Prediction of the removal efficiency of emerging organic contaminants based on design and operational parameters of constructed wetlands
Graphical abstract
Prediction of the removal efficiency of emerging organic contaminants based on design and operational parameters of constructed wetlands.
Introduction
Constructed wetlands (CWs) are nature-based treatment technologies that have been extensively investigated for the removal of emerging organic contaminants (EOCs) from wastewater [[1], [2], [3], [4], [5]]. To date, a large number of individual case studies have been published in peer reviewed journals related to the removal of EOCs by CWs. Several studies indicated that design and operational parameters such as depth, area, hydraulic loading rate (HLR), organic loading rate (OLR), and hydraulic retention time (HRT) are among the major governing factors in the removal of EOCs by CWs because these parameters considerably influence the possible removal mechanisms such as biodegradation and adsorption/sorption (e.g., [[6], [7], [8]]). The design and operational factors play an important role in the treatment process in CWs for the removal of pharmaceuticals (PhCs), personal care products (PCPs), and steroidal hormones (SHs) [[9], [10], [11]]. For instance, a significant correlation was found between the removal efficiency of some of the studied PhCs and design as well as operational parameters (depth, area, HLR, OLR, and HRT) [9].
However, the previous research did not investigate the possibility of predicting the removal efficiency by using design and operational parameters. For example, multiple linear regression analysis to develop predictive models for removal efficiency of PhCs, PCPs, and SHs with design and operational parameters was not conducted by the above-mentioned studies. Similarly, a principal component analysis (PCA) was not conducted to understand the role of different design and operational parameters of CWs in removal processes. Thus, a comprehensive statistical analysis is lacking to develop predictive models for the removal efficiency of EOCs (PhCs, PCPs, and SHs) based on design and operational parameters of CWs.
Moreover, research is needed on all aspects of modelling of EOCs removal in CWs [12]. The CWs models are broadly classified as process-based and black-box [[12], [13], [14]]. The widely studied process-based models of CWs are RTD/GPS-X [15], Diph-M [16], FITOVERT [17], HYDRUS-CW2D [18], HYDRUS-CWM1 [19], CWM1-RETRASO [20], CFD Model [21], and BIO-PORE [22]. These models mainly focus on the removal of conventional parameters such as chemical oxygen demand (COD), nitrogen, phosphorus, and ammonia. The process-based models focus on the hydraulic, chemical, and biological mechanisms occurring in CWs. While these models attempt to cover in detail the processes happening in CWs, most of them only include to model one or few processes (e.g., hydraulic, reactive-transport, biochemical, plants, and clogging), thus, partly representing several processes happening at the same times [[12], [13], [14]]. Including more processes in available CW models is still a big challenge because it increases complexity as well as the number of parameters to quantify, and lack comprehensive experimental data for calibration and validation. On the other hand, black-box models, which are data driven models, such as regression models, first-order models, time-dependent retardation model, tank-in-series model, Monod models, neural networks, and statistical approaches, mainly focus on input and output rather than processes [13]. Although these models are formed using experimental data, the application possibilities are limited to the range of data used in their development. Meyer et al. [14] suggested that all types of models (both process-based and black-box) are valuable, although some give more understanding of the scientific processes and some are more useful for engineers to design the CW systems. In general, there is a need to include EOCs in further development of CW models.
Therefore, the main objective of this study is to comprehensively analyze the possibility of developing reliable predictive models for removal efficiency of EOCs based on design and operational parameters of CWs. The specific objectives are: (1) to develop predictive models in the form of multiple linear regression equations between removal efficiency of EOCs (PhCs, PCPs, and SHs), and design and operational parameters of CWs; (2) to develop a generic predictive model for the removal efficiencies of PhCs, PCPs, and SHs based on design and operational parameters of CWs; (3) to examine the uncertainties in the prediction process; and (4) to discuss potential applications of the developed predictive models.
Section snippets
Data
In this study, the predictive models were formulated using data of widely studied EOCs (31 PhCs, 13 PCPs, and eight SHs) in CWs. The data were collected from the peer reviewed published sources. All the sources of data are acknowledged in the database compiled in our previous work [10,11,23], which formed the basis of this study. The removal efficiency is used as an independent variable to be predicted. The dependent variables were design and operational parameters: depth, area, HLR, OLR, and
Predictive models for PhCs
The first two principal components (PCs) with eigenvalue > 1.0 explained 70 % variance in the data of 31 widely studied PhCs (Fig. 1 and Table 1). The first PC could explain 49 % variance in the data. In PC1, the high positive loadings were observed for OLR, and high negative loadings for depth, area, and HLR (Fig. 2). In PC2, area indicated high positive loadings; however, HRT showed high negative loading (Fig. 2 and Table 1). The removal efficiency indicated positive loadings in these two
Further development and applications
The predictive models developed in this study could provide useful information in the decision-making process. For example, initial estimates of removal efficiencies could be generated using design and operational parameters of a CW system. Thus, the developed models could serve as useful screening tools to provide first-hand information on the expected removal of PhCs, PCPs, and SHs in CWs. Similarly, preliminary values of design and operational parameters can be generated from the proposed
Conclusions
The following conclusions are drawn from this study:
- 1
The design and operational parameters of CWs are good predictors of the removal efficiency of EOCs in CWs. Operational parameters, HLR, OLR, and HRT are the most significant predicators followed by design parameters (depth and area). Thus, plausible predictive models were formulated by combining these parameters, and acceptable models contained two-four of these parameters.
- 2
The removal efficiency of PhCs, PCPs, and SHs in CWs could be predicted
Declaration of Competing Interest
The authors report no declarations of interest.
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