Critical Analysis: The Deep Flow Technique (DFT) for Hydroponics

 Introduction:

The Deep Flow Technique (DFT) for hydroponic crop production, also dubbed deep water culture or DWC, is a conventional method that has been widely used by many commercial greenhouse growers for the past few decades. It has also been used by many indigenous peoples inhabiting swamp marshes, slow flowing rivers, and lake fronts. In controlled environments, it contributes to significant water and fertilizer usage reduction over field crop production in soil through the retention of the irrigation solution as a stagnant or recirculating pond or raceway (typically 4 cm to 40 cm deep). It is also the preferred method for aquaponic crop production, as it integrates relatively well with aquaculture systems.

DFT systems ensure plant roots are always submerged in water without necessitating that water be pumped to the root zone. Instead, it is an air compressor or injector that is needed, and this functions to force aerate and oxygenate the solution evenly and constantly, usually through the use of air stones or other perforated pipes. The solution must be maintained at a cool temperature (normally 18 C to 24 C) to allow for both maximal dissolved oxygen (DO), which is reduced at high temperatures, and maximal enzymatic rates, which are reduced at low temperatures. Floating rafts (generally expanded polystyrene or high density polyethylene) secure plants while separating and insulating the root zone from the shoots and ambient air. Net pots may be required to hold plants securely in the raft holes, depending on the type of raft used. The insulative rafts and the high ratio of solution volume to surface area make DFT systems the most thermally stable hydroponic systems.

Using DFT Systems:

DFT systems are generally designed and laid out in one of two formats (with numerous exceptions). The first is in the form of greenhouse length raceways separated by aisles that allow complete and direct access to the crop for scouting, planting, and harvesting. Space usage efficiency in the greenhouse may be as low as 50% in this format. The second is in the form of large ponds, often spanning the entire greenhouse floor or large sections thereof. In this format, space usage efficiency can exceed 95%, and direct crop access is heavily restricted. In large pond format DFT systems, crops are moved across the greenhouse by the rafts with mature plants/finished produce being removed at one end and replaced by rafts with younger seedlings pushing in at the other end. It is not advised that personnel enter the ponds even with personal protective equipment, as this risks contamination of the entire system with plant and human pathogens.

Small scale DFT systems are most often simple self-contained tubs or trays filled with fertilizer solution and topped with a raft. In these cases, solution adjustments (E.C. and pH) must be made by raising the raft for access to the solution, and forced aeration must also be localized within the systems. The advantages of using smaller units include contamination avoidance and ease of maintenance, but all in all, this is less efficient from a commercial standpoint.

Commercially, farms planning to use DFT systems should set the size of each unit to match crop harvest volumes, such that they can free each up periodically for a complete drainage and thorough cleaning, including surface sterilization and the physical removal of biofilms and other organic debris. Due to their physical conformation, DFT systems present the most challenge to frequent cleaning, and the general approach is to include a low concentration of cleaning agents (hydrogen peroxide, ozone, surfactants, biologicals, etc.) in the solution and remove the rafts for sterilization between each short crop cycle, only emptying units for full cleaning one or two times per year.

DFT systems have long been shown to be effective at growing everything from baby greens to tomatoes under ideal conditions. However, special considerations must go into using DFT for larger crops with longer cycles. The weight of large plants, even with conventional trellising, risk damaging or submerging rafts. Furthermore, large root systems occupying the fertilizer solution can slow flow rates and inhibit proper mixing of the incoming adjusted solution. Therefore, the more common DFT conformation for larger plants is an interconnected system of single plant buckets with rigid tops. Fresh solution is pumped through the system continuously, and aeration can be either localized or centralized. Reasonable root pruning can even be exercised to improve solution flow without harming the crop.

