DISINFECTION OF RECYCLING WATER IN ROSE CULTURES. Poncet C., Offroy M. and Bonnet G. Brun R. Unité Santé Végétale et Environnement

Transcription

1 DISINFECTION OF RECYCLING WATER IN ROSE CULTURES Poncet C., Offroy M. and Bonnet G. Brun R. INRA INRA-URIH Phytopathologie et Botanique Route des Colles Unité Santé Végétale et Environnement Sophia Antipolis BP Biot Antibes Cedex France France Keywords: recycling, irrigation water, disinfection systems, chlorine Abstract New laws concerning water savings and polluting throwing limitation compel rose producers to find solutions to reduce effluents especially in soilless cultures. Recycling drainage water not only answers to these environment imperatives but also permit to reduce fertiliser s costs and to maintain production quality. However this technique imposes an efficient mineral management and an assessment of microbiological contamination. Actually, recycling water is an excellent way for pathogens dissemination. Concentrations of 10 6 bacteria per ml are commonly observed in drainage water of plant cultures. To avoid potential risks for plant health, the disinfection of irrigation water is an alternative. Indeed, techniques like the thermodisinfection were shown to be effective but request substantial investments. The characteristics of the floricultural farms of the south of France (size, capacity of investment ) directed our laboratory towards the search for a methodology effective but less expensive. Analogous to drinking water disinfection, experimentation using chlorine was performed. Feasibility studies were carried out in the laboratory to evaluate the sensitivity of Agrobacterium tumefaciens that is likely to be disseminated in recycled water. These studies demonstrated that an amount of 4 mg/l of active chlorine applied over 30 minutes was necessary and sufficient to obtain disinfection of bacteria. After these tests, active chlorine disinfection was applied to rose cultures in order to test chlorine phytotoxicity and to observe chlorides concentration. No significant differences in the production yields were found and systems using chlorine gas were set up in rose farms. No plant health problem was detected after 3 years of operation. Thus, disinfecting with chlorine gas is an excellent preventive method. 1. Introduction Soilless rose cultures are intensive systems of production were a mineral solution provide plants with all elements they need. But this solution contains fertilisers and represents a polluting throwing. New laws impose to floricultural producers to reduce their effluents. Recycling drainage-water consists in reusing the solution the plants didn t absorb. It answers to these environment imperatives but also permit to reduce fertilisers costs and to maintain production quality. Though recycling drainage water has a main disadvantage: it can induce the spread of pathogens in the culture. The aim of the studies we performed was to find solutions to disinfect the drainage water in order to manage the pathologic risk. The objectives of the experimentations were to test treatments and tools which permit to recycle the drainage solutions with no yield loss and to insure a sanitary guarantee for the cultures. After experimentations using U.V. treatment or thermodisinfection, we decided to work on chlorination because this technique had the best efficiency-price ratio. The experimentations took place in two sites: at the experimental station of the IFHP (Institut Français de l Horticulture et du Paysage, La Gaude, France) and at the 121

