integrated water management - Springer Link

INTEGRATED WATER MANAGEMENT

Ramiro Neves1 , José S. Matos2 , Luís Fernandes1 and Filipa S. Ferreira2

1 Secção de Ambiente e Energia, Dept. Engª Mecânica do IST, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, [email protected], [email protected] 2

Secção de Hidráulica e Recursos Hídricos e Ambientais, Dept. Engª Civil e Arquitectura, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, [email protected], [email protected]

Abstract:

In this paper, an overview on the development and application of state of the art integrated water modelling tools to study water pollution, either from urban or agricultural origin, from source to final destination, and also of the research carried out at IST in the framework of integrated water management is described. The modelling tools are used for computing the urban load in a subcatchment of Lisbon metropolitan area for assessing the trophic levels of Tagus estuary and its relation with urban and agricultural loads. The strategy for modelling development at IST is also described, showing that modelling can be an important contribution for the integration of water management. Results have shown that modelling of the functioning of wastewater treatment plants is a mechanism for managing the urban wastewater loads and that the trophic level in the Tagus estuary is controlled by light penetration and not by nutrients. As a consequence, a reduction of the nutrient loads from urban origin or a 50% of the agricultural nutrient load would have no benefits in terms of trophic activity.

Key words:

Integrated Management, Modelling, Receiving Waters, Wastewater.

1.

INTRODUCTION

Water constitutes one of the most important limiting factors for the development of Society and, as a consequence, its management takes priority in the whole World. In the European Union, water management has been directly and indirectly subject of multiple directives, from which stand 421 M. Seabra Pereira (ed.), A Portrait of State-of-the-Art Research at the Technical University of Lisbon, 421–446. © 2007 Springer. Printed in the Netherlands.

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out the (i) Nitrates Directive, (ii) Urban Waste Water Directive, (iii) the Drinking Water Directive, (iv) Bathing Waters Directive and (v) Water Framework Directive. The Dangerous Substances Directive, the Shellfish Directive and the Habitats Directive are examples of other directives whose implementation requires the knowledge of the water dynamics. The multiplicity of legal diplomas which regulate water management is a consequence of a variety of aspects in which Water is involved, namely as a nutrient, as an habitat, as a leisure zone, as raw material for industry, as a transport vehicle and a final destiny for residues. The multiplicity of Institutions and Organisms involved in water management is a natural outcome of the different perspectives of the use of the water, but also of technological limitations, to whose resolution have contributed the R&D activities of Instituto Superior Técnico (IST), especially the ones developed in the framework of interdisciplinary integrated projects, which contribute to optimize solutions and reduce the high costs involved in pollution control. Pollution of urban origin reaches the environment through a drainage network from point discharges, being the effluent treatment level before the discharge dependent of the dimension of the town and of the receiving waters (according to the Urban Waste Waters Treatment Directive). Industrial discharges are subject to specific legislation, namely when involving dangerous substances. Pollution of agricultural origin presents diffuse characteristics, reaching the environment through surface run-off and/or underground waters. The impacts of agricultural activities in the environment are normally due to nutrient lixiviation and eventually of toxic substances (normally pesticides and/or herbicides) and due to soil erosion. Eutrophication due to nutrients excess is nowadays in Europe the main concern in terms of water quality management, requiring the integrated management of nutrients of agriculture and urban origin, taking also into consideration nitrogen atmospheric deposition. The increase of trophic activity associated to eutrophication may originate changes in species and anoxic situations which can endanger habitats. Reservoir and estuaries are systems with high residence time of water and especially of particulate matter, therefore constituting the most susceptible areas to eutrophication, especially reservoirs where residence time has an order of one year, while in the latter it can vary from days to months. Thus, the trophic level of reservoirs essentially depends on nutrients availability, while in estuaries it can also be limited by residence time and by light availability associated to sediment dynamics. Eutrophication management in reservoirs and estuaries requires the determination of the maximum nutrient loads possible that these water

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bodies can receive and the ability to control nutrient generation in the basin, which depends on the intensity of the sources and on the retention capacity of the basin, dependent on soil biogeochemical processes and in rivers. Thus, integrated water management requires the ability to know water movement and biogeochemical processes occurring in the water bodies which are subject to higher risks (reservoirs and estuaries), but also to know the movement and the biogeochemical processes occurring between the sources and the receiving waters. These processes are normally simulated with basin models, where urban areas are treated as point sources. Urban origin discharges depend on interception capacity of effluents and on Waste Water Treatment Plants (WWTP’s) efficiency. In this paper, the state of the art of integrated water modelling and the tools developed and/or used at IST are described, using as case study the Tagus estuary, to which the integrated management is particularly important, given the dimension of the urban discharges (corresponding to about 2.5 million equivalent inhabitants); and also given the dimension of the load from the Tagus river, whose basin is the biggest in the Iberian Peninsula, draining a important region from the agriculture and urban point of view, especially in Spain.

