MODELLING SAINT MALO WWTP & DEVELOPMENT OF A DECISION SUPPORT TOOL FOR THE MANAGEMENT OF THE COMBINED SYSTEM SEWER-WWTP

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1 MODELLING SAINT MALO WWTP & DEVELOPMENT OF A DECISION SUPPORT TOOL FOR THE MANAGEMENT OF THE COMBINED SYSTEM SEWER-WWTP ABSTRACT C. Printemps*, G. Manic*, M. Zug**, S. Gauthier***, J. Meinhold* *Veolia Environnement - Anjou Recherche Chemin de la Digue, BP 76 Maisons Laffitte, 78603, France ** Générale des Eaux, Service Technique 3 rue Marcel Sembat Nantes, 44100, France *** Générale des Eaux, Agence Côte d Emeraude 17 rue A. Fresnel Saint Malo, 35417, France The regulation drawn from the French law on water of 1992 imposes on communities to treat their wastewater, except in case of exceptional rainy events. The seaside towns have also to respect the directive of 1975 concerning the quality of bathing water. Anjou Recherche launched in April 2001 an integrated study on the modelling of the sewage system of Saint Malo (Brittany, France), i.e. sewer network, wastewater treatment plant (WWTP) and receiving water. This study is embedded in a global project, with the overall aim to limit the impact of direct discharges towards the sea, and thus, preserve the quality of bathing water of the city. The method applied consists of obtaining a model for each part of the sewage system based on dedicated software. In a second step, these models are associated in a decision support tool especially made for operational use. The different models have been built, calibrated and validated using several data sets. A first version of the decision support tool has been completed and installed on site beginning of Currently, only the sewer network model is included in this version. Next developments will aim at integrating the WWTP model, before to consider its extension to the receiving water part. KEYWORDS Modelling, decision support tool, WWTP, sewage system, WEST. INTRODUCTION In 2001, Anjou Recherche began an integrated study on the modelling of the sewage system of Saint Malo (Brittany, France), i.e. sewer network, WWTP and receiving water. This study is embedded in a global project, with the overall aim to limit the impact of direct discharges towards the sea, and thus, preserve the quality of bathing water of the city. 7115

2 A sequential approach has been applied (Printemps, 2004). It consists of obtaining a model for each part of the sewage system based on dedicated software (Figure 1). In a second step, these models are associated in a decision support tool especially made for operational use. The modelling stage was started in 2001 with the building of a fully validated model under InfoWorks-CS to simulate the hydraulic aspects (Gogien et al., 2002) and pollutant transfers activities of the sewer network (Gogien et al., 2004). Subsequently a model of the WWTP was created under WEST (Printemps et al., 2006; Vanhooren et al., 2003). A 2-D model of the receiving water was built under Télémac (Denot et Lang, 2000). It offers the possibility to simulate the dispersion of pollutants discharged to the sea (Gogien et Zug, 2005). Figure 1 Scheme of the sewage system modeled This paper deals with two distinct parts. First, the set-up and calibration/validation of the WWTP model are presented, including a hydraulic study of the aeration tank. The second part covers the decision support tool developed for the management of the overall sewer network. Furthermore the methodology, allowing to incorporate the WWTP model in this tool, and its potential applications will be discussed. SITE DESCRIPTION The catchment area of the city covers about 1,800 hectares with an average imperviousness coefficient of 46% and is drained by a combined sewer network. The main catchment can be divided into twelve sub-catchments. At the outlet of each one, an interceptor chamber (Figure 2) directs the dry weather flow and the first part of the wet weather flow towards a storage tank, linked to the WWTP. In this case, the interceptor is open. The second part of the wet weather flow is sent to a channeled river (crossing the city) and then to the coast. In this other case, the interceptor is closed. 7116

3 Figure 2 Operation of the interceptor chambers (Gogien et Zug, 2005) The WWTP is an extended aeration plant with carbon oxidation, nitrification and denitrification. It treats the wastewater of 122,000 PE (population equivalent) and receives mainly municipal effluent. It consists of grit, sand and grease removal, and two activated sludge trains in parallel. Each of these is made up of an aeration tank (Figure 3) and two secondary clarifiers. The effluent has to respect the following legal limits: 30 mg/l of TSS (total suspended solids), 25 mg/l of BOD (biochemical oxygen demand), 90 mg/l of COD (chemical oxygen demand) and 16 mg/l of TKN (total Kjeldahl nitrogen). These characteristics are based on daily composite samples and have to be respected 95% of the time in accordance with the French Regulation. Figure 3 Views of the aeration tank Hydraulic study In order to identify the adequate model structure to be implemented into the WEST environment, i.e. number and position of completely stirred tank reactors (CSTR), the hydraulic behaviour of the aeration tank was analyzed. Hence, a 3-D model of the tank was built under the CFD (computational fluid dynamics) code, Fluent (Fluent Inc, 1998). The simulation results obtained show that the circulating velocity (or the recycling flow rate) depends essentially on the number of active impellers. The impact of the inlet flow rate is negligible. When the six impellers are active, the average velocity is equal to 0.30 and 0.28 m.s -1 in non aerated and aerated conditions, respectively (Cartland Glover et al., 2005). The RTD (residence time distribution) curves calculated by Fluent were reproduced under WEST in associating about 90 and 20 CSTR in non aerated and aerated conditions, respectively (Figure 4, left). In spite of the oscillations during the first hour, the shape of the signal during the rest of the period reflects the one of a completely mixed system (Figure 4, right), which is in line with the results of De Clercq et al. (1999). 7117

