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7.4 Aquaculture Unit

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The fish-rearing tanks (size, numbers and design) are selected depending on the scale of production and fish species in use. Rakocy et al. (2006) used four large fishrearing tanks for the commercial production of O. niloticus in the UVI aquaponic system (USA). With the production of omnivorous or piscivorous fish species, such as C. gariepinus, several tanks should be used due to the sorting of the size classes and staggered production (Palm et al. 2016). Fish tanks should be designed so that the solids that settle at the bottom of the tanks can effectively be removed through an effluent at the bottom. This solid waste removal is the first crucial water treatment step in coupled aquaponics as is the case in aquaculture and decoupled aquaponics. The waste originates from uneaten feed, fish faeces, bacterial biomass and flocculants produced during aquaculture production, increasing BOD and reducing water quality and oxygen availability with respect to both the aquaculture and hydroponic units. In aquaculture, the solid waste consists to a large extent of organic carbon, which is used by heterotrophic bacteria to produce energy through oxygen consumption. The better the solid waste removal, the better the general performance of the system for both fish and plants, i.e. with optimal oxygenation levels and no accumulation of particles in the rhizosphere inhibiting nutrient uptake, and with round or oval tanks proving to be particularly efficient (Knaus et al. 2015).

Fish production in coupled aquaponics in the FishGlassHouse in Germany was tested at different scales in order to ascertain cost effectiveness. This was done effectively as extensive (max. 50 kg, 35 fish msup-3/sup) or intensive (max. 200 kg, 140 fish msup-3/sup) African catfish production. The semi-intensive production (max. 100 kg, 70 fish msup-3/sup) cannot be recommended due to a negative cost benefit balance. In the semi-intensive production mode, system maintenance, labour and feed input were as much as under intensive production but with reduced fish and plant biomass output, and any economic gains in the aquaculture unit did not pay off (Palm et al. 2017). This resulted from the high biochemical oxygen demands (BOD), high denitrification because of the reduced oxygen availability, relatively high water exchange rates, predominantly anaerobic mineralization with distinct precipitation, low P and K-levels as well as a low pH-values with much less fish output compared with the intensive conditions. In contrast, the extensive fish production allowed higher oxygen availability with less water exchange rates and better nutrient availability for plant growth. Thus, under the above conditions, a RAS fish production unit for coupled aquaponics therefore either functions under extensive or intensive fish production conditions, and intermediate conditions should be avoided.

7.4.1 Filtration

Clarifiers, sometimes also called sedimenters or swirl separators (also see Chap. 3), are the most frequently used devices for the removal of solid waste in coupled aquaponics (Rakocy et al. 2006; Nelson and Pade 2007; Danaher et al. 2013, Fig. 7.4). Larger particulate matters must be removed from the system to avoid anoxic zones with denitrifying effects or the development of Hsub2/subS. Most clarifiers use lamella or plate inserts to assist in solids removal. Conical bottoms support sludge concentration at the bottom during operation and cleaning, whereas flat bottoms require large quantities of water to flush out and remove the sludge. During operation, the solids sink to the bottom of the clarifier to form sludge. Depending on the feed input and retention time, this sludge can build up to form relatively thick layers. The microbial activity inside the sludge layers gradually shifts towards anaerobic conditions, stimulating microbial denitrification. This process reduces plant available nitrate and should be avoided, especially if the process water is to be used for hydroponic plant production. Consequently, denitrification can be counterproductive in coupled aquaponics.

The density of the solid waste removed by the clarifier is rather low, compared with other technologies, maintenance is time-consuming, and cleaning the clarifier with freshwater is responsible for the main water loss of the entire system. The required amount of water is affected by its general design, the bottom shape and the accessibility of the PVC baffles to flushing water (Fig. 7.4a, b). Increasing fish stocking densities require higher quantities of water exchange (every day in the week under intensive conditions) to maintain optimal water quality for fish production, which can result in the loss of large amounts of process water, also losing substantial amounts of nutrients required for plant growth. Furthermore, replacement with freshwater introduces calcium and magnesium carbonates which may then precipitate with phosphates. Therefore, the use of such manually operated clarifiers makes predictions on process water composition with respect to optimal plant growth nearly impossible (Palm et al. 2019). It would be more effective to follow Naegle’s (1977) example of separating aerobic and anaerobic sludge and gaseous nitrogen discharge with a dual sludge system.