2.

URBAN WASTEWATER MANAGEMENT

2.1

General considerations

The present concept of urban sewerage dates back some 200 years. The European cities grew at a rate and to an extent that was no longer sustainable, due to an internal handling of water and waste in a way that created foul conditions in general and unacceptable risk of waterborne diseases in particular. The development of communal, holistic approaches to handling of water in cities has been an indisputable success according to the paradigms governing city development for more than a century. The cities became well regulated. A certain standard with paved streets, gutters and sidewalks, sub-terrain water supply and drainage pipes, nicely contained rivers and lakes with stone or concrete walls were the standard that still dominates the appearance the European city [1]. With the established classical concept of sewerage, it was controlled the waterborne diseases in the city. Meanwhile, demands of society have developed, including promotion of more sustainable approaches, in terms of better performance with respect to resources, ethics and economics; new architectural features of

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water related structures in cities; necessity of control impacts on the global environment and broadening of transparency in decision processes. The news demands lead to new challenges that are being solved with new tools and new knowledge: analytical tools for a large spectrum of chemicals and pollution parameters; new database technology, including GIS (geographical information systems) and DSS (decision support systems) and new computer simulation tools. The assessment of the environmental performance of urban wastewater systems is often a crucial issue, particularly in the developing countries of the World. In Europe, this aspect assumes a special relevance in view of the objectives set by the Water Framework Directive that aims to achieve a good ecological status of all water bodies. The assessment is an important step to optimize the performance of urban wastewater systems and to evaluate proper rehabilitation measures. To properly operate and manage urban drainage systems, numerical models may be indispensable [2]. Moreover, urban drainage components, including sewer systems and wastewater treatment plants (WWTP), should be dealt with jointly, providing a holistic and more sustainable approach. In fact, the integrated operation of the sewer network and the WWTP may be required to reduce total emissions on the receiving waters [3]. Therefore, over more than a decade, several integrated modelling approaches were developed, some of them also including receiving waters [4, 5, 6]. The modelling approaches, particularly integrated modelling approaches are seldom applied by practitioners for planning urban wastewater systems, particularly due to lack of data or deficient knowledge.

2.2

Modelling the performance of sewer systems

The deterministic modelling of water motion in sewer networks is undoubtedly one of the success stories in the field. The application of the unsteady open channel flow model, based on the Saint Venant equations allowed an accurate description of the hydraulics, to an extent that "if the simulation does not fit the results very well, then the information about the system may be faulty, rather than the model” [7]. To make the Saint Venant equations applicable for surcharged flows, Preissmann introduced the concept of a hypothetical open slot at the top of the pipe. Since the solution of the Saint Venant equations (or their approximations) is computationally demanding, simpler flow routing models have been developed. These hydrological models generally respect continuity equation but replace the conservation of momentum with some conceptual


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relationship. The underlying concept is a cascade of reservoirs in series with the water being routed downstream. Due to simplicity, the reservoir cascade approach allows rapid simulation; on the other hand, effects such backwater pressurized flows cannot be simulated at least not directly. This constitutes a serious limitation, in particular for looped or flat networks. One of the main advantages of these models is that the approach is most easily extended for additional consideration of the transport phenomena. Sewer simulation, hydrological flow routing methods are applied seldom for prediction of hydrodynamics alone but usually in connection with the simulation of water quality [4]. Since the early 1970s, the most frequent modelling approach used to simulate pollutant transport in sewer systems takes into account four main steps: pollutant accumulation; pollutant wash off; pollutant transport and pollutant processes. Simulations of the hydrology and the hydraulics of sewer systems have been well accepted, especially with respect to flooding and hydraulic loads on treatment plants and receiving waters, as well simplified simulation of pollutant transport and pollution discharged from combined sewer overflows. Perhaps the most known available models are: SWMM (Storm Water Management Model, from the US Environmental Protection Agency), MOUSE (Modelling Urban SEwer Systems, developed by the Danish Hydraulic Institute), INFORWORKS (developed by Wallingford Software) and others, such as HYDRA, Sewer CAD, XP-SWMM, FLUPOL and SAMBA.

