Zero-discharge wastewater treatment system for biomass production

Evapotranspirative willow system (EWS) is a zero-discharge wastewater treatment system where all influent water is used for growth of willows and evaporation. The EWS has no pollutant emissions to the environment and enables reuse of water and nutrients i.e. closing material flows. Willow clones used in EWS may significantly affect the performance of wastewater treatment plant, therefore the clones with high biomass production and resilience to permanent flooding, increased nutrient concentrations and salinity have to be selected. In the presented study a 27 m pilot EWS was set up in November 2015 enabling to test three different willow clones from the Croatian selection namely 'V 052' (Salix alba var. calva x S. alba), 'V 093' (Salix alba x S. alba var. vitellina) x S. alba and 'V 160' (Salix alba) in three parallels. Stem height, diameter and number of shoots per stump were measured weekly in the first year of growth on site, along with water quality parameters and water level in the experimental beds during the start up operation. There were no differences in stem height between the experimental clones; however, 'V 052' and 'V 093' have 0.16 and 0.26 m higher stems in control plots compared to the experimental ones, while 'V 160' experimental plants were in average 0.20 m higher compared to control ones. Experimental 'V 160' also developed more shoots compared to the controls and had significantly higher water demand. Between the examined clones, so far 'V 160' presents the most appropriate clone for the use in EWS; however further investigation will evaluate the efficiency of nutrient transfer from wastewater to wood biomass, water demand and the biomass yield for the selected clones.