What is the best design approach for estimating effluent concentrations from horizontal flow treatment wetlands: the use of volumetric (kV) or areal (kA) 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 (kV) are traditionally used in conjunction with the theoretical hydraulic retention time. Areal removal rate coefficients (kA) 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 kV and kA. 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, kV decreased with depth of the wetland system. Regression analyses between depth and removal rate coefficients were performed, and the equations indicated that kV was approximately related to the inverse of depth, while kA 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 kV must be provided together with the depth being considered.

What is the best procedure for determining removal rate coefficients in horizontal flow treatment wetlands: influent and effluent concentrations or longitudinal concentration profiles?

First-order removal rate coefficients (k) are used in predictive equations for estimating effluent concentrations from horizontal flow (HF) wetlands. Due to limited resources, influent and effluent concentration data from existing systems are frequently used in the estimation of k values from operating systems, but another choice is to use concentration data along the longitudinal profile of the HF wetland. Based on a dataset with 41 HF wetlands/studies obtained from a literature survey, with chemical oxygen demand (COD) measurements at different sampling points, volumetric (kV) and areal (kA) removal rate coefficients for the Tanks-In-Series (TIS) model have been obtained using the two estimation methods. In general, removal rate coefficients derived from longitudinal profiles of concentrations were higher than those obtained by using data from influent and effluent concentrations, reflecting the fact that constituent removal is mostly accomplished before the wastewater reaches the outlet zone. Deriving coefficients from longitudinal profiles is more comprehensive, providing a better explanation of the internal removal taking place in the treatment wetland. However, the more widely used approach of calculating kV and kA from influent/effluent concentrations may lead to a safer design of horizontal flow wetlands, because of underestimation of the actual removal rate coefficients.

Nitrification cessation and recovery in an aerated saturated vertical subsurface flow treatment wetland: Field studies and microscale biofilm modeling.

In aerated treatment wetlands, oxygen availability is not a limiting factor in sustaining a high level of nitrification in wastewater treatment. In the case of an air blower failure, nitrification would cease, potentially causing adverse effects to the nitrifying bacteria. A field trial was completed investigating nitrification loss when aeration is switched off, and the system recovery rate after the aeration is switched back on. Loss of dissolved oxygen was observed to be more rapid than loss of nitrification. Nitrate was observed in the effluent long after the aeration was switched off (48h+). A complementary modeling study predicted nitrate diffusion out of biofilm over a 48h period. After two weeks of no aeration in the established system, nitrification recovered within two days, whereas nitrification establishment in a new system was previously observed to require 20-45days. These results suggest that once established resident nitrifying microbial communities are quite robust.