Double use of water treatment in soilless growing systems : Disinfection of recirculating solution and removal of plant protection products from discharge water

Worldwide increase of water shortage and competition for fresh water is raising the awareness that an increase in water use efficiency in horticulture is a necessity. At the same time, environmental awareness and legislation (e.g. the EU Water Framework Directive 2000/60/EC) restricts the emissions of nutrients and plant protection products (PPPs). In the Netherlands in soilless growing systems much effort is put into measures to enable maximal recycling of nutrient solutions, thereby decreasing the required amount of fresh water needed and the emission of nutrients and PPPs. Recycling of water fluxes also implicates that the drain water has to be disinfected to prevent rapid dispersal of pathogens. Heat treatment, filtration, Ozone and UV treatment are commonly used disinfection technologies. Some of these technologies (Ozone and UV) are known for their ability to break down PPPs and therefore their joint application (disinfection and end-of-pipe purification of discharged drain water) have been studied. In this paper we present the results of the integration and optimisation of ozone technology in a cucumber and a sweet pepper soilless cultivation. An Agrozone ozone unit that alternately treated the recycled nutrient solution for the elimination of pathogens and removed PPPs from discharge water was investigated. The ozone unit eliminates pathogens and PPPs by direct oxidation of organic material batchwise. It was shown that both disinfection of the recirculating nutrient solution and removal of PPPs from discharge water are possible with the same ozone equipment, if installed correctly. Preliminary tests with Tomato Mosaic Virus have shown that this technology can completely disinfect (from 402 to 0 lesions per plant), and remove up to 98% of PPPs from standardised greenhouse discharge water. For disinfection purposes shorter treatment times and lower ozone doses can be applied.