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5.5 Water Quality Requirements

5 months ago

13 min read

Aquaponics represents an effort to control water quality so that all the present life forms (fish, plants and microbes) are being cultured in as close to ideal water chemistry conditions as possible (Goddek et al. 2015). If water chemistry can be matched to the requirements of these three sets of important life forms, efficiency and optimisation of growth and health of all may be aspired to (Lennard 2017).

Optimisation is important to commercial aquaponic production because it is only through optimisation that commercial success (i.e. financial profitability) may be realised. Therefore, water chemistry and water quality requirements within the aquaponic system are pivotal to the ultimate commercial and economic success of the enterprise (Goddek et al. 2015).

There is currently disagreement within the broader aquaponic industry and community in terms of what represents good or acceptable water quality within aquaponic systems. It appears that it is universally accepted that the natural water chemistry requirements of the individual life form subsets (fish, plants and microbes) are broadly agreed upon (Rakocy and Hargreaves 1993; Rakocy et al. 2006; Goddek et al. 2015; Delaide et al. 2016; Lennard 2017). However, the presence of a broad range of approaches, methods and technology choices that are called aquaponics and the background or history of the associated, stand-alone technologies of recirculating aquaculture systems (RAS) and hydroponic plant culture (including substrate culture) appears to lead to disagreements among operators, scientists and designers. For example, taking only one single water chemistry parameter into consideration, pH, some argue that the pH requirements of hydroponically cultured plants are very different to the pH requirements of RAS-cultured freshwater fish species (Suhl et al. 2016). The hydroponic industry generally applies pH settings between 4.5 and 6.0 for water-based plant culture (Resh 2013), whereas the RAS industry typically applies pH settings between 7.0 and 8.0 (Timmons et al. 2002) to meet the requirements of the fish and the microbes present (which perform important transformations of potentially toxic fish waste metabolites to less toxic forms). The argument, therefore, is that any pH set point is a compromise between the requirements of the plants, the fish and the microbes and that therefore an optimal pH for all life forms is not achievable which leads to suboptimal plant production (Suhl et al. 2016). Others argue, however, that a closer scrutiny of the complexities of the nutrient dynamics of plant nutrient uptake may elucidate a different opinion (Lennard 2017).

Hydroponic (and substrate culture) systems feed nutrients to the plants in their basal, ionic forms by adding nutrient salts to the water that dissociate to release the available nutrient ions (Resh 2013). Research has demonstrated that these ionic nutrient forms exist in a window of availability to the plant, based on the available system water pH. Therefore, in a standard hydroponic context, with no present microbial flora (i.e. sterilised — as most hydroponic systems are), it is important to set the pH of the system water to a level that makes the mixture of ionic nutrients the plant requires as available as possible (Resh 2013). Within any hydroponic system, this is a compromise itself, because as any ionic nutrient availability chart demonstrates (see Fig. 5.4), different ionic nutrient forms are the most available at differing pHs (Resh 2013). It is this standard ionic nutrient availability association that the hydroponic industry uses as its primer for pH set points and explains why the desired hydroponic operational pH is somewhere between 4.5 and 6.0 (an acid environment) in sterilised hydroponic and substrate culture systems.

Alternatively, RAS applies a water pH set point based on what is natural for the fish being cultured and the microbes treating and converting the fish waste products

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Fig. 5.4 Example of a standard pH mediated, nutrient availability chart for aquatically cultured plants. Red line represents a normal operating pH for a hydroponic system; the blue line that for an aquaponic system

(Timmons et al. 2002; Goddek et al. 2015; Suhl et al. 2016). In natural freshwater environments, most fish species require an environmental pH (i.e. water pH) that closely matches the internal pH of the fish, which is often close to a pH of 7.4 (Lennard 2017). In addition, the major microbes associated with dissolved metabolite transformation in RAS culture (the nitrification bacteria of several species) also require a pH around 7.5 for optimal ammonia transformation to nitrate (Goddek et al. 2015; Suhl et al. 2016). Therefore, RAS operators apply a pH set point of approximately 7.5 to RAS freshwater fish culture.

