Single cell protein could lead to an undesirable change in them. This is particularly important in continuous culture, which is often used for SCP production. Here the long growth period (in principle indefinite but in practice several weeks)can be long enough for mutants to arise, compete with the parent organism and predominate In batch culture there is not enough time for this to occur vii) Advantage. For most SCP fermentation processes the running costs(costs of operating the fermentation unit) is 10-20% of the total production cost. Aeration costs contribute 30-60% to running costs. In other words aeration costs can be much as 12% of the production costs of the sCP. the lower the aeration costs the etter. Production costs for various SCP processes are discussed in more detai later on You are now familiar with the major characteristics of organisms that are useful for SCP production, and the types of substrates on which they can be grown. We are now going to consider in detail the processes that have been developed some of these processes have been developed only as far as the pilot scale, and have not reached commercial operation. Others have reached full production scale but have subsequently failed, for a variety of reasons. These have been included as well as the successes, as they show you the variety in the technology of SCP production, and also show how economic and political factors influence the success and failure of processes. These processes might also become useful and economic some time in the future Emphasis will be put on the technology involved in the fermentation and down-stream processing of each process 4. 6 SCP from carbon dioxide 4.6.1 Spirulina open lagoon Blue-green bacteria(cyanobacteria)of the genus Spirulina have been produced as SCP system in Mexico, using natural bicarbonate-rich ground-water(into which atmospheric COz readily dissolves). A flow diagram of the process is given in Figure 4. 2. The single 10 hectare(1 ha=10,000 m )open lagoon is about 0.6 m in depth and unmixed. The system is operated as a batch culture or as a semi-continuous culture (in which a proportion of the medium is removed and replaced by fresh medium intermittently ). Nitrate is added as a nitrogen source, and other minerals are present in the water. The long filaments are raked mechanically from the pond onto screens(sieves), where water is drained and either recycled or disposed of. The biomass is then de-watered by rotary vacuum filtration, dried by vacuum drying, then dried in a drum drier and ground to a powder (to make the product more appealing). The product contains 56% protein and is sold as The plant operates with an output of 10 g dry weight per square metre per What is the output of the 10 hectare lagoon per year?(Note I hectare=10,000 10 g m2'equates to 10 x 10=10'g ha The daily output is thus: 10 kg ha. day The 10ha lagoon produces 10 kg ha. day This equates to 10 x 365=3.6 x 10 kg year The output of the lagoon is thus 360 tonnes yea
Single cell protein 69 could lead to an undesirable change in them. This is particularly important in continuous culture, which is often used for SCP production. Here the long growth period (in principle indefinite but in practice several weeks) can be long enough for mutants to arise, compete with the parent organism and predominate. In batch culture there is not enough time for this to occur. Advantage. For most SCP fermentation processes the running costs (costs of operating the fermentation unit) is W20% of the total production cost. Aeration costs contribute 3040% to running costs. In other words aeration costs can be as much as 12% of the production costs of the SCP. The lower the aeration costs the better. Production costs for various s8 processes are discussed in more detail later on. vii) You are now familiar with the mapr characteristics of organisms that are useful for SCP production, and the types of substrates on which they can be grown. We are now going to consider in detail the processes that have been developed. Some of these processes have been developed only as far as the pilot scale, and have not reached commercial operation. Others have reached full production scale but have subsequently failed, for a variety of reasons. These have been included as well as the successes, as they show you the variety in the technology of SCP production, and also show how economic and political factors influence the success and failure of processes. These processes might also become useful and economic some time in the future. Emphasis will be put on the technology involved in the fermentation and down-stream processing of each process. 