Introduction of Our Aquaponic System
In our study, aquaponic systems are placed in the greenhouse facility of the Magoon Research Station, University of Hawaii at Manoa (UHM). A series of individual, identical aquaponic systems are being operated side by side. For each aquaponic system, 85-gal oval plastic tanks, filled with 52-gal of water has been operated as an aquaculture unit. The tank is stocked with a high density of tilapia fish (Oreochromis niloticus) at around 0.15-0.2 lb/gal. The fish are obtained from Windward Community College (Honolulu, Hawaii, USA). An air pump is used to provide sufficient oxygen for fish growth by aerating the tank water. The tank is covered by using a wooden board to prevent algal growth.
Fig. 1 Aquaponics at UHM
With respect to the hydroponic component, there exists three primary designs (as shown in Fig. 2), i.e., nutrient film technique (NFT), flood and drain system (also known as ebb and flow), and floating rafts. In NFT system, plants are grown in long narrow channels and a thin layer or film of water flowing over the roots of the plants in the system, providing the plant roots with water, nutrients and oxygen. Flood and drain systems use a container or grow bed containing a growing medium (e.g., gravel, perlite). The container is periodically flooded with water from the fish tank. The water then drains back to the fish tank after a period of time. For a floating raft system, plants are grown on raft (e.g., polystyrene boards) that float on top of water. The fish tank water flows continuously to the raft tank through filtration components, and the purified water flows back to the fish tank.
NFT has less efficiency than the other two designs. While flood and drain is widely used in backyard aquaponics, most of the commercial aquaponic farms prefer floating raft design.
Fig. 2 Different hydroponic designs. (A) NFT; (B) flood and drain; (3) floating raft.
Floating raft hydroponic system is employed in our study. A peristaltic pump (water pump also works) is used to pump the fish tank water to a clarifier, which is built using a 5-gal sealed bucket filled with biomedia. The clarifier captures majority of the suspended solids in the tank water to protect the plant roots in the grow bed. After passing through the clarifier, tank water flows to grow bed. A rectangle plastic tank with an effective volume of 100 gal is used as the grow bed and plants are held up by a foam raft that floats on the water. 50% shade cloth is installed over the grow bed to prevent excessive sun exposure to plants. The nutrients in the water which are essential to plant growth, will be absorbed by plant roots and the purified water then flows back into the fish tank. The water is recirculated continuously in the aquaponic system.
1. Seed Germination
Rockwool cubes are used to sprout plant before stocking into the grow bed. The cubes are soaked in water for eight hours before use. A seed is placed in each hole of the cubes and water is added daily to saturate the cubes. About 2 weeks after germination, the seedling can be transplanted to the grow bed. Cherry Tomato (Lycopersicon esculentum) and Pak Choi (Brassica campestris subsp. chinensis) have been chosen in our aquaponic system to compare the differences between fruity and leafy plants.
Fig. 3 Seed germination
2. System Startup
Leakage test should be carefully conducted before the start of the system. After seedling plants are transplanted to the grow bed, fish can be stocked to the fish tank. Tilapia is the most commonly cultured fish species in aquaponics, because of their high tolerance to fluctuation of oxygen, ammonia and dissolved solids. Thus, we use tilapia is our aquaponic system.
Fig. 4 Tilapia in aquaponics
3. Aquaponics Maintenance
Fish are fed with commercial fish feed once or twice a day, depending on fish size. Ten minutes after feeding, the number of feed particles remaining above water is counted and the next day’s fish feed rate is adjusted so that no more than 5% of the feed added remains after 10 minutes. The pH of tank water is maintained at a neutral range by periodic dosing of calcium hydroxide and potassium hydroxide. The water quality parameters are measured every other day so that the accumulation of ammonia and nitrite could be monitored. The inorganic nitrogen compounds (NH4+, NH3, NO2- and NO3-) can be measured with water quality kits (HACH company, Loveland, CO, USA). The physical parameters (dissolved oxygen, pH, temperature) are determined by using the HQ40d Portable Water Quality Lab Package (HACH company, Loveland, CO, USA). The water quality usually stabilizes after one month following establishment of beneficial nitrifying bacteria. In the floating raft system, there is plenty of surface area for the bacteria to grow. Figure 5 shows the root systems for the growth of bacteria.
Fig. 5 Pak Choi roots two weeks (A); and six weeks (B)
Aquaponics is a food production system. After a certain period of time, fish and plants are ready to be harvested. For details, please refer to CTAHR publication “On-Farm Food Safety: Aquaponics” (http://www.ctahr.hawaii.edu/oc/freepubs/pdf/FST-38.pdf).
Fig. 6 Tomato (A) and Pak Choi (B) ready to be harvested
Aquaponics provides a cost effective way of growing fish and plants. In our study, the fish weight can increase by more than 50% in weight and three batches of pak choi can be harvested during 4-month period.
Table 1 Performance of aquaponics in 135 days
Water Quality in Aquaponics
The integration of hydroponics with aquaculture is believed to be able to improve the water quality. Fig. 7 shows the comparison of water quality parameters between the steady aquaculture and aquaponic systems (at the same fish stocking density). As seen from the figure, TAN, NO2-, and NO3- concentrations are lower in the aquaponic system than that in the aquaculture system. Aquaponic systems show better water quality for healthy fish growth.
Fig. 7 Water quality parameters in steady aquaculture and aquaponic systems