Swirl and Clarifier Tanks

Swirl and clarifier tanks are intermediary water tanks that rely on the power of gravity and water movement to settle solids out of the water before it moves on to other filtration methods. They’re usually installed immediately after the fish-rearing tanks and before mechanical filtration units and bio-filters. Clarifier tanks are usually the largest units in the system after the fish and plant tanks. For the DWC/raft culture style of aquaponics, these tanks are usually 1,000 gallons or more in size. In the UVI raft system, you’ll find two 1,000  gallon clarifier tanks for every four 2,000 gallon fish tanks. In a 22,000-gallon system, this translates into the clarifier tanks accounting for approximately 11% or 1/10th of the system. If these tanks are too small, water will flow out of them before sediment and dissolved solids get a chance to settle at the bottom. These tanks require routine manual cleaning, so make sure they’re low and wide rather than tall and narrow, as it’s much harder to vacuum or scoop out the waste when balanced on a ladder.

Swirl filters work similarly to clarifier tanks, but they include a basket-shaped filter unit that traps even more sediment as the water swirls in and out of the tall and relatively narrow space. With swirl tanks being considered mechanical filters, they’re sized similarly to other static units at 1.5% of the total system volume.

Degassing Tanks for High Stocking Systems

If you choose a system with high stocking rates for the fish, you’ll need to include a degassing tank that exposes the water to a large surface area and active aeration. This turbulence causes volatile ammonia and methane gasses to evaporate rapidly, resulting in higher water quality. Systems with less than 1 lb. of fish per cubic foot of water generally don’t need degassing tanks, while those above that rate should incorporate at least one. These tanks are usually sized at half of the size of filter tanks, or .75% of the total system volume. This translates into one 185-gallon tank for a 22,000-gallon system.

Sump Pumps for Processing and Returning Water

In recirculating systems, sump pumps move water between tanks, through filters, and back to the beginning of the system after it leaves the plant tanks and biofilters. Some systems manage to do all of this with a single large pump. The exact size or power of the pump is less important than its effective volume of movement. Aim for around one full exchange of water in your fish tanks per hour. If you have 10,000 gallons among the fish rearing tanks in your system, look for a pump that can move that many gallons per hour. Don’t forget to consider the amount of head lift the pump creates as well. Setting up your system with a slight slope from the fish tanks to the end of the growing beds encourages a steady and strong flow without any additional pumps. If the total slope is five feet from top to bottom, your pump must provide the required flow at that amount of head. Every foot of lift the pump must provide lowers its hourly volume slightly, so look for a pump with 50% more head lift than necessary so you don’t lose volume during real world operation.

Aerating the Tanks

Most aquaponics systems only require active aeration in the fish tanks and degassing units. In warm climates, active aeration is often needed in the plant troughs as well. High temperatures speed up the loss of dissolved oxygen, leaving little in the water by the time it reaches the plants. Aeration in the plant troughs is also needed if the dissolved oxygen (DO) measurement of the water, leaving it and returning to the fish tanks, is below 4 mg/L. Lower water temperatures reduce the amount of aeration you must add, so coldwater systems usually only need aeration equipment in fish and degassing tanks. High stocking rates also use up the oxygen faster, increasing the need for aeration further down the line.

Air stones, similar to the ones used in home aquariums, only much larger, are the primary aeration tools used in aquaponics. Look for large, medium-pore air stones, as both large and small pore sizes are easily clogged by solid fish waste. You’ll also need a regenerative air pump to connect to the stones. Other tools include paddles, airlift pumps and rotors that trap air and release it underwater. The size and number of aeration devices you need depends on the rate of pumping. The faster you pump water through the system, the more air is added naturally and the less oxygen escapes before water reaches the end of the troughs. Areas that experience at least five gallons per minute of flow don’t need their own airstones, and areas that can maintain stable measurements of DO of 5 to 6 mg/L likely don’t need separate aeration devices either.

All aeration devices, including regenerative pumps and paddlewheels, are rated by the cubic feet of air they move per minute (CFM) at a specific depth of water. Airstones and other diffusers are rated by the number of pounds of fish they will support at an average of 6 mg/L DO and at the rated depth and an average temperature of 72 degrees. For example, many six-inch-long medium-pore airstones are rated to support around 14 lbs. of fish each. That means you’ll need an airstone for each 14 lbs. of fish, along with an air pump that can generate the total CFM once you add up the rate for each stone. Commercial quality diffusers tend to support closer to 20 to 30 pounds of fish per stone, while paddlewheels and rotors can be found that generate enough dissolved oxygen for hundreds of pounds of fish each. Don’t forget to adjust these rates up and down by the amount of your average temperatures vary from the 72 degree F baseline. For example, a system that operates at an average temperature of 65 degrees is 10% colder and will require 10% more aeration volume and diffuser output to compensate.

Designing the Plumbing to Connect Everything Together

PVC plumbing is the primary piping used to connect the various tanks and pumps in all aquaponics systems. It’s even commonly used to make the tubes for NFT-growing systems because it’s food safe, durable and its white color reduces algae growth and controls temperature gains. Unlike many other parts of an aquaponics system, most of the plumbing you’ll need to connect your pumps should be available locally at any plumbing or home supply store.

It is necessary to oversize all the pipes you use in the system to accommodate the eventual accumulation of bacterial biofilm, algae growth, wandering tree roots and other obstructions. Using the minimal diameter pipe for the flow calculations of your system will only result in constant clogs that can kill off your fish or plants before you notice the problem. Aim for four-inch pipes for parts of the system that contain high levels of solids and nutrients. Two to three-inch pipes can work just fine further down the line where filtration and settling has conditioned the water.

Minimize curves and twists as much as possible to maintain optimal flow speeds and volumes. Pipe length will vary depending on the layout and size of your system, but connecting the tanks is relatively straightforward. Planning work is based solely on the distance between individual parts of the system. Make sure to include valves after every major stage so you can always shut down the system right at the source of a problem rather than draining the entire system just for a water change or clean-up procedure. Pipe placement plays a large role in creating the desired amount of slope across the system as well. Aim to place inlet pipes high on tank sides and outlets much lower to increase water flow. This is especially important between connected plant tanks or troughs, as they tend to significantly lose speed and turbulence over a long run of open water. Water slowing down in the middle of a plant trough causes uneven distribution of nutrients and dissolved oxygen, resulting in patchy plant growth.