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Fig. 7.4 Principle of aquaponic filtration with a sedimenter (a-b) and (c) disc-filter (PAL-Aquakulur GmbH, Abtshagen, Germany) of commercial African catfish (Clarias gariepinus) RAS in the FishGlassHouse (Rostock University, Germany)

More effective solid waste removal can be achieved by automatic drum- or diskfilters which provide mechanical barriers that hold back solids, which are then removed through rinsing. New developments aim to reduce the use of rinse water through vacuum cleaning technologies, allowing the concentration of total solids in the sludge up to 18% (Dr. Günther Scheibe, PAL-Aquakultur GmbH, Germany, personal communication, Fig. 7.4c). Such effective waste removal has a positive influence on the sludge composition, improving effluent water control in order to better meet the horticultural requirements. Another option is the application of multiple clarifiers (sedimenters) or sludge-removal components in a row.

Biofilters are another essential part of RAS, as they convert ammonia nitrogen via microbial oxidation to nitrate (nitrification). Even though plant roots and the system itself provide surfaces for nitrifying bacteria, the capability to control the water quality is limited. Systems that do not have biofiltration are restricted to mini or hobby installations with low feed inputs. As soon as the biomass of fish and the feed input increases, additional biofilter capacity is required to maintain adequate water quality for fish culture and to provide sufficient nitrate quantities for plant growth.

For domestic and small-scale aquaponics, plant media (gravel or expanded clay for example) can suffice as effective biofilters. However, due to the high potential for clogging and thus the requirement for regular manual cleaning and maintenance, these methods are not suitable for larger-scale commercial aquaponics (Palm et al. 2018). Additionally, Knaus and Palm (2017a) demonstrated that the use of a simple biofilter in a bypass already increased the possible daily feed input in a backyardcoupled aquaponic system by approximately 25%. Modern biofilters that are used in intensive RAS are effective in providing sufficient nitrification capacity for fish and plant production. Because of increased investment costs, such components are more applicable in medium- and larger-scale commercial aquaponic systems.

7.4.1.1 Hydroponics in Coupled Aquaponics

In coupled aquaponics, a wide range of hydroponic subsystems can be used (also see Chap. 4) depending on the scale of operation (Palm et al. 2018). Unless labour has no significant impact on the yield (or profit) and the system is not too large, different hydroponic subsystems can be used at the same time. This is common in domestic and demonstration aquaponics that often use media bed substrate systems (sand, gravel, perlite, etc.) in ebb and flow troughs, DWC channels (deep water culture or raft systems) and even often self-made nutrient film channels (NFT). Most labourintensive are media substrate beds (sand/gravel) in ebb and flow troughs, which can clog due to the deposition of detritus and often need to be washed (Rakocy et al. 2006). Due to the handling of the substrates, these systems are usually limited in size. On the other hand, DWC hydroponic subsystems require less labour and are less prone to maintenance, allowing them to be adopted for larger planting areas. For this reason, DWC subsystems are mainly found in domestic to small/semicommercial systems, however, not usually in large-scale aquaponic systems. For larger commercial aquaponic production, the proportion of labour and maintenance in the DWC system is still seen to be too high. Even the use of water resources and energy for pumping are also unfavourable for large-scale systems.

If closed aquaponic systems are designed for profit-oriented production, the use of labour must decrease whilst the production area must increase. This is only possible by streamlining fish production combined with the application of easy-touse hydroponic subsystems. The nutrient film technique (NFT) can, at present, be considered the most efficient hydroponic system, combining low labour with large plant cultivation areas and a good ratio of water, energy and investment costs. However, not all aquaponic plants grow well in NFT systems and thus it is necessary to find the right plant choice for each hydroponic subsystem, which in turn correlates with the nutrient supply of a specific fish species integrated in a specific hydroponic subsystem design. For coupled aquaponics, the sometimes higher particle load in the water can be problematic by clogging drips, pipes and valves in NFT installations. Hence, large aquaponic systems have to contain professional water management with effective mechanical filtration to avoid recirculation blockages. When the continuous supply of water is ensured through the pipes, the NFT system can be used in all types of coupled aquaponic systems, but is most recommended for production under small/semi-commercial systems and large-scale systems (Palm et al. 2018).