What is the best design approach for estimating effluent concentrations from horizontal flow treatment wetlands: the use of volumetric (k_V) or areal (k_A) removal rate coefficients?

Effluent concentrations from horizontal flow (HF) treatment wetlands can be estimated by using the Tanks-In-Series model for describing hydraulics and first-order removal rate coefficients for describing pollutant removal. In the design of conventional wastewater treatment plants, volumetric removal rate coefficients (k(V)) are traditionally used in conjunction with the theoretical hydraulic retention time. Areal removal rate coefficients (k(A)) coupled with the applied areal hydraulic loading rate are widely used in the literature. Despite this, supporting evidence of its appropriateness is scarce in the literature. The objective of this study is to investigate the adequacy of both approaches by analyzing the influence of liquid depth on k(V) and k(A). Data from 74 HF wetlands were collected, covering biochemical oxygen demand and chemical oxygen demand, and diverse types of influents (raw sewage and primary, secondary and tertiary effluents). For these conditions, k(V) decreased with depth of the wetland system. Regression analyses between depth and removal rate coefficients were performed, and the equations indicated that k(V) was approximately related to the inverse of depth, while k(A) was almost independent of depth. These findings endorse the utilization of the areal-based approach for design purposes. The volumetric-based approach can also be used, but the value of k(V) must be provided together with the depth being considered.