Potential of microalgae as a feedstock4.1. Characteristics of microalgae for biofuel production 4.
2. Cultivation and Harvesting of Microalgae 4. Potential of microalgae as afeedstockAs the demand for fuel resourcesincreases by the growth of population and global economy new alternateresources are needed.(Razeghifard 2013).The idea about using the microalgae as feedstock for biofuelproduction is not new, but now it is discussed seriously because elimination offossil fuels and the harm of global warming is associated with fossil fuels.
(Chisti 2007).Algae could be prove a viable alternate energy resource as theyuse photosynthesis to take CO2 by changing it into carbon source as biofuel.(Razeghifard 2013)t. Now this biofuel is produced by CO2, water, and sunlight whichare all renewable sources.
(Razeghifard 2013).Algae have cells that converts CO2 into Biofuel, foods andbioactive sources.(Chisti 2007) Algae accumulate carbohydrates and lipids that make algae a goodfeedstock for biofuels such as bioethanol and biodiesel production.
Underoptimum growth conditions algae grow relatively fast than other species and infew hours, they double its number. (Ahn, Hwangbo et al. 2012) They grow at very high rate 100 time faster than other plants andtheir biomass is double in less than a day.(Tredici 2010) Algal harvesting time is very short take less than 10 days andcould be done continuously. They are not like conventional crop which can onlyharvested one or two times a year.
Most of algae need a tropical environmentalcondition however some algal species like chlorella sp. that is isolated form anarctic sea ice could grow at 4 to 32 C temperature.(Ahn, Hwangbo et al. 2012) Algae and some cyanobacteria like spirulina platensis could growin alkaline environments.(Gimmler and Degenhard 2001) Microalgae could provide many typesof biofuels including methane by digestion of algae anaerobically, biodiesel from microalgal oil and gas produced in photosynthesis.(Chisti 2007) The potential advantages of microalgae asfeedstock for biofuels are followings: I. Oilproduced by algae do not have adverse effects on other agriculture because itdoes not compete with other food crops for land.
II. Algaecan be grown in different in every environment, like seawater etc. III. Algalgrowth can be helpful in the treatment of waste water as they removephosphorous and carbon dioxide. IV.
Algaeis helpful as it could be used to produce the broad range of the biofuels suchas hydrogen, syngas and methane also.(Han, Jin et al. 2015) V. Microalgaecould be grow round all the year, so production of oil exceeds to the yield ofthe oilseed crops. VI. Algaegrow in the aqueous environment but need much less quantity of water than othercrops so they reduce the freshwater source load.
VII. Microalgaecan be harvested in saline water therefore it does not experience land usechanges.VIII.
Microalgaehave exponential growth rate that they have oil content range from 20-50%biomass (dry weight) and they double their biomass in very short time as 3.5hours. IX. Algal cultivation does not need anypesticides or herbicides applications. X.
Microalgaecould also produce many useful co products like residual biomass from oilextraction and proteins which are used as fertilizers and feed.The Combination of CO2 fixation,treatment of wastewater, biohydrogen production and potential of biofuelproduction are the potential of microalgae to use as a feedstock. (Brennan and Owende 2010) 4.1. Characteristics of microalgae for biofuel productionMicroalgae accrue carbohydratecomponents and lipid compounds which make them the most favorable feedstock ofbiofuels production such as bioethanol and biodiesel. (Razeghifard 2013) They can be grow in everyenvironment and all year-around which make them suitable for using asfeedstock.(Razeghifard 2013) Microalgae contains fatty acids andlipids as membrane components, metabolites, source of energy and storageproducts that make microalgae a viable possibility as a feedstock for biofuel.
(Demirbas and Demirbas 2011) Microalgae use photosynthesis to convert the light into chemicalenergy and as they can grow in every environment that is not possible for othercurrent feedstocks of biodiesel such as rapeseed, soybean, and palm oil.Microalgal growth rate is higher than agricultural crops, conventional forestryand other aquatic plants. They need less area than other feedstocks 49 to 132 timesless than soybean crops.
