Investigating Metrics for Wastewater Treatment Resilience

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Abstract

Wastewater Treatment Plants (WWTPs) play a critical role in society, and their susceptibility to the adverse effects of climate change jeopardizes the sanitation and health of people in their communities. The impacts of adverse climate situations on the stability of WWTPs are plentiful, underscoring the need for WWTP resilience. Therefore, the focus of this study involved assessing the consequences of climate change on the functioning of WWTPs. In addition to expounding the medium and high consequences climate change on WWTPs, the study also focused investigating the potential metrics that could form the basis of measuring the resilience of these plants to climate change impacts. the study adopted a systematic review methodology to achieve the stated objectives. The primary impacts underscored in the study include nuisance flooding odor and spills, damage to infrastructure, and deterioration of water quality as a result of increased uncontrolled discharges.

Investigating Metrics for Wastewater Treatment Resilience

1. Introduction

Wastewater Treatment Plants (WWTPs), especially in urban areas, provide an important service to individuals in their communities. Over the last 150 years, the expansion WWTPs has contributed to significant improvements in public health outcomes, with notable reductions in disease outbreaks and mortality, together with reductions in environmental pollution (Hughes et al. 2020, p. 1). Despite the existing improvements, WWTPs, like all engineered systems, can underperform or malfunction when certain conditions stress them beyond their functional conditions or design thresholds. Although the failure of these plants can stem from internal factors, such as poor conditions of materials, it is majorly caused by external hazard events associated with climate change, such as heavy rainfall events, extreme temperature increases, and rise in sea levels (Hughes et al. 2020, p. 1). Resulting disruptions can cause wide-ranging serious implications to public health and receiving environments.

Climatic changes generate multiple adverse outcomes that may be damaging to wastewater systems. Some of the effects of climate change include increased dry spells and temperatures, sea-level rise, intense storms, frequent and extreme rainfall events, and intense and prolonged westerly winds causing heavier and frequent ocean or coastal swells (Lawrence et al., 2020). The specific hazards eventually accumulate and spread across essential services in communities (Lawrence et al., 2020). The increased flooding, drought, coastal erosion, and higher groundwater levels generate consequential impacts on critical infrastructure, the built environment, and on economic and activities. Evidence shows that compound hazards, especially riverine flooding and extreme coastal waters, exacerbate flooding, leading to an increase in the vulnerability of communities and lowland coastal assets (Moftakhari et al., 2017). For instance, in the United States, sewage contamination resulted from joint sewer overflows that occurred after wet weather events (Olds et al., 2018). When these events occur, they cause extensive damage to WWTP systems, widespread contamination, and the pollution of drinking water.

Given the rapidly increasing impacts of climate change on the world’s built environment, analyzing these impacts and understanding how to promote the resilience of WWTPs to the shocks of climate hazards is becoming necessary.  Impact assessments are key in the processes of developing resilient WWTPs that can survive the shocks associated with climate change. To date, despite the importance of WWTPs, only limited studies have been carried out on how climate change affects their performance and how what metrics can be used to determine the resilience of these systems (Abdulla and Farahat 2020; Evans 2012). Most of the existing studies focus on issues related to sewer operations (Yang, Cicek, and Ilg, 2006). Also, various metrics have been developed are used to form the basis for measuring the resilience of WWTPs to shocks emanating from climate change, and the development of these metrics continues to be an active field of study. Unfortunately, there is limited guidance regarding which metric is most appropriate for measuring the resilience of WWTPs in specific situations.

Therefore, this study conducts a systematic review of the specific impacts of climate change on the performance of WWTPs and investigate the potential metrics that could form the basis for measuring the resilience of these plants. Specifically, the study focused on determining the most suitable software for investigating metrics for wastewater treatment resilience. The study compares the functioning of different software in different areas, such as data input, data output, their merits and demerits in the simulation WWTPs in different extreme climate situations, user friendliness, pricing, and other important variables. The software evaluated in the study include ASIM, AQUASIM, BioWin, GPS-X, SIMBA, STOAT, WEST, and EFOR. The evaluated of this software is based on the initial findings on the specific impacts of extreme climate situations for the performance of WWTPs. By achieving the primary objective, the study will recommend a comprehensive cost-effective, reliable, and sustainable software that can be used in evaluating the resilience of WWTPs to climate change shocks in different situations.

2. Aim and Objectives

The focus of a systematic review is on generating evidence that can be used to support practices and policies. The process adopts categorical, transparent, and systematic approaches in the appraisal and synthesis of previous empirical outcomes (Gough et al., 2013). In developing a key objective, a systemic review provides a defined methodology that can be applied in the scrutiny of past research and identification of areas that may require further attention. In this study, a systematic review was used to address the following main objectives;

1. To explore the various impacts of climate change on the performance of WWTPs.

1. To investigate the potential metrics that could form the basis of measuring the resilience of wastewater treatment plants to shocks emanating from climate change.

The systematic review focuses on finding the most suitable software for investigating metrics for wastewater treatment resilience. It adopts a review process in comparing the different aspects of software, such as the input data and output data, and analyzing their advantages and disadvantages in simulation WWTPs, especially in different extreme climate situations. Previous studies on resilience primarily focused on the use of qualitative methods (Karamouz 2016). Others were based on system characteristics (Yazdani et al. 2011). On the contrary, some studies argued that the resilience of system properties does not depict the resilience of performance. This means that resilience analysis needs to be focused on performance and not on the properties of a system (Butler et. al., 2017). The current study considered the user-friendliness of every software, the price, and other variables that determine their effectiveness. The reviewed software includes ASIM, AQUASIM, BioWin, GPS-X, SIMBA, STOAT, WEST, and EFOR.  Addressing the objectives of the study through a critical review of empirical studies helps advance in understanding how the resilience of WWTPs can be ensured, especially given their criticality in promoting human and environmental sustainability in the face of climate change.

3. Literature Review

This section reviews the existing studies related to WWTPs, impacts of climate change on the functioning of these plants, the concept of resilience in the context of climate change, and the different computer software used in the implementation of models and subsequently predicting the performance of WWTPs.

3.1 Impact of Climate Change on Wastewater Treatment Plants (WWTPs)

Wastewaters containing biological, physical, and chemical pollutants can be released from different commercial, industrial, domestic, and agricultural activities that humans indulge in as they strive to meet their needs. Wastewater treatment plants (WWTP) stand out as some of the key strategic infrastructures that help in the removal of multiple contaminants from wastewater, thus supporting environmental protection and human health (Panico et. al, 2013). However, managing these plants is a major problem worldwide because of the various associated issues, especially the unanticipated adverse weather conditions and the ever-increasing natural hazards (Fernández-Arévalo 2017; Olyaei et al. 2018). It is expected that in the future, threats stemming from adverse weather conditions will increase as a result of human activities on the environment and will negatively affect the management of WWTPs.

The prevalence of natural calamities, such as hurricanes, floods, and rises in sea levels, have led to an increase in concerns regarding the functionality of engineering infrastructure (Tung et. al., 2006). In 2002, the United Nations Environment Program (UNEP) raised a concern that untreated agricultural, industrial, and domestic wastewater severely polluted approximately half of lakes, rivers, and coastal water bodies worldwide (UNEP, 2002). Nearly two decades later, today, the problem of wastewater continues to persist due to the scathing effects of climate change, especially as the world continues globalizing and industrializing. WWTPs are developed to help in addressing the growing problem of wastewater, although the persistence of extreme weather conditions associated with climate change continues to not only compound the existing problems but also impeded the performance of WWTPs.

Also, increased urbanization resulting from the heightened economic development culminates in the degradation of ecosystems, such as deterioration of water quality, depletion of water resources, and loss of habitats. These problems are particularly prevalent in developing nations that are witnessing widespread economic transformation and globalization (Seto et al., 2012; Fezzi et al., 2015). The deterioration of water quality is of significant concern because it also causes unsustainable farmland and arable land uses, especially in most coastal regions worldwide (Li, Li, and Wu 2016, p. 1). Nevertheless, generalizing the quantitative impacts of climate change on water quality is difficult because of the complex interactions of ecological processes involved.

