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A model of bioconversion of aquacultural residues for aquaculture
Materials and methods
Results and discussion
University of the South Pacific, Suva, Fiji
Like most developing countries throughout the world, Fiji is feeling the pinch of the energy crisis with increasing shortages and rising prices of one of the modern sources of energy, fossil fuels. Alternative sources of energy in developing countries in the South Pacific need to be investigated. What is needed is a source that would be economically viable, easily accessible, and inexhaustible for a reasonable period of time. For countries such as Fiji, where there is considerable emphasis on farming in the rural areas, utilization of natural products (e.g., sugar cane, cassava, wood) for energy resources should be encouraged if the factors involved in processing them are favourable.
In 1977, total fuel consumption in Fiji was about 760,000 tons, or 1,280 kg per person, 50 per cent of which was imported commercial fuel and the remainder indigenous, non-commercial (1). Industry (mostly sugar processing) accounted for about half of all energy use, with one quarter used for transportation and the other quarter for business and domestic purposes. Commercial energy consumption was 30 per cent electric and 70 per cent non-electric. Nearly all of the transport energy and 90 per cent of the electricity generated were obtained from petroleum fuels. Table 1 illustrates the sources of energy, and table 2 the use of these sources by the different sectors of the community (1).
The Government of Fiji, itself concerned about the consequences of further increases in the price of oil, let alone the possibility of not being able to purchase it at any price, has initiated a team of consultants to conduct a feasibility study on the production of ethanol from cassava. Incidently, the governments of Papua New Guinea and the Solomon Islands have also carried out a similar study. The terms of reference for the Fiji study include consideration of small-scale production: "to investigate as a special case the feasibility of cassava-based ethanol production on a small scale (5 ha crop or less) on remote islands for use in electricity generation and outboard motors .. ." (2).
TABLE 1. Estimated Gross Energy Consumption, Fiji, 1977
|Liquid petroleum gas||2||48.0||10||0.5|
|Wood (oven dry)||208||18.6||387||18.7|
a. Calculated on the basis of an average
consumption of 350 kg/person (urban, 80 kg/person x 37.3 per
cent, and rural, 510 kg/person x 62,7 per cent), and a mid-1977
population of 595,000.
b. About 760,000 tons coal equivalent.
TABLE 2. Percentages of Gross Energy Consumption by Sector and Fuel Type, 1977
|Household, including subsistence level||6.2||-||-||18.6||24.8|
|Commercial,government, and miscellaneous||5.8||-||-||0.1||5.9|
Considering that there is a significant amount of fertile virgin land available not only in Fiji but in neighbouring countries as well (e.g., the Solomon Islands, the New Hebrides) that could be cultivated, it would be beneficial not only to the members of the rural community, but to the country as a whole, if it were practical to use cassava as an alternative source of energy. Cassava is relatively easy to grow, the villagers know how to plant it, and it does not take years for a crop to be ready for harvest.
It has been reported that the content of starch in cassava varies over the year (3). Perhaps local crops could be studied to determine the amounts of starch in cassava over a period of time.
The Fiji Sugar Corporation is also investigating the extraction of ethanol from sugar cane juice, although the possibility of using molasses had been considered earlier (4).
In determining the development of a plant to produce ethanol from either sugar cane or cassava, or both, the Government of Fiji would also have to consider that there is undeveloped land available in most of the outer islands of Fiji, and this may tend to indicate that a cassava-processsing plant would be in Fiji's best interests, although initially it may not be as economical as a sugar cane factory. It should also be noted that conditions with regard to availability of land, terrain, etc., differ throughout the 11 countries represented by the University of the South Pacific; thus, while it might be favourable to establish a cassava plant in Fiji, it would not be so in Tuvalu, for example.
The source of energy that has been used extensively over the years by the rural communities in the regional countries has been wood (5). The net fuel wood resource in Fiji in 1980 was conservatively estimated as 463 tons of coal equivalent, which is triple previous use and 17 per cent above commercial energy consumption (6). As an alternative source of energy, a wood-based system to produce a combustible gas (producer gas, wood gas) for stationary engines could be developed to complement liquid fuel from alcohol.