(Chisti 2007) Microalgae can play role as feedstock for many different kinds ofrenewable biofuels such as methane, hydrogen, biodiesel and bioethanol.Microalgal biodiesel does not contain sulfur and performs truly as petroleumfuel.(Delucchi 2003) The most significant character ofmicroalgal oil is its biodiesel yield, according to estimation studies theyield of biodiesel from algae per acre is over 200 time than the yield of othercrops and plants.(Demirbas and Demirbas 2011) If algal biodiesel is produce commercially it would reduce the costof per barrel of oil from 100$ to 20$ per barrel only. (Demirbas and Demirbas 2011)The application microalgae forproduction biofuel can also aid other purposes, some of them are thefollowings:· Eliminationof CO2 from industrial gases by algal fixation reduce the greenhousegasses effect while biodiesel production.
(Mata, Martins et al. 2010)· Treatmentof wastewater by removal of NO3, PO4, NH3 making microalgae to grow by usingthese water pollutants as nutrients.(Wang, Li et al.
2008) · Afterwardthe oil extraction the resultant algal biomass could be treated into methane,ethanol feedstock used as bio fertilizer because of its high ratio of N:P orcan be burned for energy like heat and electricity.(Wang, Li et al. 2008)· Microalgae have high value bio derivatives somicroalgae carry the potential to transform huge number of biotechnology fieldssuch as biofuels, pharmaceuticals, cosmetics, nutrition, food additives andpollution prevention.(Mata, Martins et al. 2010) 4.2. Cultivation and Harvesting of Microalgae Ø Cultivation:Microalgae can adjust or molecular structure such as physiologicaland biochemical acclimation so they grow in different enviornments.
(Richmond 2004) As they adaptmany modes for their growth so there are severel methods for microalgalcultivation. For example, photoautotrophic, heterotrophic, photoheterotrophicand mixotrophic.(Mata, Martins et al. 2010) they can adjust themselves metabolically in response ofenvironmental change so different species can grow in different environmentalconditions such as· Photoautotrophic cultivation: uses light sourceto produce the chemical energy by photosynthesis reaction, In this process, thelight is used as energy source and the inorganic carbon is taken as source ofcarbon.
(Wang, Yang et al. 2014) Currently, this method is the only process which iseconomically and technically suitable for the Viable production of algalbiomass.(Brennan and Owende 2010) Two methods are developed for the photoautotropicallycultivation of algae, (1) Open pound production.
(2) Closed photobioreactors.(Borowitzka 1999) A carbondioxide rich environment can increase the productivityof biomass to a certain level.(Chiu, Kao et al.
2008) However, the light penetration decreases rapidly bythe increment of broth turbidity this method got difficulty to achieve the verylarge-scale productivity of biomass.(Markou and Georgakakis 2011) Lipid concentration have great importance as aparameter for biodiesel production.(Demirbas and Demirbas 2011) The lipid concentration (dry weight) varies in microalgae inranging 5-68% in photoautotrophic cultivation.(Chen, Yeh et al. 2011) Large content of lipids could be obtained in the environmentin which nutrient or nitrogen is present in less amount and limitedconcentration, however the productivity of biomass that is achieved in thistype of stressed environment if usually much lower than that is obtained in thenormal conditions, which results in even lower productivity of microalgallipid.(Wang, Yang et al.
2014) The higher productivity of biomass can be obtained by thephotoautotrophic bioreactors than open pound production of microalgae. (Brennan and Owende 2010) · Heterotrophic Cultivation: In this methodmicroalgae is grown on organic source. Carbon source is used as both carbon andenergy source.(Brennan and Owende 2010) Heterotrophic method avoids the limitations that were presentin the photoautotrophy, so higher productivity of biomass can be achieved.(Wang, Yang et al. 2014) Lipid concentration in this method is much higherthan photoautotrophic method.(Wang, Yang et al.
2014) This method is light independent so simpler and smallerfermenters or reactors are used for cultivation.(Brennan and Owende 2010) These setups produce a higher level of growth andalso decrease the cost of harvesting because of high level of cell densitiesobtained.(Chen and Chen 2006) Wide range of organic substrate are utilized inheterotrophy such as organic carbon, sugar sources are mostly adopted and themost effective forms.(Liang, Sarkany et al. 2009) however some cheaper sources like glycerol and cornpowder can also provide some good yields.(Wang, Yang et al.