The processes that occur in WWTPs are significantly affected by climate change, especially extreme weather events like floods, which often generate more untreated sewer overflows (Abdulla and Farahat 2020). Extended drought conditions often reduce water, potentially affecting how WWTPs perform their designated roles. Further, temperature plays a pivotal role in both non-mechanized and natural-based treatment processes. For instance, warm temperatures have the effect of increasing removal efficiencies and ensuring that some treatment processes are feasible for use. Besides, some WWT processes, especially anaerobic reactors, may be used for diluted wastewater, like domestic sewage, only in areas with warm climates (Vo et al. 2014). Other WWT processes like stabilization ponds, may only be utilized in regions with lower temperatures, but their performance decreases significantly during winter. Also, processes like aerobic biofilm reactors and activated sludge do not largely depend on temperature because they have an enhanced mechanization level and technological input (Augustos de Lemos Chernicharo and Von Sperling 2005).

When storms increase, flooding increases, and this can potentially harm the infrastructure, especially when WWTPs are located in coastal regions or in regions where river floods are common (Abdulla and Farahat 2020). Moreover, an increase in the intensity and frequency of rainfall is one of the major immediate impacts of global warming that is currently considered to be a perennial problem for the last several years. It is expected that when the storms are severe, more severe flooding will occur. This will unavoidably lead to additional pollution of water from different sources, especially wastewater treatment (Vo et al. 2014).

The primary processes in WWTPs whose functioning is impeded by climate change include warm water biological aeration, sedimentation, waste sludge processing, chlorination, and stabilization ponds (Abdulla and Farahat 2020). According to Tolkou and Zouloulis (2016), the wastewater sector is implementing different interventions to deal with the key obstacles resulting from climate change. Some of the interventions include developing regulatory frameworks, decreasing emissions, and implementing the facilitate adaption to the drastically changing climate. Also, Plósz et al. (2009) characterized climate change-related impacts on WTTP processes as shock-conditions, that is, substantial changes in the boundary conditions of a system, happening in a relatively short period. The authors also indicated that when the influent flow rate increased during the flooding period, the hydraulic retention time in the WTTP system can decrease, leading to the deterioration of biological nitrogen elimination processes, and increment of the TN concentration in the effluent.

Given the rapidly increasing impacts of climate change on the world’s built environment, understanding the type of infrastructure that will likely be vulnerable and exposed to climate hazards is becoming necessary. In this case, vulnerability, hazard, and exposure are all overlapping factors that combine to create climate change-associated risk. Often, risk manifests in the form of impacts, which potentially cause major implications across economic, social, environmental, and cultural domains. Impacts refer to the effects of climatic changes on human and natural systems, and they involve extreme climate events, weather conditions, and overall climate change (Hughes et al. 2020, p. 2). Although there are various wastewater systems susceptible to the adverse impacts of climate change, WWTPs often suffer the greatest impacts, underscoring the need for resilience of these facilities.

3.2 Resilience of Wastewater Treatment Plants to Climate Change

Resilience is an important concept in many areas, such as ecology, sociology, and engineering technology. It can be traced back to the discussion on the resilience and stability of ecosystems by Canadian system ecologists in the late 1960s (Holling, et al. 1973). Initially, resilience is main to describe the persistence or plasticity of a natural system in the face of changes in external natural elements and human factors. There are two different definitions of resilience, first one is mainly concern with stability, its properties are quantified by its immunity from interference and the speed of back to the original equilibrium state. On the other hand, another definition is focused on instability, which could be illustrated as the conditions required to trigger a system to convert from one state to another, the magnitude of the disturbance that causes the abrupt change in the equilibrium state of the system could be used to measure its properties. (Pimm,1984) The research system of resilience in ecology includes four aspects namely, the latitude of the system from an equilibrium state to loss of resilience ability, the system’s resistance to external force disturbance to maintain an equilibrium state, the precariousness of the current state of the system approaching the critical value of collapse, and the relevance (Panarchy) of internal component levels of the disturbed system (Gunderson, 2000).

However, with the development of the resilience theory, people find that resilience could be widely adopted in various fields (Folke, 2006.), and it has different definitions in interdisciplinary discourse. In the area of engineering technology, the concept, definition, and theory of resilience are more concise and intuitive than that of ecological systems. The first definition of resilience in ecology resembles that in engineering, which is both describe stability, but because of the uniqueness of equilibrium in engineering and technology, there is barely multiple transitions and jumps of the equilibrium status, as a result, the second definition of resilience in ecology is no longer adopted in engineering. Besides, resilience is also defined as: “the capacity of a system to absorb disturbance and re-organize while changing to still retain essentially the same function, structure, identity and feedbacks” in the social-ecological systems field (Brian Walker et al, 2004).

According to Bruneau et al. (2013), a resilient system is one with reduced failure probabilities and minimized consequences from failures. In case a system fails or suffers from any form of disturbance, it has a reduced recovery time. It is primarily characterized by rapidity, robustness, resourcefulness, and redundancy (Bruneau et al., 2013). One of the most comprehensive definitions of resilience was offered by Butler et al. (2014). According to the author, resilience can be understood as the extent to which the system lessens the duration and magnitude of service failures when subjected to exceptional conditions. Resilience is also categorized into two extensive types. The first one is attribute-based or general resilience, which denotes the condition of the system that enables it to minimize failure magnitude and duration in the face of all types of threats. The second broad type is performance-based or specified resilience, which refers to the agreed performance of the system in reducing failure duration and magnitude to an identified threat (Butler et al. 2014).

To understand the application of resilience in engineered systems, four fundamental elements should be involved, which are stressors, properties, metrics, and interventions (Juan-García et al, 2017). Stressors are the pressure of the system, which is caused by human activities or nature, stressors can influence the performance of the system by affecting the system’s variables. There are two kinds of stressors, chronic stressors, and acute stressors. It is usually recurrent and can often be estimated for chronic stressors, like urbanization and ageing of infrastructure. On the contrary, the acute stressors are hard to predict, it is infrequent and can cause devastating consequences. Several properties are possessed by the resilient engineered system, include robustness, redundancy, and flexibility, which can be considered as indicators of resilience. Recent work on resilience shows that metrics to count resilience are limited without a framework to lead (Hosseini et al, 2016). Furthermore, the resilience definition adopted in a published framework (Tran, 2017) is: “the ability to prepare and plan for, absorb, recover from, and more successfully adapt to adverse events”. In this framework, investigators aim to consider the properties and stressors over the whole life of systems to evaluate the resilience of the system.

Recently, resilience has been taken more consideration as the impact of natural disasters, urban hazards, and uncertainty associated with climate change, globalization as well as urbanization, also, practical guidance on resilience’s demand increases dramatically in numerous areas, such as environmental management and urban planning as well as climate adaptation and disaster risk reduction. However, more studies are theoretical, the practical works on resilience are rare. There is a gap between the characteristics of elasticity explained by theory and how practitioners or decision-makers view its practical applicability. Kerner and Thomas (2014) believe that stakeholders of the system must first understand the local socio-ecological system, coordinate and collaborate within the system, and build resilience across the board.

The resilience of WWTPs has been explored more under the context of urban resilience rather than wastewater or water itself. The Urban Resilience has been present in the annual meeting of the Ecological Society of America (ESA) in 2002, though it has been discussed for almost 20 years, the detailed scientific definition is still not widely agreed upon because of the high complexity inside the urban system and the diversity of external disturbance factors (Meerow and Stults, 2016). There are some attractive and constructive definitions. For example, the City Resilience Index (CRI) (Arup, 2014) defined: “the capacity of cities to function, so that the people living and working in cities—particularly the poor and vulnerable—survive and thrive no matter what stresses or shocks they encounter.” Meanwhile, some of these definitions have been adopted by authorities, for instance, the Welsh Water Resilience Framework defines resilience in the water sector as ‘the capacity of individuals, communities, institutions, businesses, and systems to survive, adapt, and grow no matter what kinds of chronic stresses and acute shocks they experience’.

