Seasonal variations in hydraulic performance of rock-plant filters

Hydraulic retention time (HRT) may be one of the major factors causing seasonal variations in treatment efficiency of rock-plant filters. Six meso-scale rock-plant filters, three with narrowleaf cattails (Typha augustifolia) and three unplanted, were investigated to evaluate the influence of plant factors on HRT. Tracer studies were conducted during summer, fall, and spring to evaluate seasonal variations in hydraulic performance. Significant evapotranspiration was observed during summer, up to 38.7 mm d(-1), which was five times the corresponding Class A pan evaporation and more than 58% of the applied water. This caused effective HRT to increase by about 50%. Significant gravel expansion also was observed after 2 years of operation. Total bed volume increased up to 21.8%. This increase in bed volume may help maintain porosity and prevent dogging in wetlands. It also caused the subsurface flow pattern to vary from plug flow. High evapotranspiration significantly increased effective HRT in rock-plant filters, which consequently contributed to seasonal variations in treatment performance. Changes in effective HRT should be considered both in process modeling to account for seasonal hydraulic variations and in wetland operations to prevent potential dry-out during warm seasons.