Quantifying the log reduction of pathogenic microorganisms by constructed wetlands as a basis for QMRA of water reuse applications.
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Over the last 30 years, constructed wetlands (CWs) have been used as an alternative, cost-efficient way of treating wastewater, mainly in combination with conventional wastewater techniques. In the context of circular economy and water reuse applications, policymakers have to take precautionary measures regarding water safety, water supply and the acute effects of contamination from surface water. Consequently, water and wastewater companies in the Netherlands are obliged to perform Quantitative Microbial Risk Assessment (QMRA) every three years in order to monitor water quality, calculate the risk of infection, and improve water supply safety. Moreover, RIVM has developed a tool named QMRAspot to analyze and conduct QMRA for specific pathogens that can be found in water sources. KWR contributes to this effort by building a knowledge base regarding pathogen removal and providing RIVM with log removal values from various wastewater processes using another computational tool called Watershare Treatment Calculator. The process of constructed wetlands for wastewater treatment is currently missing from their knowledge base. The objective of this paper is to quantify the log removal of pathogens from constructed wetlands as a basis for QMRA of water reuse applications. In that way, it will contribute to the efforts of KWR for the development of a knowledge base. To do that, a systematic literature review for the creation of a complete dataset with all the necessary information regarding pathogen removal was performed. Furthermore, three predictive models (one for every type of CW) were developed in order to be incorporated into the Watershare treatment calculator. The models gave log removal values of pathogens based on specific parameters such as Hydraulic Retention Time (HRT) and Hydraulic Loading Rate (HLR) of CWs. Additionally, a storyboard for the Graphical User Interface (GUI) in the platform of the treatment calculator was created. Based on the systematic literature review, the average log removal for Free Water Surface, Subsurface Horizontal, and Vertical Flow CWs was 1.6, 1.7, and 2.12 log respectively suggesting that CWs can only be used for wastewater polishing when they are combined with conventional wastewater treatment systems. Moreover, all three types of CW showed great variability in log removal with ranges between 0.07-6.08 log, indicating that the complexity of these systems makes it difficult to draw robust conclusions on their performance. Contrary, the results of the predictive models were promising since the predictive models showed great residuals in terms of R2, RMSE, MAE, and the correlation between observed and predicted values. That means that predicted values of pathogen removal with high precision can be extracted. Overall, the created dataset, and predictive models can provide guidance to municipalities and water boards regarding the polishing ability of CWs and lay the foundations for a better understanding of the design and operational parameters of CWs since the decision makers are able to know the required values of HRT and HLR, in order to achieve a certain degree of pathogen removal from their CW.