2.3

Wastewater treatment modelling

The modelling of the wastewater treatment subsystem is quite different from the modelling of sewer systems in two respects: first, the underlying hydraulics can nearly always be approximated crudely and, second, the modelling is built up around unit processes. The mathematical description of the unit processes usually requires the specification of large numbers of components and of numerous interactions. Hunge [4] introduced a matrix form for the presentation of the model reactions which has become standard in all aspects of water quality modelling. This overview on unit processes is limited to some of the most important ones (activated sludge and clarifiers). The modelling of the activated sludge process has clearly drawn most of the unit process modelling since the 1950ties and many different approaches have been explored. Since the groundbreaking work of the IAWPRC Task Group on Mathematical Modelling of the Activated Sludge Process in the early 1980ties, most model development work has been geared around what

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called the industry standard suite of Activated Sludge Models [9]. These models have shown to adequately describe the behavior of nitrogen and biological and chemical phosphorus removal processes, more particularly in terms of the oxygen demand, sludge production and nitrogen/phosphorus removal. More recently, refinements of the models were presented in which storage processes are included. These models have also lead to the introduction of simulation software in the consulting an engineering companies and have been a driving force for a more detailed understanding of the processes, leading to considerably improved operation treatment plants. Clarifiers act on particulate matter that one either wants to prevent from entering the plant (primary clarification), or from leaving the system (secondary or final clarification). Another objective of such unit processes is the thickening, either to increase the biological activity in the bioreactors, or to prepare for waste sludge treatment. Models for these systems are classified according to their spatial resolution, going from simple 0- to complex 3- dimensional models that require application of computational fluid dynamics models. The 0- dimensional models only separate a particulate-rich stream from a (nearly) particulate-free stream and have no volume, relating to the assumption that no accumulation of mass occurs in the clarifier. The most popular clarifier models that can reasonably describe both the separation process and the dynamic mass accumulation in the clarifier are the so-called ID-models. Since usually only 10 layers are applied, the common approach is in fact a reactors-in-series approach rather than a discretization of an ID partial differential equation. Any clarifier model contains a settling velocity function that describes its dependence on the local concentration (settling is hindered increasingly with concentration above a certain threshold value) and the sludge volume index as an indicator for the settling capacity. The empirical model of Takács [10] is currently the most widely applied one. The models are available to simulate the performance of wastewater treatment plants (being based on activated sludge or biofilms). Some of the most important models in terms of application are: EFOR (developed by the Danish Hydraulic Institute), STOAT (developed by the Water Research Center), SASSPRO (developed by the Science Traveler International), BIOWIN (from Envirosim Associates, Lta) and CPS-X (from Hydromanties, Inc.).

2.4

Integrated modelling

Even though one of the first mentions of the idea of integrated modelling was made by Beck [11] and the first integrated model was applied 20 years


