4.1. Modelling of Technological, Natural and Operational Risks in the Domestic Wastewater Treatment System Based on Constructed Wetland “El Dorado”
The essential stages or sub-processes that were postulated in the process map of the domestic wastewater treatment system based on the CW are illustrated in
Figure 4.
From the analysis of the operation of each one of the stages, fifty-three initiating events were identified, two in the first stage, three in the second stage, and forty-eight in the third stage (
Table A1 in
Appendix A).
While initiating events associated with the first two stages have been fully explained, a list of 48 initiating events related to the CW has been summarized for the sake of document simplicity. Most of these events share similarities, with some specific characteristics setting them apart. For example, while many are related to plant biomass accumulation causing reversible obstruction, others relate to irreversible obstruction. Insufficient maintenance, a crucial aspect affecting the system’s performance, has been extensively examined in this study.
Insufficient maintenance often leads to anomalies in the waterproofing layer, allowing groundwater to enter the constructed wetland or allowing polluted water to drain into the groundwater. Anomalies in water levels and dikes can result from this lack of maintenance. Similar problems arise when there is insufficient maintenance of the vegetation cover, causing fluctuations in water levels and issues with the wetland dikes.
The accumulation of debris can attract rodent infestations, leading to damage in the waterproofing layer and problems with water levels, both high and low, as well as issues with the dikes. Suspended solids cause both reversible and irreversible obstruction, and the growth of microorganisms responsible for wastewater treatment can have the same effect, leading to blockages in the matrix pores.
Similar situations can also occur due to chemical precipitation and pore deposition. Inadequate maintenance, the improper regulation of the level control mechanism, and the incorrect use of sharp tools that lead to breaks in the waterproofing layer can result in high or low water levels, depending on the groundwater table at the wetland’s location. To develop the accidental sequences, the defenses, including barriers and frequency and consequence reducers, implemented in the current treatment system were identified (
Table A2 in
Appendix A).
Figure 5 shows one of the accidental sequences for the wetland, depicting the interplay between initiating event, defenses, and consequences. In the tree of events, once the high frequency initiator IE-HSC-002 (H) occurs, a decision node emerges. If barrier B-2 (VR-Very Robust) succeeds (leading upward), no consequences follow. However, if the barrier fails (leading downward), sequence 2 (@SEC2) concludes with medium-magnitude contamination (consequences C-CON (M)). Frequency reducers FR-1 and FR-3, characterized by soft robustness (S), are applicable for this initiator. There are no consequence reducers (CR) in this sequence.
The initiator IE-HSC-002(H) corresponds to the entry of fats and related substances (such as soap dish) into the system, leading to solidification and obstruction, resulting in the reflux of used water to the outside.
4.2. Evaluation of Technological, Natural, and Operational Risks in the Domestic Wastewater Treatment System Based on Constructed Wetland “El Dorado” Applying the Proposed Risk Model
Figure 6 displays the risk profile resulting from the application of the TDRM method to assess the technological, natural, and operational risks of the domestic wastewater treatment system based on a subsurface horizontal flow CW “El Dorado”.
Out of the fifty-three identified initiating events for accidental sequences, six were determined to be of very high risk (
Table 2), forty were of high risk, six were of moderate risk, and only one was of low risk. This allows for the classification of the studied system as a high-risk installation.
Besides the very high risks listed in
Table 2, the presence of 40 high risks is also a concern. To enhance risk control in the wetland, the implementation of new defenses, including frequency reducers, barriers, and consequence reducers were proposed (
Table 3). It is worth noting that these new proposals necessitate the regular presence of technical staff to manage the wetlands, a role that currently does not exist.
The simulation of how the implementation of the defenses listed in
Table 3 would affect the variation in risk levels for contamination in the impact area and the immediate area of the domestic wastewater treatment system based on CW “El Dorado” is depicted in
Figure 7.
As observed, the implementation of the proposed defenses reduces the risks to lower levels. Very high levels initiating events are no longer identified, and high-risk events decrease from forty to six, with an increase in moderate (from six to thirty-one) and low-risk events (from one to sixteen). Consequently, the system transitions from a high-risk to a moderate-risk installation.
Figure 8 provides a clearer visualization of this shift towards less significant risks resulting from the incorporation of defense measures presented in
Table 3. Notably, only six high risks would remain in this scenario, with the others falling into the moderate and low categories. Additionally, these histograms depict the distribution of consequences severity for each risk level. For example, in
Figure 6, among the six high risks displayed, three entail very serious consequences and thus demand the most attention.
