The Recovery Of Microalgae Biomass Biology Essay


Energy production from microalgae represents a third generation of biofuel with a lot of potentials because of its advantage over first and second generation biofuels which have sustainabililty issues such as their competition for land and water and threat to food security. However, generating energy from these cells has yet to reach commercial application as a result of the need to discover technologies that can make them cost-competitive and minimize the complexity associated with such generation. For the first time, "a single stage process" which will involve using Nyex®100, an adsorbent, to recover (harvest) microalgae cells from a dilute broth and subsequent electrochemical regeneration of the adsorbent with the aim of disrupting the cells is considered in this project. It is expected that such cell disruption will not only extract lipids but will also minimize the cost, high energy intensity and the difficulties inherent in other known two-stage processes which usually require either further dehydration of microalgal biomass or addition of chemicals or a combination of both methods.

2.2 Background

2.2.1 Quest for Sustainable Energy

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The International Energy Agency (2010) reported that in 2008 fossil fuel accounted for about 81.3 per cent of world total primary energy supply and this superiority in the global energy mix will continue unchallenged in the foreseeable future. However, the amount of CO2 emissions from fossil fuels mainly through combustion is 96.4 percent of world CO2 emissions by fuel usage (IEA).

With the concern of the impacts of these emissions on the climate coupled with the security of supply as a result of the finite nature of fossil fuel, global society is at crossroads; how can the needs of the rising population for increasing energy supply be met while minimizing the footprints of such increase on the environment? Renewable energy from biomass is one of the mitigation options to provide sustainable energy in which security of supply can be guaranteed and emissions of anthropogenic greenhouse gases to the atmosphere can be kept at a reasonable level in order to reduce the negative effects of climate change (IPCC, 2007).

2.2.2 Microalgae Biofuel as an Option

Microalgae represent a large and diverse group of unicellular photo- and heterotrophic organisms which have attracted much international attention in recent years as a result of the important natural products they produce, their ability to remediate effluents and for their potential as energy crops (Greenwell et al 2010). Microalgae accumulate lipids such as triacylglycerols (TAG) and depending on the kind of species; do so either naturally or under stress conditions such as high light or nutrient starvation (USDOE, 2010). Lipids along with other valuable products from the microalgae cell are potential biofuel or biofuel precursors which therefore makes understanding of the metabolic pathways and processes that generate them a significant research area in order to advance biofuels production (USDOE, 2010). Though not yet produced at commercial scale, it is believed that renewable energy from microalgae has the potential to replace liquid fuel from fossil fuel without the sustainable controversies other sources of biomass fuel have generated (Brennan and Owende 2010).

2.2.3 Present Methods of Microalgae Biofuel Production; Challenges and Prospects

Production of lipids from microalgal cells requires the following: (1) cultivation of the microalgal biomass; (2) harvest/recovery of the biomass from a relatively dilute broth; (3) extraction of the lipids from the biomass; and (4) purification of the crude extract (Grima et al 2003). The costs of harvesting microalgal biomass which generally requires one or more solid-liquid separation steps is a challenging phase of the algal biomass production process accounting for up to 20-30% of the total cost of production (Greenwell et al 2010; Brennan and Owende 2010 ). The first challenge is to concentrate cells from relatively dilute solutions and the key harvesting and dewatering operations currently used are flocculation, sedimentation in gravity field, centrifugation, flotation and filtration (Greenwell et al 2010; Brennan and Owende 2010). To efficiently extract the lipids or other valuable products, a second stage is usually needed in which the concentrated biomass cells are ruptured either through mechanical or chemical methods or a combination of both methods (Greenwell et al 2010, Brennan and Owende 2010). The mechanical disruption of cells which is preferred as it offers a methodology that avoids further chemical contamination of the algal preparation while preserving most of the functionality of the material within the cell includes the use of high-pressure homogenisers, ultrasounds, autoclaving and bead mills (Greenwell et al 2010; Brennan and Owende 2010; USDOE 2010). Chemical methods involve the use of organic solvent mixtures such as hexane/ethanol; use of organic solvents at high pressure and temperatures above their boiling point; use of water at temperatures just below its critical temperature and pressure high enough to maintain the liquid state; and utilization of supercritical fluid (Greenwell et al 2010; USDOE 2010).

