CONTRIBUTIONS TO OPERATIONAL SAFETY

Transcription

1 CONTRIBUTIONS TO OPERATIONAL SAFETY 27 Disinfecting of irrigation water Selected methods and their mode of effect Advantages and disadvantages of different disinfection methods Check list: What must be kept in mind

2 Introduction Plant pathogens (phytophthora) are easily disseminated via contaminated irrigation water in greenhouses. In particular with circulating systems complete cultivated stocks can be infected and destroyed by the germs present in the water. Accordingly systems for the disinfecting of the irrigation and drainage water are to be found ever more frequently in horticultural production units. A variety of systems have been established for the treatment of the service water over the past years while in addition new ones have come along in recent years. There are advantages and disadvantages for the cultivated stocks depending on the type of disinfecting system in existance. In certain cases hazards for the production can start from the disinfecting system itself. This contribution on operational security is intended to provide an overview on the methods of disinfecting irrigation water most often in use today. Particular attention is paid to Structure of a slow sand filter l / m 2 h operational security. The different methods are presented subdivided into filtration, physical and chemical methods. In order to ensure a high degree of effectiveness, it is often in addition advisable to combine different systems together. Filtration methods In horticultural enterprises filtration is the method most commonly found for cleaning irrigation water. Here attention must be paid to the mesh size of the filter to ensure that the suspended solids are reliably filtered out. Several pathogens can also be filtered out with a small mesh size whereby the effectiveness is supported by some methods through biological activity. Slow sand filters, for example, are frequently found in horticultural enterprises. ÖÖSlow sand filters A slow sand filter consists of a number of different layers of sand with different draining properties. The Wasser Water from vom collecting Auffangbecken tank Überstand Supernatant water Biofilm ( dirt ( Schmutzdecke ) cover ) Filterbett bed (sand) (Sand) (Schichtdecke (layer thickness circa approx. 0,8 1, m) m) Drainageschicht layer (gravel) (Kies) (Schichtdicke jeweils circa 0,15 m) (layer thickness in each case approx m) Drainagerohr pipe Filtrate (zum (to clean Reinwasserbecken) water tank) Source: Ufer et al. advantage of this filter type lies not only in its purely mechanical filtration effect but also in the biological activity with which the phytopathogens are destroyed. This activity dominates in the so-called "dirt cover" of the filter which requires a period of time to build up after the filter is first put into operation. Since the biological destruction process requires a certain period of time, the rate of flow may not be set too high. The slower the water flows through the filter, the more reliable its effect will be. A flow rate of between 100 and 300 litres per square metre of filter surface per hour is recommended. This should be checked with a flow meter. The slow sand filter possesses a very high level of effectiveness against fungal pathogens such as phytophthora or pythium as well as bacteria. The success rate against viruses and nematodes is around 90 %, this level being as a rule sufficient to prevent spreading in the plant population. Depending on the intensity with which the filter system is used, the surface of the filter must be peeled at intervals in order to prevent it becoming blocked. This filtration method can be matched to the operational needs in respect of the amount of watering required and accordingly can be employed for every form of operation. However the filter size can be a disadvantage, the system requiring a considerable amount of space depending on the amount of watering required. ÖÖCrossflow technique With this method the water is fed along a membrane. As a function of the pore size, this layer cannot be crossed by the constituents of the water. Since experience has shown that fungi form along the membrane and that these are able to grow over to the "clean side", this method has been found to be ineffective. 2 Contributions on operational safety No. 27

