High pH-induced flocculation of marine <Emphasis Type="Italic">Chlorella</Emphasis> sp. for biofuel production

Microalgae are being considered as a promising raw material for biofuel production. However, rapid, efficient, and economic technologies for harvesting microalgae are essential for successful applications. In this study, the high–pH-induced flocculation method was applied to harvest marine Chlorella sp. strains. These algae could be concentrated up to approximately 20-fold by increasing pH using NaOH, with a flocculation efficiency of 90 %. When NaOH dosage was low (1 or 3 mM), the flocculation efficiency decreased considerably with the increase of biomass concentration. At higher NaOH dosage tested (5 or 7 mM), flocculation occurred quickly and efficiently, which tended to be independent of biomass concentration. In larger volumes, all strains were flocculated with similar efficiencies (approximately 90 %) after adding 5 mM NaOH. After flocculation, the flocculated algae cells could be re-cultured as inoculum, and the growth yields in flocculated medium were slightly higher than those from fresh medium. Additionally, for each strain, there were no significant differences in lipid extraction yield and fatty acid composition according to different harvesting methods. These results showed that the high–pH-induced flocculation method could be used to harvest marine Chlorella sp. for biofuel production successfully.     

Effects of bacterial communities on biofuel-producing microalgae: stimulation,inhibition and harvesting

Despite the great interest in microalgae as a potential source of biofuel to substitute for fossil fuels, little information is available on the effects of bacterial symbionts in mass algal cultivation systems. The bacterial communities associated with microalgae are a crucial factor in the process of microalgal biomass and lipid production and may stimulate or inhibit growth of biofuel-producing microalgae. In addition, we discuss here the potential use of bacteria to harvest biofuel-producing microalgae. We propose that aggregation of microalgae by bacteria to achieve >90% reductions in volume followed by centrifugation could be an economic approach for harvesting of biofuel-producing microalgae. Our aims in this review are to promote understanding of the effects of bacterial communities on microalgae and draw attention to the importance of this topic in the microalgal biofuel field.     

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.     

A novel method to harvest <Emphasis Type="Italic">Chlorella</Emphasis> sp. by co-flocculation/air flotation

Objectives

To develop a more effective dissolved air flotation process for harvesting microalgae biomass, a co-flocculation/air flotation (CAF) system was developed that uses an ejector followed by a helix tube flocculation reactor (HTFR) as a co-flocculation device to harvest Chlorella sp. 64.01.

Results

The optimal size distribution of micro-bubbles and an air release efficiency of 96 % were obtained when the flow ratio of inlet fluid (raw water) to motive fluid (saturated water) of the ejector was 0.14. With a reaction time of 24 s in the HTFR, microalgae cells and micro-bubbles were well flocculated, and these aerated flocs caused a fast rising velocity (96 m/h) and high harvesting efficiency (94 %).

Conclusions

In a CAF process, micro-bubbles can be encapsulated into microalgae flocs, which makes aerated flocs more stable. CAF is an effective approach to harvesting microalgae.

     

Investigating the Efficacy of Integrating Energy Crops into Non-Profitable Subfields in Iowa

Within-field spatial variability reduces growers’ return on investment and overall productivity while potentially increasing negative environmental impacts through increased soil erosion, nutrient runoff, and leaching. The hypothesis that integrating energy crops into non-profitable segments of agricultural fields could potentially increase grain yield and biomass feedstock production was tested in this study using a statewide analysis of predominantly corn- and soy-producing counties in Iowa. Basic and rigorous controls on permissible soil and soil-carbon losses were imposed on harvest of crop residues to enhance year-to-year sustainability of crop and residue production. Additional criteria limiting harvesting costs and focus on large-area subfields for biomass production were imposed to reduce the impacts of energy crop integration on grain production. Model simulations were conducted using 4 years (2013–2016) of soil, weather, crop yield, and management practice data on all counties in Iowa. Miscanthus (Miscanthus x giganteus), switchgrass (Panicum virgatum), and crop-residue-based bioenergy feedstock systems were evaluated as biomass. Average energy crop and plant residue harvesting efficiencies were estimated at 50 and 60%, respectively. Because of higher potential yields, average logistics costs for miscanthus-based biomass production were 15 and 23% lower than switchgrass-based and crop residue-based biomass productions, respectively, under basic sustainability controls, and 17 and 26% lower under rigorous sustainability controls. Subfield shape, size, area, and harvest equipment size were the dominant factors influencing harvesting cost and efficiency suggesting that in areas where subfields are predominantly profitable or harvesting efficiencies low, other options such as prairie strips, buffer zones around fields, and riparian areas should be investigated for more profitable biomass production and sustainable farming systems.     

