Representing performance of horizontal flow treatment wetlands: The Tanks In Series (TIS) and the Plug Flow with Dispersion (PFD) approaches and their application to design

Horizontal flow wetlands have been designed using the so-called P-k-C* approach, which has been largely embraced by the treatment wetlands literature. P is meant to represent the equivalent number of apparent tanks in series (hydraulic factor), but also incorporates the loss of biodegradability as the wastewater undergoes treatment (kinetic factor). For design purposes, literature proposes fixed values of P. The proposal of this paper is to decouple hydraulics from kinetics and use the traditional concept of N or NTIS (number of tanks in series) as a function of geometric relationships of the wetland to be designed, leaving kinetic elements to be dealt with solely by the first-order removal rate coefficient (k). From the literature, a database with 41 wetlands with data from tracer studies was used, and a novel regression-based equation was derived relating N with the ratio length/depth of horizontal wetlands. This equation can be used at the design stage for estimating N and, hence, the output concentration of the pollutant using the traditional structure of the TIS model, with a possible inclusion of background concentration (C*). The paper presents all relevant equations, including those from the plug-flow with dispersion model (PFD), and it is shown how to convert from one hydraulic model to the other, what is also believed to be a novel approach in the treatment wetland literature. Finally, the area-based removal rate coefficients (kA) proposed by Kadlec and Wallace (2009) for designs of horizontal wetlands treating domestic wastewater based on the P-k-C* approach are converted into kA values for the TIS model in the paper.