Seasonal variations in hydraulic performance of rock-plant filters

Hydraulic retention time (HRT) may be one of the major factors causing seas onal variations in treatment efficiency of rock-plant filters. Six meso-sca le rock-plant filters, three with narrowleaf cattails (Typha augustolia) an d three unplanted, were investigated to evaluate the influence of plant fac tors on HRT. Tracer studies were conducted during summer, fall, and spring to evaluate seasonal variations in hydraulic performance. Significant evapo transpiration was observed during summer, up to 38.7 mm d(-1), which was fi ve times the corresponding Class A pan evaporation and more than 58% of the applied water. This caused effective HRT to increase by about 50%. Signifi cant gravel expansion also was observed after 2 years of operation. Total b ed volume increased up to 21.8%. This increase in bed volume may help maint ain porosity and prevent clogging in wetlands. It also caused the subsurfac e flow pattern to vary from plug flow. High evapotranspiration significantl y increased effective HRT in rock-plant filters, which consequently contrib uted to seasonal variations in treatment performance. Changes in effective HRT should be considered both in process modeling to account for seasonal h ydraulic variations and in wetland operations to prevent potential dry-out during warm seasons.