Zero liquid discharge in soilless greenhouse horticulture: solutions to save water and the environment while ensuring an optimal production

Zero liquid discharge (ZLD) should be the future for soilless production. In The Netherlands emission of nutrients and plant protection products to surface water is the main reason to investigate the potentials of cultivation methods without any discharge. In a few other countries water pollution issues is the main motive, but for a majority water shortage and the increasing competition for water of an acceptable quality is reason for the growing need for water efficient cultivation methods. During four years ZLD was investigated for cucumber (2014, 2017) and sweet pepper (2015, 2016). The primary goal was to reuse all drain water, while maintaining a similar yield and quality as in the traditional growing method with regular discharges. To achieve ZLD all practical reasons for growers to discharge the nutrient solution had to be anticipated before. In the ZLD trials technical and strategic solutions were jointly tested and further developed into an adapted growing strategy without any discharge. A number of factors could be mentioned as main reasons for discharge: sodium in the supply water, discharge of filter rinsing water, unbalances in the nutrient solution, no recirculation of the drainwater in the beginning of the cultivation, no disinfection equipment, too low drain storage capacity, appearance of calamities (technical failures) and the amount of remnant nutrient solution left in slabs and tanks at the end of the cultivation. The next step was demonstrating the technical and strategic solutions in a cultivation. In 2014 and 2015 we showed that ZLD-cultivation in stone wool substrate was possible without loss of yield and quality. In 2016 and 2017 the more challenging coir substrate was used to investigate the influence of buffering and sodium accumulation. It appeared that ZLD was possible without loss of production or quality. Appearing problems were more in the field of management of water flows than lack of technical equipment. Special care should come for technical failures of equipment, mostly not enough storage capacity is available.