Moreover, The WHO 2030 vision, IPCC 2001, and the UN-Water 2010 all came up with the importance and the specific definitions of water resilience (CWRA,2019). However, water management, namely integrated water resources management (IWRM) and approaches of adaptive management, has largely failed to give priority to resilient thinking and planning. In addition, traditional water resource management approaches regard social systems and ecosystems as respective entities, which largely leads to the failure to achieve effective, equitable, and sustainable outcomes (Bohensky, 2006). Instead, understanding and addressing the biophysical and social components is critical. To make resilience more practical and acceptable for society, the discussion of economy and assets cannot be ignored. More specific frameworks and guidelines should be proposed. In academia, several studies provide a framework or guideline towards one or more resilience crucial factors mentioned above. First of all, stressors have to be defined accurately, resilience proposed an index for the performance of wastewater treatment, and a pressure source identification method based on realistic modeling was introduced in the framework stated by Cuppens et al. (2012).

Then, resilient performance is needed, which is key to obtaining a comprehensive assessment. The second factor is the definition of the system attributes. It will also require the best efforts of all stakeholders to reach an agreement. In this connection, Butler et al. (2014) proposed a new framework for urban water management, it adopts resilience as the main tool and discussing the quality of resilience of systems. Next, establish metrics that measure system performance and relate them to system properties. Although indicators for the wastewater industry were not specifically studied, Francis and Bekera (2014) made a comprehensive recommendation that also included stakeholder involvement and uncertainty assessment. The final critical point is to provide guidance for benchmark interventions to improve resilience. There is no guideline to discuss how to decide the specific interventions using as benchmarks and the characteristics considered in various cases. (Juan-García et al, 2017).

The study of Scott et al. (2012) and Gersonius et al. (2013) illustrated that resilience was incorporated into scenario planning of WWTP as well. The first is the forerunner of resilience theory because it notes measures for better wastewater management within the range of resilience (scenario planning); The definition here is the ability to completely reduced and gradually recover from potentially disastrous internal or external perturbations, with a view to two characteristics: robustness as well as rapidity, also including reliability measurements. The second approach demonstrates a technique that deals with infrastructure uncertainty at the management level called Real In Option (RIO) analysis as a method of identifying an optimal set of adaptation tactics to improve resilience within climate change. Finally, Xue et al. (2015) regard resiliency as the main part of evaluating the sustainability of a system and emphasize the irregular resilient metrics.

As for industry and government, resilience has been taken part in the consideration of their infrastructure asset management. For example, after the destructive disaster of Hurricane Sandy in 2012, the NYC Mayor’s Office of Recovery and Resiliency (2013) was established. These frameworks and guidelines remain general and do not provide enough detail to put into practice. It also finds that the future needs to put resilience into effect in the water sector and integrate it into the wider infrastructure management framework. In 2015, the UK Water Economy Regulator (Ofwat) published two reports incorporating resilience assessments (Ofwat, 2015a, 2015b). These two reports mainly identified the role of resilience in wastewater from a supplier and regulatory perspective, explain how to implement, assess resilience and regulate it as service providers. Then the UKWIR foundation has also published several reports to support the development of business plans for water companies by helping to introduce good practice ideas and methods through flexible planning. The five properties defined are Resistance, Reliability, Redundancy, Response, and Recovery respectively.

Global platforms have begun to recognize the need to shift the focus to flexible thinking and planning. One of these projects is the Global Partnership for Drought-Resistance (GRP), which works with several agencies/organizations including the Standing Committee for Interstate Drought-Resistance in the Sahel (CLISS), the Department for International Development (DFID), and so forth. Resilience provides a new dimension for people to understand and pursue the sustainability of complex socio-economic systems (SESS) (Tai, 2015). Though the studies and frameworks are being completed, the governance of wastewater resilience or water resilience is also uneasy. Lacking incentives for stakeholders, fragmented institutions, leadership, and political willing absence would become the barriers of effective governance of water resources, besides, high cost and insignificant returns would also be negative for promotion of water resilience management (Abell, R., et al, 2017). To address these problems, Butler et al. (2017) stress that the properties of a resilient or sustainable system must be distinguished from the performance that enables it.

Over the years, scholars have adopted different forms of governance, ranging from the traditional, state-centric, hierarchical approach to problem-solving to goodness with attributes such as openness, efficiency, rule of law, fairness, transparency, accountability, broad participation, decentralization, and consultation. Other levels of governance have also emerged in theoretical research, such as new governance (society-centered, market-based rationality, economic incentives, multi-level, multi-participant arrangement, corporate social responsibility approach, code of conduct) (Steurer, 2011). The most widely discussed version of governance in the resilience literature is adaptive governance. Many scholars use adaptive governance as a model and strategy to enhance the adaptability, convertibility, and flexibility of SESS to build resilience and cope with uncertainty and complexity (Berks et al, 2017). The use of the term “adaptive governance” first showed in the work of Dietz et al. (2003) and later studies believe that adaptive governance promotes the flexibility and resilience of the management system to deal with uncertain events. (Arnold et al. 2017) emphasize that the role of adaptive governance is not only to manage resources and the environment but also to have the ability to adapt to change.

If one or more properties are missing or weak, this can lead to bottlenecks or gaps of governance. The way to achieving resilience through the realization of governance attributes is hard, including challenges such as existing policies, a country’s political dynamics, and limitations in prediction (Menard, Jimenez and Tropp, 2018). No single system of governance can guarantee success in dealing with complexity and uncertainty. While majority of the existing literature on resilience governance takes a pro-democratic stance, the further investigation of the impact of regime types, or the trade-offs between open and closed political systems in resilience building is needed (Fraser and Kerby County, 2017). Through social legitimacy, combining scientific accuracy with social components can enhance the management approach in dealing with uncertainty. This is done by understanding the governance process (Cosen and Williams, 2012., Cosen and Stow, 2014). In addition, governance contributes to understanding the dynamics of ecosystems, introducing feedback elements, enhancing the resilience of natural systems and the ability of social systems to respond to ecological problems by seeking to restore the ecosystem (Cosens, 2014).

 

3.3 Wastewater Treatment Plant Modelling Simulation Software

The most commonly used wastewater treatment technology is the Activated Sludge Process (ASP) technology because it is deemed to be highly flexible, allowing designers to adopt it in all wastewater systems. It is also considered to be cost-effective, and it can produce high effluent quality that adheres to the gradually stringent effluent standards (Arif, Sorour, and Aly 2018, p. 637). In the ASP, the degradation and removal of pollutants in WWTPs are done by microorganisms. Therefore, the ASP is considered one of the most widely used wastewater treatment technologies in the world.

However, because the WWT system is complicated, multivariable, unstable, and time-varying, there are many factors of disturbance and uncertainty in the system. To better analyze and study WWTPs, the simulation method based on a mathematical model and combined with the principle of WWT technology is often considered. In the 1950s, the traditional static models of WWT have been invented and used, for example, the Eckenfelder Model, Mc Kinney Model, and the Lawrence-McCarty Model (Chi Chunrong et al.). These models are easy to use, but their drawbacks were also obvious. For instance, they only considered the relationship between microbial growth and substrate concentration while the system is in a steady-state operation condition. As a result, problems would come up when these models are applied in a practical system. The traditional models cannot explain the phenomenon of rapid removal of organic matter at the initial stage of removal.

Over the years, changes at the operational and design conditions of conventional WWTPs have been crafted to meet increasingly rigorous performance demands (Henze et al 2008). As a result, numerous variations of the traditional WWTPs have been developed to improve system performance through the medication of reactor layouts, aeration systems, operational conditions, and influent patterns. To solve traditionally existing problems, dynamic models based on activated sludge kinetics theory have been developed. There are mainly three kinds of it, namely the Andrews Model (Andrews, 1974), WRC model, and ASM family models. Currently, ASM models are the most commonly used models for the simulation of WWT. The International Water Association (IWA) officially released the Activated Sludge Model 1 (ASM1) in 1987. ASM1 use a matrix to describe the hydrolysis of organic matter, the growth and decay of microorganisms, and other reactions in the aerobic and anoxic conditions in the wastewater treatment process, but its main defect is that it does not include the removal process of phosphorus (Henze et al. 1987b).

