Advances in the process-based models of constructed wetlands and a way forward for integrating emerging organic contaminants.

This research examines advancements in the development of process-based models of constructed wetlands (CWs) tailored for simulating conventional water quality parameters (CWQPs). Despite the promising potential of CWs for emerging organic contaminant (EOC) removal, the available CW models do not yet integrate EOC removal processes. This study explores the need and possibility of integrating EOCs into existing CW models. Nevertheless, a few researchers have developed process-based models of other wastewater treatment systems (e.g., activated sludge systems) to simulate certain EOCs. The EOC removal processes observed in other wastewater treatment systems are analogous to those in CWs. Therefore, the corresponding equations governing these processes can be tailored and integrated into existing CW models, similarly to what was done successfully in the past for CWQPs. This study proposed the next generation of CW models, which outlines 12 areas for future work: integrating EOC removal processes; ensuring data availability for model calibration and validation; considering quantitative and sensitive parameters; quantifying microorganisms in CWs; modifying biofilm dynamics models; including pH, aeration, and redox potential; integrating clogging and plant sub-models; modifying hydraulic sub-model; advancing computer technology and programming; and maintaining a balance between simplicity and complexity. These suggestions provide valuable insights for enhancing the design and operational features of current process-based models of CWs, facilitating improved simulation of CWQPs, and integration of EOCs into the modelling framework.

Prediction of the removal efficiency of emerging organic contaminants in constructed wetlands based on their physicochemical properties.

This study investigates the prediction of the removal efficiency of emerging organic contaminants (EOCs) (pharmaceuticals-PhCs, personal care products-PCPs, and steroidal hormones-SHs) in constructed wetlands based on their physicochemical properties (e.g., molecular weight-MW, octanol-water partition coefficient-Log Kow, soil organic carbon sorption coefficient-Log Koc, octanol-water distribution coefficient-Log Dow, and dissociation constant-pKa). The predictive models are formed based on statistical analysis underpinned by principle component, correlation, and regression analyses of a global data set compiled from peer-reviewed publications. The results show that the physicochemical properties of EOCs emerged as good predictors of their removal efficiency. Log Koc, Log Dow, and Log Kow are the most significant predictors, and combination with MW and/or pKa often improved the reliability of the predictions. The best performing model for PhCs was composed of MW, Log Dow, and Log Koc (coefficient of determination-R2: 0.601; probability value-p < 0.05; root mean square error-RMSE: training set: 11%; test set: 27%). Log Kow and Log Koc for PCPs (R2: 0.644; p < 0.1; RMSE: training set: 14%; test set: 14%), and a combination of MW, Log Kow, and pKa for SHs (R2: 0.941; p < 0.1; RMSE: training set: 3%; test set: 15%) formed the plausible models for predicting the removal efficiency. Similarly, reasonably good combined models could be formed in the case of PhCs and SHs or PCPs and SHs, although their individual models were comparatively better. A novel decision support tool, named as REOCW-PCP, was developed to readily estimate the removal efficiency of EOCs, and facilitate the decision-making process.

A decision tree framework to support design, operation, and performance assessment of constructed wetlands for the removal of emerging organic contaminants.

There is an increasing focus on research related to the removal of emerging organic contaminants (EOCs) from wastewater by using constructed wetlands (CWs). However, research is lacking on translating the available scientific evidence into decision support tools. In this paper, a novel decision tree framework is developed and demonstrated. The proposed framework consists of five steps: (1) generate a list of EOCs by the analysis of the wastewater; (2) select the best type of CW for each of the selected EOCs; (3) select a final type of CW for the removal of the selected EOCs; (4) identify detailed design and operational features of the proposed CW such as, depth, area, plants, support matrix, hydraulic loading rate, organic loading rate, and hydraulic retention time; and (5) assess the expected removal efficiency of EOCs in the selected CW. A novel decision support tool, named as DTFT-CW, was developed to generate data and information for the application of the proposed decision tree framework. DTFT-CW (given as a supplementary material) was developed using Microsoft Excel 2016 to support decisions on the design, operation, and performance of CWs for the removal of 59 EOCs (33 pharmaceuticals-PhCs, 15 personal care products-PCPs, and 11 steroidal hormones-SHs). The paper demonstrates the usefulness of the developed decision-making tools by considering 19 EOCs (13 PhCs, one PCPs, and five SHs) as an example, which pose high environmental risk and are on the European Union watch list (six of the 19 EOCs). An integrated design of HCW (combining vertical flow CW, horizontal flow CW-HFCW, and free water surface CW) is recommended for the treatment of multiple EOCs instead of a single type of CW such as HFCW that is most widely used in practice. The proposed tools could be useful for decision makers such as policy makers, design engineers, and researchers.

Pharmaceuticals' removal by constructed wetlands: a critical evaluation and meta-analysis on performance, risk reduction, and role of physicochemical properties on removal mechanisms.

