A Comparative Study of Two Aquaponic Methods: Raft vs. Gravel Filtration Connor Coffin BIOL 493 Winter 2011 Mentor- Dr. D. Bybee 1
Abstract Sweet basil (Ocimum basilicum) was planted and grown using both a gravel filtration and floating raft method in order to determine which produced the greatest biomass. Basil was also grown under the same conditions in soil as a control. The growth period was 30 days at which point the total biomass was measured and weighed (wet weight). The gravel filtration method produced the greatest biomass according to height (19in), and weight (459 gms). The basil grew twice as fast in the soilless system compared to the control group (P = 0.0049). Comparing only the two aquaponic methods, the gravel filtration method of aquaponics was significantly superior to the raft method when growing the herb basil (P = 0.0014). Introduction Soilless agriculture, or hydroponics, entails growing vegetation without a substrate, using chemical fertilizers (Emberger 1991). Aquaculture is the captive cultivation of aquatic organisms for human use (Howerton 2001). The combination of both practices creates a third system of soilless cultivation known as aquaponics. This system differs from hydroponics and aquaculture in that fish are grown simultaneously in the same water as the vegetation resulting in byproducts (nutrients from fish wastes) that fertilize the plants instead of chemical additives. Aquaponics creates a mutualistic relationship between fish and plants; the fish produce nutrients metabolically which the plants absorb, cleaning the water in the process. Growth continues until both fish and plants are harvested as food products (Whitton 2009). Some of the advantages of aquaponics are: control of nutrition, high yields on a small footprint, lower costs due to recirculating water, and easy management of disease and pests (Hershey 1994). The most common aquaponic method in use today is the raft system (Mackowaik 1989). This process utilizes a floating raft to suspend the roots of the plants into the water, and is considered static because the roots of the plants are always in direct contact with nutrient rich water (Figures 1 & 2). 2
. Figure 1 A floating raft with 10 mesh pots. Figure 2 Roots coming through the mesh pot into nutrient rich water. A second method is a dynamic gravel filtration system in which the gravel is used as a substrate. Water is pumped into the gravel bed which activates a negative pressure siphon draining the tank. The tank refills in a cyclical manner, aerating the gravel. Limited research comparing the two methods has been published to date (Emberger 1991). The purpose of this study was to compare the effect of these two aquaponic systems on biomass production in the herb basil (Ocimum basilicum). Methods An outdoor system was constructed using two large 55 gallon food grade, closed top, plastic barrels. One of the barrels was cut in half vertically, and the two halves were laid on their sides to serve as growth beds. The second barrel (Figure 3) was equipped with an underwater pump, and filled with approximately 10 lbs of Tilapia (54, 3 inch average) (Oreochromis mossambicus). 3
Figure 3 Complete raft, and gravel Figure 4 2 siphon with attached 3/8 tubing. filtration aquapoinics system. The first growth bed employed the raft method. In this system, a 2 x4 piece of poly styrene was cut to fit in the half barrel, and was then painted white with water base latex on the surface exposed to the sun. Holes were then cut to fit the 3 mesh pots and 6 waste collection area. In the first replicate coconut fibers were placed in the pots to act as a substrate for the seedlings. In later replicates, the coconut fibers were replaced with black cinders in order to eliminate a possible nutrient drain. The second growth bed was a gravel filtration system. In this system, an 8 standpipe was fitted with a water siphon system (figure 4). This siphon was protected with a segment of 4 pipe in order to keep the gravel out. Once in place the barrel was filled with previously washed black cinders. Both barrels were equipped with custom wood cradles in order to secure and prevent them from tipping over. The barrels were also tied down with nylon safety chords in order to prevent rotation. Both systems received water pumped at equal rates from a 500 gal/hr. underwater fountain pump located in the bottom tank containing the tilapia. Fish were grown simultaneously with basil, and the nitrogenous wastes of the fish were used as the fertilizer for the basil in both tanks. One teaspoon of anhydrous Iron was added to the water to meet the physiological needs of the fish (Tamaru pers. comm). Basil seeds were sprouted indoors using standard germination methods and grown until they achieved a height of one inch measured from ground to tip (Jones 1997). The aquaponic system was placed in an area where it would receive at least four hrs of direct sunlight per day. Nutrient levels were monitored in the fish tank until they reached the optimum concentration of 4
50-100 ppm of nitrates, at which point the seedlings were placed in the tanks. Ten seedlings each were placed in the floating raft system, the gravel filtration system, and in pots with soil next to the system for comparison. Readings were taken in the water systems every two days measuring the height, ph, nitrates, nitrites, hardness, and alkalinity throughout the growth period (30 days). At the end of the growth period, plants from the two systems were harvested, weighed, and measured. Measurements consisted of height and number of surviving plants in each group. Basil wet weight was compared statistically using a one way ANOVA, and T-test. The basil was cut at the base and depending on size, cut in the middle to facilitate weighing. Wet weight was taken immediately after cutting to prevent dehydration. The raft seedlings in the mesh pots were planted in gravel instead of the coconut fibers used in replicate one which were recommended by a hydroponics shop in Honolulu. This was done due to a concern that the non-chlorophyll containing organic materials might be a nutrient sink, stealing nitrogen from the plants roots (Bybee pers. comm). Results The actual yield in total biomass from the three methods was 722.1 grams of basil. In replicate #1 the gravel siphon method produced the highest biomass (459 grams). The raft system only produced 136.3 grams (Table 5). The gravel produced significantly higher amounts of basil (P= 0.0049) with the raft producing just a few grams more than the control (Figure 6). (Table 5) In the results of replicate 1, the gravel produced three times the basil as the other two groups. Replicate #1 Raft Height # of plants Wet weight Average 9.6 2.7 13.63 gms Gravel Average 13.1 2.9 45.9 gms Dirt Average 7.75 2.5 12.6 gms 5
