Hydroponic systems

There are three main types of hydroponic systems (see also Module 1). In media bed hydroponics the plants grow in a substrate. In nutrient film technique (NFT) systems the plants grow with their roots in wide pipes supplied with a trickle of water. In deep water culture (DWC) or floating raft systems the plants are suspended above a tank of water using a floating raft. Each type has its advantages and disadvantages which are discussed in more detail below. The evidence is somewhat contradictory in terms of their relative efficiency for crop production in aquaponic systems. Lennard and Leonard (2006) compared the three hydroponic sub-systems for lettuce production and found the highest production in gravel media beds, followed by DWC and NFT. However, subsequent studies by Pantanella et al. 2012 found that NFT performed as well as DWC, while media bed consistently underperformed in terms of yield.

As for the role of the design of the hydroponic component on the overall performance and water consumption of aquaponic systems, a literature review by Maucieri et al. 2018 found that NFT is less efficient than media bed or DWC hydroponics, although the results were not unequivocal. The hydroponic component directly influences water quality, which is essential for fish rearing, and is also the main source of water loss by plant evapotranspiration. The design of the hydroponic component therefore influences the sustainability of the entire process, either directly in terms of water consumption and/or indirectly in terms of system management costs. The choice of the hydroponic component for an aquaponic system will also influence the design of the entire system. For example, in media bed systems the substrate usually provides enough surface area for bacteria growth and filtration, while in NFT channels the surface area is insufficient, and additional biofilters will need to be installed (Maucieri et al. 2018).

Media bed hydroponics

In media bed hydroponics, a soil-less growing medium or substrate is used to help the roots support the weight of the plant. The media bed also serves as a biological and physical filter. Of the hydroponic sub-systems, media beds have the most efficient biological filtration because of the large surface area where biofilm, containing nitrifying and other bacteria, can colonize. The substrate also captures the solid and suspended fish waste and other floating organic particles, although the effectiveness of this physical filter will depend on the particle and grain size of the substrate, and the water flow rate. Over time, the organic particles are slowly broken down by biological and physical processes into simple molecules and ions that are available for the plants to absorb (Somerville et al. 2014b).

The substrate may be organic, inorganic, natural, or synthetic (Figure 1), and is housed in grow containers of different forms. It needs to have an adequate surface area while remaining permeable for water and air, thus allowing bacteria to grow, water to flow, and the plant roots to breathe. It must be non-toxic, have a neutral pH so as not to affect the water quality, and be resistant to mould growth. It must also not be so lightweight that it floats. Water retention, aeration and pH balance are all aspects that vary depending on the substrate. Water is retained on the surface of the particles and within the pore space, so water retention is determined by particle size, shape, and porosity. The smaller the particles, the closer they pack, the greater the surface area and pore space, and hence the greater the water retention. Irregular-shaped particles have a greater surface area and hence higher water retention than smooth, round particles. Porous materials can store water within the particles themselves; therefore, water retention is high. While the substrate must be capable of good water retention, it must also be capable of good drainage. Therefore, excessively fine materials must be avoided so as to prevent excessive water retention and lack of oxygen movement within the substrate. All substrates need to be cleaned periodically (Resh 2013).

Substrates can also be classified as either granular or fibrous. Granular substrates include light expanded clay aggregate, gravel, vermiculite, perlite, and pumice. Fibrous substrates include rockwool and coconut fibre. Water is mainly held in the micropore space of a substrate, while rapid drainage and air entry is facilitated by the macropores (Drzal et al. 1999). An adequate combination of large and small pores is therefore essential (Raviv et al. 2002). Granular substrates have high macroporosity (air availability) but comparatively low microporosity (water availability), while fibrous substrates have high microporosity but comparatively low macroporosity.

Light expanded clay aggregate (LECA) is very lightweight compared with other substrates, which makes it ideal for rooftop aquaponics. It comes in a variety of sizes; the larger sizes with diameters of 8-20 mm are recommended for aquaponics (Somerville et al. 2014). Larger pore spaces (macroporosity) mean better percolation of solution through the substrate and better air supply, even when biofilms cover the surfaces. However, LECA has small micropores, and thus does not have good water holding capacity.

Volcanic gravel (tuff) has a very high surface area to volume ratio which provides ample space for bacteria to colonize, and it is almost chemically inert, except for small releases of microelements such as iron and magnesium and the absorption of phosphate and potassium ions within the first few months. The recommended size of volcanic gravel is 8-20 mm in diameter. Smaller gravel is likely to clog with solid waste, while larger gravel does not offer the required surface area or plant support (Somerville et al. 2014b).