There is an obvious difference between a pH of 5.5 (an average for standard, sterilised, hydroponic plant culture) and a pH of 7.5 (an average standard for RAS fish culture). Therefore, it is argued broadly that pH represents one of the largest water quality compromises present in aquaponic science (Goddek et al. 2015; Suhl et al. 2016). Advocates of decoupled aquaponic designs often cite this difference in optimal pH requirement as an argument for the decoupled design approach, stating that fully recirculating designs must find a pH compromise when decoupled designs have the luxury of applying different water pH set points to the fish and plant components (Suhl et al. 2016; Goddek et al. 2016). However, what this argument ignores is that aquaponic systems, as opposed to hydroponic systems, are not sterile and employ ecological aquatic techniques that encourage a diverse population of microflora to be present within the aquaponic system (Eck 2017; Lennard 2017). This results in a broad variety of present microbes, many of which form intricate and complex associations with the plants, especially the plant roots, within the aquaponic system (Lennard 2017). It is well known and established in plant physiology that many microbes, associated with the soil medium and matrix, closely associate with plant roots and that many of these microbes assist plants to access and uptake vital nutrients (Vimal et al. 2017). It is also known that some of these microbes produce organic molecules that directly further assist plant growth, assist plant immunity development and assist to outcompete plant (especially root) pathogens (Vimal et al. 2017; Srivastava et al. 2017). In essence, these microbes assist plants in many ways that are simply not present in the sterilised environment applied in standard hydroponic culture.

With these diverse microbes present, the plants gain access to nutrients in many ways that are not possible in systems that rely on aquatic pH settings alone to enable plant nutrient access (e.g. standard hydroponics and substrate culture). Many of these microbes operate at broad pH levels, just like other soil-based microbes, such as the nitrification bacteria (pH of 6.5—8.0, Timmons et al. 2002). Therefore, with these microbes present in aquaponic systems, the pH set point may be raised above what is normally applied in hydroponic or substrate culture techniques (i.e. pH of 4.5—6.0) while advanced and efficient plant growth is still present (Lennard 2017). This is evidenced in the work of several aquaponic researchers who have demonstrated better plant growth rates in aquaponics than in standard hydroponics (Nichols and Lennard 2010).

Other water quality requirements in aquaponic systems relate to physical/chemical parameters and more specifically, plant nutrient requirement parameters. In terms of physical/chemical requirements, plants, fish and microbes share many commonalities. Dissolved oxygen (DO) is vital to fish, plant roots and microflora and must be maintained in aquaponic systems (Rakocy and Hargreaves 1993; Rakocy et al. 2006). Plant roots and microflora generally require relatively lower DO concentrations than most fish; plant roots and microbes can survive with DO below 3 mg/L (Goto et al. 1996), whereas most fish require above 5 mg/L (Timmons et al. 2002). Therefore, if the DO concentration within the aquaponic system is set and maintained for the fish requirement, the plant and microbe requirement is also met (Lennard 2017). Different fish species require different DO concentrations: warm water fish (e.g. Tilapia spp., barramundi) can generally tolerate lower DO concentrations than cool water fish species (e.g. salmonids like rainbow trout and arctic char); because the fish DO requirement is almost always greater than the plant roots and microfloral requirement, DO should be set for the specific fish species being cultured (Lennard 2017).

Water-carbon dioxide (COsub2/sub) concentrations, like that for DO, are generally set by the fish because the plant roots and microbes can tolerate higher concentrations than the fish. Carbon dioxide concentrations are important to optimal fish health and growth and are often ignored in aquaponic designs. Parameters and set points for COsub2/sub concentrations should be the same as for the same fish species cultured in fish-only, RAS systems and in general, should be kept below 20 mg/L (Masser et al. 1992).

Water temperature is important to all the present life forms within an aquaponic system. Fish and plant species should be matched as closely as possible for water temperature requirements (e.g. Tilapia spp. of fish like 25 ˚C plus, and plants like basil thrive in this relatively high water temperature; lettuce varieties like cooler water, and therefore, a better matched fish candidate is rainbow trout) (Lennard 2017). However, as for other water physical and chemistry parameters, meeting the fish's requirement for water temperature is paramount because the microbes have the ability to undergo specific species selection based on the ambient conditions (e.g. nitrification bacterial species differentiation occurs at different water temperatures and the species that matches best to the particular water temperature will dominate the nitrification bacterial biomass of the system) and many plants can grow very well at a broader range of water temperatures (Lennard 2017). Matching the water temperature, and maintaining it within plus or minus 2˚C (i.e. a high-level temperature control) to the fish, is an important requirement in aquaponics because when water temperature is correct and does not deviate from the ideal average, the fish achieve efficient and optimised metabolism and eat and convert feed efficiently, leading to better fish growth rates and stable and predictable waste load releases, which assists plant culture (Timmons et al. 2002).

