Optimization of Urban Wastewater Systems using Model Based ...

11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008

Optimization of Urban Wastewater Systems using Model Based Design and Control C. Vélez1*, A. Lobbrecht1,2, R. Price1,3, A. Mynett1,3,4 1.

Institute for Water Education UNESCO – IHE, Westvest 7 2611 AX Delft, The Netherlands. 2. HydroLogic BV, Stadsring 573811 HN Amersfoort, The Netherlands 3. Technological University of Delft TU Delft, Julianalaan 67 2628 BC Delft, The Netherlands 4. DELTARES, Rotterdamseweg 185 2600 MH Delft, The Netherlands *Corresponding author, e-mail [email protected]

ABSTRACT The pressure on the Urban Wastewater Systems (UWwS) increases as urbanisation continues relentlessly and climate change appears to lead to more extreme rainfall events. These pressures have a negative effect on the efficiency of UWwS to control the urban pollution reaching water-receiving systems. Thus, the urban pollution managers are being forced to optimize the design and operation of UWwS in order to deal with more pressure and new requirements for performance. Traditionally the UWwSs have been designed for steady loading, but are operating under dynamic loading. Thus, only in the case when design loading occurs, the system operates optimally. It is also general practice to design the components of the system separately (sewer network or treatment works), without considering the synergies between components and the impacts in the receiving system. In order to tackle these limitations this paper present an approach named dynamic Model Based Design and Control (MoDeCo). The approach include design of control strategies in the process of the system design, using optimisations routines based in the impact in the water quality in the receiving system and the investments cost. An analysis of the possibility to apply the approach to a case study in Cali - Colombia is presented.

KEY WORDS Integrated Urban Wastewater Systems, Wastewater Drainage, Wastewater Treatment, Model Based Design, Real Time Control.

INTRODUCTION The pressure on the Urban Wastewater Systems (UWwS: sewer network, wastewater treatment plant and water receiving system) increases as urbanisation continues relentlessly and climate variability appears to lead to more extreme rainfall events. These pressures have a negative effect on the efficiency of UWwS to control the pollution reaching water-receiving systems. The first challenging factor that increases the pressure on the UWwS is the rapid growth of the urban population. The United Nations (UN) World Population Prospects shows that virtually all the population growth expected between 2000 and 2025 will be concentrated in urban areas. Figure 1 shows some of the characteristics of the World Urbanization tendencies based in the world population online data base (UN, 2007). Around 50% of the total urban population in 2025 will live in developing countries in Africa, Asia or Latin America and the Caribbean (LA&C). Thus, the urban population changes must be seen in the context of the

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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008

Rural 62.8%

Urban 37.2%

Pop 1975: 4 076 080 000

Rural 42.5%

Urban 46.7%

Rural 53.3%

Pop 2000: 6 124 123 000

Urban 57.5%

Pop 2025: 8 010 509 000

Figure 1. World Urbanization Prospect Based in Online Data Base UN (2007).

levels of urbanization, the rate of urban growth and changing life styles that demand more consumption of water and generate more pollution stress under the urban water resources (Marsalek et al., 2001). The second factor considered is the climate variability. The effects of large urban areas on local microclimate occur because of changes in the energy regime, air pollution, air recirculation and release of greenhouse gases. These factors change the amount of precipitation and evaporation. Geiger et al (1987) in Marsalek et al, (2001) shows that in large industrialized cities precipitation is 5 – 10% higher than in the surrounding areas, and the increase in precipitation for individual storms can be as high as 30%. In developing countries, the pollution impact of UWwS on the water receiving systems is bigger comparing to the developed countries, due to the lower coverage in sanitation (Figure 2). In the global assessment on water supply and sanitation report of the United Nations (UN), the lack of one or more components of the systems in urban areas was described. Even though, during the period of evaluation there has been an increase in the provision of sanitation, the fulfilment of the international objective for the Millennium Development Goals (MDG) is a challenge. Although people served with some form of improved sanitation rose from 55% (2.9 billion people served) to 60% (3.6 billion) from 1990 to 2000, to achieve the 2015 target in Africa, Asia and Latin America and the Caribbean alone, an additional 2.2 billion people will need access to sanitation by that date. In fact, this means providing sanitation facilities to 384 000 people every day for 15 years (WHO et al., 2000). Even more, in many countries the sanitation challenge need to be faced with conditions of limited resources and financial pressures to achieve more coverage with less resource. Therefore, the urban pollution managers are being forced to optimize the design and operation of UWwS in order to deal with more pressure and new criteria for performance.