Table 4 presents the initiating events of the six accidental sequences identified as high risks in the study after the implementation of new defenses.
As part of the analysis, an assessment of the significance of defenses was conducted. This assessment was conducted on an improved version of the “El Dorado” domestic wastewater treatment system based on CW. One approach for evaluating the importance of defenses is to determine their percentage participation in accidental sequences. In this case, we focused on barriers and identified those with the highest participation in the accidental sequences, as illustrated in
Figure 9.
The three barriers with the highest percentage participation turned out to be the following:
B-17 Periodic checking of the facility records to detect deviations from the parameters established for the proper functioning of the wetland.
B-6 Periodic inspection to verify the status and operation of the wetland, the status of the surrounding areas, and the completion of the corresponding records.
B-18 Existence of a reserve cell (functional redundancy).
However, this measure of importance does not account for the impact of additional redundant barriers within each sequence. A barrier may participate in multiple sequences but have minimal significance because it is supported by others. To address this, a complementary study was carried out using the measure of importance called “increased risks when defenses disappear”.
Figure 10 displays the results of the risk assessment study when removing barriers, indicating the number of accidental sequences that experience increased risk when a specific barrier is eliminated. Similarly,
Figure 11 pertains to the study involving the removal of frequency reducers, while
Figure 11 addresses the removal of consequence reducers.
The first study confirmed the importance of barriers B-6, B-17, and B-18, as previously identified by the percentage method. In this case, there were no significant differences, given the low redundancy of barriers in the analyzed sequences. However, it is worth noting that the order of importance changed between the percentage participation and increased risks for subsequent barriers. Therefore, more attention should be given to barriers B-3 (Parshall channel to monitor the flow rate at the system’s output) and B-21 (weekly inspection to verify the wetland’s status and operation, along with record-keeping), which are considered insignificant in the percentage representation.
The second study, shown in
Figure 11, allowed the identification that the most important frequency reducers are as follows:
FR-9 Periodic check of the facility’s records to detect breaches of maintenance, training, and inspection plans.
FR-11 Training of workers who attend the installation and filling out the corresponding record.
Finally, the third study showed that the most important consequence reducers identified were the following (
Figure 12):
CR-1 Replant the vegetation coverage and remove dead plants, if they exist.
CR-13 Repair the waterproofing layer.
Through this study, the most crucial defenses that need to be implemented to control the risk of the “El Dorado” treatment system were identified.
As far as we know, there has not been any study of the risk management of wastewater treatment plants based on constructed wetlands. Nevertheless, recently, Analouei et al. have performed a risk assessment of a wastewater treatment and reclamation plant not fulfilling the effluent requirements by applying the bow-tie method [
25] and the dynamic Bayesian network [
26]. However, these proposed methodologies are more complex than the Three-Dimensional Risk Matrix. On the other hand, they concluded that human errors were responsible for the most risks in WWTP failure, while in the wastewater treatment plants based on constructed wetlands with all the new defenses implemented, the high risks identified were mostly related with natural processes.
4.3. Discussion on the Proposed Model for Risk Management in Domestic Wastewater Treatment Systems Based on Constructed Wetland
The Federal Roundtable on Remedial Technologies (FRTR) recognizes that the long-term effectiveness of constructed wetlands in containing or treating certain contaminants is not well understood [
27]. This publication also emphasizes that, like other biological methods, constructed wetlands are constrained by the biota’s ability to withstand exposure to its environmental factors, such as weather events, wildlife, and pollutant concentrations. The challenges in establishing these systems align with the findings in [
28]. These situations were considered in the risk model designed for the wetland in this investigation.
Another issue highlighted in the reference [
27] is the limitation of remediation for metals in constructed wetlands. Wetlands do not destroy metals; instead, they restrict their mobility through sorption or plant bioaccumulation. In the context of this investigation, the treatment is specifically designed for domestic wastewater, so the situation described earlier is not anticipated. It is important to avoid using exotic and invasive species when developing a CW, and a plan should be in place for their removal if they appear [
25]. This aspect is taken into account in the proposed risk model, which is designed based on defenses implemented through an appropriate wetland vegetation cover maintenance policy.