Later in 1995, the ASM2 model has been published, which includes the anaerobic reaction, anoxic reaction and aerobic hydrolysis of organic matter, the polyphosphate storage of phosphorous accumulating bacteria, the growth of phosphorous accumulating bacteria, the growth of nitrifying bacteria and denitrification, and other reaction processes (Gujer et al. 1995). The ASM3 model was released in 1999, the expert group fixed several problems of ASM1. ASM3 emphasizes the internal activity process of microbial cells, and instead of focusing on the hydrolysis of organic matter, it introduces the storage and endogenous respiration process of organic matter in microorganisms. The main processes in ASM3 and ASM1 are the same, such as organic matter removal and biological (Gujer et al. 1999).

In practical application, the simulation of the whole system is not always necessary, so some improved models based on the ASM family are developed. For example, Kim and Hyunook et al. (2001) delete some data that is not involved in the biochemical reactions directly, such as nitrogen and Total Suspended Solids (TSS), they set up an SLM model, and simulate the contaminant removal process in SBR technology with SLM, the results show that the simulated values are very close to the measured ones. It is shown that the simplified model has a good prediction effect on the simulation of nitrification, ammonification, denitrification, and biological phosphorus removal in the SBR process. Compared with the original ASM2 model, the simplified model has fewer model parameters and simpler model correction, so the simulation calculation time is greatly shortened. Moreover, the Mantis Model which is widely used on GPS-X is also an advanced model based on ASM1. There are some modifications in Mantis. For instance, two additional growth processes were introduced to allow the growth of heterotrophic biomass fed by nitrate. To meet the demand of different situations, more and more models are being created, and more software programs are being used in practice.

The WWTP simulator environment can be described as software that allows modelers to simulate WWTP configurations. This is an essential part of the simulation, and various software programs can operate the model. Common examples of such programs include GPS-X, WEST BioWin, STOAT, ASIM, and AQUASIM. In the simulation process, data input is required. Generally, many parameters can be measured directly, while others are based on experimental data taken from the literature. Those parameters that cannot be measured directly or estimated from the literature are usually determined using nonlinear dynamic optimization techniques based on actual plant records and/or experimental data collected at the plant or in the lab. It is recognized that the reliability of the calibrated model degrades with increasing numbers of mathematically optimized parameters.

First of all, the physical plant data should be provided, which includes the size of every process flow sheet, flow pattern, sludge collection, and withdrawal locations, and the dimensions of the various reactors. Then, the operational plant data, flow, control variables (independent variables), and responsive variables (dependent variables), as for activated sludge system, MLSS, Volatile Suspended Solids (VSS), COD of the mixed liquor, DO, and Oxygen Uptake Rate (OUR) are required to calibrate the activated sludge portion of the model. Besides, water quality constituents such as BOD5 (inhibited), Total Suspended Solids (TSS), Total Kjeldahl Nitrogen (TKN), ammonia (NH3), and nitrates (NO3) are necessary for the calibration of the various unit processes.

The basic parameters of influent wastewater are also important (BOD5, BODu, COD, TSS, VSS, and TKN), these are the essential part for us to establish the mass balances across the system. Also, the kinetic and stoichiometric model parameters for organic, nitrogenous, and phosphoric compounds and settling parameters should be involved. There are numerous stoichiometric and kinetic parameters in ASM models, and many of the default model parameters can be used with a high degree of confidence. Site-specific model parameters include the maximum growth rate and the yield coefficient of the heterotrophs.

Different models could be adopted according to the actual condition while simulating the various processes of WWT. Normally, the system is started from the influent process, in this stage, we should consider the influent objects, for example, wastewater influent or batch influent. The chemical dosage influent also should be considered. In GPS-X, there would be an influent advisor, so we can choose the suitable model with the data we have. Besides, the other part of the system also could be simulated respectively, include Attached-Growth, Sedimentation and Flotation, Sand Filtration, and Anaerobic Digestion. Moreover, some special technologies could also be simulated, such as Membrane Filter, UV Disinfection Object, and pipe or pumps as well. The specific comparisons of different simulation software are shown below in Table 1 below.

As the impacts of climate change continue to escalate, one of the most challenging steps in WWTP design is the selection of a treatment process that combines unit operations and has processes that meet effluent permit requirements (Tchobanoglous, Burton, and Stensel 2003). Often, designing a WWT process is done after following various procedures, including is selecting unit processes from numerous alternatives and interrelating them to create an effective process flow diagram that meets predefined cost and performance criteria (Arif, Sorour, and Aly 2018, p. 637). Conventionally, designing WWTPs was done using simplified systems descriptions and empirical equations found in guidelines (Park et al. 2015). These guidelines were utilized for design purposes through the identification of influent wastewater operating conditions and characterizations and developing the effluent requirements. As a consequence, the WWTP units were sized and estimated in terms of pump capacity, reactor volume, aeration capacity, sedimentation tanks, among others (Arif et al. 2018, p. 638).

 

Table 1 Comparison of common simulation software

Simulation Software
Feature	BioWin	GPS-X	STOAT	WEST	SIMBA	Comment
Price	Annual license
Model building	Simple. It is based on a graphical interface	Simple. It is based on a graphical interface	Simple. It is based on a graphical interface	Simple. It is based on a graphical interface.	Somehow complicated. Based on graphical interface.
Primary settler models	Two types of Models, which include a modified model of Takacs et al. (1991).	Two types of Models, which include a modified model of Takacs et al. (1991). It is also reactive.	Model developed by Lessard and Beck (1993). It has two biochemical processes.	Two types of models, which include one developed by Lessard and Beck (1994). It has two biochemical processes.	Simplified model developed by Otterpohl and Freund (1992).
Secondary settler models	Three types of Models, which include one developed Takacs et al. (1991). It is also reactive.	Two types of models, which include one developed Takacs et al. (1991). It is reactive.	Model by Takacs et al. (1991). Not reactive.	Three types of models, which include one by Takacs et al. (1991). It is reactive.	Three types of Models, which include the model by Takacs et al. (1991). Not reactive.
Activated sludge models	ASM family models and general biokinetic model. .	ASM family model, “New General ”.	ASM family model, own model grounded on the BOD balance.	ASM family model, ASM3P, TUDP.	ASM family model, ASM3P.	All have ASM models but the built-in models are different.

 

Table 1 (cont.) Comparison of common simulation programs

 

Simulation Software
Feature	BioWin	GPS-X	STOAT	WEST	SIMBA	Comment
Reactor hydrodynamic model	CSTR or various CSTRs.	CSTR or various CSTRs	CSTR or various CSTRs.	CSTR or various CSTRs.	CSTR or various CSTRs.
Anaerobic digestion models	ADM1 and general biokinetic model.	ADM1 and simplified.	Simple and more complex models for digesting mesophillic.	Three types, which include ADM1.	ADM1 and two simplified Models.
Sewer system model	Not included, but it is possible to link it with such programs.	Very simple.	Simplified, but it is possible to link it with such programs.	Simplified model (KOSIM) included, it is possible to link it with such programs.	Includes a very complex model.
Petersen matrix editor	Yes	Yes	No	Yes	Yes
Chemical P precipitation	In all points. It is based on the chemical equilibrium.	In all points. The kinetic model is adopted from ASM2.	In all points. The kinetics is adopted from ASM2 and the model is founded on the chemical equilibrium.	In all points.  The kinetics is adopted from ASM2, and the model is predicated on the chemical equilibrium.	In all points. The kinetic model is adopted from ASM2.

 

 

 

 

Table 1 (cont.) Comparison of common simulation software

Simulation Software
Feature	BioWin	GPS-X	STOAT	WEST	SIMBA	Comment
Introduction of input data	Directly in the software or import from MS Excel.	Directly in the software or text files. It can be copied in the input data module.	Directly in the software or import from MS Excel.	Directly in the software of import from data bases or MS Excel	Directly in the software, text files, import from data bases or MS Excel	GPS-X has Influent Advisor tool that supports users to input data.
Presentation of output data	Various types of graphs, such as Sankey’s diagrams. It includes a distinct module for formatting graphs.	Various types of graphs. It is not possible to format graphs.	X-Y graphs or tabular form.	Various types of graphs. It includes a distinct module for formatting Graphs. .	Has one type of graphs (SIMBA Monitor) and Sankey’s Diagrams. It is also possible to conduct better visualization in MATLAB.
Parameter estimation	No.	Yes	Yes	Yes	Yes
Sensitivity analysis	No. It is possible to present most simulations in one graph.	Yes	Yes	Yes	Yes
Controllers	Several. It is possible to control any measured variable.	Several. All measured variables can be controlled. Includes an advanced control module .	Two types of controllers. Not linked to MATLAB or related programs.	Several. Possible to design user’s own control algorithms.	Two types of controllers. Possible to design additional controllers in MATLAB.

