The effects of flow rate variation and vegetation ageing on the longitudinal mixing and residence time distribution (RTD) in a full-scale constructed wetland

Abstract

A field-based experimental study has been undertaken within a full-scale constructed wetland, designed to treat runoff from agricultural land in Knapwell, Cambridgeshire, UK. The effects of flow rate variation and natural vegetation ageing on the mixing characteristics are investigated over a eight month period. Detailed fluorometric measurements were made to examine the longitudinal spreading of a solute within the wetland. Between a UK November winter period, and June summer period, 125 tracer tests were undertaken for a range of dry weather and storm flow conditions, using an automated daily injection tracer system. The longitudinal dispersion results show that the dispersion is influenced by the flow rate for low discharge conditions, however, for higher discharges, the longitudinal dispersion becomes independent of discharge. Residence Time Distribution (RTD) curves are examined through a series of flow conditions for each testing month, ranging from transitional (Re ~2000) to turbulent (Re ~7000) flow conditions. For the conditions measured, differing flow rates produce changes in the RTD, demonstrating that higher flow rates induce shorter mean residence times, generating predominantly an advective flow regime. The effects of plant age are prominent on the mixing pattern. Towards the end of the annual plant cycle, in February/March, mixing pattern approaches complete mixing, longitudinal mixing increases significantly due to long tails on the RTDs, and mean flow velocity is retarded. This indicates that the dormant plant period, which normally takes 5–6 months (October to March), alters progressively the mixing pattern in the system in such a way that it is significantly different from the mixing pattern during the growing plant season.