This paper presents a comprehensive and critical analysis of the removal of pharmaceuticals (PhCs), the governing physicochemical properties, and removal mechanisms in constructed wetlands (CWs). The average removal efficiency of the most widely studied 34 PhCs ranges from 21% to 93%, with the exception of one PhC that exhibited negative removal. Moreover, CWs are effective in significantly reducing the environmental risk caused by many PhCs. Based on risk assessment, 12 PhCs were classified under high risk category (oxytetracycline > ofloxacin > sulfamethoxazole > erythromycin > sulfadiazine > gemfibrozil > ibuprofen > acetaminophen > salicylic acid > sulfamethazine > naproxen > clarithromycin), which could be considered for regular monitoring, water quality standard formulation and control purposes. Biodegradation (aerobic and anaerobic) is responsible for the removal of the majority of PhCs, often in conjunction with other mechanisms (e.g., adsorption/sorption, plant uptake, and photodegradation). The physicochemical properties of molecules play a pivotal role in the elimination processes, and could serve as important predictors of removal. The correlation and multiple linear regression analysis suggest that organic carbon sorption coefficient (Log Koc), octanol-water distribution coefficient (Log Dow), and molecular weight form a good predictive linear regression model for the removal efficiency of PhCs (R2 = 0.65, P-value <0.05).

Performance Comparison of Different Constructed Wetlands Designs for the Removal of Personal Care Products.

This research investigates the performance of four types of constructed wetlands (CWs): free water surface CW (FWSCW), horizontal flow CW (HFCW), vertical flow CW (VFCW), and hybrid CW (HCW) for the removal of 20 personal care products (PCPs), based on secondary data compiled for 137 CWs reported in 39 peer reviewed journal papers. In spite of considerable variation in the re-moval efficiency of PCPs, CWs prove to be a promising treatment technology. The average removal efficiency of 15 widely studied PCPs ranged from 9.0% to 84%. Although CWs effectively reduced the environmental risks caused by many PCPs, triclosan was still classified under high risk category based on effluent concentration. Five other PCPs were classified under medium risk category (triclocarban > methylparaben > galaxolide > oxybenzone > methyl dihydrojasmonate). In most of the examined PCPs, adsorption and/or sorption is the most common removal mechanism followed by biodegradation and plant uptake. The comparatively better performance of HCW followed by VFCW, HFCW, and FWSCW might be due to the co-existence of aerobic and anaerobic conditions, and longer hydraulic retention time enhancing the removal of PCPs (e.g., triclosan, methyl dihydro-jasmonate, galaxolide, tonalide, and oxybenzone), which are removed under both conditions and by adsorption/sorption processes.

Performance comparison of different types of constructed wetlands for the removal of pharmaceuticals and their transformation products: a review.

This paper presents a comprehensive and critical comparison of four types of constructed wetlands (CWs): free water surface CW (FWSCW), vertical flow CW (VFCW), horizontal flow CW (HFCW), and hybrid CW (HCW) for the removal of 29 pharmaceuticals (PhCs) and 19 transformation products (TPs) using a global data compiled for 247 CWs reported in 63 peer-reviewed journal papers. Biodegradation (aerobic being more efficient than anaerobic) is the major removal mechanism for 16 out of 29 PhCs besides the influence of other processes (e.g., adsorption/sorption, plant uptake, and photodegradation). The HCW performed better followed by VFCW, HFCW, and FWSCW. The comparatively better removal in HCW might be due to the coexistence of aerobic and anaerobic conditions and longer hydraulic retention time considering more than one compartment enhances the removal of PhCs (e.g., diclofenac, acetaminophen, sulfamethoxazole, sulfapyridine, trimethoprim, and atenolol), which are removed under both conditions and adsorption/sorption processes. The augmentation in dissolved oxygen by the application of artificial aeration improved the removal of PhCs, which are degraded under aerobic conditions. Furthermore, the better performance of aerated CWs could be due to the establishment of various microenvironments with different physicochemical conditions (aerobic and anaerobic), which facilitated the contribution of both aerobic and anaerobic metabolic pathways in the removal of PhCs. The removal of some of the PhCs takes place by the formation of their TPs and the nature of these TPs (persistent or non-biodegradable/biodegradable) plays a major role in their removal process.

An exploration of disinfection by-products formation and governing factors in chlorinated swimming pool water.

This paper investigates disinfection by-products (DBPs) formation and their relationship with governing factors in chlorinated swimming pools. The study compares concentrations of DBPs with WHO guidelines for drinking water quality recommended to screen swimming pool water quality. The statistical analysis is based on a global database of 188 swimming pools accumulated from 42 peer-reviewed journal publications from 16 countries. The mean and standard deviation of dichloroacetic acid and trichloroacetic acid were estimated as 282 ± 437 and 326 ± 517 µg L-1, respectively, which most often surpassed the WHO guidelines. Similarly, more than half of the examined pools had higher values of chloral hydrate (102 ± 128 µg L-1). The concentration of total chloramines (650 ± 490 µg L-1) was well above the WHO guidelines in all reported cases. Nevertheless, the reported values remained below the guidelines for most of the studied pools in the case of total trihalomethanes (134 ± 160 µg L-1), dichloroacetonitrile (12 ± 12 µg L-1) and dibromoacetonitrile (8 ± 11 µg L-1). Total organic carbon, free residual chlorine, temperature, pH, total nitrogen and bromide ions play a pivotal role in DBPs formation processes. Therefore, proper management of these governing factors could significantly reduce DBPs formation, thereby, contributing towards a healthy swimming pool environment.