Current research on energy in the Department of Chemistry at the University of the South Pacific has been concentrated on the formation of biogas. The anaerobic digestion of a mixture of field grasses (mainly Axonopus compressus, Eleusine indica, and Digitaria longiflora), water hyacinths (Eichhornia crassipes), and seaweeds (Gracaleria, Verrucosa, and Sargassum) has been investigated as a potential input supplement for biogas digesters currently in operation in Fiji and elsewhere in the South Pacific (7).
In conclusion, it is noted that alternative sources of energy in the South Pacific can be obtained from sugar cane, cassava, and wood. However, taking into account the socio-economic factors involved, it may not be possible to single out any one source as the only choice.
1. P. Johnston, at the Seminar on Woods as an Alternative Energy Resource, University of the South Pacific, Suva, Fiji, 3-4 July 1978.
2. Terms of Reference for Ethanol Study, as approved by Cabinet Sub-committee, Fiji Government.
3. V. Yang, W.N. Milfont, Jr., A. Scigliano, C.O. Massa, S. Sresnewsky, and S.C. Trinidade, "Casava Fuel Alcohol in Brazil," in Proceedings of the 12th Intersociety Energy Engineering Conference (19771,1 : 4453.
4. South Pacific Island Business News, October 1979, p. 7,
5. S. Siwatibau, A Survey of Domestic Rural Energy, Energy Use and Potential in Fiji, report to the Fiji Government and the international Development Research Centre (IDRC), Ottawa, Canada (University of the South Pacific, Suva, Fiji, 1978).
6. P. Johnston, A Preliminary Study of Fuel Wood for Rural Electrification in Fiji (Commonwealth Regional Consultive Group on Energy, New Delhi, India, 1978).
7. R.K. Solly, The Production of Biogas from Water Hyacinth (Commonwealth Science Council/ United Nations Environment Programme, New Delhi, India, 1978).
H. Hirata and S. Yamasaki
Kagoshima University, Kagoshima, Japan
This paper will describe an attempt to maintain a steady-state zooplankton community in a feedback culture system. A transparent, round, 550-litre tank connected to a 150-litre zigzag stream unit was used for multi-species culture of Brachionus plicatilis and Tigriopus japonicus. The water in the system was recirculated about 20 times per day by air-lift pumps. The animals were fed frozen baker's yeast daily, and faeces were removed from the bottom of the stream and transferred into a sludge activator. Marine chlorella cultured in the sludge were then fed back to the zooplankton. Approximately 10 to 20 per cent of the animals were harvested each day of the 480-day experiment. The relative proportion of B. plicatilis and T. japonicus in the community was maintained at a steady-state ratio of 82:18 by body volume. The proportion of nauplii, copepodites, and adults of T. japonicus was also constant during the culture period at a ratio of 28:47:25. The food conversion efficiency was calculated to be 26.7 per cent.
Recently, because of rapid development of mass production of zooplankton as food for cultured fish and prawn larvae, two serious ecological problems have emerged: energy loss in feeding and water pollution from excretion 11-3). For example, about 10 kcal of yeast must be fed to produce 1 kcal of rotifers by culturing (4); thus, about 90 per cent of the energy source is lost. Also, rotifers produce large amounts of faeces, causing gradual pollution of the culture medium. This type of water pollution is becoming common. Self-purification is carried out smoothly in natural sea water but is almost impossible in an accumulation microcosm because of over-biodeposition by the cultured animals. Therefore, supplements must be added to promote the energy flow (5).
Our experiment was conducted to determine how to maintain a steady-state zooplankton community in an accumulation microcosm by a feedback culture system. Steady-state zooplankton communities have also been studied by some other microcosm researchers (6-11). The idea of the feedback system was initiated in about 1930 and has been developed in the field of electronic circuits (12). Since the term "feedback" is convenient, it has been used in several scientific fields, e.g., biochemistry, nerve physiology, and ecosystem research (13). The feedback system discussed here refers to feeding and excretion, then excretion to feeding again to regulate the energy flow in the system.