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As discussed above, the harvesting of microalgal biomass and extraction of lipids/metabolites usually involves two stages. However, because the harvested microalgal biomass slurry is perishable and must be processed rapidly after harvest; dehydration or drying is commonly used to extend the viability depending on the final product required (Brennan and Owende 2010). Likewise, the use of mechanical disruption increases the cost of production as optimum metabolites extraction can only be achieved when the microalgal biomass is dried (Grima et al 2003).Apart from increasing the costs of production, dehydrating or drying is linked to a risk of material loss during movement of microalgal biomass (Brennan and Owende 2010). A disadvantage of using solvents is that it can cause denaturing of cellular materials which ultimately affects the quality of biofuel oil produced (Greenwell et al 2010). In addition, the use of solvents at a large scale requires additional costs due to the very high standard of plant design criteria because of the risk of fire and explosion hazards (Greenwell et al 2010). Hence, there is still need for further research to reduce the costs, minimize the process energy intensity and complexity associated with using present methods in harvesting of microalgal biomass and extraction of lipids (Grima et al 2003; Greenwell et al 2010; USDOE 2010).

2.2.4 Our Proposed Methodology

The method we intend to research on is "a single stage extraction process" which is aimed at developing an efficient lipid extraction process from a harvested microalgal biomass. It is an application of an adsorption process and electrochemical regeneration of an adsorbent (Nyex®100 is the adsorbent in this case) that had been successfully used in the water industry (Brown et al 2004a; Brown et al 2004b). Adsorption Process

It is a mass transfer process of accumulating materials (adsorbate) in solution (liquid phase) on a suitable solid phase referred to as the adsorbent (Metcalf and Eddy, 2003). The need for a better quality of treated wastewater effluent has led to the use of this process where in most cases activated carbon is the adsorbent. The characteristics and concentration of adsorbate and the temperature determines the amount of adsorbate taken up by the adsorbent (Metcalf and Eddy, 2003). To establish the adsorptive capacity of the adsorbent to take up quantity of adsorbate, an adsorption isotherm is developed and prepared for the adsorbate/adsorbent system by exposing a given amount of adsorbate in a known volume of liquid to varying quantities of adsorbent (Metcalf and Eddy, 2003). The adsorbent phase concentration is then determined using equation (1) which are then utilized to develop adsorption isotherms (Metcalf and Eddy, 2003).

Where = adsorbent phase concentration after equilibrium, mg adsorbate/g adsorbent; = initial concentration of adsorbate, mg/L; = final equilibrium concentration of adsorbate after adsorption has occurred, mg/L; = volume of liquid, L; = mass of adsorbent, g.

Freundlich, Langmuir, and Brunauer, Emmet, and Teller (BET) isotherms are developed equations often used to describe experimental isotherm data. Electrochemical Regeneration

Regeneration implies a process used to recover the adsorptive capacity of the "exhausted" adsorbent (Metcalf and Eddy, 2003). For instance, rather than dispose exhausted activated carbon by landfill or incineration, regeneration is the most commercially viable and environmentally acceptable option (Brown et al 2004a). Electrochemical regeneration was developed as a consequence of the need to minimize energy intensity, high cost and loss of material associated with other widely known methods of regeneration (Brown et al 2004a; Brown et al 2004b). It involves the regeneration of loaded adsorbent inside an electrolytic cell which result into desorption and/or destruction of the adsorbed organic matter; thereby restoring the adsorptive capacity of the adsorbent (Brown et al 2004a). However, the limitation of electrochemical regeneration of activated carbons is that it requires long adsorption and regeneration periods because the rate of adsorption and desorption of organics from activated carbons is often governed by intra-particle diffusion (Brown et al 2004b). Nyex®100

This is an alternative adsorbent material developed to remove the limitations inherent in the electrochemical regeneration of activated carbons. Nyex®100, a graphite intercalation compound, is a low cost, highly-conducting carbon-based adsorbent material consisting of non-porous particles with no internal surface area (Brown et al 2004a). Though its low surface area gives rise to a low capacity adsorbent, other properties such as (1) quick adsorption rate; (2) ability to achieve low discharge concentrations; (3) its high electrical conductivity; and (4) improved adsorption efficiency after electrochemical regeneration makes it a cost effective and efficient adsorbent material in the removal/ treatment of contaminants such as atrazine, chlorinated effluents and organic dye (Brown et al 2004a; Brown et al 2004b).

Hypothesis and Objectives of Proposed Project

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The overall aim of this project is to develop an efficient process for microalgal biomass recovery and lipid extraction employing the adsorption process and electrochemical regeneration of the adsorbent which has been successfully applied in the waste water industry. It is expected that when Nyex®100, an adsorbent, is mixed with a dilute broth of microalgal cells, the latter will accumulate on the surface of the adsorbent, thereby harvesting the cells from the dilute broth. It is believed that subsequent electrochemical regeneration of the adsorbent can lead to oxidation of the adsorbed microalgal biomass and which can therefore cause the disruption of the microalgae cells leading to release of lipids while the remaining biomass sinks to the bottom of the electrochemical bed which then makes the recovery of the extracted lipids easier. To verify the robustness of our proposed method, different microalgal cells will be explored. Hence, the objectives of this project are to;

Evaluate and develop adsorption isotherms for microalgae cells/Nyex®100 systems.