Perspectives on microalgal CO₂-emission mitigation systems--a review

The problem of climate change arising mainly from CO? emission is currently a critical environmental issue. Biofixation using microalgae has recently become an attractive approach to CO? capture and recycling with additional benefits of downstream utilization and applications of the resulting microalgal biomass. This review summarizes the history and strategies of microalgal mitigation of CO? emissions, photobioreactor systems used to cultivate microalgae for CO? fixation, current microalgae harvesting methods, as well as applications of valuable by-products. It is of importance to select appropriate microalgal species to achieve an efficient and economically feasible CO?-emission mitigation process. The desired microalgae species should have a high growth rate, high CO? fixation ability, low contamination risk, low operation cost, be easy to harvest and rich in valuable components in their biomass.     

Perspectives on microalgal CO2-emission mitigation systems — A review

The problem of climate change arising mainly from CO₂ emission is currently a critical environmental issue. Biofixation using microalgae has recently become an attractive approach to CO₂ capture and recycling with additional benefits of downstream utilization and applications of the resulting microalgal biomass. This review summarizes the history and strategies of microalgal mitigation of CO₂ emissions, photobioreactor systems used to cultivate microalgae for CO₂ fixation, current microalgae harvesting methods, as well as applications of valuable by-products. It is of importance to select appropriate microalgal species to achieve an efficient and economically feasible CO₂-emission mitigation process. The desired microalgae species should have a high growth rate, high CO₂ fixation ability, low contamination risk, low operation cost, be easy to harvest and rich in valuable components in their biomass.     

An ultrasonically enhanced inclined settler for microalgae harvesting

Microalgae have vast potential as a sustainable and scalable source of biofuels and bioproducts. However, algae dewatering is a critical challenge that must be addressed. Ultrasonic settling has already been exploited for concentrating various biological cells at relatively small batch volumes and/or low throughput. Typically, these designs are operated in batch or semicontinuous mode, wherein the flow is interrupted and the cells are subsequently harvested. These batch techniques are not well suited for scaleup to the throughput levels required for harvesting microalgae from the large‐scale cultivation operations necessary for a viable algal biofuel industry. This article introduces a novel device for the acoustic harvesting of microalgae. The design is based on the coupling of the acoustophoretic force, acoustic transparent materials, and inclined settling. A filtration efficiency of 70 ± 5% and a concentration factor of 11.6 ± 2.2 were achieved at a flow rate of 25 mL·min^?1 and an energy consumption of 3.6 ± 0.9 kWh·m^?3. The effects of the applied power, flow rate, inlet cell concentration, and inclination were explored. It was found that the filtration efficiency of the device is proportional to the power applied. However, the filtration efficiency experienced a plateau at 100 W L^?1 of power density applied. The filtration efficiency also increased with increasing inlet cell concentration and was inversely proportional to the flow rate. It was also found that the optimum settling angle for maximum concentration factor occurred at an angle of 50 ± 5°. At these optimum conditions, the device had higher filtration efficiency in comparison to other similar devices reported in the previous literature. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 31:414–423, 2015     

Microbial flocculation, a potentially low-cost harvesting technique for marine microalgae for the production of biodiesel

Microbial flocculation is investigated as a separation technique for harvesting marine microalgae for the production of biodiesel. Organic carbon (acetate, glucose or glycerine) was used as substrate for the growth of flocculating microbes in situ. Under stress, due to nutrient depletion, these microbes produced extracellular polymeric substances that promote flocculation of the coccolithophorid alga, Pleurochrysis carterae. Maximum recovery efficiency was achieved at low concentration of organic substrate (0.1 g L⁻¹) and with a long mixing time (24 h); an average recovery efficiency of over 90% and a concentration factor of 226 were achieved. The recovery efficiency is positively correlated with mixing time (R ² = 0.90). The concentration factor is negatively correlated to the product of substrate concentration and mixing time (R ² = 0.73). The microalgae cells were not under stress and remained viable, thus potentially allowing media to be reused in large-scale processes without further treatment. Other advantages of the process are that no metallic flocculants were required and the organic substrates are readily available, e.g. glycerine is a by-product of biodiesel production and acetic acid may be produced by anaerobic digestion of the biomass residue after lipid extraction. Further research is required to optimise the process.     