3 ÖÖOther types of filter In addition to the aforementioned ones, there are also a multitude of other types of filter. These include the inclined filter, this being generally employed as a prefilter. Of decisive importance for the efficiency of the particular filter system is the pore size. Table 1 gives an overview of the filtering capacity provided as a function of the pore size. UV irradiation Table 1: Pore sizes necessary as a function of the pathogens and constituents Microfilter Ultrafilter Nanofilter Reverse osmosis Pore size > 0.1 µm µm µm < µm Type of particles Suspended solids, turbid materials, fungal germs Bacteria, viruses, proteins Multivalent ions, organic compounds of low molecular weight Univalent ions (salts) Source: Changed in accordance with Wohanka When ultraviolet light is used, the killing effect is to be attributed to the high energy content of the radi ation. The optimum wavelength for killing plant pathogens lies in the UVC range at approximately 254 nm. Of decisive importance is the radiation dose. This is stated in joules per square metre (J/m²) or, as the case may be, in microwatt seconds per square centimetre (µws/cm²). A large proportion of plant-damaging bacteria, for example, are killed with 300 J/m². For fungal spores, on the other hand, a dose of 1,000 J/m² is needed. In order to be certain that the spores and hypha are killed, a multiple of this level may be necessary in certain cases. Accordingly the dose must be selected in accordance with the plant pathogens to be killed. Further details on this are to be found in table 2 on page 4. A direct hazard to the plant popula tion as a result of over-dosing can be excluded. the absorption of the UV irradiation should necessarily be determined in advance so that the system can be set correctly. In addition the irradiation is also absorbed by iron chelates. This causes them to be destroyed and manifestations of iron deficiency on the plants can develop. Since the UV radiator becomes dirty being used for some time, it is necessary that it is cleaned at regular intervals. In addition attention must be paid to the fact that as a rule a UV radiator should be replaced after approximately 10,000 operating hours. This is a considerable cost factor that arises over the complete service life of the system. Thermal methods With the heating-up or pasteurisation of irrigation water the water is heated up to between 95 and 97 C for approximately 30 seconds. This method possesses a wide spectrum of effect in respect of a variety of pests and brings about a level of disinfection that is completely adequate. A similarly good effect is achieved at some what lower temperatures but with a longer exposure time (85 to 95 C for approx. 3 minutes). Following the treatment the heat is recovered as much as possible with the aid of heat exchangers. Nevertheless the input of energy is considerable since approximately 1.5 m³ Prefiltration of the water can be advisable in order to improve the effectiveness of the UV irradiation. Through this the turbidity (transmission) of the irrigation water is reduced to a minimum and thereby the absorption of the energy by reason of this is prevented. Also the origin of the water is playing a role here because the quality of the water can be very different and at the same time its absorption properties. The transmission of the water which leads to Figure 1: UV system Contributions on operational safety No. 27 3

4 d isinfectin g of irri g ation w ater Table 2: Lethal doses of UV irradiation for different plant pathogens Bacteria 300 J/m2 sufficient Fungi - Colourless spores/hypha - Dark spores/hypha - Individual fungi are resistant to more than 30,000 J/m². Viruses, nematodes approx. 1,000 J/m2 3,000-6,000 J/m2 Mode of function and effect: > 1,000 J/m2 of natural gas is required for each 1 m³ of irrigation water. This method is relatively reliable but as stated is energy- and thus cost-intensive. Chemical methods Chemicals can be added to the irrigation water which kill the germs contained therein. The chemicals are either produced on the spot or are purchased as a finished product. With all chemical methods it makes sense to filter the water first in order to remove the organic material in it as far as possible in advance. A consequence to not undertaking this process will lead to the chemicals being consumed so that they are nullified. The chemical ingredients consist frequently of oxidizing agents which are added to the irrigation water. The oxidation of these substances with the organic substance leads to the killing of all micro-organisms present in the water. Fundamentally this method of oxidation can be used to combat all types of pathogens. One exception are nematodes which are eliminated less well. In addition with some oxidizing agents the effectiveness may depend on the ph of the water. Disinfection with chlorine dioxide (schematic) Plant tables ilter Rainwater, mains water, well water Ret rn Remo al or man al tests low olle tin Probes tank Water tank Rea tor ertili er tank io i e s stem Source: Changed in accordance with ProMaqua 4 Contributions on operational safety No. 27 al e lorine io i e eas rement water line lorine ÖÖChlorine dioxide The disinfection of irrigation water with chlorine dioxide has only been employed in horticulture in recent years. However many findings on the prospects and risks associated with this method have already been obtained. The method is based on the two star ting chemicals, namely sodium chlorite (NaClO2) not to be confused with cooking salt (NaCl) and hydrochloric acid (HCl). A reaction unit admixes the required concentration of chlorine dioxide directly or indirectly to the irrigation water with a precision of +/- 2 %. The admixing of the concentrate is carried out with a flowmeter with impeller or as a magnetically inductive flow measurement and is thus regulated in a quantity-proportional manner. With this method the potential for errors is reduced in comparison with measurementcontrolled dosing. As a rule a control measurement is carried out after the actual dosing process. The application is possible at two locations: a) The germs are killed in the irrigation water itself; b) the germs present on the surface of the plants of the cultivated stock are killed and in this way dissemination via the irrigation water is prevented. The quantity to be dosed has to be matched to the type of cultivated stock and in so far as the killing of the germs is desired on the plants themselves to the length of the pipes to the point where the irrigation water is being applied. The chlorine dioxide is very unstable and decomposes into different subproducts some minutes following the dosing. Following this, the effect terminates. Areas of application and risks: The chlorine dioxide method is employed in particular against bacterio-