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ago [12], it took until the early 1990s before the concepts started to be disseminated in larger scale. Whereas early approaches considered only total emissions from sewer system and treatment plant, the work by Schütze [13] and the work of Vanrolleghem [14] were the first to include deterministic models of the total system. These studies revealed the importance of consideration of both, treatment plant effluent as well as Combined Sewer Overflows (CSO) discharges, for a proper assessment of impacts of storm events on the receiving water body. The Danish Hydraulic Institute (DHI) and Water Research Center (WRc) developed an “Integrated Catchment Simulator (ICS)” in a large EU-funded “Technology Validation Project”. ICS is basically a graphical interface for setting up and running integrated models with feed forward feed back of information. The present ICS version includes existing models for sewers (MOUSE), rivers (MIKE 11), wastewater treatment plants (STOAT) and coastal areas. During the course of this project, then fairly complex constituent models were linked in various stages; first in a sequential way, later in a simultaneous way. The complexity of the sub-modules, however, currently limits the application of ICS. The simulator platform WEST follows a different pathway. Although originally developed for wastewater treatment modelling, it can be seen as a general simulation environment for computing. The concept puts a limit to the description of water motion and transport processes in the elements but allows to implement more or less freely different conversion models for the different elements (representing catchments, CSO structures, reactors and clarifiers. WEST is predominantly an environment for the development of fast surrogate models for the purpose of long term simulation. SIMBA® is a simulation platform running on top of MATLABTM/SIMULINKTM. Models are available for sewer systems, treatment plants and rivers. The general principle is similar to the network concept already presented for the example of WEST, however, the use of the general purpose simulation environment MATLABTTM/SIMULINKTTM allows the user to add its own modules to fit the actual modelling. Thereby, the distinction between model developer and model user is largely removed. This system is also a convenient tool for optimization of the overall performance of the system. Basically, it can be stated that today a number of tools are available which allows the urban wastewater system to be considered in simulation as what it indeed is - one single system [4]. Nevertheless, and due to the systems complexity, numerical models generally require a large amount of data in order to build the physical representation of the system and to calibrate and validate all the significant model parameters. Data requirements include the catchments surface

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characterization (i.e., imperviousness area, ground slopes and usage), data on sewer system characteristics (i.e., geometry and dimensions of pipes and structures, storage volumes, pumping capacities) and hydraulic loads (namely from dry weather flow and runoff). Errors or omissions in the database contribute to model structure uncertainty, which is seldom accounted for and may lead to incorrect decisions if models are not properly calibrated. Also due to the systems complexity and the commonly severe lack of data, models and particularly integrated approaches are seldom applied by practitioners for planning urban drainage systems [15]. Furthermore, there is commonly an incompatibility between modelling time requirements and the time demands of decision makers. In view of these limitations, a simplified integrated concept for assessing and grading the environmental performance of urban drainage systems was developed in IST/UTL [16]. The Integrated Simplified Approach (ISA) focuses on situations in which the application of complex models is particularly difficult or involves a high level of uncertainties. Considering the simplicity of the ISA concept, it should be specially applied in cases of scarceness of data and during initial phases of the planning processes. The ISA concept can be considered a management support tool that is intended to assess the integrated environmental performance of urban wastewater systems (including combined, separate or partially separate sewers and WWTP). The ISA concept can be applied to simple drainage basins or to basins in series or in parallel with the sewer lines and was already applied to the Lisbon wastewater system. On chapter 3 of the paper, a case study is presented, where a detailed integrated approach was followed, using MOUSE for transport in sewers and EFOR for treatment purposes. The models were calibrated with real data.

3.

CASE STUDY: THE URBAN DRAINAGE SYSTEM OF S. JOÃO DA TALHA

3.1

System characteristics

The urban drainage system of S. João da Talha serves the civil parishes of Bobadela, S. Iria da Azóia and S. João da Talha, in the municipality of Loures. The system includes a wastewater treatment plant (WWTP) and two main gravity interceptors, namely the South Interceptor and the North Interceptor.


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These interceptors were built in very unfavorable conditions due to the characteristics of the foundation soils and to the high freatic levels, having suffered subsequent differential settlements. Recent topographic surveys demonstrate the existence of uneven slopes along the interceptors, different from the designed ones. Some sewer stretches present upward sloping. Consequently, self-cleansing velocities are seldom verified, thus leading to the deposition of sediments and the frequent occurrence of surcharges. The interceptors transport the effluents of combined and separate domestic sewer systems. Therefore, combined sewer overflows (CSO) take place during rain storms, discharging into the Tagus estuary and contributing to the pollution of receiving waters. The North Interceptor drains the effluent of most of the regions’ industries effluent. It length is nearly 3,8 km and it presents an initial stretch of 315 mm diameter, intermediate stretches of 400, 600 and 800 mm diameter and, after the connection with South Interceptor, a small stretch that leads to the WWTP entrance of 1000 mm diameter. The South Interceptor is around 2 km long and its diameters vary between 400 and 600 mm. Each interceptor has a weir through which the overflows are discharged into the receiving waters. As illustrated in Figure 1, S. João da Talha wastewater treatment plant is located in Bobadela between the national road EN 10 and the railway. The WWTP, operating since 1997, was designed to serve 130 000 equivalent population (e.p.) in the design period. Nowadays, more of 65% of the treated wastewater has an industrial origin (IST, 2005).

