2.4 Plant Culture or Hydroponic Subsystem

The hydroponic portion of the system encompasses the majority of the facility footprint. Three primary designs are used: media beds, deep water culture (DWC), and NFT.

Media-based systems: The design of media-based systems, sometimes called flood-and- drain, is fairly straight forward. A container filled with substrate is periodically flooded with water from the fish tank. Water then drains back to the sump (or fish tank) drawing oxygen into the substrate for plant roots and nitrifying bacteria. The media bed supports the plant as it grows and serves as a solids and biological filter (Figure 6). Due to relatively few components and ease of construction and operation, these systems are popular for hobbyists and in developing regions. However, it is uncommon to find commercial production using only media beds as they are less productive than other types discussed below. Rule of Thumb for media systems are detailed in Table 1.

Table 1: Rules of thumb for media-based aquaponic systems.

A variety of materials can be used as substrate, including pea gravel, lava rock, expanded clay pebbles, or other inert media; practitioners may be limited by what is locally available. Water flow in the system is controlled by either a timer or siphon. Using the timer method, water is pumped for a set amount of time, allowing the bed to fill.

When the timer shuts off, water drains until the timer engages the pump again. The siphon method is often implemented using an automatic bell siphon (Figure 7a) or loop siphon (Figure 7b). In both siphon methods, the pump runs continuously, controlling how fast the bed fills and drains. Fox et al. (2010) gives comprehensive, step-by- step instructions for building, operating, and troubleshooting an automatic bell siphon.

Constant-flow media systems offer an alternative to the flood-and-drain method. Heavily aerated water flows into the media bed. Instead of a flood- and-drain cycle, the water level stays constant by using a standpipe. This drastically reduces the size of the sump needed for this type of growing system.

Deep Water Culture: This growing method involves suspending plants in a floating raft, allowing the roots to hang down into the water (Figure 8). Plant roots are in constant contact with nutrient-rich water from the fish tank.

Effective solids filtration is a requirement in these systems to prevent solids from entering the plant bed and clogging plant roots. Aeration must also be provided in the plant troughs to maintain adequate oxygen levels for plant roots and beneficial bacteria. Along with their large water holding capacity that keeps water quality parameters more stable, the underside of the rafts and lining of the troughs provide adequate space for nitrifying bacteria to colonize. The design itself also provides a cushion against power outages, as roots stay submerged in water despite loss of water or air flow.

Table 2: Rules of thumb for DWC in aquaponics.

*Exception is in early life stages where fish can consume 5-10% of their body weight in food per day.

Deep water culture (DWC) is more productive (kg of produce/m^2^ growing space) than media-based systems; however, it can be more difficult to manage on a smaller scale. These systems are well researched by the hydroponics and aquaponics industry and are commonly implemented in commercial settings.

Leafy greens and herbs, such as basil, do well in this production system. Fruiting crops like tomatoes, cucumbers, and peppers can be successful with appropriate nutrient densities and structural support. The DWC technique may not be suitable for areas where access to supplies or equipment is limited. Rules of thumb for DWC in aquaponics are listed in Table 2.

Nutrient Film Technique: Nutrient Film Technique (NFT) technology comes directly from the hydroponics industry. In this method, plants are inserted into the top of shallow horizontal channels. A small film of water is pumped through the channel, coming into contact with plant roots that utilize those nutrients for growth (Figure 9). NFT systems, like DWC, require sufficient solids filtration to prevent contamination of plant roots. In contrast to DWC, NFT systems need a separate biological filter, as the channel alone does not provide enough surface area for sufficient growth of nitrifying bacteria.

These systems are more complex to design, build, and manage than media-based systems. If channels are not sized correctly, plant roots can disrupt water flow by clogging the pipes. This design assumes a degree of risk, as pump failure can result in large crop loss if water flow does not resume quickly. However, NFT can be a great system for urban areas or rooftops as they are lightweight, use very little water, and can be made from easily sourced materials. Rules of thumb for NFT in aquaponics are listed in Table 3.

Table 3: Rules of thumb for NFT in aquaponics

*Exception is in early life stages where fish can consume 5-10% of their body weight in food per day.

Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.