Maintaining water clarity (low turbidity) is another important parameter in aquaponic culture (Rakocy et al. 2006). Most water turbidity is due to suspended solids loads that have not been adequately filtered, and these solids may affect fish by adhering to their gills, which may lower potential oxygen transfer rates and ammonia release rates (Timmons et al. 2002). Suspended solids loads less than 30 mg/L are recommended for aquaponically cultured fish (Masser et al. 1992; Timmons et al. 2002). High suspended solids loads also affect plant roots because they have the ability to adhere to the roots which may cause nutrient uptake inefficiency, but more commonly provides increased potential for pathogenic organism colonisation, which leads to poor root health and ultimate plant death (Rakocy et al. 2006). These suspended solids also encourage the prevalence of heterotrophic bacteria (species that break down and metabolise organic carbon) which, if allowed to dominate systems, may outcompete other required species, such as nitrification bacteria.

Electrical conductivity (EC) is a measure often applied in hydroponics to gain an understanding of the amount of total nutrient present in the water. It, however, cannot provide information on the nutrient mix, the presence or absence of individual nutrient species or the amount of individual nutrient species present (Resh 2013). It is not often applied in aquaponics because it only measures the presence of ionic (charged) nutrient forms, and it has been argued that aquaponics is an organic nutrient supply method, and therefore, EC is not a relevant measure (Hallam 2017). However, plants generally only source ionic forms of nutrients, and therefore, EC can be used as a general tool or guide to the total amount of plant-available nutrient in an aquaponic system (Lennard 2017).

For fully recirculating aquaponic systems, in terms of physical and chemical parameters, it is the fish that are more exacting in their requirements, and therefore, if systems are managed to maintain the requirements of the fish, the plants and microbes are having their requirements more than satisfied (Lennard 2017). The difference when it comes to the plants, however, is their requirement for the correct mixture and strength of nutrients to be present to allow optimised nutrient access and uptake (whether stand-alone or microbial-assisted) which leads to efficient and fast growth. Decoupled aquaponic systems may therefore be more attractive because of the perception that they allow more exacting nutrient delivery to the plants (Goddek et al. 2016). Fish feed and, therefore, fish waste do not contain the correct mixture of nutrients to meet the plant requirements (Rakocy et al. 2006). Therefore, the aquaponic system design must account for those missing nutrients and supplement them. Fully recirculating aquaponic systems generally supplement nutrients by adding them in the salt species used to manage the daily pH buffering regime; the basic portion of the salt adjusts the pH and the positive portion of the salt allows the supplementation of missing plant nutrients (e.g. potassium, calcium, magnesium) (Rakocy et al. 2006). Decoupled aquaponic designs take the waste water and associated solid wastes from the fish component and adjust the water to contain the nutrients required for plant production by adding nutrients in different forms (Goddek et al. 2016). These nutrient additions are generally based on using standard hydroponic salt species that do not necessarily provide any pH adjustment outcome (e.g. calcium phosphate, calcium sulphate, potassium phosphate, etc.).

The pathway to efficient plant growth in aquaponic systems is to provide an aquatic nutrient profile that provides all the nutrients the plant requires (mixture) at the strengths required (concentration) (Lennard 2017). In fully recirculating aquaponic designs, or decoupled aquaponic designs that do not apply sterilisation methods, there appears to be less of a requirement to meet the nutrient concentrations or strengths applied in standard hydroponics, because the ecological nature of the system associates many diverse microflora with the plant roots and these microflora assist plant nutrient access (Lennard 2017). For decoupled, or other, aquaponic designs that apply sterilisation to the plant component and follow a standard hydroponic analogue approach, there appears to be a requirement to try and approach standard hydroponic nutrient concentrations (Suhl et al. 2016; Karimanzira et al. 2016). The compromise, however, with the decoupled approach is that it leads to external supplementation ratios far beyond those of fully recirculating aquaponic designs; European decoupled designs currently average 50% or greater external nutrient additions (COST FA1305 2017; Goddek and Keesman 2018), while the UVI method supplies less than 20%, and other systems may supply less than 10% external nutrient supplementation (Lennard 2017).

No matter the method, all aquaponic systems should strive to supply the plants with the nutrition required for optimised growth so as to provide the enterprise with the greatest chance of financial viability. In this context, the nutrient content and strength of the water being delivered to the plants is very important and regular nutrient testing of the water should be employed so that nutrient mixture and strength may be maintained and managed as a very important water quality requirement.