Figure 1. Location of S. João da Talha wastewater treatment plant.

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The WWTP is an activated sludge plant that includes the following treatment stages for the liquid phase: x

x x x x

preliminary treatment (after the wastewater enters the treatment plant collection wet-well, it is pumped by six Arquimedes screw pumps and enters the preliminary treatment, which includes screening, sand, grit and fats removal, flow measurement and a homogenation tank); physical-chemical treatment and primary settling; biological treatment by activated sludge; secondary settling; final discharge into the estuary of Tagus river.

Sludge is treated in thickeners, anaerobic mesophilic digesters and mechanical centrifuges and, subsequently, land applied.

3.2

Field experiments in S. João da Talha WWTP

The experimental work was carried out in order to characterize quantitatively and qualitatively the wastewater in S. João da Talha WWTP, during dry weather. Two experimental campaigns were carried out in the days 12/13 and 26/27 of January 2005. The campaigns included the collection of wastewater samples in the following sections: at the entrance of the WWTP (SA1 (CJ1)), downstream of the primary treatment (SA5), in the final effluent (SA7), in the sludge supernatant stream (SAE), in the aeration tanks (SL6.1) and in the recirculation stream (SLR). Samples were collected at 22 h, 0 h, 3 h, 5 h, 10 h, 12 h, 14 h, 16 h, 18 h and 20 h, and the following quality parameters were determined: temperature, pH, dissolved oxygen (DO), conductivity, COD, BOD5, TSS, total nitrogen, nitrites, nitrates, Kjeldahl nitrogen, total phosphorus and total coliforms (TC). At the same time, effluent, influent, recirculation and supernatant flows were continually measured. In Figure 2, the major analytic results of the experimental campaigns that took place on 12/13 of January 2005 are presented.

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3.3

Integrated modelling of the S. João da Talha drainage system

The mathematical simulation of the environmental and hydraulic performance of the interceptor system was made using the program MOUSE (Modelling package will go Urban drainage and SEwers systems), developed by the Danish Hydraulic Institute (DHI). This program carries out the computation of unsteady flows in pipe networks and models both the hydrological and the hydrodynamical aspects of the urban drainage systems. Initially, the detailed physical characterization of all the components of the drainage system (including sewers, manholes and overflow weirs) was made. Drainage catchments were also described, including parameters such as the area of the catchments, population served, percentage of impervious areas, times of concentration and locations of the nodes where the catchments are connected. The model of the system included, besides the North and South Interceptors, the final stretch that connects the treatment plant collection wet-well to the stretch located immediately before the screening equipment. The performance of the weir wall located in the collection wet-well and of the final sewer that discharges the treated effluent into Tagus river (or the wastewater that exceeds the WWTP capacity) were also simulated. In the node representing the estuary of the Tagus river, the variation of the outlet water level due to tidal effects was taken into account.

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In Figure 3 a schematic representation of the interceptor systems’ model is presented. The zoom in refers to the final sections of the North and South Interceptors, next to the WWTP, and includes the Arquimedes screw pumps (stretch ETAR – OE), the general by-pass of the WWTP (stretch ETAR – OE-jus) and the final discharge into the estuary of the Tagus river (stretch OE-jus – Cx.1).

Figure 3. Schematic representation of the interceptor systems’ model.

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The mathematical simulation of the WWTP performance was made using the software EFOR, developed in the decades of 80’s and 90’s by a group of Danish consultants (Krüger TO/S and Emolet Date) in collaboration with the “Technical University of Denmark”. In December 2000 the software was included in the package of the DHI. In the present case study, EFOR was used in an integrated way with the model MOUSE. The program EFOR comprises blocks that can be interconnected by links and that represent the inflow to the WWTP, reactors, settlers, dosing units which allow adding organic or chemical additives to the activated sludge system, pumps, valves, rotors and diffusers, outlets and the excess sludge leaving the system. The characteristics of influent wastewaters can be introduced by the user or edited (based on pre-definite types) and are submitted to mass balances: the values not explicitly specified are estimated through algorithms that consider the relationships between different parameters. The program allows the implementation of control loops referring to aeration, excess sludge, sludge recirculation and chemical additives dosage. The controllers may be configured in order to activate or deactivate a control device in response to the values measured by sensors associated to the WWTP units. Different types of controllers may be used, such as timer, step, on/off and PID (proportional, integral, derivative) controllers. To simulate the biological reactors of the S. João da Talha WWTP, the CNDP model was considered. This model is based in the IWA models ASM1, ASM-2 and ASM-2d and is the only model in EFOR that takes into account the dosage of chemical additives. The primary and secondary settlers were simulated considering a simple model of two layers and a flux model, respectively. The model of the WWTP, presented in Figure 6, was developed in view of the physical characteristics of each treatment unit and its equipment, as well as the operation criteria and methodologies implemented in this WWTP.

