scenario was entirely based on primary data obtained from algal cultivation (in pilot scale raceway pond), harvesting, and biogas production; while only the downstream processing involved in biodiesel production namely drying, reaction and purification were based on secondary data. Overall, eight scenarios were modeled for the integrated process involving: algae-based CO₂ capture and downstream processing scenarios for biodiesel and biogas along with impact assessment of nutrient addition and extent of recycling in a life cycle perspective. The LCA results indicated a huge energy deficit and net CO₂ negative in terms of CO₂ capture for both the biodiesel and biogas scenarios, majorly due to lower algal biomass productivity and higher energy requirements for culture mixing. The sensitivity analysis indicated that variability in the biomass productivity has predominant effect on the primary energy demand and global warming potential (GWP, kg CO₂ eq.) followed by specific energy consumption for mixing algal culture. Furthermore, the LCA results indicated that biogas conversion route from microalgae was more energy efficient and sustainable than the biodiesel route. The overall findings of the study suggested that microalgae-mediated CO₂ capture and conversion to biodiesel and biogas production can be energy efficient at higher biomass productivity (> 10 g m⁻² day⁻¹) and via employing energy-efficient systems for culture mixing (< 2 W m⁻³).

     

Magnetophoretic harvesting of freshwater microalgae using polypyrrole/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocomposite and its reusability

A magnetophoretic harvesting agent, a polypyrrole/Fe₃O₄ magnetic nanocomposite, is proposed as a cost and energy efficient alternative to recover biomass of the microalgae Botryococcus braunii, Chlorella protothecoides, and Chlorella vulgaris from their culture media. The maximal recovery efficiency reached almost 99 % for B. braunii, 92.4 % for C. protothecoides, and 90.8 % for C. vulgaris. The maximum adsorption capacity (Q ₀) of the magnetic nanocomposite for B. braunii (63.49 mg dry biomass mg^?1 PPy/Fe₃O₄) was higher than that for C. protothecoides (43.91 mg dry biomass mg^?1 PPy/Fe₃O₄) and C. vulgaris (39.98 mg dry biomass mg^?1 PPy/Fe₃O₄). The highest harvesting efficiency for all the studied microalgae were at pH 10.0, and measurement of zeta-potential confirmed that the flocculation was induced by charge neutralization. This study showed that polypyrrole/Fe₃O₄ can be a promising flocculant due to its high efficacy, low dose requirements, short settling time, its integrity with cells, and with great potential for saving energy because of its recyclability.     

Inexpensive non-toxic flocculation of microalgae contradicts theories; overcoming a major hurdle to bulk algal production

There are two major energy and cost constraints to bulk production of single cell microalgae for biofuels or feed: expensive culture systems with high capital costs and high energy requirements for mixing and gas exchange; and the cost of harvesting using high-speed continuous centrifugation for dewatering. This report deals with the latter; harvesting by flocculation where theory states that alkaline flocculants neutralize the repelling surface charge of algal cells, allowing them to coalesce into a floc. It had been assumed that with such electrostatic flocculation, the more cells to be flocculated, the more flocculant needed, in a linear stoichiometric fashion, rendering flocculation overly expensive. Counter to theory of electrostatic flocculation, we find that the amount of alkaline flocculant needed is a function of the logarithm of cell density, with dense cultures requiring an order of magnitude less base than dilute suspensions, with flocculation occurring at a lower pH. Various other theories abound that flocculation can be due to multi-valent cross-linking, or co-precipitation with phosphate or with magnesium and calcium, but are clearly not relevant with the flocculants we used. Monovalent bases that cannot cross-link or precipitate phosphate work with the same log-linear stoichiometry as the divalent bases, obviating those theories, leaving electrostatic flocculation as the only tenable theory of flocculation with the materials used. The cost of flocculation of dense cultures with this procedure should be below $1.00/T algae for mixed calcium:magnesium hydroxides.     

Rapid harvesting of freshwater microalgae using chitosan

In this study, chitosan was used as a flocculant to harvest freshwater microalgae Chlorella vulgaris. The recovery efficiency of C. vulgaris was tested at various chitosan concentrations. 120 mg/L of chitosan showed the highest efficiency (92 ± 0.4%) within 3 min. The maximum concentration factor of 10 was also achieved at this dose of chitosan. The harvesting efficiency was pH dependent. pH 6.0 showed the highest harvesting efficiency (99 ± 0.5%). Measurement of zeta-potential confirmed that the flocculation was induced by charge neutralization. This study showed that a biopolymer, chitosan, can be a promising flocculant due to its high efficacy, low dose requirements, and short settling time.     

The influence of different flocculants on the physiological activity and fucoxanthin production of Phaeodactylum tricornutum

Phaeodactylum tricornutum is an economically important species of microalgae that is widely used in aquaculture, and it is rich in bioactive substances including eicosapentaenoic acid and fucoxanthin. The major bottleneck for industrialization of this species is harvesting. Flocculation is used to harvest microalgae, thus the selection of flocculants is of great importance. In this study, we compared the flocculation effect of four different chemicals (ferric chloride, aluminum sulphate, polyaluminum chloride, and aluminum potassium sulphate) on P. tricornutum. Microexamination showed that ferric and aluminum salts had similar flocculation effects on the algae. Growth and chlorophyll fluorescence measurements showed that P. tricornutum can be re-cultured after flocculation. Pigment analysis showed that flocculation did not result in degradation of fucoxanthin, which suggests that the four flocculants tested may be useful for industrial applications. The results also showed that ferric chloride was the best flocculant for harvesting P. tricornutum when the target product was fucoxanthin, as it had the least influence on the physiological activity of P. tricornutum and it did not lead to degradation of cell components. In contrast, aluminum is poisonous to the nervous system of animals and humans. In addition, the culture medium can be recycled after flocculation by ferric chloride.     

On the Properties of the Starch Granules from Unicellular Green Algae

Starch granules from Chlorella, Chlamydomonas and Scenedesmus, grown heterotro-phically in a medium containing organic carbon sources, were isolated by means of the toluol treatment of the sonicate of alga. The toluol treatment separated the starch granules in the water layer from the cells and cell debris coagulated in the upper toluol layer.

The starch granules of Chlorella vulgaris and Chlamydomonas sp. were composed of amylose (12 to 3%) and amylopectin. The amylose content of the starch granules of Scenedesmus basilensis was 22 %. All the X-ray diffraction patterns of algal starch obtained in this investigation were of the A-type, identical to that of corn starch.     

Modeling of microalgal shear‐induced flocculation and sedimentation using a coupled CFD‐population balance approach

In this study, shear‐induced flocculation modeling of Chlorella sp. microalgae was conducted by combination of population balance modeling and CFD. The inhomogeneous Multiple Size Group (MUSIG) and the Euler–Euler two fluid models were coupled via Ansys‐CFX‐15 software package to achieve both fluid and particle dynamics during the flocculation. For the first time, a detailed model was proposed to calculate the collision frequency and breakage rate during the microalgae flocculation by means of the response surface methodology as a tool for optimization. The particle size distribution resulted from the model was in good agreement with that of the jar test experiment. Furthermore, the subsequent sedimentation step was also examined by removing the shear rate in both simulations and experiments. Consequently, variation in the shear rate and its effects on the flocculation behavior, sedimentation rate and recovery efficiency were evaluated. Results indicate that flocculation of Chlorella sp. microalgae under shear rates of 37, 182, and 387 s^?1 is a promising method of pre‐concentration which guarantees the cost efficiency of the subsequent harvesting process by recovering more than 90% of the biomass. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 34:160–174, 2018