The effects of different aeration strategies on the performance of constructed wetlands for phosphorus removal.

The effects of different aeration methods such as tidal flow (TF), effluent recirculation (ER), and artificial aeration (AA) on the performance of vertical-flow constructed wetland (VFCW), horizontal-flow constructed wetland (HFCW), and hybrid constructed wetland (HCW) are extensively and critically evaluated in this review paper. Aerated constructed wetlands (CWs) demonstrate superior performance compared with non-aerated systems. The removal of total phosphorus (TP) showed substantial variation among different types of CWs and aeration strategies, with mean and standard deviation of 68 ± 20% estimated from all reviewed studies on aerated systems. The TF-VFCW designated the highest removal efficiency and removal rate of 88 ± 6% and 2.6 ± 2.5 g m-2 day-1, respectively, followed by the ER-HCW with values of 79 ± 18% and 1.3 ± 0.7 g m-2 day-1, respectively. The superior performance of TF-VFCW could be attributed to a positive effect of TF in rejuvenating the wetland with fresh air, thus enhancing dissolved oxygen (DO) in the system, and augmenting phosphorus precipitation and adsorption to the substrate. A positive correlation of TP and orthophosphate (PO43--P) with DO indicates that the improvement in DO levels due to redox manipulation with aeration strategies facilitates the phosphorous removal processes (e.g., through precipitation and adsorption to the substrate). The conflicting results on the impact of AA and ER reported by many studies need the cautious interpretation of their impact and require further studies. Only few studies have examined the impact of oxidation-reduction potential on phosphorous removal, which requires more attention in future research, as it appears as an important factor in enhancing the phosphorus removal.

The performance of the intensified constructed wetlands for organic matter and nitrogen removal: A review.

The effects of different aeration strategies including tidal flow (TF), effluent recirculation (ER) and artificial aeration (AA) on performance of vertical flow constructed wetland (VFCW), horizontal flow constructed wetland (HFCW) and hybrid constructed wetland (HCW) are comprehensively and critically reviewed in this paper. The removal efficiencies of nine types of intensified constructed wetlands (CWs) were examined in detail and their mean and standard deviation were estimated at 89 ± 11%, 84 ± 12%, 81 ± 17% and 63 ± 20% for total suspended solids (TSS), chemical oxygen demand (COD), ammonium-nitrogen (NH4+N) and total nitrogen (TN), respectively. From the studied CWs, ER-HCW, TF-HCW, AA-VFCW and ER-VFCW emerged as the four best performing systems. The overall removal efficiency of TSS, COD, NH4+N and TN by ER-HCW was 98 ± 2%, 85 ± 11%, 83 ± 15% and 73 ± 11%, respectively. Specifically, the ER enhances the interactions between pollutants and micro-organisms, consequently, the efficient removal of NH4+N and TN has been achieved in ER-HCW. The TF has a positive effect in refreshing the wetland with fresh air to enhance the dissolved oxygen (DO) in the system. In case of AA, intermittent aeration is more effective than continuous aeration, as it facilitates the establishment of aerobic and anaerobic conditions suitable for nitrification and denitrification. Statistical analysis shows that DO, organic loading rate and specific surface area requirement are the most significant factors that influence the performance of intensified CWs.

Intensification of constructed wetlands for land area reduction: a review.

The large land area requirement of constructed wetlands (CWs) is a major limitation of its application especially in densely populated and mountainous areas. This review paper provides insights on different strategies applied for the reduction of land area including stack design and intensification of CWs with different aeration methods. The impacts of different aeration methods on the performance and land area reduction were extensively and critically evaluated for nine wetland systems under three aeration strategies such as tidal flow (TF), effluent recirculation (ER), and artificial aeration (AA) applied on three types of CWs including vertical flow constructed wetland (VFCW), horizontal flow constructed wetland (HFCW), and hybrid constructed wetland (HCW). The area reduction and pollutant removal efficiency showed substantial variation among different types of CWs and aeration strategies. The ER-VFCW designated the smallest footprint of 1.1 ± 0.5 m2 PE-1 (population equivalent) followed by TF-VFCW with the footprint of 2.1 ± 1.8 m2 PE-1, and the large footprint was of AA-HFCW (7.8 ± 4.7 m2 PE-1). When footprint and removal efficiency both are the major indicators for the selection of wetland type, the best options for practical application could be TF-VFCW, ER-HCW, and AA-HCW. The data and results outlined in this review could be instructive for futures studies and practical applications of CWs for wastewater treatment, especially in land-limited regions.