5 ses such as acidovorax. In addition it is also used for combating particular types of fungi, in particular fusarium. Chlorine dioxide prevents the spreading of the bacterial slime via the nutrient solution or, as the case may be, keeps the density of the fusarium spores at a low level. That way the dissemination of the pathogens is prevented. The feedback from horticultural enterprises on the efficiency of the method are consistently very positive. Thus, for example, all the nurseries that are insured with Gartenbau-Versicherung and who have this technology installed confirm containment of the rate of infestation in combination with long-term low levels of infestation. Safety regulations for the insurability of technically caused damage to plant stocks when using chlorine dioxide systems 1.1 Commissioning shall be carried out by the supplier. Here particular attention is to be paid to the consumption of chlorine dioxide, e.g. by lines. A commissioning report shall be handed over. 1.2 The system shall be subjected to a check by expert personnel at least once a year. 1.3 In addition to the dosing measurement (generally carried out on the mixing reactor), a second and independent control unit must be installed to check the chlorine dioxide content of the water. This must be able to signal incorrect dosing as well as a shortage of any one of the components to be mixed in. In addition a manual measurement must be carried out in the plant population once a week and a hand-written memorandum prepared. 1.4 Logging of the reference and set alarm values as well as electronic data-storing of the control measurement values over a period of at least four weeks is necessary. An important field of application lies in combating of xanthomonas in young plant nurseries. On the one hand the boxes for young plants are treated with highly concentrated Figure 2: Damage to begonia caused very probably by ClO 2. chlorine dioxide before being loaded. In addition combating is carried out within the plants at a low dosage. With this type of application the great est attention has to be paid to the operational safety. The reason is that should the system break down the bacteria can proliferate very considerably under certain circumstances within a period of between a few hours and two days. Hence it is essential that the disinfection system is incorporated into the existing alarm chain. The situation in regard to the safety of the chlorine dioxide method has not been fully clarified up to the present time. More experience is needed. However it is essential to ensure that watering is continued if the disinfection unit fails because due to the low "buffer times" of modern cultivation methods such as rockwool cultures the plants must be provided with water all the time. The sensor of a control measurement unit should be installed as close as possible to the points of application of the irrigation water to the surface of the plants. The water for the weekly control measurement should be taken manually from the watering operation running at the application point. Continuous monitoring in accordance with the manufacturer s recommendations is obligatory as is regular servicing of the system by qualified specialist firms. Practical problems: ¾ Water circuit volume too small With ebb-flood systems using rainwater this problem arises during extended periods without precipitation so that there are smaller quantities in the watering tanks. In addition changes to the water circuits or the source of the irrigation water (munici pal water, well water, rainwater) can have negative effects under certain circumstances. ¾ Polyethylene incompatibility This is listed as a general problem in the operating instructions of chlorine dioxide systems. Problems with PE distribution pipes have already been reported by horticultural enterprises. Contributions on operational safety No. 27 5

6 Investigations still have to be carried out as to whether the PE inlays of steel tanks may also be affected. ¾ Storage of sodium chlorite (NaClO 2 ) If the temperature in unheated rooms falls below 12 C, flocculation of the starting chemical NaClO 2 can cause gas emission. The emission of gas from an over-concentrated master solution can lead to considerable damage to the plants. ¾ Absence of link to the monitoring system In general the systems run in an autarchic manner side by side with previously installed control mechanisms such as watering and climate control computers! ¾ Failure of the disinfection system Attention has to be paid that the flow of irrigation water remains ensured. ÖÖElectrolysis A method that has been applied in horticultural nurseries only since 2012 is that of the disinfecting of the irrigation water with the aid of electrolysis. Here the chloride present in the irrigation water is converted into a chlorine compound with a disinfecting effect through the application of a potential difference. For "active" chlorine (substances with a reactive chlorine compound) to be produced in adequate quantities, the chloride content of the water to be treated must generally be increased through the addition of salt. Mainly it is hypochlorous acid (HOCl) that is produced here as an active agent. Arising in addition are hypochlorites (ClO - ), ozone (O 3 ), chlorine dioxide (ClO 2 ) and other substances; the latter can be ignored in respect of their effect. As a rule the active agents are stored in a tank following their production by electrolysis and are metered from this into the irrigation water. This is generally carried out without measurement. It has to be criticised that most of the systems are installed completely outside the normal irrigation water regulation system and that there is no form of monitoring or of an alarm system. Since up to present time only very few scientific studies have been carried out there are still a lot of questions to be answered in connection with this method. However first investigations have shown that the products of the electrolysis react with the ammonium fertilizers in the irrigation water. That way depending on the ammonium content of the water a large proportion of the disinfecting agent is consumed. Consumption of the active agent also takes place if there is a high level of organic matter, e.g. in the form of particles of peat. In addition the ph must be within the acid range (maximum ph 6.5) in order to ensure effectiveness. The afore-mentioned aspects might be the reason why no positive effect could be recorded in many cases with the first investigations in the horticultural sector. Before an electrolysis system is installed in a nursery or at the latest after the installation (but then even more thoroughly) the operator should concern himself with the following questions: ¾ What reactions can take place between fertilizer and disinfection agent? ¾ What happens if the "salted" drain water is used again? ¾ Are there particular reactions that occur if the water quality changes? ¾ Are there risks if only very small cultivated areas (partial areas) are treated? ¾ What is the situation with the electrolysis at times when the requirement for watering is very low? ÖÖCopper ions A further possibility for disinfecting irrigation water is the use of copper ions. Copper has been employed for many years now for combating fungal pathogens in the areas of viticulture and fruit-growing. This has not yet been the case for the treatment of irrigation water for greenhouse horticulture. With this method the complete amount of irrigation water is led between two copper plates. A potential difference is applied between the plates. This leads to the situation that copper ions, which have a positive effect against fungal pathogens, accumulate in the irrigation water. The effect of copper ions on bacteria and viruses is very small or non-existent. The dosage of the copper, whereby the concentration should be around 1 ppm, is achieved by the application of the particular voltage. Up to the present time Gartenbau-Versicherung is not aware of any damage to plants by using copper systems although this might be due to the fact that only a few systems have been installed to date. In addition attention should be paid to the fact that if the system should break down watering can be carried out with untreated water in order to avoid heat damage in the height of summer. To summarise: A separate control measurement must be carried out with all chemical disinfection measures! The control measurement must be incorporated in the monitoring carried out by the climate control/irrigation system computer. According to the security prescriptions this also requires that alarms are given in the case of system failure and that the measurement, alarm and reference values are documented. 6 Contributionson operational safety No. 27