(Figure 6) The growth comparison between raft and gravel methods 24hrs before harvesting. The tape measure (right) is set at 12in for scale. Data analysis from the three replicates produced significant P-values. The one-way ANOVA presents the P-values for each replicate performed (Table 7). P=0.00002 for replicate #1 P= 0.0134 for replicate #2, and P= 0.0012 for replicate #3. All three replicates were statistically different. The T-test performed examined only the raft and gravel methods to determine which was more efficient. Compiling the weights of all three replicates, the T-test result was P= 0.0014. (Table 7) With no mortality and a p value of less than.05, data from replicate one was easy to compare. Analysis of Variance (One-Way) Replicate 1 Summary Groups Sample size Sum Mean Variance Raft 10 136.3 13.63 79.649 Gravel 10 459.4 45.94 386.13156 Dirt 10 126.8 12.68 31.15733 ANOVA Source of Variation df MS F p-level Between Groups 2 3,585.11033 21.64321.0000245 6
In all three replicates; the gravel, raft, and dirt s growth curve rates were similar (Figure 8). Within the first two weeks after planting all three groups performed equally (Figure 8). The gravel method produced the fastest growth rates in the 2 nd replicate (Figure 8). 12 Growth rates average height (in) 10 8 6 4 2 Gravel raft dirt 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 30 days (Figure 8) The results of replicate #2 after 30 days of growth. Nitrate levels varied throughout the three month period. The seedlings were planted on day eight when the nitrogen levels reached 50-100 ppm (Figure 9). Nutrient levels dropped to 30 ppm before rebounding. After this initial drop the levels never fell below 100 ppm, and were usually around 200 ppm for the second two growth periods. 7
250 200 Nitrate ppm 150 100 50 0 0 5 10 15 20 25 30 35 40 Days (Figure 9) The average nitrate level running the system with fish was 175 ppm. Discussion The total amount of basil harvested in the first replicate was 722 gms, 460 gms of which were from the gravel method. This means that with three gravel beds the theoretical yield could be as high as 1377gms. Statistically this experiment allowed for multiple comparisons. Comparing gravel, raft, and dirt the average P value was 0.0049 showing that all the methods were different. Looking at only the raft and gravel methods however, showed that the differences were small (P= 0.0014). Average heights and weights remained consistent with the gravel producing slightly more biomass than the raft method. On day 15 of 30 the basil began to grow at a rapid rate (Figure 8). After day 15 the two aquaponic methods surpassed the soil group. The reason for this may be because they were sprouted indoors using standard germination techniques, and they had to become accustomed to their new environment. Weather was also a factor. During the second replicate growth period, a heavy rain storm came two days after planting. Seven seedling groups in the three methods were killed. This made comparison with this group difficult because its final biomass was much lower than the other periods. Replicate 3 also had a high mortality rate with four seedling groups 8
dying due to heavy rains. By the end of 30 days the gravel group was still twice as tall as the dirt group. Nitrate levels are also important to include because an aquaponics system as a whole, is dynamic (Figure 9). It takes time for the buildup of nutrients to occur (Tyson et. al 2007). The system keeps building on itself growing with beneficial bacteria, nutrients, and natural water filters (Whitton 2009). With the water moving up and down in the gravel system, it creates an ideal environment for beneficial bacteria to eliminate solid wastes from the fish (Tamaru pers. comm). The nutrient cycle of the system may be a reason why the mean biomass in the third group would have been the highest had four of the plants not died. With the basil absorbing nitrogen in the running system, the average nutrient capacity while in use was 175 ppm. When no plants were in the system the water became more nutrient rich achieving as high as 350-400 ppm which caused a small amount of mortality in the fish. In conclusion, both raft and gravel filtration forms of aquaponics are effective methods in the cultivation of basil. In this study, the dynamic gravel method produced the highest basil biomass. With the extra cost of purchasing the gravel, the greater biomass produced would still be worth the initial cost. The gravel would also far outlast the brittle Styrofoam rafts. One of the benefits of the raft method is greater ease of access to the water for cleaning and maintenance. The perfect aquaponics garden might have a combination of both gravel and raft methods. Perhaps two gravel beds and two raft beds. This ratio could possibly create the most biomass with the lowest maintenance and costs. 9
Sources cited Bybee, D. 2011. Pers Comm. July 8 th. Biology teacher/mentor. BYU-H Laie, HI. Emberger, G. 1991. A simplified integrated fish culture hydroponics system. The American Biology teacher 53(4): 233-235. Hershey, D. R. 1994. Solution culture hydroponics: History & Inexpensive Equipment. The American Biology teacher. 56(2):111-118. Howerton, R. 2001. Best management practices for Hawaiian aquaculture. Waimanalo HI, Center for tropical and subtropical aquaculture press. Pp. 32. Jones, J. B. 1997. Hydroponics: a practical guide for the soilless grower. Boca Raton, Florida. St. Lucie Press. Pp 230. Mackowaik, C. L. 1989. Continuous hydroponic wheat production using a recirculating system. John F. Kennedy space center, Fla. National aeronautics and space administration press. Pp 60. Tamaru, S. C. 2011. Pers Comm. June 13 th. Aquaculture specialist. Kaneohe, HI. Tyson, R. V.; Simonne, E. H.; Davis, M. 2007. Effect of Nutrient Solution, Nitrate-Nitrogen Concentration, and ph on Nitrification Rate. Journal of Plant Nutrition. 30 (6):901-913. Whitton, K. 2009. Liquid engineering: Organic farming goes big by getting small. Green: Hawaii s sustainable living magazine 1(3):41-45. 10