DISINFECTION OF RECIRCULATION WATER FROM CLOSED CULTIVATION SYSTEMS WITH OZONE

In the Netherlands most fruit vegetables and some flower crops in glasshouses are grown in soilless cultures. The most common system currently in use is the open drainage system, where plants grow in rockwool slabs which are enveloped in a polyethylene film. Excess nutrient solution drains from slits in the cover directly to the soil and then to the surface water. For environmental reasons re-use of the drainwater is necessary. By the year 2000 all crops in glasshouses must be grown in closed cultivation systems. In order to exclude any risk of dispersal of root pathogens the drainwater has to be disinfected before re-use.

During the last decade a lot of research has been carried out to achieve reliable methods for water disinfection. A heating installation was developed which found its way to some 300 nurseries.

Initially ozonisation was not effective, mainly due to the poor distribution of the ozone gas in the drainwater. A new ozone installation was tested for the efficacy against plant pathogens. The water is pumped via an ozone injector into a treatment tank. At the same time nitric acid is applied. The pH of the water is lowered to 4 as better inactivation can be achieved than under alkaline conditions because of greater stability of ozone in the water. When the treatment tank is full the water is recirculated along the ozone injector during a certain exposure time.

Cucumber green mottle mosaic virus was eliminated completely in 75 minutes with 6 g ozone per hour. Tomato mosaic virus was eradicated after 60 minutes of ozonisation with 20 g ozone per hour. Microsclerotia of Verticillium were more resistant to ozone. After 210 minutes with 20 g ozone per hour, some infectivity was left. However it is unknown if microsclerotia do occur in recirculation water. It is likely that conidia of Verticillium play the most important role in the dispersal of the pathogen. These conidia are killed by ozone. Non-pathogenic bacteria and fungi were as sensitive to ozone as tomato mosaic virus. For practical applications a bacterial test can be used to check the ozone installation. At the moment about 150 growers use ozone installations for the disinfection of recirculation water in closed cultivation systems.