Evaluate the effectiveness of Nyex®100 for the adsorption (harvesting) of microalgae cells.

Evaluate the quantity and quality of the lipids produced.

Evaluate different microalgal species in order to identify the species that give the optimum lipids yield using the proposed methodology.

Evaluate the optimum conditions to achieve high lipids extraction from microalgal biomass.

Carry out a life cycle assessment of the proposed and conventional methods.

Programme and Methodology

2.4.1 Methodology

The specie that will be initially investigated is Chlamydomonas reinhardtii because it can be cultivated easily in the laboratory (Hu et al 2008). This specie and other microalgae species to be investigated will be cultivated and their growth monitored at the Faculty of Life Sciences because its research facilities are designed and adapted for such cultivation. Before the adsorption process starts, the harvesting of microalgae cells will be evaluated through monitoring the growth of the cells in terms of dry cell weight and lipid content (Kim et al 2005). This will be achieved by measuring the optical density (O.D) of the culture using a spectrometer and the O.D is plotted against number of days of cultivation. Harvesting can be done when the O.D remains relatively constant for 1 or 2 days before death of the cells start (Kim et al 2005). This is to ensure that harvesting is carried out when microalgae cells can give the maximum harvest yield (Kim et al 2005).

Once the harvesting time is estimated, a batch adsorption experiment will be carried out whereby a known dose of the Nyex®100 will be added and mixed with a dilute solution of the microalgal cells for the purpose of harvesting the microalgae cells. At regular intervals, samples will be taken and spectroscopically analysed to measure the rate of adsorption (harvesting) of the microalgal cells by Nyex®100. From this, the time required to achieve equilibrium can be determined and will enable us to understand the adsorption kinetics.

Consequently, adsorption studies will be investigated by adding various known weights of adsorbent to a dilute solution of the microalgal cells which will be mixed for the time measured to achieve equilibrium. Using the equation (1) stated above, adsorption isotherms can be evaluated and developed for the Chlamydomonas reinhardtii/Nyex®100 system. Outcome from these experiments can assist us to evaluate the effectiveness of Nyex®100 to harvest microalgal biomass.

An electrochemical regeneration of the adsorbent will then be achieved by placing a mixture of the filtered, wet adsorbent and NaCl in the anode compartment of a batch electrochemical cell. A direct current will be applied to regenerate the adsorbent which in turn can lead to the oxidation of the adsorbed microalgal biomass. It is expected that this process will disrupt the microalgal cells leading to release of lipids. The volume of total lipid released will be measured using the modified Bligh and Dyer method. The latter, a relatively quick solvent extraction method, involves quantifying the total lipid released using a mixture of chloroform, methanol and water. (Bligh and Dyer, 1959). The outcome from this stage will enable us to evaluate the feasibility of extracting lipids from microalgal cells using this method.

Because Nyex®100 has a low adsorbing capacity (Brown et al 2004a), the process may only be commercially viable and competitive with other known processes for extracting lipids from microalgal biomass if the adsorbent is capable of being cheaply regenerated many times and can still extract lipids. Hence, regenerated Nyex®100, without any further treatment, will be used to harvest more microalgal biomass and more lipids can be extracted following the procedure described above. The performance of the adsorbent, time required to possibly achieve 100% regeneration, regeneration cycles within which the adsorbent is still effective and the total quantity lipids extracted will be evaluated at this stage. A conventional method which involves harvesting by centrifugation and extracting lipids from harvested biomass by sonification method (the application of ultrasound) will also be carried out (USDOE, 2010). Results obtained will then be compared with results of our proposed method in order to verify whether the proposed method is more effective and efficient.

It has been reported that the amount of lipids extracted is a function of the type of process employed and on the kind of harvested microalgae specie (Kim et al 2005). Consequently, it will be reasonable to evaluate different microalgal species in order to identify the species that gives the optimum lipids yield using proposed methodology. Other microalgal species that will be assessed include Chlorella vulgaris, Dunaliella and Nannochloropsis oculata. Each of them has different physiology and lipid accumulation characteristics. For instance, Dunaliella and Nannochloropsis oculata are marine species which are very high lipid producing species but unlike the former, the latter has a very thick cell wall (Sheehan et al 2008); hence, their breakage properties coupled with their growth in salt water will be interesting to study for our proposed methodology.

