Soil Nematodes as an Indicator of the Efficiency of Hydrophytic Treatment Plants with Vertical Wastewater Flow

This paper presents the role of soil nematodes as bio-indicators of the functioning of soil-plant beds in hydrophytic vertical-flow constructed wetland (VFCW) wastewater treatment plants. This study aimed to determine the abundance and trophic composition of nematode populations in seven soil-plant beds, the third component of plant-based wastewater treatment plants designed as Nature-Based Solutions (NBSs), in line with blue–green infrastructure and the closed-loop economy. The technology of this type of treatment plant is also in line with the idea of sustainability due to the very low energy requirements of the wastewater treatment system. In addition, soil nematodes were analysed in the soil adjacent to the WWTPs to assess the differences in trophic structure between these environments. The average nematode abundance in the soil-plant beds ranged from 606,000 [N·m−2] to 1,982,000 [N·m−2], with bacterivorous nematodes being the most abundant trophic group (61–73% of the population). This study’s results confirmed that soil-plant beds are abundantly populated by bacteria participating in key organic matter decomposition processes and nitrogen and phosphorus compound transformations, contributing to adequate wastewater treatment. The dominance of bacterivorous nematodes indicates a practical support of physicochemical and biological processes that reduce pollutant concentrations and eliminate pathogenic bacteria flowing into the deposits with the wastewater.