Figure 1 is a flow chart of the feedback culture system. Baker's yeast is fed to zooplankton; organic matter from the faeces and the uneaten food is mineralized by bacteria into inorganic nutrients for algae; then the algae are fed to the zooplankton. Marine zooplankton organisms, Brachionus plicatilis and Tigriopus japonicus, were cultured together as the consumers. Microalgae, Chlorella sp. (probably Chlorella saccharophila var. saccharophila, as reported by Tsukada and co-workers  ) and Nitzschia spp., and macro-algae, Enteromorpha intestinalis, were cultured as the producers at the beginning of the experiment. Ten to 15 species of marine bacteria that grew naturally in the tanks acted as decomposers.
FIG 1. Feedback Culture Systems: (a) Mono-feedback System, and (b) Multifeedback System. The monotype system is used for the culture of rotifers, which are herbivorous. The more complicated multi-type system is suitable for carnivorous animals. A-1, herbivorous animals; A-2, carnivorous animals; B-1 and B2, decomposers; C-1 and C-2, algae reproduced by excess nutrients in the culture system.
FIG. 2. Process of the Rotifer Culture in the Feedback System. Biodeposits and water are reused for chlorella culture as by-product nutrients.
Two round polycarbonate tanks were used for the zooplankton culture: tank A for the feedback experiment and tank B for batch culture. Tank A was connected to a 150-litre zigzag stream unit, but tank B was not (fig 2). The water in tank A was recirculated to the stream unit by an air-lift pump at a rate of about 20 times per day, resulting in a water current in the stream of approximately 1 m/mint The feedback producer, marine chlorella, was cultured in two 60-litre transparent tanks used alternately at two-day intervals. Every three or four days the faeces and uneaten food were siphoned from the bottom of the stream unit and transferred to the decomposer tank (modified from Fujiwara et al.  ).
Water removed from tank A after harvesting of the zooplankton and from the decomposer tank was transferred to the chlorella culture tanks. Macro-algae (E. intestinalis) were grown, together with the zooplankton, in the zigzag stream unit.
The experiments were conducted for 480 days, from 12 March 1976 to 4 July 1977, under laboratory conditions. Water temperatures ranged from 16.4° to 30.5°C and were maintained within tolerable limits by electric heaters that came on when the temperature fell below 17°C. In addition to natural illumination, white-beam fluorescent lamps (eight 40-watt and sixteen 20-watt lamps) were used to maintain a 15-hour light and 9-hour dark photo-period (16).
During the first 30 days, 552 g of activated sludge composed of soycake particles and dried yeast (17), 608 g of heads and bony parts of fish, 60 g of marine chlorella, 18 g of diatoms, and 308 g of baker's yeast were supplied to tank A and to the stream unit. After the fifty-fourth day of culture, 30 g of wet weight marine chlorella reproduced in the chlorella culture tanks was also supplied to the zooplankton daily as feedback food. The zooplankton population density, pH, and phosphate content were measured each morning before feeding. Special caution was taken to control the zooplankton density within a range of 50 to 100 individuals per millilitre by harvesting.
Population Density of B. plicatilis in the Feedback Culture
Seasonal variations of the zooplankton population density are shown in figure 3; figure 4 shows the algal feedback rate; and water temperature, pH and PO4-P content are given in figure 5. Figure 6 shows algal productivity. The feedback rates were calculated as amount of chlorella fed times 100 divided by total food input ( kcal ).
FIG. 3. Population Densities of Zooplankton throughout the Culture Experiment in the Feedback Culture System
FIG. 4. Chlorella Feedback Rate - Calculated as the Amount of Chlorella Fed Multiplied by 100 and Divided by the Total Food Supplied (kcal)
FIG. 5. The Culture Medium in the Feedback Culture System.
B: water temperature.
C: Phosphate content.
The initial B. plicatilis population density was only 3.7 individuals per millilitre, but it increased about tenfold during the first month of culture. Population density was maintained at about 45/ml from day 30 to day 70 by daily harvesting. Beginning at day 70, when chlorella was fed back to the animals and E. intestinalis started to grow in the stream unit (fig. 4), the B. plicatilis population density increased to 60 to 65/ml. The density was then maintained at 57 to 65/ml until day 185. Thereafter the density fluctuated from about 50 to 75/ml through the winter season (days 195 to 365) because of the lower temperature (16° to 24° C).