Another aspect to consider is the control measures suggested by [
27], which must be implemented in the wetland and that were included as part of the defense measures in the model [
27,
29]:
Maintenance of any water distribution system, including clogging prevention;
Frequency and type of vegetation monitoring;
Need for vegetation management, odor control, and pest control;
Frequency and scope of monitoring of conventional parameters and contaminants.
On the other hand, the US Environmental Protection Agency notes that using constructed wetlands as a treatment technology entails certain risk for several reasons [
30]. Firstly, constructed wetlands are not uniformly accepted by all state regulators. Some authorities encourage their use based on misconceptions regarding their proven efficiency, simultaneous aerobic and anaerobic treatment capacity, and their potential for oxygen and phosphorus treatment due to insufficient modeling data. In this regard, this research considers the wetland as a proven treatment technology for the pollutants that require treatment, as supported by previous experimental research [
31]. Secondly, although there is no evidence of harm to the environment associated with the use of CW, some regulators have raised significant concerns about these systems attracting wildlife. Unfortunately, there has been limited research on the potential risks to wildlife when constructed wetlands are used. Despite their distinct habitat type, there is also a lack of evidence regarding wildlife risks in treatment pond systems. The risk model developed in this research does not specifically address harm to wildlife. However, it does recognize that such interactions can act as initiators affecting the wetland’s operation.
Finally, the absence of a substantial body of scientifically validated data makes the design process complex. It relies primarily on empirical observations rather than scientific theories. Because many factors in CW, such as climatic effects, influent wastewater characteristics, design configurations, construction techniques, and operation and maintenance practices, exhibit variability, disagreements may persist regarding certain design issues and performance over time. Coincidentally, this research aims to comprehensively address various aspects of risk that remain to be investigated. It employs a multidimensional model that incorporates multiple variables related to risk and the factors listed. This provides the model with a unique characteristic, allowing us to examine risks from a simulation perspective. Through this approach, we can execute tests involving changes in the configuration of inputs, simulating alterations in risk-related variables.
The US Environmental Protection Agency’s reference document, “EPA, Risk Assessment, 2000” [
32], outlines a sequence of activities for conducting risk assessments related to human health from environmental effects. The stages considered are as follows:
- -
Harm identification: to identify the types of adverse health effects that may be caused by exposure to any agent(s) in question, and to characterize the quality and weight of evidence supporting this identification.
- -
Dose response to document the relationship between dose and toxic effect. A dose-response relationship describes how the probability and severity of adverse health effects (responses) are related to the amount and conditions of exposure to an agent.
- -
Exposure assessment: the process of measuring or estimating the magnitude, frequency, and duration of human exposure to an agent in the environment.
- -
Risk characterization: to summarize and integrate the information from the previous steps of the risk assessment to synthesize an overall conclusion about the risk.
It should be clarified that the previous analysis combines deterministic and probabilistic approaches, to which experimental results are added. While there are similarities, it’s important to emphasize that the developed risk model primarily follows a probabilistic approach. In the EPA methodology, the identification of harm involves explicit studies of toxicokinetics and toxicodynamics, which analyze how damages occur in interaction with humans. All of this research is incorporated into the risk model developed for the domestic wastewater treatment system based on a constructed wetland, particularly in the determination of consequences.
The questions related to dose response are implicit in the descriptive definition of the model concerning accidental consequences. The results are reflected in the varying magnitudes of modeled consequences, with probabilities of harm established based on the magnitude of the doses received. To define these consequences, the corresponding step as described in the EPA methodology must have been completed. It is worth noting that the relationship between dose and the probability of harm may require experimental data, possibly involving animal studies, in the absence of human data. All of these aspects are implicitly incorporated into the definition of consequences within the designed risk framework in this research. Exposure assessment is achieved by modeling various scenarios (accidental sequences), which illustrate different risk exposure situations. This modeling allows us to determine the magnitude and duration of exposure (grouped as consequences) as well as frequency (captured by the parameter of the same name associated with the initiating event). Notably, the method employed in this research incorporates management capabilities through the inclusion of defenses within the model.
The convolution of the results of the three previous stages makes it possible to calculate the individual risk, as well as the collective risk. The manner in which results are presented here differs from the approach taken in the probabilistic risk model of this research. These are two distinct risk assessment methodologies, each with its unique perspective on risk. It’s worth noting that once the probabilistic risk pattern of the wetland has been established, obtaining its results is a relatively quick process, which underscores the model’s practicality.