2 C.R.E.A.T (Centre de Recherches Economiques et d Actions Techniques, La Gaude, France). 2. Materials and methods 2.1. U.V. treatments Water disinfection by U.V treatments was tested in several places in France. The principle consists in exposing water to a U.V. radiance of 254 nm, which injures microorganisms DNA (Poncet, 1998). We tested a low-pressure system on Agrobacterium tumefaciens in 1995 at the C.R.E.A.T. The drainage water was pumped in the stocking tank and passed several times throw a lamp UV TZ 66 with an exposition dose of microw s./cm 2. Different flows were tested: 6 litres per minute, 2 litres per minute, 2 litres per minute (recycled in closed circuit during 24 hours). We over contaminated the solution with a strain of Agrobacterium tumefaciens at bacteria per litre in order to compare to a reference, which was not contaminated Thermo-disinfection The experimentation took place at the IFHP in The principle consisted in heating the drainage water in order to obtain a temperature of 95 Celsius during thirty seconds. We used L ien thermodisinfection co system. The drainage water successively passed in two heat exchangers. The treated solution went back in the first exchanger to be cooled by the solution that would be treated. The system efficiency was tested directly on Agrobacterium tumefaciens in a short circuit before the installation in a recycling culture. The flow used was 3 m 3 per hour. The total bacteria were counted and Agrobacterium tumefaciens was detected from an over-contaminated and a non-contaminated drainage water before and after the treatment Chlorination Chlorine is the most widely used bactericidal agent to disinfect drinking water. In aqueous environments, there is equilibrium between two forms: hypochlorous acid (HOCl) and hypochlorite ions (ClO - ). Their ratio depends on ph and temperature. HOCl is the more reactive of these two forms and it is called active chlorine. The concentration and the chlorine form in solution determine the disinfecting power. That s why we refer to the different forms of chlorine in water as: free chlorine (Cl 2, HClO, ClO - ), active free chlorine (Cl 2, HClO), combined chlorine (chloramines) and total chlorine (Doré, 1989) Determination of the efficient concentration The chlorine disinfection efficiency was first tested in our laboratory. We over contaminated a drainage solution coming from a roses soilless culture with a strain of Agrobacterium Tumefaciens at a concentration of 10 8 bacteria per litre. We used sodium hypochlorite solutions at a concentration of 48 chlorometric degrees (1 chlorometric degree corresponds to 3,17g of free chlorine per kg). Usually, these solutions contain 12,5% of active chlorine Phytotoxicity test The aim of this trial was to valid the chlorine disinfection system on rose cultures. The variety used was Anna. The Sodium hypochlorite disinfection was applied to the drainage in a stocking tank. There was no active chlorine left in the irrigation pipes. A culture irrigated by a recycled solution was compared to a culture irrigated by a recycled and chlorine disinfected solution. Flowers were counted and the production quality was 122

3 observed. Furthermore, chlorides concentration was measured Utilisation of gaseous chlorine in rose floricultural farms A pilot experimentation of recycling and disinfection with gaseous chlorine was directly installed in a rose culture of one hectare (May 1997). One of the main points of the system was its working at a constant flow. The station had to fill a buffer tank. This tank avoided the variations of volume, which required an adjustment during the fertilisers injection. In the tank there were a ph electrode and a conductivity electrode, which permitted the correction of the solutions by a regulation system. After the drainage recovering, an hydroejector disinfected the solution with gaseous chlorine. In this case, the tank permitted to obtain a longer time of contact between the recovering solution and the disinfectant. Concerning the microbial counts: samples of drainage water were taken before and after disinfection by gaseous chlorine in order to test the efficiency of the system in real conditions. The total microbiological flora was spred on a non-selective medium L.P.G.A and was counted after 3 days of incubation. Two other gaseous chlorine installations were set up in rose farms (October 1997, April 1998). Chlorides concentration was controlled each week. 3. Results 3.1. U.V. treatments Results of the CREAT experimentation are reported in the table 1. In our experimental conditions, we observed only a very slight efficiency of the system Thermo-disinfection The IFHP experimentation results are reported in table 2. Results obtained during tests with Agrobacterium tumefaciens were excellent. Even with 10 8 bacteria per litre before disinfection we found no detectable cell at the end. We could notice that he solution went out at a temperature of 5 Celsius more than it went in and that this process permitted to treat high flows with an important reliability. It had no secondary effect on the environment Chlorination Determination of the efficient concentration We determined that the minimal efficient concentration to disinfect a solution containing a strain of Agrobacterium tumefaciens at a concentration of 10 8 bacteria per litre was 4 mg/l of active chlorine during 30 minutes Phytotoxicity test The vegetation observation didn t reveal any influence of the chlorination on the production quality. There was no significant difference between the number of flowers produced in the disinfected and non disinfected crop. The evolution of chlorides concentration is reported in figure 1. The concentration of chlorides stayed between 0,5 and 2 milliequivalent per litre without any increasing tendency in the chlorine disinfected solution. 123