Limestone gravel is not recommended as a substrate, although it is sometimes used. Limestone has a lower surface-to-volume ratio than volcanic gravel, it is comparatively heavy, and it is not inert. Limestone is composed primarily of calcium carbonate (CaCO₃), which dissolves in water. This will increase the pH, and it should therefore only be used where water sources are very low in alkalinity or acidic. Nevertheless, a small addition of limestone can help to counterbalance the acidifying effect of nitrifying bacteria, which can offset the need for regular water buffering in well-balanced aquaponic systems (Somerville et al. 2014b).

Vermiculite is a micaceous mineral which expands when heated above 1000 ⁰C. The water turns to steam, forming small, porous, sponge-like kernels. Vermiculite is very light in weight and can absorb large quantities of water. Chemically, it is a hydrated magnesium-aluminium-iron silicate. It is neutral in reaction with good buffering properties, and has a relatively high cation exchange capacity and thus can hold nutrients in reserve and later release them. It also contains some magnesium and potassium, which is available to plants (Resh 2013).

Perlite is a siliceous material of volcanic origin, mined from lava flows. It is heated to 760 ⁰C, which turns the small amount of water into steam, thereby expanding the particles to small, sponge-like kernels. Perlite is very lightweight and will hold three to four times its weight of water. It is essentially neutral, with a pH of 6.0–8.0, but with no buffering capacity; unlike vermiculite, it has no cation exchange capacity and contains no minor nutrients. It should not to be used on its own, but rather mixed with another substrate in order to improve drainage and aeration and thereby prevent nutrient build-up and subsequent toxicity issues while providing an oxygen-rich environment for the roots to thrive in (Resh 2013).

Pumice, like perlite, is a siliceous material of volcanic origin and has essentially the same properties. However, it is the crude ore after crushing and screening, without any heating process, and therefore it is heavier and does not absorb water as readily, since it has not been hydrated (Resh 2013).

Rockwool is made from basalt rock that is molten in furnaces at a temperature of 1500 ⁰C. The liquid basalt is then spun into threads and compressed into wool packets which are cut into slabs, blocks, or plugs. Most of the rapid expansion of the greenhouse industry over the past two decades has been with rockwool culture. However, in recent years concerns have been raised about its disposal, as it does not break down in landfills. Now many growers are turning to a more sustainable substrate – coconut fibre (Resh 2013).

Coconut fibre (or coir) is an organic substrate derived from frayed and ground coconut husks. It is close to pH neutral and retains water while allowing for a good amount of oxygen for the roots (Resh 2013).

Table 1: Characteristics of different growing media (after Somerville et al. 2014b)

Depending on the type of substrate, it will occupy roughly 30-60 percent of the total media bed volume. The depth of the media bed is important because it controls the amount of root space volume in the unit, which in turn determines the types of vegetables that can be grown. Large fruiting vegetables such as tomatoes, okra and cabbage will need a substrate depth of 30 cm to allow sufficient root space and to prevent root matting and nutrient deficiencies. Small leafy green vegetables only require 15-20 cm of substrate depth (Somerville et al. 2014b).

Figure 2: Tomato transplants growing in a media bed container system with drip irrigation and LECA substrate https://commons.wikimedia.org/wiki/Category:Hydroponics\#/media/File:Hydroponic_Farming.jpg

There are different techniques for delivering nutrient-enriched water to media beds. It can be simply trickled from drippers attached to pipes uniformly distributed on the medium (see Figure 2). Alternatively, a method called flood-and-drain (or ebb-and-flow) causes the media beds to be periodically flooded with water which then drains back to a reservoir. The alternation between flooding and draining ensures that the plants have fresh nutrients and adequate air flow in the root zone, which replenishes the oxygen levels. It also ensures that enough moisture is in the bed at all times so the bacteria can thrive in their optimal conditions. The nature of a flood-and-drain media bed creates three separate zones which are differentiated by their water and oxygen content (Somerville et al. 2014b):

• The top 2-5 cm is the dry zone, which functions as a light barrier, minimizing evaporation and preventing the light from directly hitting the water which can lead to algal growth. It also prevents the growth of fungus and harmful bacteria at the base of the plant stem, which can cause collar rot and other diseases.