The regeneration of the adsorbent in the anodic compartment of the electrochemical cell implies that electrolysis of water is likely to be a significant side reaction generating hydrogen ions (Brown et al 2004a; Brown et al 2004b). The implication of this may be that at lower pH, the performance of the adsorbent may be hampered. It has been shown from earlier works that acidic/alkalinity characteristic of microalgae cells affect the ease of their recovery because naturally microalgae cells have negative charges on their surface which tends to keep them in suspension (Greenwell et al 2010); hence an investigation of the effectiveness of the adsorbent to recover microalgal biomass under acidic/basic condition will be carried out. Kim et al (2005) reported that addition of NaCl enhances the recovery of certain microalgae species with gas vesicle by flotation method, as a result, we will carry out an investigation of how NaCl affects the performance of our process since the latter is used as an electrolyte in the electrochemical regeneration of the adsorbent which is then used to achieve additional lipid extraction. The outcome at this stage will enable us understand the optimum conditions to achieve high lipids yield from microalgae cells using our proposed methodology. Also, depending on the outcomes of the experiments carried out above, further investigations may be required to ascertain the robustness of our proposed method.

Lastly, a life cycle assessment of the proposed and conventional methods will be carried out to determine the environmental impacts of these methods in the recovery of microalgae biomass and lipids extraction. The CCaLC tool will be employed in this regard to measure the carbon footprinting of these methods and to identify opportunities for process improvement.

2.4.2 Milestones

Adsorption isotherms for Chlamydomonas reinhardtii/Nyex®100 system.

A report on our proposed method to harvest C.reinhardtii and extract lipids.

Adsorption isotherms for other microalgal species/Nyex®100 systems.

A report on our proposed method to harvest other microalgal species and extract their lipids.

An LCA report on our proposed and conventional methods.

A final report for Supervisor's review.

Dissertation report for submission.

Each report will also include presentation of experimental data and results.

2.4.3 Programme Management

The student carrying out the research will be mostly based in the laboratory as the project largely involves carrying out experiment. Due to the interdisciplinary nature of the project, he will be working from laboratories of the Faculty of Life Sciences (LF) and the School of Chemical Engineering and Analytical Sciences (CEAS). A weekly meeting will be held where the student will provide updates (either through report or presentation of results) to the supervisor so as to discuss and analyse results, to monitor the progress of the project and resolve challenges that may be encountered during the duration of the project. Researchers (either Dr Pittman or Olumayowa or both) will also be available at LF to analyse the outcomes of experiment and make occasional visits to CEAS so that results can be further analysed while also making use of their expertise in microalgae biofuel to make relevant suggestions that can aid the success of the research.

The work plan for the project is attached with this proposal which shows that an estimate of about 14 weeks. A start date of June 6, 2011 is suggested (though theoretical studies have started long before this date) and a deadline date for submission of a dissertation report by the student to the School of Chemical Engineering and Analytical Sciences (CEAS) is September 9, 2011.

It is likely that all the stated objectives may not be achieved during the duration of this project considering the short period involved and possibility of unexpected challenges that may arise along the way; which is not uncommon for application of novel methods. Nevertheless, the final report that will be submitted will be comprehensive enough to detail the challenges and prospects of this method for future research.

2.5 Relevance to Beneficiaries

We will be working closely with Dr. Jon Pittman of the Faculty of Life Sciences and Olumayowa Osundeko of the Sustainable Consumptive Institute both of whom have been carrying out research on optimizing and improving the lipid content of microalgae cells through sustainable cultivation. We will be utilizing their research facilities to cultivate microalgae cells, monitor their growth and measure the total lipids extracted. These researchers have genuine interest in the outcome of this project as a result of the need to develop a downstream process that will ease harvesting of microalgal biomass especially from a wastewater effluent and subsequent lipid extraction. This implies that our proposed method, if successful, will benefit researchers within the Faculty of Life Sciences, Sustainable Consumptive Institute, and their UK and international networks.

Furthermore, generated experimental data and results and developed adsorption isotherms for microalgae cells/ Nyex®100 systems will be the first of its kind which can be utilized by the research community to enable them understand the recovery principles of microalgal biomass and extraction of lipids using adsorption and electrochemical processes respectively. The research community has been desirous of a technology that is cost-effective and simple in operating; our proposed method has the potential to achieve this status. In addition, new skills and knowledge will be generated that will empower the research community towards discovering a sustainable energy option; a sine qua non for a secured future.

The benefit of the research will also include development of a methodology, if successful, will mean that a novel and innovative technique has been introduced for the first time which can then be further exploited to evaluate the possibility of its commercial application.

This method is a single stage extraction process that will not require the need to transport harvested microalgal biomass thereby removing the risk of material loss. Furthermore, as it involves neither the use of additional solvent nor use of mechanical equipment to cause cell disruption, it is expected to be relatively cheaper than present methods. In addition, it can lower the energy intensity associated with present methods as intracellular products such as lipids will be extracted from the wet biomass without need for drying. Furthermore, an essential consideration in the process of microalgal cells, as with handling biological materials, is that it should take place as rapidly as possible to preserve the value of materials in the source cells (Greenwell et al 2010); undoubtedly, this may be a key advantage of our process as it involves a single stage procedure. All these advantages will obviously be to the benefits of the microalgae biofuel industry.