4 Utilisation of gaseous chlorine in rose floricultural farms Concerning the tests in the rose culture of one hectare recycled and disinfected by gaseous chlorine and compared to a recycled and non-disinfected culture, results are reported in table 3. Gaseous chlorine permitted an efficient disinfection of the water even if an adjusting had to be done at the beginning of the installation. Chlorides concentration in the recycled and disinfected solutions of three rose farms is reported in figure 2. Analyses showed that chlorides concentrations are stable (between 1 and 2 meq/litre). Concerning the yields, recycling and disinfecting the drainage didn t decrease the quantitative and qualitative performances of roses. Concerning the installation no problem was detected concerning the pressure pumps, the dosing pumps and the conductivity and ph control system, which guarantees the reliability of the equipment and the method used. Concerning chlorine disinfection, the ejector position was modified. Actually, at first, it was set before the fertilisers injection. Because of the risks of iron and manganese precipitation with chlorine, recycled solutions were chlorinated before their stocking in the tank. 4. Discussion Our results showed that U.V. treatment was not satisfying in our experimental conditions. On the other hand, thermodisinfection is an efficient system but the investment is important and the functioning cost is not insignificant. That s why this technique is not really appropriate for floricultural farms of little dimensions. Chlorine treatments appeared to be the most adapted disinfection system for French roses producers. If we compare the two systems sodium hypochlorite is the cheaper than gaseous chlorine. Nevertheless during its stocking, sodium hypochlorite isn t stable which causes problems to the producers who can t control active chlorine concentration regularly. Concerning gaseous chlorine, the totality of the chlorine injected is changed in active chlorine. Moreover, it leads to a little decrease of the ph, auspicious factor for the bactericide action. Gaseous chlorine has another advantage: it doesn t need to be manipulated because the installation is totally automated; however security norms have to be respected because of the product toxicity. The reliability of the system installed in a rose culture for 3 years proved that producers could use this disinfection technique in routine. Thanks to this recycling and disinfection system, rose farmers can save money, respect laws and their environment. References Brun. R Les bases du recyclage : gestion minérale. Le recyclage de l eau en horticulture. Les journées de l astredhor. p : Doré. M Chimie des oxydants et traitement des eaux. Tec et doc Lavoisier. Paris Poncet C., Maglione P., Recyclage et désinfection par le chlore gazeux des solutions de drainage. Vie Agricole 62 : Poncet C., Antonini C., Bettachini A., Bonnet G., Hericher D., Julien P., Recyclage des eaux de drainage en culture hors sol : prise en compte du risque pathologique. PHM revue horticole 396 :

5 Tables 1. Total microflora of different samples from drainage water treated by U.V. SAMPLING Nb of bacteria /l Drainage water reference Drainage water over contaminated by Agrobacterium tumefaciens (10 9 bacteria/l) Solution treated by U.V.: 6 l/min Solution treated by U.V.: 2 l/min Solution treated by U.V.: 2 l/min (recycled during 24h) Bacteria counts before and after thermodisinfection (Laboratoire de phytopathologie, INRA, Antibes, 1996) Nb of bacteria/l Before treatment After treatment (30 seconds at 95 C) In the tank Contaminated water Just after the After cooling, contaminated After cooling, non contaminated water treatment water Agrobacterium Total flora Total microflora of drainage water and stocking tanks from disinfected or nondisinfected cultures (Laboratoire de phytopathologie, INRA, Antibes, 1999) Nb of bacteria/l 25/06/99 5/08/99 30/08/99 24/09/99 drainage (non , , , disinfected circuit) drainage (disinfected , , circuit) stocking tank (non 3, , , disinfected) stocking tank (disinfected)

6 Figures 3,00 2,50 2,00 disinfected solution 1,50 1,00 0,50 non disinfected solution 0, days 1. Evolution of chlorides concentration in disinfected and non-disinfected crops Chlorides concentration 4 3,5 3 2,5 2 1,5 1 0, weeks 2. Chlorides concentration in crops with chlorine disinfection systems 126