DWC or Raft Aquaponics

What is the Raft Aquaponics System?

The raft system of aquaponics, also known as deep water culture (DWC) or floating system, is one of the most efficient aquaponics techniques. This system is generally implemented on large-scale or commercial aquaponics because of its mass production capability.

How Raft Aquaponics System Works

In a raft system, the nutrient-rich water circulates through the long canals while rafts (polystyrene or foam board) float on top. The plants are grown on the raft boards supported within holes by net pots. Plant roots hang down in the nutrient-rich, oxygenated water, where they absorb oxygen and nutrients to grow rapidly. The nutrient-filled water flows continuously from the fish tank through the filtration process, (the beneficial bacteria primarily live in the biofilter) , then to the raft tank/growbeds/cannals where the plants are grown, and finally back to the fish tank. 

Since the development of aquaponics raft systems (DWC) on a commercial scale by Dr. Rakecy at the University of the Virgin Islands,many commercial aquaponics farms use this system because it allows the plants to grow faster and yield more crops. 

Advantages and Disadvantages of the Raft System

Advantages:

• Cost-effective.
• The roots are more exposed to water nutrients.
• Simple and can be the most economical to build of all aquaponics systems at a commercial scale .
• Plants are easier to harvest since roots are submerged in water and not in any media.
• The water volume in a raft system ensures it is more stable in water quality and temperature than in other systems.
• Easy maintenance as raft beds are easy to clean.
• It is suited for home gardens, hobby applications, and commercial production.
• It allows for a higher stocking of fish.

Disadvantages:

• It is limited to growing small leafy greens like lettuce and basil.
• Unsuitable for roots, some fruits, and a few other plants.
• It can be a mosquito breeding ground if not designed correctly. (these may be controlled by using guppies or mosquito fish)
• Water can evaporate at the gaps between raft edges and the tank/cannals it is kept in.
• It needs filtration, since the roots are entirely immersed.
• The filtration process increases costs, besides requiring regular filter cleanups.
• Roots are susceptible to microbial attacks .
• Because the roots are completely submerged, the addition of oxygen through aeration is definitely required.

Components of A Raft System

 1. Fish Tank

The fish tank is where your fish will live, and the tank acts as the collection reservoir for fish waste. T.

2. Grow Canals/Beds ( Hydroponic Component ) 

Like the fish tank, canals should be made of  robust and inert materials that can hold a large water volume. Canals can vary in length, but it is generally recommended that the width should be the standard width of a polystyrene sheet. ( About 4 feet but this varies)  The recommended depth is 30 cm to allow adequate plant root space. The canals’ retention time should be 1 to 4 hours, regardless of size, to allow adequate replenishment of nutrients in the canal. Plants grow faster from a faster water flow rate because the roots will hit many more ions.

3. Floating Rafts

This is the place for the plants to grow. Floating rafts are constructed from styrofoam or other lightweight material lined with foam. Plants are placed in holes on the rafts, allowing their roots to dangle into the water. Net pots are often used for added stability and to prevent the plants from falling through rafts into the system.

4. Mechanical /Swirl Filters

Filtration in aquaponic systems is essential because the solid particles, residual fish foods, and fish waste can create a severe problem in your aquaponic system. The filters capture the solid waste from the fish, plant material, and anything that might get into the system. They can clog your piping, nozzles, and plant roots and cause unwanted damage to your system.

5. Biofilter 

A Biofilter is a place for the bacteria to colonize. This is where the beneficial bacteria turn fish waste into usable, nutrient-rich food for the plants.

6.Mineralisation tank 

 Mineralization tanks are used to house other beneficial bacteria that further breakdown waste into essential nutrients over a period of time 

7. Water Pump

 Pumps are used to recirculate the water in raft systems and keep the water moving.

7. Aeration Method

Aeration is the addition of oxygen into the water. Adding more oxygen to the water is essential for the fish and the plant’s health and growth. Air pumps, diffusers and  other methods can be used to aerate the system.

Planting in A Raft System

Planting in a raft system of aquaponics involves suspending plants in the floating rafts (foam or polystyrene sheets), with their roots hanging down in the water. The floating rafts have holes drilled to fit the net cups used for supporting plants. The amount and location of holes depend on the vegetables and the distance between plants.

Seedlings can be started in a plant nursery. Once the seedlings are large enough to handle, they can be transferred into net cups and planted in the floating rafts. The net cup should be filled with grow media such as gravel or expanded clay to support the seedling. 

It is essential to remove the whole plant from the canal during final harvest, including roots and dead leaves, to prevent clogging and solid waste buildup. The rafts should also be cleaned but not left to dry to prevent killing the nitrifying bacteria on the submerged surface of the raft.

Plants To Grow In A Raft System

Small and lightweight plants are the most suitable for growing in a raft system. Vegetables that grow best in this system enjoy a moist environment and have a small root system and structure.

Factors For Choosing Raft System Plants

• Weight

The best plants for the raft system or DWC are small and lightweight. The rafts are generally durable and affordable, but they can only support a limited weight. Top-heavy plants can fall over or break at the stems without root anchoring provided by a dense media.

• Volume

Raft systems function on a single horizontal plane since they are too heavy to stack. If you have limited space, consider an approach to facilitate denser growing plants. Be sure to match your plant site spacing on your rafts to your plant size.

• Water Friendly

Choose plants that are thirsty plants that will thrive in a raft system, so pick seed varieties that love water, like lettuce. Drought-loving plants will not do well in this system.

• Harvest Potential

Ensure that the plant will provide you with a profit if you’re growing commercially and a good harvest if you’re growing for home use or as a hobby.

The Plants That Grow Best in Raft Systems are:

Lettuce and Leafy Greens

Basil

Collard Greens

Kale

Chard

Bok Choy

Raft Systems Tips

The raft system can be excellent for mass production and easy to maintain with these simple tips. Here are a few tips on running and maintaining a raft system.

• Aeration is the key. This should be maintained regularly to ensure your plants and fish have plenty of oxygen to live and grow. The water needs air added either by pumps or air stones or some methods to increase the dissolved oxygen content.
• Test the water’s pH level and temperature regularly to ensure that it is within the appropriate range for nitrification and nutrient uptake .
• Check the pumps and piping regularly for proper functions and remove any solid materials from the filters that might clog or block.
• To ensure maximum yield from your system, use plants and fish that are easy to care for, grow quickly, and are disease resistant.

Conclusion

The raft systems are one of the most straightforward aquaponics systems to set up and are most promising for commercial and mass production. The rafts can be reused for many planting cycles once the plants are harvested.