4.5 Disinfection of the Recirculating Nutrient Solution
To minimize the risk of spreading soil-borne pathogens, disinfection of the circulating nutrient solution is required (Postma et al. 2008). Heat treatment (Runia et al. 1988) was the first method used. Van Os (2009) made an overview for the most important methods and a summary is given below. Recirculating of the nutrient solution opens possibilities to save on water and fertilizers (Van Os 1999). The big disadvantage of the recirculation of the nutrient solution is the increasing risk of spreading root-borne pathogens all over the production system. To minimize such risks, the solution should be treated before reuse. The use of pesticides for such a treatment is limited as effective pesticides are not available for all such pathogens, and if available, resistance may appear, and environmental legislation restricts discharge of water with pesticides (and nutrients) into the environment (European Parliament and European Council 2000). In addition, in AP systems, the use of pesticides exerts negative effects on fish health and cannot be carried out, even if hydroponic and AP parts of the system are in different rooms, because spraying of chemicals may enter the nutrient solution via condensation water or via direct spraying on the substrate slabs. In view of this, a biological control approach can be adopted to manage pest diseases, and this can be accessed via the EU Aquaponics Hub Fact Sheet (EU Aquaponics Hub). At the same time, similar problems can be observed for fish treatment using veterinary drugs that are not compatible with the plant’s cycle.
4.5.1 Description of Disinfection Methods
Disinfection of the circulating nutrient solution should take place continuously. All drain returned (10—12 h during daytime) has to be treated within 24 h. For a greenhouse of 1000 msup2/sup in a substrate cultivation (stone wool, coir, perlite), a disinfection capability of about 1—3 msup3/sup per day is needed to disinfect an estimated needed surplus of 30% of the water supplied with drip irrigation to tomato plants during a 24-h period in summer conditions. Because of the variable return rate of drain water, a sufficiently large catchment tank for drain water is needed in which the water is stored before it is pumped to the disinfection unit. After disinfection another tank is required to store the clean water before adjusting EC and pH and blending with new water to supply to the plants. Both tanks have an average size of 5 msup3/sup per 1000 msup2/sup. In a nutrient film system (NFT), about 10 msup3/sup per day should be disinfected daily. It is generally considered that such a capacity is uneconomical to disinfect (Ruijs 1994). DFT requires similar treatment. This is the main reason why NFT and DFT production units do not normally disinfect the nutrient solution. Disinfection is carried out either by non-chemical or chemical methods as follows:
4.5.1.1 Non-chemical Methods
In general these methods do not alter the chemical composition of the solution, and there is no build-up of residuals:
Heat treatment. Heating the drain water to temperatures high enough to eradicate bacteria and pathogens is the most reliable method for disinfection. Each type of organism has its own lethal temperature. Non-spore-forming bacteria have lethal temperatures between 40 and 60 ˚C, fungi between 40 and 85 ˚C, nematodes between 45 and 55 ˚C and viruses between 80 and 95 ˚C (Runia et al. 1988) at an exposure time of 10 s. Generally, the temperature set point of 95 C is high enough to kill most of the organisms that are likely to cause diseases with a minimum time of 10 s. Whilst this may seem very energy intensive, it should be noted that the energy is recovered and reused with heat exchangers. Availability of a cheap energy source is of greater importance for practical application.
UV radiation. UV radiation is electromagnetic radiation with a wavelength between 200 and 400 nm. Wavelengths between 200 and 280 nm (UV-C), with an optimum at 254 nm, has a strong killing effect on micro-organisms, because it minimizes the multiplication of DNA chains. Different levels of radiation are needed for different organisms so as to achieve the same level of efficacy. Runia (1995) recommends a dose which varies from 100 mJ cmsup-2/sup for eliminating bacteria and fungi to 250 mJ cmsup-2/sup for eliminating viruses. These relatively high doses are needed to compensate for variations in water turbidity and variations in penetration of the energy into the solution due to low turbulence around the UV lamp or variations in output from the UV lamp. Zoschke et al. (2014) reviewed that UV irradiation at 185 and 254 nm offers water organic contaminant control and disinfection. Moreover, Moriarty et al. (2018) reported that UV radiation efficiently inactivated coliforms in AP systems.
Filtration. Filtration can be used to remove any undissolved material out of the nutrient solution. Various types of filters are available relative to the range of particle sizes. Rapid sand filters are often used to remove large particles from the drain water before adding, measuring and control of EC, pH and application of new fertilizers. After passing the fertilizer unit, often a fine synthetic filter (50—80 um) is built in the water flow to remove undissolved fertilizer salts or precipitates to avoid clogging of the irrigation drippers. These synthetic filters are also used as a pretreatment for disinfection methods with heat treatment, ozone treatment or UV radiation. With reductions in filtration pore size, the flow is inhibited, so that removal of very small particles requires a combination of adequate filters and high pressure followed by frequent cleaning of the filter(s). Removal of pathogens requires relatively small pore sizes (\10 μm; so-called micro-, ultra- or nanofiltration).