 

Today, the application of simulation and dynamic modeling is a common practice in wastewater treatment efforts (Copp et al. 2009). There are many models or software available for WWTPs. including those for degrading organic carbon material, nitrifying and denitrifying, and removal of biological phosphorus. Also, other models are available for modifying activated sludge, such as membrane bioreactors and moving bed bioreactors (Mannina et al. 2011). These models are supported by various platforms for simulation of WWTPs, such as SIMBA, GPS-X, EFOR, AQUASIM, STOAT, WEST, and BioWin (Gernaey et al. 2004). As show in Table 1 above, these simulation software programs have different characteristics that make them perform differently as metrics for the resilience of WWTPs, especially as climate change continues to topple the performance of these facilities. They have been used in evaluating process alternatives Hao et al. (2001) and Larrea et al. (2007) optimizing design Rivas, Irizar, and Ayesa (2008), and analyzing and evaluating costs. Simulation and modeling software are applied in evaluating suggested process alternatives for newly developed WWTP units, checking and validating WWTP designs to confirm sludge concentrations, process unit sizing, effluent compliance to standards, recirculation rates, and evaluating the performance of the WWTP under changing conditions.

1. Methodology

This study adopted a systematic review methodology to explore the impacts of climate change on the performance of WWTPs and to determine the potential metrics that could form the basis for measuring the resilience of WWTPs to shocks emanating from climate change. On the first objective, the approach involved grilling peer-reviewed academic and professional literature on how the performance of WWTPs is impeded by the changing climate. The researcher examined the potential impacts of climatic changes on WWTPs by assessing the severity of each climate impact and categorizing it as either, high, medium, or low. Also, on the second objective, the approach adopted involved extensive searches of academic databases to identify all the relevant literature on the simulation software and how they have been used in different contexts to measure the resilience of WWTPs. The assessment process adopted was based on an in-depth literature review to compare and contrast findings from previous studies on the effectiveness of different software applications.

4.1 The Search Process 

The systematic review process in this study utilized an unbiased aggregation procedure to identify extant empirical studies related to the phenomenon under study. This study collected and analyzed the data in three fundamental steps. First, an initial review of the literature on the impacts of climate change was conducted. Second, a detailed review of the literature on the subject was conducted while integrating the findings with the existing professional literature. Third, the researcher conducted an analysis and sense-making of the identified impacts together with the appropriate metrics that could form the foundation for measuring the resilience of WWTPs. The primary data was sourced from bibliographic databases, internet search engines, professional literature, peer-reviewed journals, and reference lists. All the obtained results during the search were only limited to peer-reviewed articles written in English.

For consistency, this study used software for systematic literature review (EPPI-Reviewer 4). The EPPI-Reviewer application is often used as a tool for effective reference management, coding of documents, storage, annotation of sources, extraction of data, quantitative and qualitative analytical processes, and secure exportation of review data. This tool provided a single web location that the researcher utilized in storing, evaluating, and monitoring the process of review. Also, using the EPPI- Reviewer 4 application allows other users to access, improve, remove, and review all the academic documents without compromising their integrity. Fig. 1 below presents the systematic process adopted in searching for the sources used for this review.

4.2 Article Inclusion and Exclusion Criteria

During the initial search, the articles included were empirical studies that were published in peer-reviewed journals, written in English language, and use “WWTPs,” “modeling simulation,” and “climate change and wastewater” as some of the keywords. Although there is extensive literature on WWTPs and climate change, this systematic review was limited to the publications of current and updated sources on the impacts of climate change on the performance of WWTPs and WWTP simulation software. The year of publication for the articles was restricted between 2005 and 2021. With the increased climatic changes worldwide, this review period was deemed appropriate to unravel how WWTPs have responded to the adverse changes in the global environment and how different software tools have been applied as metrics that determine the resilience of these plants.

In the initial review process, all the articles with the selected keywords were identified and recorded. The next process involved examining the presence of any external duplicates from the database under search. All the duplicated journals articles were identified and deleted from the last database searched. The overall number of articles found was updated at every stage of the process. After all the possible empirical studies had been retrieved, a second screening procedure was conducted to assess their eligibility against a developed inclusion criterion. Only the full-text articles that met the inclusion criteria were retrieved. In the second screening process, the inclusion criteria required that the peer-reviewed publications meet four conditions. First, they should be written in English. Second, the article needed to be an empirical study. All essays, books reviews, letters, journalistic articles, editorials, or opinion articles were excluded. Third, the main topic discussed in the article needed to be either WWTPs and how they have been affected by climate change. Finally, the article required to use of either qualitative or quantitative methods in examining the resilience of WWTPs in the face of adverse weather conditions. If any of these conditions were not addressed in the abstract, findings, or discussions of the article, it was excluded during the review process.

The design of the review process was influenced by the practical impediments such as access to relevant publications, time limitations, feasibility, and the designated scope of the systematic review. For example, although extensive grey literature on WWTPs and resilience is available, these materials were not considered in the review because of the concerns relating to their quality and reliability. Grey publications have not been exposed to a rigorous peer-review process, which often improves the quality of a source before publication. The study also considered practicality issues in omitting grey publications. It was not possible to review or access all the existing materials around the world.

4.3 Sampled Peer-Review Articles

In total, 915 articles were retrieved using the keywords. After the deletion of the duplicates, 403 items remained. After all the initial screening processes were completed, 30 relevant articles remained. Upon the execution of a secondary screening process, 15 more articles were removed because they failed to satisfy the stipulated screening criteria. The final sample of the peer-reviewed articles was constituted by 14 empirical studies and one book published between 2005 and 2021. These sources were collected from various journals and databases. The primary databases used include Taylor & Francis, Emerald, ScienceDirect, Sage, Google Scholar, Environmental Studies, and Climate Change abstracts. Including more databases in the search process provided extensive opportunities for the collection of useful literature related to WWTPs, how climate change affects their performance, and how to determine their resilience.

 

Fig. 1: The systematic review process for the study

4.4 The Weight of Evidence and the Quality of Empirical Studies

During the review process, the methodology adopted in examining the primary theme in each publication was evaluated. The review utilized a Weight of Evidence (WoE) framework in assessing that appropriateness, relevance, and quality of all the articles that met the inclusion criteria. For qualitative empirical studies, a critical appraisal tool developed by Letts et al. (2007) was used. This critical appraisal tool was used to create and apply a three-point scale reflecting the quality of each study. These levels included high, medium, and low. Also, two distinct categories were created and used to assess the soundness of the empirical research and its relevance of its thematic area to the systematic review. All the articles that met the inclusion criteria were required to obtain a higher score in all the categories to be considered for the final systematic review. Table 2 below shows the WoE framework and definitions used in this study

Table 2: Weight of Evidence (WoE)

WoE	Description
Soundness of empirical study	High: For quantitative articles, the study is clearly focused, a sufficient background and basis have been provided, planned well, an appropriate methodology adopted, and measures used are justified. Also, an existing WWTP has been used as the basis of the study, the data analysis process is adequately rigorous with sufficient use of statistical methods, and the results are clearly stated. The discussions and interpretations have been done based on the findings of the study, and critical comparisons with other similar studies have been made. For quantitative studies, the purpose is clearly stated, relevant background literature is reviewed and the design used is appropriate. Also, there is procedural rigor in the application of data collection and analysis techniques. The author (s) have comprehensively described the evidence of all the four elements of trustworthiness (confirmability, credibility, dependability, and transferability). Medium: For quantitative studies, a clearly defined and satisfactory methodology has been used in the study. The data analysis process is straightforward, and the findings well stated. The interpretations and discussions were partially based on the results of the study. The measures used in the study have not been cleared validated. For qualitative studies, relevant background literature has been reviewed, the objectives are clear, and an appropriate design utilized. There is a limited procedural rigor in applying data collection, and analysis strategies and the evidence of all the four elements of trustworthiness is partially provided. Low: For quantitative studies, the study is not focused, has an insufficient background, poorly planned, and an inappropriate methodology is adopted. The measures used are invalidated, the context is inapplicable in the analysis of WWTPS, and the data analysis process is inefficiently rigorous. The statistical methods used are scarce, and the overall results are unclear. The findings are not interpreted based on the outcome of the study. For qualitative studies, the purpose is vaguely formulated, an insufficient background has been provided, the descriptions of the context are unsatisfactory, and the trustworthiness elements are inadequately addressed. The study does not have a clear description of the data gathering and analysis process, and the findings are unclear.
Relevance of the study’s scope and focus to the review	High: WWTPs, the effect of climate change on WWTPs, resilience, and simulation programs are the main topics discussed. Medium: WWTPs are discussed as the main topic but not squarely related to climate change, resilience, and simulation programs. Low: There is a partial discussion of WWTPs and the article is not related to climate change, resilience, and simulation software.

