Advanced oxidation to eliminate growth inhibition and to degrade plant protection products in a recirculating nutrient solution in Rose cultivation

The EU Water Framework Directive demands a sound ecological and chemical basis for ground and surface waters. This has motivated the Dutch greenhouse industry to seek more sustainable water management procedures which will enable a zero-emission of nutrients and plant protection products (PPP) in the year 2027. Although closed soilless growing systems are widely applied in The Netherlands, it appears that discharge of nutrients varies between 5 and 40%. Discharge based on salinity is only a minor part, up to 15%. In rose cultivation, growth inhibition is the major reason for discharge of the nutrient solution. Former research could not find a proper reason for growth inhibition, but it is most likely of organic origin. The water treatment method of advanced oxidation, at which first hydrogen peroxide is added, directly followed by an exposure to UV-C light (200-280 nm), is known to degrade large organic molecules. Therefore this method has been investigated to eliminate growth inhibition and also to degrade PPPs. Among all methods advanced oxidation has been a first choice because many rose growers already possess a UV installation for elimination of pathogens from the recirculating solution. Trials with advanced oxidation have been performed at two rose nurseries at which the dosages of hydrogen peroxide (0-25 mg/L) and UV lighting (0-900 mJ/cm2) have been varied to search for indications for elimination of growth inhibition and the degradation of PPPs. After treatment samples were taken to test for growth inhibition of the solution in a bioassay, Phytotoxkit; the chemical composition; the residual amount of PPPs and the presence of micro-organisms. Preliminary results show that (1) growth inhibition exists and can be decreased, (2) plant protection products can be degraded, (3) pathogens have been eliminated and (4) composition of the nutrient solution is unchanged except for iron