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Figure 6. Schematic representation of the WWTP model.

The model was run for the periods of time coincident with the experimental campaigns and for the rain event that occurred in 30-10-1988. The simulation results include flows in links, pollutant concentrations and process rates in all the simulated units. In Figure 7, Figure 8 and Figure 9 the simulation results obtained from 22h of 12/Jan/05 to 22h of 13/Jan/05 are presented. Figure 7 refers to the WWTP inflow (Inlet1), final effluent flow (Outlet1), recirculation flow (SS1->AS1) and excess sludge flow (WS1 – secondary sludge; WS2 –primary sludge). In Figure 8 the variation of the DO and TSS in the aeration tank (i.e., MLSS) are presented. Figure 9 refers to the final effluent characteristics in terms of the following parameters: COD, TSS, total phosphorus, DO and total nitrogen.

Figure 7. Influent, effluent, recirculation and excess sludge flows.


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Figure 8. Variation of the DO and TSS in the aeration tank.

Figure 9. Final effluent COD, TSS, total phosphorus, DO and total nitrogen concentrations.

3.4

Conclusions

The S. João da Talha case study has demonstrated the ability for integrated modelling of the performance of sewer systems and treatment plants, with acceptable simulation of flows and pollutant concentrations along the treatment units. Anyway, special difficulties were faced, in terms of simulations of suspended solids in the second clarifier. Modelling of this case study was expected to be especial difficult, taken into account the relevant industrial origin of the influent.

4.

MODELLING RECEIVING WATERS

In the previous chapter, the models for simulation and management of urban waste waters and its application to the S. João da Talha WWTP were

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described. The drainage network models require solving a two-dimensional problem (urban areas can be considered as bi-dimensional), but which locally are one-dimensional (if one considers streets as lines). As a consequence, these models require an important empirical knowledge and are normally developed by institutions of applied character giving origin to commercial modelling packages. The same occurs with WWTP’s, where treatment involves well known biogeochemical and separation processes and where the success depends on the application of practical details whose study includes a high experimental empirical component. The difficulties associated with the simulation of receiving waters are related to the great number of processes involved and to its spatial-temporal variability and also to the effects of contaminants over the biota, directly and indirectly through habitat changes. As a consequence of this complexity, the management tools require the simulation of processes and thus its development it’s normally associated to research institutions. Hydrodynamic modelling was initiated in the early 1960’s, with the birth of computation, a decade where the first temporal discretization methods for flows with hydrostatic pressure were published [17, 18] and developed for two-dimensional vertically integrated models. In the 1970’s, the number of applications was multiplied and extensive research on numerical methods was carried out, namely on forms to minimize numerical diffusion introduced from solving advection terms (e.g. [19, 20]). Three-dimensional models necessary to simulate oceanic circulation had a high development in the 1980’s, benefiting from the increase in computing capacity and in the breakthroughs in turbulence modelling based on work since the 1970’s which had in Rodi [21] one of its main pioneers. In the 1990’s, hydrodynamic models were consolidated and several models with great visibility started to emerge, e.g. POM [22], MOM [23] but also from European schools, e.g. GHER model [24]. Benefiting from technological advances, including both hardware and software (e.g. compilers, data management, graphical computation), from the second half of the 1990’s, the dawn of integrated models, coupling modules developed by several authors, was witnessed. Turbulence modelling packages like GOTM [25] constitute one of the first examples of this integration, but coupling GOTM to other models constitutes a second level integration example. Together with the development of hydrodynamic models, ecological models were also developed. Among the pioneer models one can mention WASP developed at EPA [26] and BOEDE model developed at NIOZ [27]. These models were developed in boxes and in former times used a time step of one day, being the short term variability of flow (e.g. tidal) accounted using diffusion coefficients. Ecological models have improved a lot during the 1980’s and 1990’s, benefiting from the scientific and technological