 

4.5 Quality Assurance Processes

This systematic review followed standard EPPI-Centre processes to maintain quality. At the scoping review phase, consistency was assured in the application of the article selection criteria. The reviewer conducted a double screening of select publications to pilot the inclusion or exclusion criteria. The researcher also worked alongside two other review team members to jointly develop a review procedure with insightful contributions from discussions with various climate change and WWTPs experts. This initial process aimed to enhance the completeness and thoroughness of the entire review exercise and expand the evidence base of the review outcomes. The researcher then conducted the remainder of the screening processes.

During the later stages, a third reviewer was involved in interrogating all the identified articles to consider them for inclusion in the final stage. The third reviewer was conversant with WWTPs and simulation software, and the reviewer validated the selected articles to confirm their relevance to the scope of the systematic review. Overall, the two review groups involved in this study conducted a detailed qualitative assessment of all the identified peer-reviewed articles, and each of the articles was considered on an individual basis to determine its merit. The second reviewer acted as the quality assurance person by scrutinizing all the assessed articles. All the reviewers that took part in the processes had expert knowledge of WWTPs, modeling simulation, and resilience. In the final stage, the individual reviewer then revised the findings to ensure that the interpretation was done consistently.

5. Results

The process of this systematic review involved the initial transcription of data, rereading of the selected articles, development of appropriate search codes, and review of the primary themes. A total of 15 peer-reviewed articles one book were selected and used to answer the primary questions of the study. All the articles addressed WWTPs and how their performance is affected by climate change as well as the application of different simulation software programs in different contexts. Data were synthesized using narrative methods to reflect the increasing need for the promotion of WWTP resilience to enable them survive the adverse effects off climate change. All the articles were categorized as sound and relevant to the thematic area of the systematic review. The developed conclusive statements were all based on the synthesis of results from each article. On objective one of the study, Table 3 summarizes the potential impacts of climate change on the functioning of WWTPs together with the assessment of their severity as grilled from the existing literature. In the process of quality assessment, six empirical studies were categorized as high, one as medium, and one as low quality.

 

Table 3 Summary of impacts of climate change on the performance of WWTPs

Result theme	References	Main conclusions
Reduced rainfall	Hughes et al. (2020) (Qualitative high-quality study); Marleni et al. (2012) (Quantitative low quality).	–          An increase in the strength of influent, causing a breach of toxicity levels. –          Low wastewater volume due to reduced rainfall enhances the formation of hydrogen sulphide gas, which causes corrosion and odor in the WWTP.
	Tran et al. (2017) (Qualitative high-quality); Benotti et al. (2010) (Qualitative high-quality).	–          Extreme drought causes a decline in natural flows in rivers, and wastewater discharges increase in the surface water flow, leading to the deterioration of water quality. –          Higher constituent concentrations in WWTP influent causes increased constituent concentrations in the effluent.
Increased rainfall	Hughes et al. (2020) (Qualitative high-quality study); Langeveld et al. (2013); (Qualitative high-quality study).	–       Increased inflows causing frequent bypassing and combined sewer overflow. –       Storm events impeded the functioning of WWTPs.
	Plosz, Liltved, & Ratnaweera (2009) (Quantitative high-quality study, longitudinal data)	–       When inflows into activated sludge secondary systems increase, there can be deterioration in treated effluent quality.
	Boholm & Prutzer (2017) (Qualitative high-quality study).	–       Heavy rain causes flooding, leading to the overfilling of the waste water system and landslides that result infiltration of chemicals into the water supply.
	Tolkou and Zouboulis (2015) (Qualitative medium quality)	–       Severe storms associated with increased rainfall lead to more severe flooding, which inevitably causes increase in wastewater volumes that require treatment. This affects WWT systems.
Wind/storms	Hughes et al. (2020) (Qualitative high-quality study).	–       Increased breakages and blockages linked with strong storms or rainfall events
	Tolkou and Zouboulis (2015) (Qualitative medium quality)	–       An increase in storm events due to rising sea levels causes flooding, which harms infrastructure, especially for WWTPs built in coastal regions or places affected by river floods. –       Strong waves during rising sea-levels can damage effluent pipes, leading to an increase in maintenance needs.
Temperature	Hughes et al. (2020). (Qualitative high-quality study)	–       Temperature causes variations in the performance of oxidation ponds, biological systems, and sludge management. –       Odors resulting from to increased temperatures
	Boholm & Prutzer (2017) (Qualitative high-quality study). Marleni et al. (2012) (Quantitative low quality).	–       An increase in temperature results into microbial growth and further contamination in the WWTPs. –       High temperature enhances the formation of hydrogen sulphide, which lead to the odor and the corrosion of WWTPs.
Sea-level rise	Hughes et al. (2020) (Qualitative high-quality study) Hummel et al. (2018) (Qualitative high-quality study).	–       WWT networks in coastal or low-lying areas may significantly be disrupted due to the flooding resulting from seas-level rise –       Elevated groundwater table that prevents sludge management dewatering. –       Outfalls may be affected.
	Tolkou and Zouboulis (2015) (Qualitative medium quality)	–       Sea level rise by 2050 in some regions, jeopardizing the location of most WWTP –       Rising downstream levels of water make pumping effluent a requirement, causing an increase in the plant’s energy needs.
	Boholm & Prutzer (2017) (Qualitative high-quality study).	Leads to the infusion of salty water into the water supply systems and compromising the effectiveness of WWTPs.

 

On the second objective of the study, Table 4 provides a summary of the application of different simulation software in the modelling of WWTP performance in different contexts.

Result theme	Reference	Main conclusions
AQUASIM	Reichert (1994) (Reichert 1994, p. 25).

 

6. Discussion

In this section, the discussions of the impacts identified to have high or medium severity on the functioning of WWTPs have been provided. The researcher acknowledges that the climate change impacts will be felt differently depending on location of the WWTPs due to the geographical variations in climate change and climate.

6.1 How Climate Change affects WWTPs and Processes

Accelerated and reduced rainfall, increased temperature extremes, and rise in sea levels are anticipated to exert a high or medium impacts on WWTPs as shown in Table 1. The concentration of contaminants in the wastewater influent is a mixture of the volume and the load water with which the contaminant is mixed (Henze et al. 2008). The design and type of WWTP processes determine how wastewater effluent features will change under extreme climatic changes. The changing water temperature, water conservation or usage measures adopted, and flow patterns often determine the magnitude of the potentially adverse effects within receiving environments, such as eutrophication. Ultimately, the adverse climatic changes impeded the capacity of the receiving environment to accommodate pollutant loads, also known as the “assimilative capacity” (Chapra, 2008).

6.1.1 Impacts of increased rainfall on WWTPs and processes

The literature review process identified power outages and increased inflows as the most substantial impacts likely to affect WWTPs and processes. When rainfall increases, there will be higher volumes and peak inflows into WWTPs because of inflow and infiltration and flow from combined systems (Hughes et al. 2020, p. 6). While increased stormwater infiltration can cause an increase in “flow” or volume of wastewater, there will be no changes in the Total Suspended Solids (TSS) of the wastewater, diluting the influent to the WWTP. When this happens, it affects the biological treatment processes. Empirical evidence shows that when inflows into activated sludge secondary systems increase, there can be deterioration in treated effluent quality. This condition has been primarily linked to the reduced detention times in the treatment processes (Plosz, Liltved, & Ratnaweera, 2009).