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100% 22% 90%

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% Sanitation Coverage

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0% Africa

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Total No access

0% Africa

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Figure 2. a. Sanitation Coverage by Category of Service and b. Median Percentage of Wastewater Treated by Effective Treatment Plants. Source: (WHO et al., 2000)

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Optimization of Urban Wastewater Systems using Model Based Design and Control

11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008

DESIGN OF URBAN WASTEWATER SYSTEMS According with Harremoës and Rauch (1999), there are two extreme approaches for the design of the components: the empirical iterative approach and the prediction-design approach. Empirical Iterative Approach – Steady State Design In the empirical iterative approach, structures for pollution abatement are built on simplified assumptions and their performance is subsequently evaluated through monitoring. When the monitoring system proves that the performance is inadequate, then an improved plan of action is implemented. This approach advocates purely inductive interpretation of information from experience gained from operating the systems, from which a pattern can be identified and responded to in an empirical, iterative approach to design and operation (Harremoës and Rauch, 1999). The experience gained and patterns identified became design rules that are typically used for setting up urban water facilities and could be considered as the oldest and simplest models. With some safety factors, these simplified models are often applied for design purposes. However, if these design models are applied to cases outside of their validity errors can be made. One example of this approach is the rational method which works relatively well in systems with moderately steep, dendritic (tree-like) urban drainage systems for intensive rainfall. If it is applied on a flat system with a looped sewer network in a less urbanised catchment during winter situation, the results will deviate significantly from reality (Vanrolleghem and Schilling, 2004). Many of the existing sewer networks and wastewater treatment plant (WwTP) were designed using this approach with relative success. However, one of reasons of the limited efficiency of UWwSs to control pollution is that they have been traditionally designed for steady state loading but are operating under dynamic loading. Hence, only in the rare case of the design loading occurring does the system operate optimally. In all other operational scenarios, the built-in capacity of the system is not used, or it is used in such a way that the objectives cannot be met. In the first situation, the invested capital is not productive; in the second, the damage occurs: receiving waters are polluted (Harremoës and Rauch, 1999). Prediction Design Approach – Dynamic Design In the prediction-design approach, models play an essential role in the prediction of performance and evaluation of competing alternatives for design. This approach is dominated by a deductive interpretation of the problem. It is based on the idea that if the problem is reduced to its basic components and tied together in a system of physical, chemical and biological laws of natural science, the future can be predicted with sufficient accuracy to warrant a safe design and operation. In principle, the prediction approach has more universal applicability than the empirical approach because it looks for the cause – effect relationships through investigations and monitoring. However, eventually the predictive approach cannot avoid significant elements of pragmatism because investigations and monitoring provide the empirical basis for the structure of reasoning and parameters of the models (Harremoës, 2002). Thus, a static design and then a dynamic assessment is perhaps one of the most common ways to include models in the design. First, the component (Sewer or WwTP) is statically designed and then the model is built to assess the functioning of the system under different scenarios. Another conventional practice has been to design and operate various components of the UWwS in isolation. For example, the study of various design options for the sewer system often ends at the overflow structures and the WwTP inlet, whereas the role and function of the

