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Aqu@teach: Classification of aquaponics

8 months ago

11 min read

The delineation between aquaponics and other integrated technologies is sometimes unclear. Palm et al. (2018) proposed a new definition of aquaponics, where the majority (> 50%) of nutrients sustaining plant growth must be derived from waste originating from feeding the aquatic organisms.

Aquaponics in the narrower sense (aquaponics sensu stricto) is only applied to systems with hydroponics and without the use of soil. Some of the new integrated aquaculture systems which combine fish with algae production would also fall under this concept. On the other hand, the term aquaponics in the wider sense (aquaponics sensu lato) can be applied to systems which include horticulture and crop production techniques which utilize the mineralization processes, buffer and nutrient storage function of the different substrates, including soil. Palm et al. (2018) propose the term 'aquaponic farming' for these activities.

Table 2: A classification of aquaponics according to different design principles with examples for each category (adapted from Maucieri et al. 2018)

Design goal Categories Examples Objective or main stakeholder Commercial crop production ECF Farm Household sufficiency Somerville et al. 2014 Education Graber et al. 2014a Junge et al. 2014 Social enterprise Laidlaw & Magee 2016 Greening and decoration Schnitzler 2013 Size L large (>1000 m2) Monsees et al. 2017 M medium (200-1000 m2) Graber et al. 2014b S small (50-200 m2) Roof Water Farm XS very small (5-50 m2) Podgrajšek et al. 2014 XXS micro systems (<5 m2) Maucieri et al. 2018 Nozzi et al. 2016 Operational mode of the aquaculture compartment Extensive (allows for integrated sludge usage in grow beds) Graber & Junge 2009 Intensive (obligatory sludge separation) Schmautz et al. 2016b Nozzi et al. 2018 Water cycle management Closed loop (‘coupled’ systems): water is recycled to aquaculture Graber & Junge 2009 Monsees et al. 2017 Open loop or end-of pipe (‘decoupled’ systems): after the hydroponic component, the water is either not or only partially recycled to the aquaculture component Monsees et al. 2017 Water type Freshwater Schmautz et al. 2016b Klemenčič & Bulc 2015 Salt water Nozzi et al. 2016 Type of hydroponic system Grow beds with different media Roosta & Afsharipoor 2012 Buhmann et al. 2015 Ebb-and-flow system Nozzi et al. 2016 Grow bags Rafiee & Saad 2010 Drip irrigation Schmautz et al. 2016b Deep water cultivation (floating raft culture) Schmautz et al. 2016b Nutrient film technique (NFT) Lennard & Leonard 2006 Goddek et al. 2016a Use of space Horizontal Schmautz et al. 2016b Klemenčič & Bulc 2015 Vertical Khandaker & Kotzen 2018

Aquaponics can address various goals or stakeholders, from research and development, educational and social activities, to subsistence farming and commercial scale food production. It can be implemented in various ways and environments, such as on arid and polluted land, backyard production, urban agriculture, etc. While a system can simultaneously fulfill several objectives, including greening and decoration, social interaction, and food production, normally it cannot achieve all of these at the same time. To perform satisfactorily for each of the possible goals, the components of a system have to fulfil different, sometimes contrasting, requirements. The choice of a suitable aquaponic system for a particular situation should be based on realistic assessments (including a sound business plan, where appropriate) and should result in a tailor-made solution. If we follow the classification of Maucieri et al. (2018), which categorizes aquaponic systems according to various different categories (e.g. type of stakeholder, operational mode, size, type of hydroponic system, etc.), several distinct options for choosing a suitable aquaponic system emerge (Table 2). Any decision has to be made within the limits of the available budget, though it is possible to construct a system at very low cost.

Classification according to operational mode: extensive (with integrated sludge usage) and intensive (with sludge separation)

One part of the aquaponic system is the fish tank, where the fish are fed and, through their metabolism, faeces and ammonia are excreted into the water. However, high concentrations of ammonia are toxic for fish. Through nitrifying bacteria, ammonia is transformed to nitrite and then into nitrate, which is relatively harmless to fish and is the favoured form of nitrogen for growing crops such as vegetables. Extensive production integrates the biofilter as well as the sludge removal directly within the hydroponic unit, by using substrates that provide the appropriate support for the growth of the biofilm, such as gravel, sand, and expanded clay. Intensive production uses a separate biofilter and sludge separation system. Both operational methods have their advantages and disadvantages. Whilst integrated sludge usage allows for complete nutrient recycling, the negative aspects include turbid water, and rather low biofilter performance, which only allow limited fish stocking. Separate sludge removal and biofilter, on the other hand, allow intensive fish stocking of up to 100 or more kg/m3. The positive aspects include clear water, lower BOD (biochemical oxygen demand) concentration, lower microbial load, and optimized biofilter performance. However, these systems only allow for partial nutrient recycling. An additional sludge treatment step (on-site or off- site), such as connecting sludge biodigesters or vermicomposting, may be necessary (Goddek et al.2016b).

Figure 4: Aquaponic system with integrated sludge usage

Figure 5: Possible arrangement of an aquaponic system with sludge separation

Water cycle management

Closed loop (coupled) systems: aquaponic systems can be constructed and operated as a recirculating loop, with the water flow moving in both directions, from fish basin to the hydroponic unit, and vice versa. Water is constantly circulated from the RAS to the hydroponic unit, and back to the RAS.