The effect of accelerated rainfall when mixed with an extended dry season can further exacerbate the impacts of increased rainfall on WWTPs. According to Langeveld et al. (2013), this contributes to the extended biologic overloading, stressing the WWTP processes, ultimately increasing the combined sewage overflow. Increased water flows come along with debris linked with storm events, which can infiltrate the WWTP and damage screens or cause blockages (Hughes et al. 2020, p. 6). Increased inflows from excessive rainfall can have an adverse effect on the hydraulic functioning of the system or overwhelm the infrastructure entirely. In the event of extreme weather conditions, system bypasses can occur, causing flows to be diverted past parts or all of the treatment processes. This leads to entrance of untreated or partially treated wastewater to the receiving environment (Langeveld et al., 2013). When this happens, it can cause serious public health concerns, contamination of drinking water supplies, and the closure of social amenities, such as swimming beaches (Boholm & Prutzer, 2017).

As with other infrastructure, storminess and high winds associated with increased rainfall can heighten the possibility of windfall from trees. This increases the risk of unexpected power outages, leading to disruptions to the operations of WWTPs and dependence on back-up generator systems. Further, storm-related road closures can prevent access to WWTPs (Hughes et al. 2020, p. 7).

6.1.2 Impacts of reduced rainfall on WWTPs and processes

A reduction in drought and rainfall conditions increases the amount of water flowing into WWTPs, through less inflow and infiltration. This potentially lowers household water consumption, especially as a result of the effort to implement water conservation strategies. When the occurrence of low flows increases, the contaminant dilution capacity decreases, causing higher pollutant concentrations (Tolkou and Zouboulis, 2015). Although the water volume or “flow” decreases, the waste load remains constant, forming “high-strength wastewater”, whose flow potentially causes problems for WWTPs. Also, when the concentration of oils, fats, grease, solid and organic matter increases, blockages, early system corrosion, and severe environmental and health impacts may occur.

Higher concentrations of these products and decreased flow can lead to the occurrence of interrelated problems. First, extended detention times during the process of conveyance can lead to the occurrence of extensive anaerobic decomposition before the WWTP. This causes changes in influent properties which the WWTP may process inefficiently (Benotti et al. 2010). Second, First, the depositing of solids in the reticulation pipes of the WWTP can be swiftly released as a slug upon the resumption of flows. This can cause problems since most WWTPs strive to process erratic loads. Third, there may be an increase in the salinity levels due to an increase in the influent pollution concentrations. This has an adverse effect on the treatment effectiveness, operational costs, and effluent quality (Tran et al., 2017).

The effect of reduced flows and increased wastewater strength on WWTP is influenced by the capacity and type of the individual plants. On the one hand, it is expected that increased pollutant concentrations will significantly affect plants relying on trickling filters, especially during the winter period when efficiencies decrease as a result of slower rates of biological reactions at lower temperatures. On the other hand, WWTPs with activated sludge systems may be affected positively because the retention times in process tasks increase’ leading to an increase in the removal of solid within the clarifiers (Benotti et al. 2010).

6.1.3 Impacts of rising sea-level on WWTPs and processes

The study found that raised groundwater tables, flooding, and reduced outflow capacity are all related to sea level rise, and they can affect WWTPs significantly. Typically, WWTPs are established in low-lying coastal land to lower the wastewater collection discharge of treated effluent costs. As a result, these plants are largely exposed to coastal flooding caused by sea-level rise (Hummel, Berry, & Stacey, 2018). Also, the risk of compound flooding increases with sea level rise, especially due to combined raised groundwater levels, combined fluvial, coastal, and pluvial events. Compound flooding increases the potential for direct damage to WWTPs, causing disruptions to WWTP processes, necessitating protection or repair works, and ultimately leading to desertion of stranded assets (Hummel et al. 2018). The rise in the receiving-water levels may necessitate the need change from gravity to pumped effluent, or higher heads for pumped systems, leading to an increase in the overall energy requirements (Tolkou and Zouboulis, 2015). Also, some WWTPs are involved in land-based de-watering of wastewater sludge. De-watering depends on a groundwater table at a sufficient depth below ground level to allow for the infiltration of excess water. When the groundwater table rises, it compromises the dewatering capacity.

6.1.4 Impacts of increased temperature on WWTPs and processes

 The performance of WWTPs varies with temperature, and this study found that increased temperature extremes cause an increase in odors. Fresh wastewater, especially from residential areas, produces a benign stale odor. When the composition of wastewater changes in the wastewater network, malodorous compounds are produced (Marleni et al., 2012). An increase in the wastewater strength and transformations in the performance of WWTP may occur when the temperatures are warmer, and can potentially lead to an increase in odor, or change effluent properties or the WWTP process inputs (Hughes et al. 2020, p. 7). An increase in temperature may modify WWTP processes since higher temperature naturally cause a drastic increase in biological reactions. Given that the secondary treatment stage within WWTPs depends on biological reactions, land requirement would decrease, conversation processes would be enhanced, and removal efficiencies will increase when the temperatures are warmer. Also, warmer temperatures can have the effect of increasing the feasibility of utilization of some treatment processes (Tolkou and Zouboulis, 2015).

It is a requirement that sludge is heated to 37 ◦C during “sludge digestion.” An increase in ambient temperature would imply that lees energy will be required for this heating to occur. Different processes, such as aerobic biofilm reactors and activated sludge are less reliant on temperature due to the advanced mechanization and technological input. As a result, these processes will be less affected.

Also, warmer climatic conditions can potentially lead to higher evaporation rates. This is because increased temperature causes a simultaneous increase in the atmosphere’s water-holding capacity. For WWTPs, the implication is that there will be stricter final discharge standards for effluent, especially as salinity in receiving water catchment areas increases.

Wastewater treatment modelling software packages

ASIM

The findings show that the Activated Sludge SIMulation (ASIM) can be used for the simulation of different activated sludge systems. For instance, the examined plant layout may be made up of ten dissimilar reactors in series (anoxic, aerobic, and anaerobic), including internal MLR streams and RAS, chemostat reactors, batch reactors, among others (Makinia and Zaborowska 2020). The reactor building blocks found in ASIM can be used to build very simple models for both primary and secondary clarifier (Gujer, 1995). The user may define, store, and edit the biokinetic models freely, although various pre-defined models are available in a model library. Various biokinetic models is also provided with the software, either with or without denitrification, nitrification, and phosphorus removal. They include ASM1 (adapted), ASM2d, and ASM3. Furthermore, several controllers can be simulated using ASIM, such as internal recirculation, return and waste sludges, aeration, and it is possible to control a second influent flow rate. It is also possible to implement control loops in the program using on/off type and simple proportional controllers (Makinia and Zaborowska 2020).

Both dynamic and steady-state simulations can also be run in ASIM. A “variation” file is used to introduce the dynamic input data, especially process alternation, loads, and operational and parameters (excess sludge removal, recycle rates, and aeration intensity). Besides, analysis of data is supported by the possibility of likening measured data with simulation outcomes, which can then be exported to spreadsheets for additional treatment.

However, ASIM is only used in modelling the ASP and related control options (Schütze, Butler, and Beck 2011, p. 293). This means that ASIM does not have any primary clarification module. In its original form, ASIM can only allow for a restricted or limited timesteps to be simulated.