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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008 WwTP and the receiving water body should also be taken into account (Butler and Schütze, 2005). The transfer across the interfaces of each subcomponent is characterized by static rules. For example, the flow to the WwTP under wet-weather conditions is limited to a value of the order of twice the peak dry-weather flow or the number of CSO discharges per year is restricted to a certain number (Rauch et al., 2005). However, in the contrary to static rules, the water quality-based approach to manage the pollution in urban systems demands the evaluation of the cause–effect relationships between loads from the wastewater system and effects in the receiving water. In addition, this water quality-oriented approach offers greater degrees of freedom for improving the wastewater system’s performance, because the choice of measures is not constrained by prescribed guidelines. Thus, the potential synergy originating from the interactions between the subsystems may be beneficially used to reduce the pollution impact. Optimal Design Identifying the best performance with a minimum cost design is an important issue when constructing sewer networks or WwTPs, and it implies having an “optimal design”. Designing sewer networks can be a time-consuming task that is largely based on trial and error where suitable pipe diameters and slopes combinations for all pipelines between manholes must be identified. Since there is a large range of possible slopes, diameters and roughness coefficients of pipes, only a small number of combinations of these parameters are usually analyzed in traditional design processes. The design of the WwTP also faces the same problem; usually a small number of combinations of loading rates and mean values of water quality in the influent are used to estimate the volumes of the reactors. Even more, in the case of activated sludge process, the possibilities of combinations of internal recycle of sludge; chemical dosage and aeration rate is enormous. Thus, the design, even using dynamic modelling approach, ends in the evaluation of a few scenarios that will depend on the expertise and skills of the designer. In addition to an already complex problem, using integrated approach and water quality-based objectives the design of an UWwS becomes a complex multiple – objective problem. Not many application of the integrated design of systems are found in the literature, most of the cases are oriented to optimize one component at the time or to optimize for example the sewers in a “predefined layout” of a sewer system or a static design of the WwTP. In the case of wastewater systems, Vojinovic et al (2006) posed the problem as a multi-extremum (global) optimisation problem. The multi-extremum optimisation is also referred as global optimisation, or, more generally, can be named global search. One of the widely used methods of global optimisation is genetic algorithm. For instance, Muschalla et al (2006), illustrates a case study in which multi-objective evolutionary algorithms were used to optimize the design of an urban drainage system according to water-quality oriented criteria. Control of Urban Wastewater Systems The degree and sophistication of the operation of most urban wastewater systems is generally low. Most sewer systems are operated passively, requiring little or no outside intervention. Where control is provided, it tends to be local such as in pumping stations. WwTPs sometimes have Supervisory Control and Data Acquisition (SCADA) systems with control centers containing mimic panels, but the level of automatic control are still normally low (Butler and Schütze, 2005). The lack of performance of a large number of existing WwTPs is due, on the one hand to infrastructure and equipment not allowing for the implementation of modern and effective treatment processes; and on the other hand, to inefficient management rules and a lack of control of the processes. The management of the plants is often based on empirical

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Optimization of Urban Wastewater Systems using Model Based Design and Control

11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008 and static rules that do not take into account the complexity and the dynamics of the influent water quality and quantity and the biological treatment process (Feller, 2002). The disturbing feature of this sub-optimum operation is that parts of the system might be idling while, at the same time, other parts are overloaded. This is where real time control becomes an option. Manipulate the system such that its capacity could be used better in order to achieve improved performance of the system (Schütze et al., 2003). Real Time Control (RTC) is defined by EPA (2006) in general as: a system that dynamically adjusts the operation of facilities in response to online measurements in the field to maintain and meet the operational objectives, both during dry and wet weather conditions. In other words, an urban wastewater system is controlled in real time if process variables are monitored in the system and, (almost) at the same time, used to operate actuators during the flow process (Schütze et al., 2004). The basic elements of an RTC system are sensors, which monitor the process evolution, actuators, which influence the process, controllers, which adjust actuators to achieve minimum deviations of the controlled process variable from its desired value (set-point), and data transmission systems, which are transmitting data between the different devices. Figure 3 shows a representation of the UWwS and the basic control elements. The design practices for sewer networks have been historically conservative and have included significant safety factors that result in for example, larger pipes in the collection system than really needed. As UWwS design does not normally include consideration of RTC, there are often opportunities to optimize the function of the existing system through operational strategies (EPA, 2006). However, the question is: why not design the system including Control strategies? In summary, there is a need for design and operation of sewer system, treatment plant and water receiving systems in an integrated way. There is also a need for design and operation to be based on a more realistic set of water quality criteria to be met by the performance of the system in total. This implies the use of integrated modelling tools in a dynamic way and the development of optimization routines that cope with multi-objective requirements in the modern UWwS.