Flocculation of microalgae using cationic starch

Due to their small size and low concentration in the culture medium, cost-efficient harvesting of microalgae is a major challenge. We evaluated the potential of cationic starch as a flocculant for harvesting microalgae using jar test experiments. Cationic starch was an efficient flocculant for freshwater (Parachlorella, Scenedesmus) but not for marine microalgae (Phaeodactylum, Nannochloropsis). At high cationic starch doses, dispersion restabilization was observed. The required cationic starch dose to induce flocculation increased linearly with the initial algal biomass concentration. Of the two commercial cationic starch flocculants tested, Greenfloc 120 (used in wastewater treatment) was more efficient than Cargill C*Bond HR 35.849 (used in paper manufacturing). For flocculation of Parachlorella using Greenfloc 120, the cationic starch to algal biomass ratio required to flocculate 80% of algal biomass was 0.1. For Scenedesmus, a lower dose was required (ratio 0.03). Flocculation of Parachlorella using Greenfloc 120 was independent of pH in the pH range of 5 to 10. Measurements of the maximum quantum yield of PSII suggest that Greenfloc 120 cationic starch was not toxic to Parachlorella. Cationic starch may be used as an efficient, nontoxic, cost-effective, and widely available flocculant for harvesting microalgal biomass.     

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.     

Carbon debt and carbon sequestration parity in forest bioenergy production

The capacity for forests to aid in climate change mitigation efforts is substantial but will ultimately depend on their management. If forests remain unharvested, they can further mitigate the increases in atmospheric CO₂ that result from fossil fuel combustion and deforestation. Alternatively, they can be harvested for bioenergy production and serve as a substitute for fossil fuels, though such a practice could reduce terrestrial C storage and thereby increase atmospheric CO₂ concentrations in the near‐term. Here, we used an ecosystem simulation model to ascertain the effectiveness of using forest bioenergy as a substitute for fossil fuels, drawing from a broad range of land‐use histories, harvesting regimes, ecosystem characteristics, and bioenergy conversion efficiencies. Results demonstrate that the times required for bioenergy substitutions to repay the C Debt incurred from biomass harvest are usually much shorter (< 100 years) than the time required for bioenergy production to substitute the amount of C that would be stored if the forest were left unharvested entirely, a point we refer to as C Sequestration Parity. The effectiveness of substituting woody bioenergy for fossil fuels is highly dependent on the factors that determine bioenergy conversion efficiency, such as the C emissions released during the harvest, transport, and firing of woody biomass. Consideration of the frequency and intensity of biomass harvests should also be given; performing total harvests (clear‐cutting) at high‐frequency may produce more bioenergy than less intensive harvesting regimes but may decrease C storage and thereby prolong the time required to achieve C Sequestration Parity.     

Harvesting <Emphasis Type="Italic">Chlorella vulgaris</Emphasis> via rapid sedimentation induced by combined coagulants and tapered shear

Objectives

In this study, a rapid sedimentation induced by combined coagulants and gradual shear was developed to harvest Chlorella vulgaris.

Results

The microalgal harvesting efficiency was observably promoted by the synergistic effect between FeCl₃ and PAM, especially in the first 10 min. A higher harvesting efficiency, 95.61%, could be achieved within approximately 3 min due to the large and dense flocs generated by the combined coagulants. In contrast, the efficiencies were only 54.25 and 60.20% with FeCl₃ and PAM, independently. When coagulation was performed under gradually reduced shear (from 50 to 30 rpm), smaller clusters or cells filled the pores of the aggregates via interception, which caused the flocs to become larger and more compact.

Conclusions

The sedimentation time was shortened to 30 s for microalgal coagulation induced by the simultaneous use of combined coagulants and tapered shear, providing an effective approach to harvesting microalgae.