After 365 days of culture, the B. plicatilis population density was increased by more feeding and less harvesting to about 100 individuals per millilitre as a trial. The density decreased gradually, however, to 40 to 50/ml during the last four months when the growth rate of algae decreased from about 700 to 200 kcal/day and PO4-P contents in the water increased from 2 to 5 mg/litre at the same time (see fig.6). On the basis of these results, we estimated that the optimum population density of B. plicatilis for maintaining a steady state in this system is 59.8 ± 5.4 individuals per millilitre.
FIG. 6. Total Amount of Algae Harvested (kcal) and Dissolved Phosphate Content during the Last Half of the Experiment
Population Density of T. japonicus in the Feedback Culture
The population density of T. japonicus was about 25 per cent that of B. plicatilis. The initial density of T. japonicus was only 1.1 individuals per millilitre; this was the lowest level in the experiment. The greatest density was 21.2/ml, observed on day 335. The density ranged from 6 to 17, averaging about 11/ml throughout the culture period. Fluctuations in T. japonicus population density followed a pattern similar to that of B. plicatilis, with the peak density occurring approximately 10 to 20 days later. The population density of B. plicatilis increased rapidly at approximately day 75, whereas T. japonicus density peaked near day 95 (fig. 3).
Population densities of T. japonicus nauplii were closely related to the amount of chlorella fed back, but were not related to the growth of E. intestinalis. The reverse was true for the copepodites of T. japonicus; i.e., they were affected by the growth of E. intestinalis but not by the amount of chlorella fed back. The adult T. japonicus population density was not affected by either of these factors, but was affected by the reproduction rate of B. plicatilis. This may be a result of the different feeding habits of T. japonicus in each developing stage (18).
Food Conversion Efficiencies
Efficiencies of food conversion in the feedback system are shown in figure 7. Two methods were used to calculate the rates - Z conversion:
[ zooplankton harvest x 100] / total food supplied
and Z+E conversion:
[ total zooplankton and E. intestinalis harvested x 100 ] / total food supplied
FIG. 7. Food Conversion Efficiencies during the Experiment, Calculated by Different Methods (see text)
All the data presented here come from measurement of caloric contents of materials by bomb calorimetry.
The Z conversion efficiency was only 5.2 per cent between days 11 and 20 of the experiment. It increased to 23.2 per cent between days 141 and 150 (fig. 7), and thereafter varied between 23.0 and 27.2 per cent. An interesting result was also observed for the Z+E conversion efficiency. Harvesting of E. intestinalis began on day 80, and the Z+E conversion efficiency increased gradually from 16.4 per cent at the first harvesting to 60.1 per cent between days 400 and 410. Thus, the total Z+E food conversion efficiency was 57.7 per cent.
Composition of the Zooplankton Community in Feedback and Batch Culture Tanks
The B. plicatilis and T. japonicus proportions of the zooplankton community, by body volume, are given in figure 8. Variations in the density proportions of each develop mental stage - nauplius, copepodite, and adult - of T. japonicus, in numbers of individuals per millilitre, are shown in figure 9. The balance between the two zooplankton species was maintained in a steady state throughout the 480-day culture experiment in the feedback system; i.e., the dominant species was always B. plicatilis (82.2 per cent), and T. japonicus made up only 16.8 per cent of the community. The highest ratio of B. plicatilis to T. japonicus was 92:8 on days 121 to 130, and the lowest was 68:32 on days 391 to 400.
The average proportions of T. japonicus developmental stages (nauplii:copepodites: adults) in the feedback culture system were constant at 28:47:25 throughout most of the 480-day experiment (fig. 9). During the last five months of the culture period, however, the copepodites gradually became dominant, and the proportion during the final ten days (days 471 to 480) was 18:66:16 (nauplius:copepodite:adult).