Controller Data Communication System

Actuator Overflow

Actuator Air Pump

WQ Sensor

Flow Sensor Tank

Sewer CSO

Actuator Pump

WwTP WL & WQ Sensors

Water Receiving System

Figure 3. Urban Wastewater System including Control Components

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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008

DYNAMIC MODEL BASED DESIGN AND CONTROL Most sewers and WwTPs are still designed and operated on an empirical basis. Permissible discharges from the system to the receiving waters are still formulated much on an equally empirical basis. This is not only due to the lack of knowledge but also due to the significant conservatism in the profession. In order to tackle the discrepancy between the planned “design load” and the real operation of an UWwS, the proposed approach considers the improvement of the design of UWwSs using dynamic Model based Design and Control (MoDeCo). The model-based design, favours an understanding of the performance of the whole system concerning the dynamic nature of the load. Furthermore, the control strategies included in the design, help to avoid costly capital investments by improving the capability of the system to adapt, in real time, to the dynamic loads. The optimisation based on the water quality in the receiving system using evolutionary algorithms contributes to meet new regulations and higher performance requirements. The proposed combination of effective UWwS design and optimal control of their performance can bring benefits through the reduction in the pollution impacts and the reduction in the total capital invested. To evaluate the benefits from including the control strategies in the design process, a case study for Cali, Colombia was proposed. The aims of the case study are, therefore, to: • •

Determine what are the main benefits and drawbacks of using MoDeCo for UWwSs, Explain why this innovative approach brings about those benefits

The main output is a technological framework for applying dynamic Model based Design and Control in UWwS. The conceptual framework is presented in the Figure 4.

Static Design

Design Parameters

Expert Knowledge Control Strategy Control Module Input Data Sewer WwTP

Optimisation Module for Design

Input Data River

Sewer Model

WwTP Model

River Model

Strategy Parameters

Optimisation Module for Control

Performance Indicators Value Control Objective Function Value Design Objective Function

Optimum Design

Figure 4. Conceptual Scheme of Model Based Design and Control – MoDeCo

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Optimization of Urban Wastewater Systems using Model Based Design and Control

11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008

CASE STUDY OF CALI - COLOMBIA Cali is a city of 2.07 millions inhabitants and is planning the expansion of the city in an area of 1652 ha, located in the south part of the city. The expansion zone requires the development of the whole system including stormwater drainage, sanitary drainage and a new WwTP for the south UWwS. Planning the expansion zone is a very sensitive project because it increases the risk of discharge polluted water into the Cauca River. The main concern is existing discharge of wastewater through the South Canal, upstream of the inlet for the water supply system of Cali (Figure 5).

Melendez River

Lili River

South Canal

Expansion zone

CALI Inlet for Water Supply

Location of future WwTP

Cañaveralejo WwTP

Discharge point from South Canal Juanchito station

Cauca River

Figure 5. Components of the Urban Wastewater System of Cali - Colombia

Figure 6 shows the flow variation patterns in the Cauca River and the impact generated by the flush of the South Canal during a rainfall event in the oxygen dissolved of the river. Effect like the one depicted in the Figure 6 are nowadays more commune with disturbing consequences: fish kill and closing inlet for the water supply of the city. Thus, the new development zone not only increases the risk of discharge polluted water into the Cauca River, but also enhances water quality and ecosystem deterioration and, thus, imposes health risk for 1.8 million inhabitants that drink the treated water from the river. Therefore, planning and developing of the zone is an excellent opportunity to implement new concepts and best practices in combination with the traditional solution for wastewater management in the city.