     

絮凝微生物采收微藻的作用机制研究进展

微藻广泛分布于自然界,其易培养,生长快且应用价值高,普遍用于生物燃料、医学原料、优质食品源及畜牧养殖业等。近年来,通过对光生物反应器改造设计、高产藻株筛选、代谢通路基因改造等方法实现微藻产量的提高,而在微藻处理的下游过程的研究与创新不足,特别是微藻采收已经成为其产业发展的瓶颈。本文综述了絮凝法在微藻采收中的作用,重点讨论了絮凝微生物在微藻采收中的作用,并对絮凝微生物对微藻的絮凝机制进行广泛探讨,为絮凝微生物采收微藻提供理论依据。     

Membrane technology in microalgae cultivation and harvesting: A review

Membrane processes have long been applied in different stages of microalgae cultivation and processing. These processes include microfiltration, ultrafiltration, dialysis, forward osmosis, membrane contactors and membrane spargers. They are implemented in many combinations, both as a standalone and as a coupled system (in membrane biomass retention photobioreactors (BR-MPBRs) or membrane carbonation photobioreactors (C-MPBRs). To provide sufficient background on these applications, an overview of membrane materials and membrane processes of interest in microalgae cultivation and processing is provided in this work first. Afterwards, discussion about specific aspects of membrane applications in microbial cultivation and harvesting is provided, including membrane fouling. Many of the membrane processes were shown to be promising options in microalgae cultivation. Yet, significant process optimizations are still required when they are applied to enable microalgae biomass bulk production to become competitive as a raw material for biofuel production. Recent developments of the coupled systems (BR-MPBR and C-MPBR) bring significant promises to improve the volumetric productivity of a cultivation system and the efficiency of inorganic carbon capture, respectively.     

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.     

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.     

微藻在废水处理和生物质回收再利用方面的研究进展

随着经济的发展和人口的增加,环境污染和水资源短缺已经成为不可避免的全球性问题。基于微藻的废水处理技术不仅可以净化废水、解决环境污染问题,还可以利用废水中的营养元素合成生物质,现如今这种技术已经受到越来越多的关注。为了进一步提高废水处理效果、降低废水处理成本,有必要了解微藻去除废水中营养物质和污染物的机理,开发下游低成本收获技术,提升微藻高价值副产物的生产。本文综述了微藻去除碳、氮、磷、重金属、抗生素和有机物的机理和影响因素,总结了微藻的不同收获方式和微藻生物质在各个领域的应用。最后,分析了不同微藻共培养体系和微藻固定化技术的优缺点,并展望了微藻废水处理技术未来的发展方向。     

微藻采收技术的研究进展

微藻作为一种有巨大应用前景的生物质资源,在环境保护、废水处理和清洁能源等领域广泛应用。但是微藻采收成本过高严重限制了微藻产业的发展,因此,寻找一种经济、环保、高效的采收技术对促进微藻产业的发展具有十分重要的意义。本文分析了常用微藻采收技术的优缺点,包括离心分离、沉降、过滤、浮选和絮凝技术,重点论述絮凝技术在微藻采收方面的研究进展,以期为微藻高效、低成本采收方案的选择及其研究方向提供参考。     

Efficacy of microalgae for industrial wastewater treatment: a review on operating conditions,treatment efficiency and biomass productivity

Sustainable, clean, renewable energy without negotiating contiguous environment is a challenging task mainly comprises of natural resource management which involves operational efficiency, waste minimisation and energy recovery. Disposal of untreated industrial wastewater with chemical nutrients especially compounds containing nitrogen and phosphorous lead to eutrophication and related environmental issues that affect the recycling processes of bio system. Biotransformation of pollutants using microalgae has proven to be proficient and economic method of wastewater treatment due to their adaptability of growing in various wastewater streams and also useful in the process of CO₂ fixation. Moreover this technology has the competence of producing bio fuels as an alternative energy resource in the form of bio diesel, bio ethanol and biogas. In this review paper, the applicability of microalgae cultivation in industrial wastewater treatment has been discussed extensively including the processes involved, influencing operational parameters such as study mode, cultivation mode and time, method of aeration, pH and intensity of light. Further, the cultivation methods, harvesting techniques involved in the treatment process have been presented. In addition, the analysis on removal efficiency of algal treatment, biomass productivity and lipid content of the cultivated biomass has been discussed widely which possibly will be helpful in adopting the process integration in industrial wastewater treatment with bio energy production.