4.6 SCP from carbon dioxide 4.6.1 Spirulina Blue-green bacteria (cyanobacteria) of the genus SpiruZina have been produced as SCF’ in Mexico, using natural bicarbonaterich ground-water (into which atmospheric CG readily dissolves). A flow diagram of the process is given in Figure 4.2. The single 10 hectare (1 ha = l0,OOO m’ )open lagoon is about 0.6 m in depth and unmixed. The system is operated as a batch culture or as a semi-continuous culture (in which a proportion of the medium is removed and replaced by fresh medium intermittently). Nitrate is added as a nitrogen source, and other minerals are present in the water. The long filaments are raked mechanically from the pond onto screens (sieves), where water is drained and either recycled or disposed of. The biomass is then dewatered by rotary vacuum filtration, dried by vacuum drying, then dried in a drum drier and ground to a powder (to make the product more appealing). The product contains 56% protein and is sold as food. The plant operates with an output of 10 g dry weight per square metre per day. n What is the output of the 10 hectare lagoon per year? (Note 1 hectare = l0,OOO m 1. 10 g m-2 equates to 10 x lo4 = leg ha? The daily output is thus: 10’ kg ha:’ day-’. The lOha lagoon produces Id kg ha:’ day-’. This equates to Id x 365 = 3.6 x Id kg year-’. The output of the lagoon is thus 360 tonnes year -’. open lagoon system
Chapter 4 sunlight inoculum CO, from the pond cultivation groundwater biomass disposal wate screenIng rotary vacuum filtration vacuum drying drum drying (bead milling packaging SCP as dried powder Figure 4.2 The production of Spirulina maxima for SCP 4.6.2 Algae Eukaryotic algae(Chlorophyceae) of the genera Chlorella and Scenedesmus have been used for SCP production. Several types of cultivation systems have been considered, depending on the substrate used and whether the sCP is intended for use as food or
70 Chapter 4 Figure 4.2 The production of Spirulina maxima for SCP 4.6.2 Algae Eukaryotic algae (Chlorophyceae) of the genera Chefla and Scenedesmus have been used for SCP production. Several types of cultivation systems have been considered, depending on the substrate used and whether the SCP is intended for use as food or feed
Single cell protein For use as food or feed, algae are grown in pure or mixed culture in a mineral salts richment medium containing NHt or NO, and supplied with air or gaseous CO(air contains only 0.03% CO, so COenrichmentis required for high output). A flow diagram of suc a process is essentially the same as that shown in Figure 4. 2. Open systems have been developed with organisms growing as continuous cultures in open lagoons or circulation ditches, similar in design to oxidation ponds and ditches used in sewage treatment. The ponds can be lined with clay, concrete, brick or plastic sheeting and are 20-50 cm in depth. Mixing can be mechanical, using motor driven paddles, or can be manual, and it is necessary to prevent sedimentation of cells and uneven exposure to phototrophs sunlight. Capital costs(excluding land costs)are lower than with other SCP systems, at s(US)20,000-30,000 per hectare(1989 prices). The organisms grow as phototrophs using sunlight as an energy source and atmospheric Co as carbon source. Such systems are relatively simple (low-tech) but, as they are open, they are liable to contamination by vild algae and bacteria. Heterotrophic bacteria can grow in the ponds using organic materials released into the water by the algal cells Write down three s why the requirement for sunlight limits the use of algal ponds in sCP pro Firstly, the lagoons need to be open to allow the sunlight to penetrate. This means that pond culture is non-aseptic and contamination usually limits the use of sCp to feed, where a higher degree of contamination is acceptable. The micro-biological standards of SCP as food and feed have been defined by the PAG and are based on comparisons with conventional foods and feeds. Secondly, the technology can only be applied between latitudes 35 North and South where there is sufficient sunlight to give high outputs. Thirdly, in dense algal cultures sunlight does not penetrate more than 40-50 cm, so lagoons are limited to such depths. This limits output, ie you cannot produce more algae by making the lagoon deeper centrifugation Cell recovery presents a problem in algal culture, as centrifugation( the most effective rum arying calcium hydroxide and sedimentation can be employed for feed purposes.Waste water is recycled and recovered cells are dried preferably by drum drying, which breaks up cell walls making the product easier to digest by humans. The product contains 40-50%6 protein and 4-6% nucleic adid and has been produced in relatively small quantities at a elling price $4-10 kg"(1990 prices). This compares to soya protein concentrate(70% protein)and milk powder(36% protein) at about $3 kg ocee Cost (Uss per Concentratlon thousand m of culture) Centrifugation Flocculation/Flotation Flocculation/Sedl 320 Table 4.2 Costs of alga! harvesting processes(1989 prices
