Towards zero-liquid discharge in hydroponic cultivation

Competition for freshwater resources, and legal restrictions on discharge of agrochemicals (plant protection products and fertilisers) increase pressure on agricultural practice to improve water-use-efficiency of crop production globally. Zero-discharge greenhouse crop production can be an effective solution to both problems. Preliminary results of experiments with cucumber and sweet pepper on stone wool substrate in two greenhouse compartments showed that productivity did not differ in zero-discharge growing compared to standard growing strategies. In this paper we present the final evaluation of the sweet pepper trial (December 2014-November 2015). In the reference compartment a total amount of 465 m3 ha-1 year-1 (169.3 kg N) was discharged. In the zero-discharge compartment there was no discharge during cropping. The nutrient composition of irrigation and drain water was analysed weekly (compared to two-weekly in reference) and nutrient stock composition was changed accordingly. Drain water was filtered with a flatbed filter (no rinsing water) and upon changes in fertigation, the new fertigation solution was first distributed throughout the entire irrigation system by a circulation pump, before application. An irrigation strategy was developed for the end of the cropping season, aiming to drain substrate slabs and to empty drain tanks as much as possible and reduce the amount of remaining nutrients in the slabs, without compromising productivity. Water-use-efficiency of the zero-liquid discharge strategy could be optimised by steering towards more generative crop growth.