Maintaining adequate DO is critical when using a DFT system. With no shallow film, frequent complete drainage, or air pore space at the root zone, the only DO in the system comes from what the operator adds. It is necessary to maintain a bare minimum DO concentration of 4 ppm to support the growth of the majority of non-aquatic plant crops, and stagnant water can quickly run out if plant roots are drawing oxygen out. When using biological agents or organic fertilizers in the solution, the microorganisms will compete for oxygen at the root zone and potentially create near anaerobic conditions conducive to root deterioration and pathogen infestation.

Advances in ultra fine bubble generation and injection have resulted in a few companies creating unprecedented aeration solutions for hydroponic crop cultivation. Whereas conventional venturi devices and compressors with air stones can raise DO concentrations to around 8 ppm under optimal conditions, they can be inefficient and even generate bubbles the size of which can cause pump cavitation and failure if they are not set up correctly. Micro- and nano- bubble generators can raise DO to in excess of 20 ppm while populating the solution with suspended bubbles that can last hours or even days in hydroponic systems. Pumps destroy a small portion of these ultra fine bubbles, and the bubbles in turn reduce pump efficiency slightly, however no damage is done to the pump, and the resulting contribution to root health and plant growth rates are well worth it. This method for solution aeration is highly power efficient and provides unparalleled surface area for oxygen dissolution as well as physical air pockets for roots and microorganisms in the root zone to make use of. Solution supplementation with hydrogen peroxide or ozone can also be effective at raising DO concentrations while helping to keep pathogens at bay, but these are incompatible with organic fertilizers or biological agents.

What DFT Looks Like in Nature:

In nature, DFT hydroponics can be observed in ponds, swamps, and lakes on which floating aquatic and semiaquatic plants grow using the fertilizer produced by fish waste and bacteria as well as the dissolved oxygen produced by algae during the day. Most of our favorite cultivated greens and herbs prefer more concentrated dissolved oxygen and mineral nutrition than is generally provided in the types of natural environments mentioned above, and some are quite sensitive to root zone pH.

Advantages of DFT:

-Very thermally stable due to volume:surface area and insulation of raft, saving energy on solution heating or cooling

-Young crops can be moved around without friction

-Ponds or raceways can be made to any dimension and specification and are generally low cost custom frames with liners

-No chance of accidental dry outs and power failure does not disrupt irrigation

-Raft units are ergonomic and easy to handle (usually 0.5 to 1 m2)

-Irrigation hardware is relatively simple and low cost

Disadvantages of DFT:

-System is heavy and presents added expense and challenge to raising off of ground level, mounting on rolling benches, or stacking vertically—in which case it can even be a hazard

-Distributing dissolved oxygen and nutrients and maintaining pH uniformly throughout the system requires good circulation, creating extra CAPEX and OPEX

-Forces the designer to choose between space usage efficiency and crop access in a greenhouse

-Large units make regular maintenance difficult without clearing and draining entire sections, and this creates the risk of plant and human pathogen build up if not treated preventatively

-No capacity for precision irrigation, as system submerges roots constantly

Conclusions and Future Directions:

The Deep Flow Technique for hydroponic and aquaponic crop production has been serving farmers well for a long time and is a well established cultivation method. Its future, however, lies in making it safer, lighter, and more versatile, especially for use in vertical farms. Hence, modern DFT systems will become more shallow and solution adjustment and aeration will become more centralized and well controlled. We will undoubtedly see an increase in the manufacture of more durable rafts made from materials such as HDPE that have more longevity and thereby reduce long term costs. Modern micro- and nano-bubble generators designed for use with hydroponic systems help maintain healthy roots, inhibit algae and pathogens, and reduce the system’s reliance on dissolved oxygen, since bubbles under 50 um in diameter suspend in the solution and physically persist for a relatively long period of time. The DFT will become even more specialized for the crops that benefit from its format and high potential for automation in certain scenarios. It is encouraging to see hybrid systems emerging based on conventional systems like DFT and the improvements that can be made to affect performance, cost, and efficiency. Research will undoubtedly continue to explore how to best implement the next generation DFT systems around the world.