The succession of the zooplankton community in the feedback and batch culture tanks differs greatly (fig. 8). At the beginning of the experiment, the composition of the zooplankton population was approximately 50 per cent B. plicatilis and 50 per cent T. japonicus in the feedback system. The respective percentages were 98 and 2 in the batch culture tank because of a difference in the density during initial inoculation. Approximately 20 days after inoculation, the proportions were the same in both systems (82 per cent B. plicatilis and 18 per cent T. japonicus). This density was maintained until the end of the experiment in the feedback culture system (see fig. 8). The proportion of B. plicatilis in the batch culture tank decreased, however, after day 90, and all the zooplankton in the control tank disappeared after day 120.
The PO4-P contents of the culture water at day 120 were 2.4 mg/litre in the feedback and 5.1 mg/litre in the batch culture tank. The excess nutrients in the feedback system were used by the algae, marine chlorella, and E. intestinalis.
FIG. 8. Composition of the Zooplankton Community (Percentages of Tigriopus japonicus and Brachionus plicatilis). A: In the batch culture system. B: In the feedback culture system.
FIG. 9. Relative Proportions of the Different Developmental Stages - Nauplius, Copepodite, and Adult - in the Composition of the Tigriopus japonicus Population
Comparison of the results from the feedback and batch (the traditional method) culture systems of zooplankton shows the very significant role of producers (Chlorella and E. intestinalis) and decomposers in maintaining a steady state in an ecosystem. Throughout 480 days of culture, with almost no change of culture medium, the population of both B. plicatilis and T. japonicus was maintained in the feedback culture system. It is significant that the zooplankton in the batch culture tank totally disappeared after only 120 days of culture. A stable zooplankton community was maintained in the feedback system for the duration of the culture period, whereas the batch culture had deteriorated by day 90 (fig. 8).
At day 120 the inorganic phosphate content was only 2.4 mg/litre in the feedback culture system, whereas that of the batch culture was 5.1 mg/litre (figure 5). This indicates that successful mineralization of organic wastes and their subsequent utilization by algae were responsible for maintaining a favourable water quality in the feedback system.
The food conversion efficiency in the feedback system was 25.6 per cent for the duration of culture; this is much higher than that reported by other investigators (4, 19). The high conversion efficiency was a result of the feedback culture, since about 20 per cent of the total food supplied to the zooplankton was chlorella. This means that the recycling of energy in the feedback culture system gives a food saving of 20 per cent. It might be suggested, therefore, that the feedback culture system acted to purify water and to conserve energy.
The harvesting effect is also important in maintaining a steady state in the feedback culture. The results of our experiment suggest that it is possible to maintain a steady-state zooplankton population for long periods of time by harvesting in a feedback system. We hope to develop a simpler and more efficient feedback system for conservation in accumulation microcosms.
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2. H. Hirata, "Principle of Feedback Culture System and Its Developmental Experiment," Yoshoku, 15 (1): 34 (1978).
3. H. Hirata, "Seed-Production of Prawn, Penaeus japonicus, with Reference to Application of Active Sludge Method," Food Ind. (Tokyo), 21 110): 29 (1978).
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13. T. Yamada et al., Biological Dictionary, 2nd ed. (Iwanami Shoten, Tokyo, 1977).
14. O. Tsukada, T. Kawahara, and H. Takeda, "Good Growth of Chlorella saccharophilia on the Basis of Dry Weight Under NaCI Hypertonic Condition," Bull. Japan Soc. Sci. Fish., 40: 1007 (1974).
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16. H. Hirata, "Preliminary Report on the Photoperiodic Acclimation for Growth of Chlorella Cells in Synchronized Culture," Mem. Fac. Fish. Kagoshima University (Kagoshima, Japan) 24: 1 (1975).
17. H. Hirata, A. Kanazawa, T. Yamamidori, and K. Yasuda, "Preliminary Studies on Sludgezation of SoyCake Particles and Yeasts," Mem. Fac. Fish., Kagoshima University (Kagoshima, Japan). 22:107 (1973).
18. H. Hanaoka, "Cultivation of Three Species of Pelagic Micro-crustacean Plankton," Nihon Purankuton Gakkaiho, 20: 19 (1973).
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