A review study on the association between hydraulic performance and treatment effectiveness in free surface flow constructed wetlands

The removal of contaminants by constructed wetland (CW) is typically related to the residence time and CW design is based on a stipulated residence time, usually 72 h. Studies have shown that residence time is related to hydraulic performance. Hydraulic performance is an important factor in studying the treatment behaviour of CWs and is influenced by length-to-width ratio, scale and sizing, wetland shape and configuration and the distribution of vegetation. Indices such as nominal hydraulic residence time (tn), tank-in-series (N), hydraulic efficiency (λ), short-circuit (S) and effective volume ratio (e) have been used to characterize hydraulic performance. Most studies on CW emphasise on treatment performance, however, very few studies investigate the link between treatment effectiveness and hydraulic performance, although such a link has been suggested. This review was conducted to establish correlations between treatment performance and hydraulic behaviour as a better understanding of this knowledge gap will lead to better design. Studying hydraulic performance is resource intensive and costly, and for these reasons such studies are often not undertaken. Given the paucity of data, estimates on hydraulic performance were obtained from results obtained in the literature of λ for published studies where wetland configurations were provided. The dataset was divided into CW treating stormwater and wastewater. It is found out that there is a significant (p < .05) correlation between λ with TSS removal for both wastewater (r = 0.86) and stormwater (r = 0.86) applications. For stormwater, the removals of TN (r = 0.08) and TP (r = 0.56) were not found to be significantly (p > .05) correlated, while BOD/COD were strongly correlated (r = 0.97, p < .05), with λ. For wastewater, TN and TP removals were not correlated (r = 0.18 and r = −0.04, respectively, with p > .05 for both parameters). BOD/COD was also not significantly correlated (r = 0.64, p > .05). Given that this is a first study attempting to draw relationships between treatment effectiveness and hydraulic performance, and that there is a general lack of data on hydraulic efficiency, it is recommended that future field-based wetland studies include hydraulic efficiency measurements or at the very least, provide detailed information of the physical features of the constructed wetland under investigation and the planting configuration. Further recommendations for future work include conducting laboratory experiments as a cost-effective way to study hydraulic efficiency and to study the role of vegetation distribution on hydraulic performance.