Single cell protein 71 a2 enrichmenV phototroophe centrifugation flocarlation drum drying For use as food or feed, algae are grown in pure or mixed culture in a mineral salts medium containing or NQ- and supplied with air or gaseous COz (air contains only0.0396 Ca so Co2 enrichment is required for high output). A flow diagram of such a process is essentially the same as that shown in Figure 4.2. Open systems have been developed with organisms growing as continuous cultures in open lagoons or circulation ditches, similar in design to oxidation ponds and ditches used in sewage treatment. The ponds can be lined with clay, concrete, brick or plastic sheeting and axe 20-50 an in depth. Wng can be mechanical, using motor driven paddles, or can be manual, and it is necessary to prevent sedimentation of cells and uneven exposure to sunlight. Capital costs (excluding land costs) are lower than with other SB systems, at $(US)20,000-30,000 per hectare (1989 prices). The organisms grow as phototmphs using sunlight as an energy source and atmospheric COz as carbon source. Such systems are relatively simple (low-tech) but, as they are open, they are liable to contamination by wild algae and bacteria. Heterotmphic bacteria can grow in the ponds using organic materials released into the water by the algal cells. Write down three reasons why the requirement for sunlight limits the use of algal n ponds in SCP production? Firstly, the lagoons need to be open to allow the sunlight to penetrate. This means that pond culture is non-aseptic and contamination usually limits the use of SB to feed, where a higher degree of contamination is acceptable. The micro-biological standards of SCP as food and feed have been defined by the PAG and are based on comparisons with conventional foods and feeds. Secondly, the technology can only be applied between latitudes 35O North and South where there is sufficient sunlight to give high outputs. Thirdly, in dense algal cultures sunlight does not penetrate more than 40-50 an, so lagoons are limited to such depths. This limits output, ie you cannot produce more algae by making the lagoon deeper. Cell recovery presents a problem in algal culture, as centrifugation (the most effective method) can be prohibitively expensive (Table 4.2). Methods such as flocculation with calcium hydroxide and sedimentation can be employed for feed purposes. Waste water is recycled and recovered cells are dried preferably by drum dryins, which breaks up cell walls making the product easier to digest by humans. The product contains 40-5096 protein and 44% nucleic acid, and has been produced in relatively small quantities at a selling rice $4-10 kg-’ (1990 prices). This compares to soya protein concentrate (70% protein P and milk powder (36% protein) at about $3 kg-’. Process Cost (US $ per Concentration thousand m3 of wtture) factor Centriiugation 380 40-1 00 Flocculation/Flotation 340 85 Flocculatlon/Sedlmentation 320 50 Table 4.2 Costs of algal harvesting processes (1 989 prices)
Chapter 4 In algal pond cultures cell concentrations of 2 g dry wt I are possible, corresponding to outputs of 15 g dry wt per square metre per day. What would the protein output of such a system be (as kg protein per hectare per year), suming the dry algal biomass contains 45% protein? (Use a piece of paper to do this calculation and then compare it with ours 15 g dry wt 'corresponds to 15x 104=1.5 x 10 g had The annual output is thus 1.5 x105x365-55x 10 g ha 'yr 'or 5.5 x 10 kg ha 'yr At 45% protein this corresponds to 5.5 x 10*x 45/100=2.5 x 10* kg protein ha" yr With increased CO concentrations, by the injection of CO2 gas into ponds, outputs of 3 x kg dry protein per hectare per year are possible. This compares very favourable with conventional sources of protein(Table 4.3), note the low yields of meat and milk. Sources kg protein per hectare per year Algal ponds 30,000 Farmed fish 1.000 Peanuts 450 Table 4.3 Output of protein from algal ponds and conventional sources lbular loop A novel fermentation system, a tubular loop reactor(Figure 4.3), has been developed at reactor laboratory scale which is capable of producing algal cultures with densities of 20 g dry wtI. This system converts 18% of incident solar energy, far in excess of the 7% in algal ponds and 1-2% in agriculture. Such a system gives a theoretical output in European roten as the water is conserved and can be recycled