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Figure 6: a. Flow variation of Cauca River. b. Effect of organic mater flushed from the South Canal in the dissolved oxygen of Cauca River at the gathering station Juanchito. Source (Vélez et al., 2006).

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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008 The use of MoDeCo approach will bring the possibility to optimise the design based in the ultimate goal of the system, “avoid the pollution of the Cauca River”. Performance indicators such as duration of oxygen concentration bellow a threshold value in the Cauca river can be used to optimise the design and operation of the system. The design of integrated control strategies can improve the overall behaviour of the system in response to the variability of perturbations, such as flush during rainfall – runoff events. The multi-objectives optimisation routines ensure a synthesis of optimum operation with optimum design of the urban wastewater system, leading to a system with the best performance at minimum cost. However, some counter-arguments may suggest not to implement MoDeCo approach for a city in developing countries. It appears to be a significant conservatism in the profession with respect to the use of hydroinformatics tools, namely integrated mathematical models, control strategies and optimisation algorithms. The design practices for sewer, for example, have historically been conservative and include significant safety factors that results in larger pipes in the collection system than really needed. As sewer network design usually does not include control strategies, they are often used only to optimise existing systems during their operation. However, the question is: can we afford a system that is sub-optimum designed and has to be adjusted during the operation? Or design the system including the optimum control strategies from the planning phase? One example of the lack of integrated design and operation is the existing Cañaveralejo WwTP, which nowadays is working with half of the build-in capacity because part of the sewer system was not connected to it. There is also apprehension from the practitioners and designers to include components in the wastewater systems that appear to be out of the conventional practice like monitoring sensors in sewers or adjustable weirs. Including those types of new elements in the systems can increase the risk of failure when there is no resilience in the system. However, this type of elements also bring the possibility to increase the capacity of the system to adjust on time to the variability of the perturbations (rainfalls, loading rates, etc). In Cali, the operators of the UWwS have gain experience in the use of hydroinformatics and control components to operate an already complex system. An example is the SCADA system, which is being developed for the UWwS. The implementation of new paradigms that consider sustainability, new architecture and greener upstream solutions are important component for the new design of UWwS. The MoDeCo approach has not only the potential to analyse the design and operation of centralised systems, and optimise them as an integrated system, but should be able to combine centralised systems with new approaches, such as local infiltration, local treatment and storage basins. The development of wastewater system for the expansion zone of Cali has critical consequences, because the failure of the system will lead to the impairment of the already very sensitive Cauca River. Thus, MoDeCo can bring more benefits in terms of optimal design with the best performance in the case of complex problems, were a reliability of the system is crucial.

CONCLUSIONS In this paper have been discussed the need for design and operation of sewer system, wastewater treatment plant and water receiving systems in an integrated way. Also have been discussed the need to design and operate UWwSs based on water quality criteria to be met in the receiving system.

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Optimization of Urban Wastewater Systems using Model Based Design and Control

11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008 The inherited concept of steady state design of UWwSs is challenged by new concepts. An innovative approach named Model based Design and Control (MoDeCo) is proposed. This implies the development and use of integrated modelling tools in a dynamic way and the development of optimisation routines that cope with multi-objective requirements of modern UWwS. The combination of effective UWwS design and optimal control of their performance bring benefits through the reduction in the pollution impacts and the reduction in the total capital invested. Thus, integrated design and control is indeed promising approach to increasing levels of service protection of the environment and enhanced sustainability. The next step is to prove the hypothesis proposed and for with that purpose the case study of Cali presented in this document is under development.

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