7 Table 3: Spectrum of effect of different disinfection systems and their advantages and disadvantages Method Spectrum of effect Advantages Disadvantages Slow sand filter Good effect against fungi and bacteria In many cases sufficient affect against viruses and nematodes Cost-favourable Simple handling Does not filter out all pathogens Large filters must be installed if the water requirement is high "Peeling" of the filter necessary at regular intervals UV irradiation Fungal pathogens (wide range of reactions depending on the state of development of the fungi) Bacteria and viruses (good effect) Good general effect Prefiltration can be necessary Cost intensive No effect on plant population Thermal treatment Good/sufficient effect against all phytopathogens Wide spectrum of effect High (energy) costs No effect on plant population Chlorine dioxide Good effect against fungi and bacteria but not 100 % Good effect Low operating costs Effect also within plant population High acquisition costs High risk potential if used incorrectly Consumption by organic matter Electrolysis Effect against fungi and bacteria similar to that with chlorine dioxide Study results are not yet available In respect of acquisition frequently more costfavourable than comparable methods Low operating costs Effect also in plant population Unknown risks with control systems that are often autarchic At present there are neither study results nor empirical values obtained over an extended period Reaction with ammonium no disinfecting effect! Effect depends on ph Consumption by organic matter Copper ions Good effect against fungi Little experience up to present: both in positive and negative directions Contributions on operational safety No. 27 7

8 Checklist Points to be observed in connection with the installation of an irrigation water disinfection system ¾ Check which plant pathogens cause the greatest problems in operational practice; select the system on the basis of this analysis. ¾ The quality of the water is a decisively important factor that must be taken into account when selecting the disinfection method. ¾ Take account of variations in the water quality; these can for example occur through a change in the source of the water or through quality variations at the water source. ¾ Match the system to the particular needs of cultivation. ¾ Clarify the amount of specialist knowledge needed. Who within the company can operate the system? ¾ Establish the amount of water required in order to decide on the size of the system needed. ¾ Consider a combination of different methods in order to improve the overall effect (e.g. first filter the irrigation water and then treat it chemically). ¾ Compare the acquisition and life-cycle costs; these may differ in accordance with the particular requirement so that an individual decision is always required. ¾ Have installation and commissioning carried out by a qualified specialist firm. ¾ Incorporate the system in the existing irrigation water control system. ¾ Install a control sensor and integrate this into the existing alarm chain. ¾ It must be ensured that watering will be continued even if the disinfection system fails. ¾ Ensure general operational reliability (keep susceptibility to disruptions as low as possible). ¾ Check the water quality at regular intervals following installation. Gartenbau-Versicherung VVaG Von-Frerichs-Straße Wiesbaden Germany Phone +49 (0) 611 / Fax +49 (0) 611 / Authors: B. Sc. (hort) Johannes Schmidt, M. Sc. (hort) Christian Senft Photos: Archive GV Gartenbau-Versicherung 05/2014 service@gevau.de A company of the AGRORISK Group