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progress and have been coupled to physical (hydrodynamic) models thus generating the present integrated models. Current research on modelling is oriented towards operational modelling, integrating different disciplines and assimilating as much field data as possible, with especial emphasis for remote sensing. Modelling at UTL followed the world trends and benefited from high investments on computing systems in the 1980’s. The development of MOHID system (http://www.mohid.com) was initiated at that time [28] as a 2D hydrodynamic model and was subsequently developed for becoming an integrated modelling system for tidal flow in estuaries and progressively generalized to waves [29], water quality [30], three-dimensional flows [31], new numerical methods [32], extended set of different open boundary conditions [33] and finally to be reorganized in an integrated perspective in order to accommodate alternative modules for different processes [34]. The model evolution enabled to couple alternative modules to compute biogeochemical and water quality processes [35, 36, 37], the broadening to flow through porous media [38], model water flow in a river basin [39], and ocean circulation [40]. This model is a working tool of the environmental modelling group of MARETEC research centre, having been used in more than 30 research projects, 50% of which with European funds and currently has around 500 registered users in its online website.

4.1

Integrated modelling

An ideal integrated modelling system should consider the water cycle from the moment water is evaporated from the ocean until it returns to it through the rivers, and should also consider the biogeochemical processes which occur during this path, from the atmosphere to the ocean itself. Presently, there are models that study the different compartments within the water cycle, which integration allows stepping towards the ideal integrated model. Every time two models of adjacent compartments are integrated, one boundary condition is eliminated. Boundary conditions are normally a source of uncertainty to the models. Thus, an integrated model should include: a meteorological model, a basin model (including surface waters, the vadose zone and aquifers), a model for the estuaries and coastal areas, a model for ocean circulation and an urban area model, as described in chapter 2. If this model does not exist, the coupling is made admitting the fluxes are exclusively determined by one of the compartments (e.g. the meteorological model provides winds, heat fluxes and precipitation to the watershed and ocean circulation models). The basin model produces river flows which are used as a boundary condition in reservoir and estuarine

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models. In estuaries, subjected to tide, the downstream boundary condition is more complex, as there, the flow is reversible. Thus, the study of estuaries demands the dynamic coupling of ocean models and estuarine models. The challenges when imposing boundary conditions when the flow is reversible require the consideration of high resolution models nested into large scale models with a coarser resolution of the computational grid. Figure 10 presents an application using nested models to simulate flow in the Western and Eastern Scheldt (The Netherlands). A coarse grid model simulates the southern area of the North Sea computing boundary conditions to be imposed in the maritime boundary of the estuaries. Figure 11 schematically represents the processes integration structure in MOHID model, which has two main modules, one for the water column and another for the sediments. Between these two modules and between the water column and the atmosphere there are interface modules. Between the water column and the sediments the interface is dynamic, allowing information to pass in both ways. The figure also presents the processes included in each module and the atmosphere where the processes can be simulated by a meteorological model. Figure 12 shows the wind velocity and temperature fields used to force MOHID calculated by the MM5 model operated at Secção de Ambiente e Energia of DEM - IST (http://meteo.ist.utl.pt). The dynamical coupling of these two models is in progress and will allow in the future to improve meteorological and oceanic forecast associated with small scale processes.

Figure 10. Example of an application of MOHID model in the Scheldts estuaries (The Netherlands) and in the southern North Sea using a nested models system to impose boundary conditions in the sea open boundary.


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Figure 11. Schematic representation of MOHID modules structure: in the upper part the water column is represented and the sediments below. For each subsystem the main modules of the model are indicated.

Figure 12. Example of a wind velocity and temperature field calculated by the MM5 model operated at Secção de Ambiente e Energia do DEM/IST (http://meteo.ist.utl.pt).

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Figure 13. Schematic representation of the processes simulated by hydrologic model of MOHID.

Figure 13 schematically represents a hydrographical basin, the water fluxes and the equations that represent the flow in each component of the basin (surface run-off, rivers and soil). This model is forced at the surface by the atmosphere module (precipitation, radiation, heat and evapotranspiration) and topographic information, soil properties and use to calculate the fluxes and the properties of the water reaching reservoirs and estuaries downstream.