AQUASIM

In the studies that have been analyzed ibn Table 4, AQUASIM has been used in for the simulation and analysis of the dynamics of aquatic systems. This means that the program is specifically designed to perform simulations and other analyses for natural and technical aquatic systems. Using AQUASIM, users can define the spatial configuration of the system they seek to investigate as a set of compartments (Olsson and Newell 2005). These compartments can be connected to each other by two types of links. Advective links indicate advective substance transport and water flow and between compartments, such as junctions and bifurcations. Diffusive links represent membranes or boundary layers, which can be infiltrated selectively by some substances. The different types of compartments available include mixed reactors, biofilm reactors, advective-diffusive reactors, saturated soil columns, river sections that describe water flow and substance transport in open channels, and lakes, which describe the stratification and substance transport in lake water columns and in adjacent sediment layers. When compartments are used in this program, they limit its generality, but allow for the selection of efficient numerical algorithms (Olsson and Newell 2005, p. 230). The user formulates all the internal dynamic procedures within a compartment without any restrictions.  Also, the program is made up of built-in tools for identifiability analysis, uncertainty analysis, and parameter estimation. The presentation of outcomes is done as are conventional two-dimensional plots (not on-line) or saved as American Standard Code for Information Interchange (ASCII) files. ASCII is code that provides information compatibility between digital devices (Olsson and Newell 2005, p. 666). AQUASIM is available for multiple platforms, such as Solaris, DEC-VMS, HP-UX, Apple PowerMac, DEC-Unix, NT, and Windows 98 (Olsson and Newell 2005, p. 231). The software has been freely available for interested users since 2013.

The user of the program is free in specifying any set of state variables and transformation processes to be active within the compartments. For the model as defined by the user, the program is able to perform simulations, sensitivity analyses and parameter estimations using measured data (Reichert 1994, p. 30). The sensitivity analysis module calculates linear sensitivity functions of random variables while considering all the parameters included in the analysis. When provided with a model structure and measured data, AQUASIM uses the weighted least-squares technique to perform automatic parameter estimation.

Because of the possibility of starting with a simple model and progressively increasing model complexity later on by adding processes and variables, AQUASIM is also suited for use by students conducting environmental modelling exercises (Reichert 1994, p. 25). The ability of the program to compare different models and its capability to estimate the uncertainty of calculated results also arouse discussions on the reliability of model predictions. Today, three versions of AQUASIM are available for use. The first one is the window interface version which uses the graphical user interface of the Microsoft Windows operating system. The second one is the character interface version which can be used in a primitive teletype terminal. The final one is the batch version, which is designed for long and complicated calculations to be submitted as batch jobs. The character interface and batch versions can be compiled on all operating systems with any C++ compiler (Reichert 1994, p. 22). Due to its distinctive features, AQUASIM is an extremely useful research tool.

On the downside, AQUASIM lacks any real-time capabilities. There is no technical support for the program, although there is an electronic user group that facilitates communication among program users. AQUASIM courses are also organized on a regular basis.

STOAT

In assessing the impact of climate change on the effectiveness of a Jordanian WWTP, Abdulla and Farahat (2020) used the Sewage Treatment Operation Analysis Over Time (STOAT) software in simulating the performance of the WWTP under. STOAT is one of the most dynamic wastewater treatment process modelling package with the ability to model activated sludge systems, sludge treatment, and bio film-based processes. Abdulla and Farahat (2020) used the STOAT software has because it has relatively simple data requirements and it is freely available.

 

 

 

AQUASIM

• It is freely available.
• It was developed by the Swiss Federal Institute for Environmental Science and Technology.
• It integrates spatial configurations to assist in forecasting hydrological effects in simulations
• It can be installed in non-Windows based computers (Reichert, 1995).

 

WEST

• Developed by DHI in Denmark and is accessible through MIKE, which is powered by DHI.
• It includes integrated greenhouse gas emissions, urban water systems, and anaerobic ammonium oxidation modeling (MIKE Powered by DHI, 2019).

GPS-X

• Developed by Hydromantis in Canada
• It was developed for both wastewater and water and WWTPs
• It also includes standard packages for carbon footprints and greenhouse gas emissions.

STOAT

• It was developed and maintained by Water Research Centre Limited in the United Kingdom (Water Research Centre Limited, 2019).
• It is a freely available program WWTP modeling and simulation.
• STOAT 5.0

BioWin

• It was by EnviroSim Associates Limited in Canada.
• It uses patented biological models complemented with other process models such as mass transfer gas liquid interactions and water chemistry.
• As a unique biological model, it is based on ASM and general models
• It was further improved through extensive research interventions and ultimately designed to decrease the amount of calibration that users require.
• It uses four-populations to model anaerobic systems. They include heterotrophs for hydrolysis and fermentation; acetogens for acetogenesis; and both acetoclastic and hydrogenotrophic methanogens for methane generation.
• It has an extensive parameters and state variables used in calculations and tracking.
• Variables are easily modified, and the program also permits users to build and utilize their own models.
• It also allows for biogas generation and the subsequent reuse of the gas for combined heat and power integrated with electricity and chemical costs available for process elements.
• Simulation reports can be created and edited within BioWin, but export options are also available for use with external programs such as Microsoft Excel or Word (EnviroSim Associates Ltd., 2017).
• It is frequently used in the U.S industry.
• Although BioWin makes access to the model values representing the processes more user friendly, they are extremely complex compared to GPS-X (Callahan 2019, p. 23).
• For instance, the program contains 21 fractions for characterizing influent wastewater, more than 50 state variables, 20 non-state variables, and more than 200 kinetic and stoichiometric values. These numbers exclude the additional add-ons available in the physical characteristics or model builder (Callahan 2019, p. 23).

Arif et al. (2018) applied the simulation and modeling software to verify and optimize the performance of WWTPs using the GPS-X software. The researchers used the software in simulating and verifying the performance of WWTP processes in shock loading conditions (increase of effluent flow and double organic load) and normal conditions. Generally, the findings revealed that the GPS-X software is an extremely helpful tool for use in the verification of the pre-design of WWTPs. Also, this application is helpful in the understanding of the plant’s performance under varied conditions and in deciding the future expansion works required for increased organic and hydraulic loadings (Arif et al. 2018).

7. Conclusion

The review is conducted with the sole objective of determining the effectiveness of these software in determining the resilience of WWTPs in extreme climatic conditions.

This study sought to present and demonstrate systematic approaches used in evaluating the performance of WWTPs under changing climatic conditions.

Worldwide, most of the WWTPs are vulnerable to the often uncertainly occurring hazards associated with climate change. The nature of WWTPs, often designed based on previous weather conditions and sea-level criteria, imply that they be largely affected in various ways, including the prevalence of cascading impacts and compounding hazards. Overall, the damages to WWTPs are anticipated to result from the seas-level rise, increased rainfall, increased temperatures, and reduced rainfall. When these damages occur, they will require increased repair, maintenance, and water services disruption caused by system component failures. This study has shown that climate change will adversely affect all elements of WWTPs, and the critical roles they perform in protecting the human health and the natural environment in which they operate. Changes in rainfall patterns and sea-level increase are anticipated cause highly severe impacts, with other moderately or lowly severe impacts expected from to arise from temperature and wind increases. Most of the effects are anticipated to be concentrated in low lying regions that are heavily exposed inland and coastal flooding. However, these effects will also be distributed across WWTP systems since an increase in temperature and changes in groundwater generate system-wide impacts.

Most WWTPs are ageing and although all systems need to be maintained on an ongoing basis, it only becomes challenges when the impacts of climate change persist. When the effects of climate change become adverse, existing WWTPs are rendered incapable of providing adequate service. In essence, the climate change impacts will likely accelerate the existing issues associated with the poor performance of WWTPs, which are often poorly maintained. Therefore, there is a growing need to ensure that the WWTPs are resilient to the shocks associated with climate change.

Software

Apart from allowing the user to specify a model, AQUASIM also provides different functionalities for perimeter estimation and sensitivity analysis. When defining a desired model, the user can select from various “compartments,” which act as building blocks of the model.

8. Future work

Much of this study focused on analyzing the impacts of changes in climate on the performance of WWTPs. however, the implications associated with these impacts have not been analyzed in this study. Therefore, future studies should also focus on assessing the implications arising from the impacts of climate change on wastewater systems. All implications associated with the environmental, cultural, social, and economic need to be explored to understand the urgent need for the establishment of resilient WWTPs.

List of References

 Li, Y., Li, Y. and Wu, W., 2016. Threshold and resilience management of coupled urbanization and water environmental system in the rapidly changing coastal region. Environmental Pollution, 208, pp.87-95.

Jafarinejad, S. (2020). A framework for the design of the future energy-efficient, cost-effective, reliable, resilient, and sustainable full-scale wastewater treatment plants. Current Opinion in Environmental Science & Health, 13, 91-100.