4.5.1.2 Chemical Methods
Ozone (O<sub3/sub). Ozone is produced from dry air and electricity using an ozonegenerator (converting 3Osub2/sub → 2Osub3/sub). The ozone-enriched air is injected into the water that is being sanitized and stored for a period of 1 h. Runia (1995) concluded that an ozone supply of 10 g per hour per msup3/sup drain water with an exposure time of 1 h is sufficient to eliminate all pathogens, including viruses. The reduction of microbial populations in vegetable production in soilless systems managed with ozone has also been observed by Nicoletto et al. (2017). Human exposure to the ozone that vents from the system or the storage tanks should be avoided since even a short exposure time of a concentration of 0.1 mg Lsup-1/sup of ozone may cause irritation of mucous membranes. A drawback of the use of ozone is that it reacts with iron chelate, as UV does. Consequently, higher dosages of iron are required and measures need to be taken to deal with iron deposits in the system. Recent research (Van Os 2017) with contemporary ozone installations looks promising, where complete elimination of pathogens and breakdown of remaining pesticides is achieved, with no safety problems.
Hydrogen peroxide (Hsub2/subOsub2/sub). Hydrogen peroxide is a strong, unstable oxidizing agent that reacts to form Hsub2/subO and an O˙- radical. Commercially so-called activators are added to the solution to stabilize the original solution and to increase efficacy. Activators are mostly formic acid or acetic acid, which decrease pH in the nutrient solution. Different dosages are recommended (Runia 1995) against Pythium spp. (0.005%), other fungi (0.01%), such as Fusarium, and viruses (0.05%). The 0.05% concentration is also harmful to plant roots. Hydrogen peroxide is especially helpful for cleaning the watering system, whilst the use for disinfection has been taken over by other methods. The method is considered inexpensive, but not efficient.
Sodium hypochlorite (NaOCl). Sodium hypochlorite is a compound having different commercial names (e.g. household bleach) with different concentrations but with the same chemical structure (NaOCl). It is widely used for water treatment, especially in swimming pools. The product is relatively inexpensive. When added to water, sodium hypochlorite decomposes to HOCl and NaOH and depending on the pH to OClsup-/sup ; the latter decomposes to Cl and Osup./sup for strong oxidation. It reacts directly with any organic substance, and if there is enough hypochlorite, it also reacts with pathogens. Le Quillec et al. (2003) showed that the tenability of hypochlorite depends on the climatic conditions and the related decomposing reactions. High temperatures and contact with air cause rapid decomposition, at which NaClOsub3/sub is formed with phytotoxic properties. Runia (1995) showed that hypochlorite is not effective for eliminating viruses. Chlorination with a concentration of 1—5 mg Cl Lsup-1/sup and an exposure time of 2 h achieved a reduction of 90—99.9% of Fusarium oxysporum, but some spores survived at all concentrations. Safety measures have to be taken for safe storage and handling. Hypochlorite might work against a number of pathogens, but not all, but at the same time, Nasup+/sup and Clsup-/sup concentration is increased in a closed growing system which will also lead to levels which decrease productivity of the crop and at which time the nutrient solution has to be leached. Despite the abovementioned drawbacks, the product is used and recommended by commercial operatives as a cheap and useful method.
4.5.2 Chemical Versus Non-chemical Methods
Growers prefer disinfection methods with excellent performance in combination with low costs. A good performance can be described by eliminating pathogens with a reduction of 99.9% (or a log 3 reduction) combined with a clear, understandable and controllable process. Low costs are preferably combined with low investments, low maintenance costs and no need for the grower to perform as a laboratory specialist. Heat treatment, UV radiation and ozone treatment show a good performance. However, investments in ozone treatment are very high, resulting in high annual costs. Heat treatment and UV radiation also have high annual costs, but investments are lower, whilst the eliminating process is easy to control. The latter two methods are most popular among growers, especially at nurseries larger than 1 or 2 ha. Slow sand filtration is less perfect in performance but has considerably lower annual costs. This method could be recommended for producers smaller than 1 ha and for growers with lower investment capital, as sand filters can be constructed by the grower themselves. Sodium hypochlorite and hydrogen peroxide are also cheap methods, but performance is insufficient to eliminate all pathogens. Besides it is a biocide and not a pesticide, which means by law, in the EU at least, it is legally forbidden to use it for the elimination of pathogens.
4.5.3 Biofouling and Pretreatment
Disinfection methods are not very selective between pathogens and other organic material in the solution. Therefore, pretreatment (rapid sand filters, or 50—80 um mechanical filters) of the solution before disinfection is recommended at heat treatment, UV radiation and ozone treatment. If after disinfection residuals of the chemical methods remain in the water, they may react with the biofilms which have been formed in the pipe lines of the watering systems. If the biofilm is released from the walls of the pipes, they will be transported to the drippers and cause clogging. Several oxidizing methods (sodium hypochlorite, hydrogen peroxide with activators, chlorine dioxide) are mainly used to clean pipe lines and equipment, and these create a special risk for clogging drippers over time.