1 Introduction

The main questions in the planning and decision-making process of WWTPs are the following: What is an appropriate future treatment concept of an existing or newly built WWTP , and when is the best time to invest in new technologies under rapidly changing conditions to avoid expensive disinvestments? Moreover, what are useful WWTP upgrades or expansions to fulfil all future requirements, and how can these expansion steps be controlled and adapted before final decisions on long-lasting technological installations are made? These questions arise for both European countries and China, who are subject to different WWTP planning challenges. In Germany, many of the structural components of existing WWTPs will reach the end of their originally designed lifetimes in the upcoming years. Around 9300 public WWTPs are operated to treat an average daily amount of wastewater of approximately 26.9 million m3 (in 2013) – this corresponds to 97% of the population being connected to the mostly centralized wastewater system (Destatis 2015). Operators and planners are currently facing the task to upgrade their existing plants and make corresponding long-term reinvestments in the starting investment cycle. In contrast, planning and design of entirely new WWTPs is one of the major topics in urban water management in China . Fast-growing cities and economic growth result in rapidly increasing amounts of wastewater and, therefore, the need to build appropriate wastewater infrastructure to treat it. By the end of 2013, 3508 WWTPs had been built in 31 provinces and cities with a total treatment capacity of 148 million m3/d (Zhang et al. 2016). The technologies used most often in WWTPs are AAO and oxidation ditches, which together account for over 50% of the existing WWTPs (Zhang et al. 2016).

In both planning situations, challenges arise due to the long service life of around 30 years and the high capital commitments required. Moreover, European countries as well as China are increasingly confronted with higher construction and design risks due to future uncertainties due to high system complexity and rapidly changing conditions in the WWTP environment. These risks are amplified due to the wide variety of already available and innovative technological options, e.g. for extensive carbon recovery and utilization, elimination of micro-pollutants, disinfection as well as the deammonification process for N-elimination.

To integrate long-term future uncertainties in decision-making processes, strategic planning approaches have become more and more popular in various water and wastewater infrastructure projects. Different methodologies, tools and implementations are available in literature, e.g. the “Dynamic Adaptive Policy Pathways approach” (Haasnoot et al. 2013; Kwakkel et al. 2015), the “Regional Infrastructure Foresight method” (Störmer and Truffer 2009) and others (Dominguez 2008; Dominguez et al. 2011; Truffer et al. 2010; Lienert et al. 2006; Henriques et al. 2015). This chapter presents a novel methodology of future-oriented strategic planning, focusing on the decision-making process on WWTP upgrades and expansions. The general idea is to integrate an additional future-oriented strategic planning level as internal controlling process into existing organizational structures of the WWTP operator. In this way, existing traditional WWTP design procedures will be complemented by a more strategic technological perspective. The main goal is to continuously support WWTP operators and local decision-makers in the definition of potential long-term technological directions of the overall WWTP. Moreover, specific transformation paths from the current plant situation to future-oriented technological plant concepts can be defined. The following issues will be addressed in this chapter: (1) overview of traditional WWTP planning methods , (2) main principles of the developed future-oriented strategic WWTP planning process and (3) overview of potential influencing factors.

2 Overview of Traditional WWTP Planning Methods

The existing and established WWTP planning methods enable a detailed technical planning of preselected technological alternatives using state-of-the-art guidelines (e.g. design of activated sludge tanks using the German guideline DWA-A 131 2016) and state-of-the-art technologies. Cost analysis and comparisons of the designed technological alternatives are used as basis for the final decision-making (e.g. German guideline DWA 2011).

Beside investments (as capital costs including depreciation and interest rates), it is of crucial importance to consider operational costs (energy, personnel, materials and effluent charges) in the economic evaluation of different technological alternatives. The significance of operational costs is illustrated in Fig. 1, which contains exemplary annual cost structures of two German WWTPs.

Fig. 1
figure 1

Exemplary distribution of annual costs of two German WWTPs

Full size image

The distribution of annual costs of the two exemplary WWTPs shows that operational costs can vary significantly with treatment concept and location. Although the variations among percentages of operational costs can be wide, capital costs are maintained around 50%, while operational costs are a significant portion of annual costs.

This detailed technical planning process is usually initiated after the identification of a specific planning reason (event-driven planning action). These planning reasons include, among others, the fact that effluent requirements cannot be met anymore or technical components have passed their technical service life. Moreover, the traditional WWTP planning process is completed with the successful implementation of one selected technological alternative (one-time planning action).

The required database of the traditional WWTP planning process usually includes real past and present data received during plant-specific process monitoring. Furthermore, the preselected technological alternatives are usually oriented to the existing WWTP concept (tendency to a more present-oriented planning). The established WWTP planning is traditionally applied as a stepwise design approach that is usually based on point forecasts of specific design parameters including loads and fixed effluent requirements. Figure 2 illustrates the principles of the traditional WWTP planning process.

