Aquaponics: Part II

The recirculating aquaculture system (RAS) at Aprovecho was designed mainly to raise as many tilapia as possible while keeping external inputs to a minimum. Some RAS systems balance hydroponic vegetable production with fish production (Aquaponics). Some of those systems can rely heavily on commercially produced fish food. Initially, ours will favor aquatic plants grown for fish food (duck weed, water hyacinth), over vegetable production. Having more space dedicated to water plants increases the filtering capacity of the system, which increases the total fish bearing capacity.

Aquaculture system plan view

Based on initial research and consultations with experts in marine biology, aquarium sciences, and hydroponics, we determined that the maximum stocking capacity is approximately one fish per gallon of water.

Aprovecho’s system consists of two circular eight-foot-diameter by four-foot-deep tanks that have a combined capacity of 1,220 gallons. These tanks will support approximately 1,200 tilapia. Water is drawn out from the tanks by a 1,200 gallon-per-hour in-line pump. The pump sends the water from both tanks into the top tray of our hand-made wooden water tables.

The tables consist of upper and lower troughs that are 2.5’ wide x 8” deep. Pond liner is laid and secured into the troughs to form a watertight seal. The troughs will be filled with a shallow layer of gravel, aquatic and wetland plants that will serve to filter the solids as well as remove the nitrates (pollutants) created in the fish tanks.

The water that is pumped into the upper trough must travel through 68 linear feet of this wetland environment before it reaches the end of the upper trough and drops down into the lower trough. The lower trough recreates a similar environment as the upper troughs, and the water must travel another 68 feet back to the tanks before dropping through bulkhead fittings and returning to the fish tanks.

All of this is contained in a 20’ x 30’ greenhouse. The entire system holds 4,500 gallons of water between the tanks and troughs. The water is fully exchanged between the tanks and troughs once every hour, 24 hours / day.

We didn’t want to completely pass up an opportunity to begin working with hydroponic vegetable production, so an additional hydroponic component was added. Being that most vegetable plants cannot grow in the constantly flooded environment in the troughs, we built a separate 26’ long by 3.5’ wide trough that will be filled 3” deep with pea gravel.

Part of the water being pumped out of the fish tanks is diverted from main troughs to fill up a 55-gallon drum that is set up with a toilet float valve. Every time the drum fills up, the float valve opens, releasing the 55 gallons of water into the gravel filled trough. The bottom of this trough is sloped downward to encourage the water to slowly work its way from one end to the other. This way, the plants will receive a watering 4 – 5 times per day, without completely “drowning” their roots. The frequency of the watering is controlled by how quickly the 55-gallon drum is filled up (valve control).

Our plan is to begin cycling the system this winter by adding ammonia to foster beneficial bacteria growth in the gravel, and introducing the cold tolerant plants into the troughs. The bio-filtration system needs to be working before the fish are introduced in March. Once the fish are stocked in the spring, the wetland system will grow as they grow with the fish meeting the increasing demand from the plants for nutrients, and the plants filtering the increasing volume of wastewater from the fish. Aqua-ponics indeed!

By the fall 2010 I hope to be reporting on our bountiful and tasty tilapia harvest. Perhaps we will share in the bounty and celebrate with our members by instituting an annual fish fry. Another great reason to become a member! Stay tuned.

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