• The dry/wet zone has both moisture and high gas exchange. This is the 10-20 cm zone where the media bed intermittently floods and drains. If not using flood-and-drain techniques, this zone will be the path along which the water flows through the medium. Most of the biological activity occurs in this zone.

• The wet zone is the bottom 3-5 cm of the bed which remains permanently wet. The small particulate solid wastes accumulate in this zone, and therefore the organisms that are most active in mineralization are also located here, including heterotrophic bacteria and other micro-organisms which break down the waste into smaller fractions and molecules that can be absorbed by the plants through the process of mineralization (Somerville et al. 2014b).

Nutrient Film Technique (NFT)

NFT is a system of solution culture where a thin film (two to three millimetres depth) continually flows along the base of small channels in which the root systems sit. With NFT, the objective is that part of the developing root mat is in the nutrient flow, but the other roots are suspended above this in the moist air, accessing oxygen without being submerged (Somerville et al. 2014b).

Figure 3: NFT round pipe system https://commons.wikimedia.org/wiki/File:Hydroponics_(33185459271).jpg

Figure 4: NFT rectangular pipe system https://commons.wikimedia.org/wiki/File:Hydroponics_(33185459271).jpg

The channels are often in the form of pipes (Figure 3). Pipes with a rectangular section (Figure 4) are best, with a width larger than the height, as this means that a larger volume of water hits the roots, thereby increasing nutrient uptake and plant growth. Larger fruiting vegetables and polycultures (growing different types of vegetables) require larger pipes than those needed for fast-growing leafy greens and small vegetables with small root masses. The length of the pipe can vary, but it is worth bearing in mind that nutrient deficiencies can occur in plants towards the end of very long pipes because the first plants have already stripped the nutrients (Figure 5). White pipes should be used as the colour reflects the sun’s rays, thereby keeping the inside of the pipes cool. The channels must be positioned on a slope (Figure 5) so that the nutrient solution flows at a good flow rate, which for most systems is around one litre/minute (Somerville et al. 2014a).

Figure 5: Sloping NFT channels. The NFT channel is 12.5 m long and was fed with water from the adjacent fish tank. No nutrients were supplemented. One can observe the increasing nutrient limitation along the channel

NFT systems are mostly used for producing rapid-turnover crops such as lettuce, herbs, strawberries, green vegetables, fodder, and microgreens.

Deep water culture (DWC)

DWC or floating raft system is a type of hydroponic system in which the plants are suspended above a tank using a floating raft, and the roots are submerged in nutrient solution and aerated via an air pump. However, unlike in NFT systems, where the nutrients in the small film of water flowing at root level quickly become depleted, the large volume of water contained in the DWC canals allows for considerable amounts of nutrients to be used by the plants. The length of the canals is therefore not an issue, and they can range from one to ten metres. The recommended depth is 30 cm to allow for adequate plant root space, although small leafy greens such as lettuce only require a depth of 10 cm or even less. The flow rate of the water entering each canal is relatively low, and generally each canal has a retention time (the amount of time it takes to replace all the water in a container) of 1-4 hours. This allows for adequate replenishment of nutrients in each canal, although the volume of water and the amount of nutrients in deep canals is sufficient to nourish the plants over longer periods (Somerville et al. 2014b). On the other hand, additional aeration might be required, because the flow rates are not high enough to provide sufficient oxygen.

Some plants, such as lettuce, thrive in water and are commonly grown using deep water culture. DWC is the most common method for large commercial operations growing one specific crop (typically lettuce, salad leaves or basil), and is more suitable for mechanization.

Figure 6: Basil and other plants growing in the DWC system in the CDC South Aquaponics greenhouse in Brooks, Alberta (https://commons.wikimedia.org/wiki/File:CDC_South_Aquaponics_Raft_Tank_1_2010-07.jpg)

Aeroponics

In aeroponic systems the plants are grown and nourished by suspending their root structures in air and regularly spraying them with a nutrient solution. There are two main types of aeroponic systems: high pressure aeroponics and low pressure aeroponics, the main difference being the droplet size of the mist used in each case. Low-pressure aeroponics uses low-pressure, high-flow pumps, whereas high-pressure aeroponics uses high-pressure (around 120 PSI), low-flow pumps to atomize water and create water droplets of 50 microns or less. In the case of extremely fine mist that resembles fog, the term ‘fogponics’ is used to denote a third type of aeroponic system. Plants grown using an aeroponic system tend to grow faster than those grown in other types of hydroponic system because of their ample exposure to increased oxygen (Li et al. 2018).

Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).