Fig. 2
figure 2

Principles of traditional WWTP planning. (According to Manig 2018)

Full size image

The traditional WWTP planning process , with its strong orientation towards the current situation, usually favours an adherence to the existing predominant technological system with a tendency to focus on replacement investments. A systematic consideration of potential long-term technology directions that are independent from the current WWTP concept is usually not included in traditional WWTP planning. Therefore, it is difficult to include the wide range of rapidly changing conditions or unexpected future changes in these planning approaches (e.g. changing socio-economic, environmental or political conditions).

Overall, traditional WWTP planning is essential to design and construct technological alternatives in a detailed and comprehensive manner. But to integrate a higher flexibility in the decision-making process and a long-term planning perspective considering the increasing future uncertainties, additional methods are needed to support WWTP operators and planners. This is especially important as WWTP operators and local decision-makers in European countries and particularly China are increasingly confronted with uncertain planning conditions that require a more flexible approach.

3 Main Principles of a Future-Oriented Strategic Planning Process

The developed methodology of future-oriented strategic WWTP planning can be seen as a supplement to existing traditional WWTP planning approaches . The main goal is to continuously support WWTP operators and planners by integrating a more future-oriented planning perspective in their decision-making processes. This means that broader long-term visions of possible technical future concepts of a specific WWTP are systematically integrated in the decision-making process considering a wide range of possible future uncertainties. In general, the methodology is a plant-specific planning process addressing individual catchment characteristics as well as local conditions of a specific WWTP. This is important, as these characteristics and conditions are strongly regionally dependent and WWTPs in different locations will face different future developments and challenges.

The main principles of the future-oriented strategic WWTP planning process are illustrated in Fig. 3. The definition of future-oriented WWTP expansion strategies independent from the current plant situation is of significant importance in this methodology. These WWTP expansion strategies include on the one hand different potential long-term technology concepts and on the other hand specific transformation paths acting as bridge between the current WWTP and potential future concepts. Instead of taking into account a single point forecast, a wide range of possible future developments at catchment and global scale (as scenario funnel) are considered in the planning process as strategic assumptions. The methodology allows for an ongoing control of these assumptions and an early adaptation of the defined long-term technological concepts to current conditions before starting the construction of a WWTP upgrade or expansion.

Fig. 3
figure 3

Principles of future-oriented strategic WWTP planning. (According to Manig 2018)

Full size image

As basis for the developed methodology, already established strategic management methods mainly developed for commercial companies are used (e.g. Götze and Rudolph 1994, Krystek and Müller-Stewens 2006). However, some of the basic principles have been adapted to the special situation of wastewater treatment infrastructures. As the methodology of future-oriented strategic WWTP planning can be seen as a continuous and forward-looking planning and controlling process (see also Beier and Manig 2017; Manig et al. 2018), the major steps form a control loop that is presented in Fig. 4. Beside this control loop, the interfaces between the superordinate strategic planning level and the traditional WWTP planning methods are illustrated.

Fig. 4
figure 4

Major steps of the future-oriented strategic WWTP planning level as complement to the traditional WWTP planning process. (According to Manig 2018)

Full size image

The main output of the strategic WWTP planning process is the continuous analysis of potential future-oriented WWTP technological directions of a specific WWTP under different future scenarios. In addition, specific recommendations on the next WWTP upgrade or expansion step can be derived to support the traditional WWTP planning process.

The major steps of the control loop are explained briefly in the following:

  1. 1.

    Definition of strategic goals

In this step, long-term objectives which the WWTP aims to achieve in the next decades are defined. These strategic goals directly determine the criteria for the evaluation of future-oriented WWTP expansion strategies. For example, a strategic goal can be an increasing overall economic efficiency of the future WWTP. Hence, minimum annual costs consisting of investment and operational costs are identified as evaluation criteria. Beside economics, possible strategic goals could include increasing energy efficiency by minimizing the external energy demand.

  1. 2.

    Continuous analysis of real and forecast data

In this step, the basis for continuous collection and processing of planning data and assumptions required in the strategic planning process is provided. This includes real data and information on the specific WWTP to describe the current WWTP and catchment situation including, for example, available technologies, WWTP inflow conditions, but also the current cost situation. On the one hand, this real data is used as basis to develop scenarios for potential future developments at catchment or global scale. On the other hand, real data is required to compare the actual situation with strategic assumptions made during the controlling process. Besides real data, it is of crucial importance to provide appropriate forecasting data describing potential future developments (using scenario planning). For scenario planning, the main influencing factors in a specific region must be identified, and their future development must be predicted.

  1. 3.

    Definition of future-oriented WWTP expansion strategies

In this step, potential future-oriented WWTP expansion strategies are defined. Based on developed scenarios, potential long-term technology concepts that are independent from the current WWTP concept must be developed based on expert knowledge. Moreover, the WWTP expansion strategies include specific transformation paths acting as bridge between the current WWTP and their potential future concepts. These transformation paths describe the chronological order in which the selected technologies should be introduced and implemented. To integrate already established or innovative technologies in the strategy formulation and evaluation (see next step), it is necessary to collect and document relevant technical and economic information on potential technologies in the form of “technology profiles”. These “technology profiles” include information such as specific energy consumption and necessary specific investment costs of the technology.