2014) Large number of organic compounds and fermenters are required forheterotrophic cultivation so the cost of this method is higher than the photoautotrophy.(Wang, Yang et al. 2014) The other disadvantage of heterotrophy is that susceptibilityto contamination to the other microorganisms which may lower the quantity andthe quality of the products.(Chen, Yeh et al. 2011) · Photoheterotrophic cultivation: this mode is alsoknwn as photoassimilation, photometabolism and photoganitrophy, the mode inwhich the light is needed to use the organic compounds as the source of carbon.
(Mata, Martins et al. 2010) · Mixotrophic Cultivation: Mixotrophiccultivation is the method in which organism are capable of using both modeeither autotrophic or heterotrophic.(Brennan and Owende 2010) Microalgae canutilize both organic carbon and inorganic source.(Wang, Yang et al.
2014) Means that thay can photosynthesis or ingestorganic compounds.(Zhang, Zhang et al. 1999) Inorganic compounds are fixed by photosynthesis andorganic sources are adjusted by aerobic respiration that is affected by theamount of available organic carbon.(Hu, Min et al.
2012) Main difference among the Mixotrophy and thephotoheterotrophy is that the photoheterotrophy need light as the main energysource where the mixotrophy could use the organic components to attain that,photoheterotrophy need light and the organic compounds at the same time. And itis rarely adopted to produce the microalgal diesel.(Chen, Yeh et al. 2011) Some studies show that the microalgal growth ratein mixotrophic is almost the total sum of growth under heterotrophic andphotoautotrophic modes.(Marquez, Sasaki et al. 1993) Meanwhile mixotrophic cultivation utilized the organiccompounds, the growth or algae does not rest on photosynthesis: light is not alimiting aspect for growth of microalgae, therefore photo-inhibition orphotolimitation can be decrease in the mixotrophic culture when the light levelsare too high or too low.
(Wang, Yang et al. 2014). Mixotrophic cultivation can enhance the growthrate, reduce the growth cycle, decrease the biomass loss when light is notpresent due to clean respiration and enhance the productivity of biomass.(Andrade and Costa 2007) Lipid content can be increased as well, that canlead to higher productivity of lipid and it have much importance for theproduction of the microalgal biodiesel.(Wan, Liu et al.
2011) Furthermore the carbondioxide released via aerobicrespiration through microalgae can be reused for the further photosynthesis inmixotrophic cultivation, which increased the productivity of lipids and biomass.(Mata, Martins et al. 2010) Mixotrophic cultivation of the microalgae producehigher yields of organic carbon, In view of the required carbon sources donate largelyto the cost/energy inputs of algal cultivation.(Zhang, Yan et al.
2013) Mixotrophic cultivation reduce the cost of cultivation ofmicroalgae so this mode is much beneficial for the production of microalgalbiodiesel.(Wang, Yang et al. 2014). Ø Harvesting: Microalgal harvesting takes 20-30% cost from the total production ofbiomass.
(Grima, Belarbi et al. 2003) Different harvesting methods could be adopt depending on thedensity, size and the value of the product.(Olaizola 2003) Generally harvesting methods are consist of twostages i) Bulk harvesting: isolate microalgae from suspension like floatation,flocculation and gravity sediments.(Lam and Lee 2012) ii) Thickening: To concentrate the algal slurry after the bulking technique.(Lam and Lee 2012) Techniques that are used in this stage are Centrifugation, ultrasonic aggregation and filtration. So, this is more concentratedstage then bulking.(Brennan and Owende 2010)Four key methods are used for harvesting regardingto biofuel production, belt filtering, sedimentation, flotation and mirceostaning.The methods depend on the density and size of the microalgae.
Microstrainersare striking method for harvesting because its mechanical simplicity and the largesize unit availability. The new and better accessibility polyester screens haverevitalized their interest for their use in harvesting microalgae. Recentstudies resolve that it would be essential to flocculate the cells before themicrostraining process.(Grima,Belarbi et al. 2003)Till now centrifugation and filtration are not feasible mode for microalgalharvesting at commercial level, they need high energy and their high level ofmaintenance cost make then unfavorable for long term use. While on the otherside, flocculation requires less energy for microalgal harvesting.
This happenbecause the microalgal cells carry -ve charge and due to this they repel eachother and could be suspended in the medium for long period when mixing is notforced to them. By adding coagulant having +ve charge into the medium, thecharge of microalgal cell would neutralized. Meanwhile flocculant could beneutralized and form dense flocs which settle under the natural gravity.(Lam and Lee 2012)