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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.

Determining the prevalence of antimicrobial resistance (AMR) in non-clinical settings is vital for better management of the global AMR crisis. Untreated and even treated wastewaters are important sources that release AMR into the environment. Methodologically, it is difficult to generate a comprehensive in situ profile of antibiotic resistance gene hosts. Here, we used epicPCR (emulsion, paired isolation, and concatenation PCR) as a cultivation-independent method to reveal the host profiles of the AMR indicator genes intI1, sul1, sul2, and dfrA1 in two constructed wetlands treating municipal wastewater. Overall, the epicPCR analysis revealed a profile of AMR indicator gene hosts that is consistent with literature data from cultivation-based approaches. Most carriers of antibiotic resistance (AR) genes and likely of class 1 integrons belonged to the Gammaproteobateria, particularly the Burkholderiaceae and Rhodocyclaceae families, followed by members of the Campylobacterota, Desulfobacterota, and Firmicutes. The analysis also identified several novel hosts for the indicator genes widely distributed in the wetlands, including the genera Legionella and Ralstonia. Therefore, the application of epicPCR has produced an expanded insight into the in situ indicator gene host profile, while highlighting the role of the environment as a reservoir for AMR.

The performance of an aerated horizontal subsurface flow treatment wetland was investigated before, during and after a simulated aeration failure. Conventional wastewater parameters (e.g., carbonaceous biological oxygen demand, total nitrogen, and Escherichia coli) as well as selected micropollutants (caffeine, ibuprofen, naproxen, benzotriazole, diclofenac, acesulfame, and carbamazepine) were investigated. Furthermore, the removal of biological effects was investigated using in vitro bioassays. The six bioassays selected covered environmentally relevant endpoints (indicative of activation of aryl hydrocarbon receptor, AhR; binding to the peroxisome proliferator-activated receptor gamma, PPARγ; activation of estrogen receptor alpha, ERα; activation of glucocorticoid receptor, GR; oxidative stress response, AREc32; combined algae test, CAT). During the aeration interruption phase, the water quality deteriorated to a degree comparable to that of a conventional (non-aerated) horizontal subsurface flow wetland. After the end of the aeration interruption, the analytical and biological parameters investigated recovered at different time periods until their initial treatment performance. Treatment efficacy for conventional parameters was recovered within a few days, but no complete recovery of treatment efficacy could be observed for bioassays AhR, AREc32 and CAT in the 21 days following re-start of the aeration system. Furthermore, the removal efficacy along the flow path for most of the chemicals and bioassays recovered as it was observed in the baseline phase. Only for the activation of AhR and AREc32 there was a shift of the internal treatment profile from 12.5% to 25% (AhR) and 50% (AREc32) of the fractional length.

Aerated treatment wetlands have become an increasingly recognized technology for treating wastewaters from domestic and various industrial origins. To date, treatment wetland aeration is provided by air pumps which require access to the energy grid. The requirement for electricity increases the ecological footprint of an aerated wetland and limits the application of this technology to areas with centralized electrical infrastructure. Wind power offers another possibility as a driver for wetland aeration, but its use for this purpose has not yet been investigated. This paper reports the first experimental trial using a simple wind-driven air pump to replace the conventional electric air blowers of an aerated horizontal subsurface flow wetland. The wind-driven air pump was connected to a two-year old horizontal flow aerated wetland which had been in continuous (24 h) aeration since startup. The wind-driven aeration system functioned, however it was not specifically adapted to wetland aeration. As a result, treatment performance decreased compared to prior continuous aeration. Inconsistent wind speed at the site may have resulted in insufficient pressure within the aeration manifold, resulting in insufficient air supply to the wetland. This paper discusses the lessons learned during the experiment.

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.

This paper outlines recent advances in the design, application, and operations and maintenance (O&M) of aerated treatment wetland systems as well as current research trends. We provide the first-ever comprehensive estimate of the number and geographical distribution of aerated treatment wetlands worldwide and review new developments in aerated wetland design and application. This paper also presents and discusses first-hand experiences and challenges with the O&M of full-scale aerated treatment wetland systems, which is an important aspect that is currently not well reported in the literature. Knowledge gaps and suggestions for future research on aerated treatment wetlands are provided.

One of the most frequently encountered operational problems in subsurface-flow constructed wetlands is clogging. Traditionally, the restoration procedure is to remove the clogged gravel or sand and replace it with clean material. This method, while effective, is costly and may require sections of the facility to be taken offline for extended periods of time. Another common remediation strategy is to have a resting period for each wetland cell, although this is not an option for very small systems which often consist of only one treatment cell. Recently, a more radical approach has been tested on a number of lab-scale and pilot-scale setups which consists of an aggressive oxidation of organic matter by means of hydrogen peroxide (H2O2). Results indicate that after treatment, clogging was substantially reduced and that H2O2 did not appear to have a long-term negative effect on plants and biofilms. The outcomes of two full-scale tests are discussed in this paper.

Community-level physiological profiling (CLPP) using BIOLOG® EcoPlates™ has become a popular method for characterizing and comparing the functional diversity, functional potential, and metabolic activity of heterotrophic microbial communities. The method was originally developed for profiling soil communities; however, its usage has expanded into the fields of ecotoxicology, agronomy, and the monitoring and profiling of microbial communities in various wastewater treatment systems, including constructed wetlands for water pollution control. When performing CLPP on aqueous samples from constructed wetlands, a wide variety of sample characteristics can be encountered and challenges may arise due to excessive solids, color, or turbidity. The aim of this study was to investigate the impacts of different sample preparation methods on CLPP performed on a variety of aqueous samples covering a broad range of physical and chemical characteristics. The results show that using filter paper, centrifugation, or settling helped clarify samples for subsequent CLPP analysis, however did not do so as effectively as dilution for the darkest samples. Dilution was able to provide suitable clarity for the darkest samples; however, 100-fold dilution significantly affected the carbon source utilization patterns (CSUPs), particularly with samples that were already partially or fully clear. Ten-fold dilution also had some effect on the CSUPs of samples which were originally clear; however, the effect was minimal. Based on these findings, for this specific set of samples, a 10-fold dilution provided a good balance between ease of use, sufficient clarity (for dark samples), and limited effect on CSUPs. The process and findings outlined here can hopefully serve future studies looking to utilize CLPP for functional analysis of microbial communities and also assist in comparing data from studies where different sample preparation methods were utilized.

Despite recent developments in process-based modeling of treatment wetlands (TW), the dynamic response of horizontal flow (HF) aerated wetlands to interruptions of aeration has not yet been modeled. In this study, the dynamic response of organic carbon and nitrogen removal to interruptions of aeration in an HF aerated wetland was investigated using a recently-developed numerical process-based model. Model calibration and validation were achieved using previously obtained data from pilot-scale experiments. Setting initial concentrations for anaerobic bacteria to high values ( 35–70 mg L−1) and including ammonia sorption was important to simulate the treatment performance of the experimental wetland in transition phases when aeration was switched off and on again. Even though steady-state air flow rate impacted steady-state soluble chemical oxygen demand (CODs), ammonia nitrogen (NH4–N) and oxidized nitrogen (NOx–N) concentration length profiles, it did not substantially affect corresponding effluent concentrations during aeration interruption. When comparing simulated with experimental results, it is most likely that extending the model to include mass transfer through the biofilm will allow to better explain the underlying experiments and to increase simulation accuracy. This study provides insights into the dynamic behavior of HF aerated wetlands and discusses assumptions and limitations of the modeling approach.