  1. 4.

    Evaluation and selection of defined WWTP expansion strategies

In this step, the defined WWTP expansion strategies must be evaluated based on the defined evaluation criteria (see first step) and the strategic assumptions made (see second step). In order to estimate the evaluation criteria of each WWTP expansion strategy under different scenarios, a simplified plant-wide mass balancing/design model is utilized. This simplified model must be extended by a rough dimensioning of all considered technologies. Furthermore, a cost estimation tool is required in order to calculate cost effects of strategies as one major input criterion for the evaluation process. It is of crucial importance to keep the model as simple as possible as there can be a wide variety of possible strategies and future developments, and, hence, a higher number of required simulation runs.

  1. 5.

    Continuous control of strategic assumptions

The final step of the control loop describes the continuous control of the selected future-oriented WWTP expansion strategy. This step is critical as it allows an ongoing control and early adaptation of the selected WWTP strategy according to future changes, before the design and construction planning of a next WWTP expansion step begins. A careful and systematic scanning of the main strategic assumptions is needed in order to identify whether it is reasonable to continue to pursue the selected strategy or reject it due to expected changing conditions.

Table 1 summarizes the main characteristics of both planning principles, the traditional and the future-oriented strategic WWTP planning. A significant difference of both planning procedures is the overall reason for planning. Traditional planning is usually a noncontinuous process with the clear objective to plan and design appropriate technology alternatives. It ends with the final decision for one alternative and its realization. In comparison, the methodology of strategic planning can be seen as a continuous process which encourages WWTP planners and operators to constantly focus on possible long-term technology options for the specific WWTP. In this manner, the strategic planning highlights resulting technological and economic consequences due to changing future developments in the specific catchment and global environment. The longer time horizon of strategic planning and the wide range of possible future developments in terms of technology options or catchment conditions allow WWTP planners and operators to gain a deeper understanding of existing or newly created technological path dependencies.

Table 1 Characterizations of future-oriented strategic WWTP planning compared to the traditional design and construction WWTP planning
Full size table

4 Overview of Potential Influencing Factors

In the second planning step of future-oriented strategic WWTP planning (Step 2: Continuous analysis of real and forecast data), it is of crucial importance to identify relevant influencing factors and their potential future developments using scenario analysis. These influencing factors are used as “warning” indicators for ongoing control of selected future-oriented WWTP expansion strategies and the assumptions made therein. If a warning indicator is no longer within the assumed range, then the recommendation on the next WWTP expansion step must be adapted. Generally, influencing factors and their corresponding critical values are highly plant-specific depending on the existing WWTP concept and catchment situation, the predefined strategic goals and the selected long-term technology concept.

Figure 5 shows a schematic view of both (1) key parameters of the traditional WWTP design and construction process including investment and operational costs of defined technology alternatives for economic evaluation and (2) additional exemplary influencing factors that should be integrated in the strategic WWTP planning process.

Fig. 5
figure 5

Examples of required key parameters for traditional planning processes and influencing factors for future-oriented strategic planning

Full size image

The uncertain influencing factors of the strategic planning process can be classified into two major categories. The first category includes future trends in the specific catchment area, e.g. social, economic and environmental conditions as well as possible infrastructure management measures. These future trends have a significant impact on future WWTP inflow volumes and composition, which in turn influence the required WWTP technologies and their dimensions (investments) and the plant performance including operational costs. The second category includes future trends of the global environment such as political and legal standards as well as technical and financial conditions. Further requirements as well as rising concerns over scarce resources can also lead to changing design values. Conditions in the catchment area and global environment are highly dynamic, and some of them may change drastically over long WWTP lifetimes, for example, population development due to demographic change or the number of extreme rainfall events as a result of climate change . Because the number of influencing factors can be huge, it is crucial to reduce the number to only consider the dominant parameters by prioritization. Dominguez et al. (2006) take a first step in the development of a method to guide the practitioner through the identification of project-specific driving forces.

5 Summary

This chapter presents a brief overview of the developed methodology of future-oriented strategic planning, focusing on WWTP upgrades and expansions. The continuous planning and controlling process should be implemented as an additional superordinate planning level in already existing organizational structures of the WWTP operator. The implementation can be time-consuming and can vary with the plant operator as responsibilities must be clarified and internal planning processes adapted. To evaluate potential long-term technology concepts, a simplified plant-wide mass balancing/design model is required. Moreover, there is a need to build up a systematic data management system to manage the required databases of real and forecast data.

Overall, the developed methodology can be seen as a new approach before the detailed traditional WWTP design and construction planning phase begins. The idea of the methodology is to continuously support WWTP operators and planners in their long-term investment decisions by getting a deeper understanding and transparency of long-term investment consequences and technological path dependencies.