4.2

Tagus estuary example of a MOHID application

The Tagus estuary is one of the largest in Europe and is subjected to important urban and agricultural loads (transported by the Tagus and Sorraia rivers), thus being worthy of attention by the scientific community and environmental managers from which result high quantities of data and a great deal of questions. The Tagus estuary is therefore an excellence case for mathematical modelling. In the Tagus, the model has been applied in the framework of national and international research projects and in the framework of consulting projects for companies and state authorities, from which can be pointed Instituto da Água (Portuguese National Water Authorities), SIMTEJO and SANEST. The study of trophic processes and nutrient dynamics in the estuary, with the intention of assess eutrophication risks is particularly interesting to illustrate the potential of integrated modelling.


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Figure 14 shows the current velocity fields during ebb. The figure presents the maximum velocity area in the exit channel, which give origin to an ebb jet that together with vertical mixing processes controlling the mixing of the Tagus river water with ocean water. Based on hydrodynamics the main ecological processes occurring in the estuary were simulated as well as the fate of the nutrients loaded into the estuary. In order to integrate the results, the estuary was divided into 10 boxes and the fluxes along the interfaces of those boxes were integrated along one year.

Figure 14. Velocity field in the Tagus estuary during ebb

Figure 15 shows the computed nitrate and phytoplankton fluxes along the boxes interfaces represented in the figures and integrated along one year [36]. The figure shows that the quantity of nitrate which is exported by the estuary (15300 tons/year) during one year is almost similar to the amount of imported nitrate (14900 tons/year, from which 11600 tons/year from the Tagus river). The figure also shows that the estuary is a net producer of phytoplankton (around 7000 tons of carbon and consequently 2000 tons of nitrogen). The combined analysis of these results, and also of ammonia and particulate organic nitrogen, show that the estuary imports nitrogen in the form of nitrate and ammonia, and that it exports it in the form of phytoplankton and dissolved organic nitrogen.

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Figure 15. Nitrate and phytoplankton fluxes in the Tagus estuary during 1 year simulated with the MOHID model [36].

Figure 16 represents nitrate field measurements function of salinity (data from the 2004/2005 estuary monitoring program promoted by SIMTEJO). The figure shows the approximately linear evolution of nitrate along the salinity gradient, in a trend line with a negative slope with a river concentration of 1.5mgN/l and a sea concentration of 0.2 mgN/l. The figure demonstrates that in the lower salinity areas the points tend o be below the trend line, indicating uptake and in the higher salinity areas the points are above the line, indicating regeneration, which is consistent with model results. The conservative behavior of nitrate in the estuary is a consequence of primary production limitation by light, which penetration in the water column is limited by the turbidity associated with fine sediment resuspension on the tidal flats, which sum up to 30% of the estuary’s area.

Figure 16. Nitrate in the Tagus estuary function of salinity. The linear trend suggests a conservative behavior, with some uptake in low salinity areas and regeneration in higher salinity areas.


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This model was used to study management scenarios, having been concluded that there are no advantages in removing nutrients of urban origin because primary production is not limited by nutrients but by light availability. A scenario of 50% reduction of nutrient loads from the river (e.g. reformulation of agricultural practices) was also tested, being verified that this reduction is not sufficient to alter the trophic activity of the estuary.

5.

FINAL REMARKS

In this paper the work done at IST is described in relation to the contribution to integrated management of water, with special attention to urban wastewater modelling and eutrophication of inland and coastal surface waters. Special attention was also paid to the issue on loads of urban origin and to the ability of the receiving waters to receive and assimilate these loads without creating risks of eutrophication. The text is not in-depth in terms of capacities of the presented models or in terms of the existing capacities available at IST in this subject. However it is illustrative of the potential of integrated water management and of the contribution of IST to provide this objective. The paper is also illustrative of the advantage of using modelling tools in water management. The case study, Tagus estuary, shows that integrated modelling is one of the most efficient ways to contribute to a sustainable management of the estuary, namely in terms of nutrients loads. MOHID has been used to study other management scenarios, namely in terms of water microbiologic contamination and heavy metal sediment contamination, two areas where the interaction with the urban and industrial effluent management is particularly important.

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