Open loop systems: recently there have been developments towards independent control over each system unit, mostly because of the different environmental requirements of fish and plants. Such systems, where aquaculture, hydroponics and, if applicable, fish sludge remineralization can be controlled independently, are called decoupled aquaponic systems (DAPS). Decoupled aquaponic systems consist of a RAS connected to the hydroponic unit (with additional reservoir) via a one-way valve. Water is separately recirculated within each system and is supplied on-demand from the RAS to the hydroponic unit, but it does not flow back (Goddek et al. 2016a; Monsees et al. 2017).

Figure 6 shows a schematic illustration of coupled and decoupled aquaponics. In the coupled (closed loop) system consisting of a RAS (blue: rearing tanks, clarifier and biofilter) directly connected to the hydroponic unit (green: NFT-trays), water is constantly circulated from the RAS to the hydroponic unit and back to RAS. In the decoupled (open-loop) aquaponic system consisting of a RAS connected to the hydroponic unit (with additional reservoir) via a one-way-valve, water is separately recirculated within each system and water is supplied on-demand from the RAS to the hydroponic unit, but does not go back to the RAS.

Figure 6: Schematic illustration of coupled (left) and decoupled (right) aquaponics.

Types of hydroponic system used in aquaponics

Nutrient Film Technique

In Nutrient Film Technique (NFT) systems, water from a fish tank is passed through the bottom of a horizontal PVC pipe, in a thin film. These pipes have holes cut into the top, in which plants are grown in such a way that their roots dangle in the water flowing on the bottom. Nutrients from the tank water are absorbed by the plants, and as their roots are only partly submerged, this allows them to be in contact with atmospheric oxygen as well.

Table 3: Advantages and disadvantages of NFT

Advantages Disadvantages Constant water flow Small sump tank needed Ease of maintenance and cleaning Require smaller volume of water Light hydroponic infrastructure, well-suited for rooftop farming Requires prior filtration to prevent clogged roots Expensive materials Less stable system (if there is less water) Only suitable for growing leafy vegetables and herbs which have smaller root systems Sensitive to temperature variations

image-20210210134210648

Figure 7: Nutrient film technique (NFT). Left – Diagram of an entire system. Right – Photo of the system (Photo ZHAW)

Media bed technique

Media-filled bed units are the most popular design for small-scale aquaponics. These designs use space efficiently, have a relatively low initial cost, and are suitable for beginners because of their stability and simplicity. In media bed units, the medium is used to support the roots of the plants and functions as a mechanical and biological filter.

Table 4: Advantages and disadvantages of media bed technique

AdvantagesDisadvantagesBiofiltration: medum serves as a substrate for nitrifying bacteriaActs as a solids filtering mediumMineralization takes place directly in the grow bedSubstrate can be colonized by a broad range of microflora, some of which can have beneficial effectsSome media and infrastructure are very heavy: not always suitable for rooftop farmingCan become unwieldy and relatively expensive at a larger-scaleMaintenance and cleaning are difficultClogging can lead to water channelling, inefficient biofiltration and thus also inefficient nutrient delivery to the plantsMedia can become clogged if fish stocking densities exceed the beds’ carrying capacity, and this can require separate filtrationWater evaporation is higher in media beds with more surface area exposed to the sunIf flood and drain method is implemented, sizing is important, and a large sump tank is needed

image-20210210134233776Figure 8: Media bed technique. Left – Diagram of an entire system. Right – An example from ZHAW Waedenswil (Photo: Robert Junge)

Deep Water or Floating Raft Culture

Deep Water Culture (DWC) systems use a polystyrene 'raft' which floats on about 30 cm of water. The raft has holes in which plants are grown in net pots, such that their roots are immersed in the water. The raft can also be placed to float directly in the fish tank, or it can have water pumped from the tank to a filtration system and then to channels containing a series of rafts. An aerator provides oxygen to both the water in the tank and that containing the raft. Since the roots have no medium to adhere to, this system can only be used to grow leafy greens or herbs, and not larger plants. It is the most popular system for commercial purposes, due to the speed and ease of harvest.

Table 5: Advantages and disadvantages of Deep Water Culture

Advantages Disadvantages Constant water flow Small sump tank needed Ease of maintenance and cleaning Separate biofilter needed Requires a high volume of water Heavy hydroponic infrastructure Device for root aeration necessary

image-20210210134331481

Figure 9: Deep water or floating raft culture. Left – Diagram of an entire system. Right – Lettuce growing in a styrofoam raft with roots suspended in water

Use of space: horizontal and vertical systems

Most aquaponic systems use horizontal grow tanks or beds, emulating traditional land-based arable growing to produce vegetables. However, over the years, new living wall and vertical farming technologies have arisen and evolved which, when linked to the aquaculture part of the aquaponic system, may allow more plants to be grown vertically rather than horizontally, and thus make the systems more productive (Khandaker & Kotzen 2018).

Horizontal systems have the advantage of efficiently using daylight, and may well function without additional lighting, even in winter. Therefore they have low electric energy consumption. The initial investment costs are medium/low, especially if the land price is low.

Vertical systems present an optimal space-saving solution, making them very suitable for urban facilities, either for decoration of for hyper-local food production. However, they require grow lights above the grow beds. They also require fewer water pumps, but of higher power, which all adds up to higher electric energy consumption. The initial investment costs are also high.

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).

Please see the table of contents for more topics.

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