Wu, Gao, Wu, Zhu, Xiong, and Ye [
33] argue that the risk of groundwater contamination increases in areas where constructed wetlands are utilized. They suggest considering the use of waterproofing layers to protect groundwater and strengthening management. These issues are considered among the defenses suggested by the risk model developed.
Finally, many of the references consulted deal with cases of wetlands built for the treatment of pollutants other than domestic ones [
34,
35]. It should be clarified that these wetlands necessitate specific risk models tailored to their required treatment capacities. These scenarios are beyond the scope of the risk model for the treatment system based on a constructed wetland presented in this research.
Current environmental regulations in the US demand a level of detail that has not been applied to the studied wetland. However, if it becomes necessary to seek authorization using a method similar to the one discussed in this work, it is advisable to establish strict controls for very high risks as a guideline regarding risk levels. These risk levels can be achieved through the additional defense measures proposed in the previous section (refer to the improved case). This approach is optimistic and draws inspiration from the field of radiation safety [
13], where prolonged exposure to high risks is not permitted. Considering the challenges associated with achieving such a goal for facilities of this nature, it is recommended to focus solely on prohibiting very high risks within the scope of this issue.
While matrix methods for estimating risk have been used in water treatment facilities (see references [
2,
3,
15]), they have not been widely applied to treatment facilities using constructed wetlands. One limitation of the two-dimensional approach in the risk matrix, as seen in these references, is its lack of systematicity in studying the impact of defenses. This study employs a three-dimensional risk matrix, addressing this limitation and distinguishing it from the approach found in the international standard ISO-31000 [
34], explicitly dedicated to risk management but similarly limited when considering defenses.
The use of TDRM in this investigation implicitly integrates defenses into the prioritization of the most critical sequences, enhancing the objectivity of the analysis, which has not been achieved in similar studies [
2,
3,
15,
36]. Moreover, this method identifies the most effective defense measures for controlling technological risks in the wetland. The application of this approach to the results reveals that specialized defense measures should be prioritized for very high-risk accident sequences. These sequences, as indicated by the results, primarily result from inadequate maintenance and surveillance (IE-CW-04, IE-CW-019, IE-CW-048), along with insufficient operational policies (IE-CW-023, IE-CW-031, IE-CW-032).
The measures recommended in
Table 3 effectively reduce the risk to lower levels, demonstrating their efficacy. Among these measures, those depicted in
Figure 9,
Figure 10 and
Figure 11 should be highlighted. These measures primarily address operational issues (B-17: Periodic checking of the facility records to detect deviations from the parameters established for the proper functioning of the wetland, B-6: Periodic inspection to verify the status and operation of the wetland, the status of the surrounding areas, and the completion of the corresponding records, FR-9: Periodic check of the facility’s records to detect breaches of maintenance, training, and inspection plans, CR-1: Replant the vegetation coverage and remove dead plants, if they exist), maintenance (CR-1: Replant the vegetation coverage and remove dead plants, if they exist), installation design (B-18: Existence of a reserve cell), and personnel training (FR-11: Training of workers who attend the installation and filling out the corresponding record).
Finally, we want to express how our research contributes to sustainability. Constructed wetlands represent a nature-based solution for water treatment, and this paper’s risk assessment methodology, particularly the three-dimensional risk matrix approach, offers a novel and practical means to enhance the sustainability of such systems. By identifying the most critical initiating events and defenses, with a focus on human factors, the research highlights the vulnerabilities within constructed wetland operation. The suggested defense measures and the transition from a high-risk to a moderate-risk facility demonstrate a path towards improving the reliability and effectiveness of wastewater treatment. This, in turn, contributes to environmental sustainability by reducing the potential risks and impacts associated with the discharge of inadequately treated wastewater, ultimately safeguarding water quality and ecosystem health. Moreover, this paper introduces the concept of using risk analysis as a tool for optimizing the operation of wastewater treatment technology based on constructed wetlands. This innovative approach can lead to more efficient and cost-effective water treatment processes, contributing to the sustainability of water resources and the preservation of natural ecosystems. In conclusion, the research offers a valuable framework for enhancing the sustainability of wastewater treatment systems based on constructed wetlands, addressing an urgent problem of humanity while emphasizing the role of nature-based solutions in environmental stewardship and open new directions for further research on the risk analysis of constructed wetlands for wastewater treatment and water purification.