72 Chapter 4 In algal pond cultures cell concentrations of 2 g dry wt 1-1 are possible, corresponding to outputs of 15 g dry wt per square metre per day. What would the protein output of such a system be (as kg protein per hedare per year), assuming the dry algal biomass contains 45% protein? (Use a piece of paper to do this calculation and then compare it with ours). n 15 g dry wt m-2d-' corresponds to 15 x lo4 = 1.5 x lo5 g ha-'d-'. The annual output is thus 1.5 x Id x 365 = 5.5 x 107g hi' yf' or 5.5 x lo4 kg ha-'yf'. At 45% protein this corresponds to 5.5 x lo4 x 45/100 = 2.5 x lo4 kg protein ha-' yr-'. With increased C@ concentrations, by the injection of C@ gas into ponds, outputs of 3 x lo4 kg dry protein per hectare per year are possible. This compares very favourable with conventional sources of protein (Table 4-31, note the low yields of meat and milk. I Sources kg proteln per hectare per year I Algal ponds 30,000 Farmed fish Potatoes Rice Peanuts Wheat Milk Meat 1,000 800 600 450 360 120 80 Table 4.3 Output d protein from algal ponds and conventional sources tubular loop A novel fermentation system, a tubular loop reactor (Figuxv 4.31, has been developed at laboratory scale which is capable of producing algal cultures with densities of 20 g dry wt 1-'. This system converts 18% of incident solar energy, far in excess of the 7% in algal ponds and 1-2% in agriculture. Such a s stem 'ves a theoretical output in European climates of 1oO,OOO-15O,ooO kg protein ha' year -rand could be operated in arid mons as the water is conserved and can be recycled
Single ce‖ protein o。+cO degasser lobular reactor nutrients Figure 4.3 Tubular-loop reactor for the production of algal biomass The photosynthetic production of Chlorella sp can be written as: CO+ Ho+ NH3 Biomass +o high-rate algal Development work is being carried out growing algae for feed on municipal effluents onds and animal slurries(from intensive animal farms). This is carried out in high-rate algal ponds(shallow aerated lagoons operated at high dilution rates). Aerobic bacteria oxidise organic materials in the effluents, producing CO which is used by the algae growing as photoautotrophs (using CO and sunlight). The algae in turn produce o which further stimulates the aerobic bacteria. Such systems are able to produce feed on the one hand and to reduce BOD, nitrate and phosphate (ie pollutants)from effluents on the other hand In Japan Chlorella spp has been produced for food in continuous aseptic systems in conventional bioreactors. The organisms are grown in the dark as heterotrophs usting sucrose(in the form of molasses)or glucose as carbon and energy source. Production has been 2,000-3,000 tonnes per year at a selling price of $(US)10-22 kg(1990 prices). This product is sold as a high-value health food
Single cell protein 73 Figure 4.3 Tubular-loop reactor for the production of algal biomass The photosynthetic production of Chlorella sp can be written as: light COZ + H20 + NH3 > Biomass + 02 Development work is being carried out growing algae for feed on muniapal effluent8 and animal slumes (from intensive animal farms). This is carried out in high-rate algal ponds (shallow aerated lagoons operated at high dilution rates). Aerobic bacberh oxidise organic materials in the effluents, producing COZ which is used by the algae growing as photoautotrophs (using COZ and sunlight). The algae in turn produce 0, which further stimulates the aerobic bacteria. Such systems are able to produce feed an the one hand and to reduce BOD, nitrate and phosphate (ie pollutants) from effluents on the other hand. In Japan CWmZla spp has been produced for food in continuous aseptic systems in conventional bioreactoxs. The organisms are grown in the dark as heterotrophs udng sucrose (in the form of molasses) or glumse as carbon and energy source. Producth has been 2,ooCr3,000 tonnes per year at a selling price of $(US)10-22 kg-' (1990 PriCJes). This product is sold as a high-value health food. high-rate algal pond*