Technical and economic aspects of purification strategies to minimise discharge water from companies with closed soilless cultivation systems

The aim of the research project was to achieve closure by two complementary means: 1) maximising reuse of the nutrient solution by solving problems in recircula-tion that leads to discharge, and 2) purification of the left over discharged water. In this paper the technical and economic aspects of purification strategies for single companies will be discussed. An evaluation of different techniques concluded that reverse osmosis (RO) and membrane distillation (MD) offer the best perspectives to purify the discharge water. Subsequent laboratory and onsite studies showed that ca. 80% of the discharge water can be recovered for reuse. For both technologies the water may be pre-treated with nano-filtration (NF) to increase the recovery of nutrients, and with advanced oxidation (AOP) to breakdown growth inhibition products for longer recirculation, and (in a higher dosage) to break down plant protection products in order to gain concentrate with an increased value. Based on these four technologies a range of purification strategies were formulated for which the treatment costs were estimated for two crops (rose and tomato) and two company sizes (5 and 30 ha) and three volumes of discharge (250, 500 and 1,250 m3/ha/yr). Between the two crops (rose and tomato) only little differences in costs of purification were found. Costs per ha decrease rapidly with decreasing volume of the discharge water. However, a scale advantage was found for large companies (30 ha) compared to the sector average scale of 5 ha. For the short term AOP (only reduction of plant protection products by about 80%) and reverse osmosis can be used economical. For the long term nano-filtration and membrane distillation may have prospects.