4 The short delay between both RTD curves (Figure 4, left) is due to the position of reactors in the systemic model. About a quarter of the CSTR considered has to be moved in the internal recycling loop, in order to describe the distance existing between the inlet and the outlet of the ditch (Figure 3). However, important part of the oscillation observed is present only within a small time interval (below some minutes), which probably will have a minor effect of the biokinetics, such as autotrophic biomass growth. Hence, simulation with only one CSTR could be sufficient. This is validated by comparing simulations with one and several CSTR and a recycling flowrate of 94,500 m 3.d -1. The results obtained show no significant differences in the effluent components (Printemps et al., 2006), and hence, it was decided to describe each aeration tank by one CSTR under WEST. Figure 4 Comparison of RTD (left), impact of recycling rate on the RTD and E(t) obtained with WEST for one CSTR (right) WWTP modelling The WWTP has been modelled using an extended ASM1-model (Lesouëf et al., 1992; Printemps, 2004), for the activated sludge conversion part, while the settling model of Takács et al. (1991) was used to describe the behaviour of secondary clarifiers. The model was calibrated and then validated on the basis of two intensive measurements campaigns (three and two weeks, respectively). In addition to the routine monitoring of the plant, daily composite samples were analysed at the WWTP influent and effluent. In parallel, an in-situ UV/Visible spectrometer (S::CAN TM Spectrolyzer 2 mm; Gruber et al., 2004) was applied to the influent, and two in-situ ammonia and nitrate sensors (WTW NitraLyt and AmmoLyt ) were located in one of the aeration tanks. Note that some additional analyses, such as COD and TN (total nitrogen), were performed on the mixed liquor and the return activated sludge (RAS). Figure 5 shows some results obtained during the validation stage of the model. Very high ammonia concentrations at the beginning of the period are due to the configuration of the plant. Indeed, from March 2 to 23, 2005, only one of both oxidation ditches was in operation in order to replace the air diffusers of the basin not used. Even if the model is not able to reproduce all the 7118

5 daily ammonia peaks measured in the ditch, we have judged that the results obtained on the global period, allowed us to consider the model correctly validated. The following stage of the project consisted of developing a decision support tool especially made for use by field-operators. Figure 5 Validation of the WWTP model - comparison between measured and simulated results (1 : mixed liquor suspended solids (MLSS) and SS concentration in the RAS ; 2 : nitrate concentration in the effluent ; 3 and 4 : ammonia concentrations in the aeration tank and in the effluent) Existing decision support tool Currently, only the sewer network model is included in the existing tool. It integrates the graphical interface programmed in Microsoft Visual Basic and the according model itself. The link between both elements is established based on an API module (application programming interface) of InfoWorks-CS. The architecture of the tool is divided into three layers (Manic et al., 2006). The first one links the process simulator with the in-situ instrumentation to collect, filter and treat raw data. The second layer uses the API module to handle simulations and calculations aspects. Finally, the third layer displays results through graphs or reports. The main interest of this architecture is to obtain a user-friendly interface without any compromise on the calculation capacities. The tool includes four user modes: (1) simulation using historical data, (2) simulation with new operational set-points, (3) forecast and (4) model update (Figure 6). 7119

6 The first one enables the simulation of a past period using archived data. For example, it supplies an estimation of discharged TSS and COD loads during rainfall events at each interceptor chamber (Figure 2). This will permit to be in line with the French legal requirement, which allows to use a model approach for monitoring of CSO (especially for cases where measurements are difficult to be accomplished). Furthermore it enables to improve the comprehension of what happened during a specific rainfall event. The second user mode offers the possibility to test different management strategies of the sewer network during a past period, in order to determine the better one in terms of wastewater overflow and intercepted pollution. The third mode allows the user to test the behaviour of the sewer network in a given situation (e.g. specific rainfall event with a structural modification of the sewer), aiming at forecasting and anticipation. A design rainfall generator was developed to simplify the creation of rainfall events. Finally, the fourth mode provides the possibility to modify parameters of the model and/or to adapt it, if needed, to future sewer network configuration/extensions. Extension at the combined system sewer/wwtp As WEST includes also an API module (Printemps et Amerlinck, 2004), it will be possible to integrate the WWTP model into the existing tool. A sequential approach has been chosen. In fact, both simulations will run one after the other, respecting the direction of the flow, output variables of the sewer model being used as input variables of the WWTP model. The future version of the tool will still include the same four user modes. The first one will allow the operator to verify the quality of both models on a study period, which will then be used to optimise the operation of units (mode 2) or anticipate impacts of specific events on the combined system sewer/wwtp (mode 3). For example, after having determined the management strategies of the sewer network to decrease the direct discharges towards the sea, it will be possible to ensure that the WWTP is able to treat this supplementary pollutant flow, and to test new management set-points. 7120

7 Figure 6 Screenshots of the decision support tool of Saint Malo sewer network (A : preview of historical datasets; B : definition of simulation settings in mode 1; C : comparison of sewer operation strategies in mode 2; D : rainfall events building in mode 3) CONCLUSIONS After having built a model for each unit of Saint Malo sewage system (e.g. sewer network, WWTP and receiving water), and then developed a decision support tool dedicated to the sewer network and made for an operational use, Anjou Recherche achieves now the extension of the existing tool at the combined system sewer/wwtp. ACKNOWLEDGMENTS The authors would like to thank the Urban Community of Saint Malo, which allows Veolia Environnement to study their sewage system. 7121

8 REFERENCES Cartland Glover, G., Printemps, C., Essemiani, K. et Meinhold, J. (2005). Modelling of wastewater treatment plants - How far shall we go with sophisticated modelling tools? 3rd IWA leading-edge conference on water and wastewater treatment technologies, Sapporo, Japan. (accepted for publication in Water Science & Technology). De Clercq, B., Coen, F., Vanderhaegen, B. et Vanrolleghem, P.A. (1999). Calibrating simple models for mixing and flow propagation in waste water treatment plants. Water Science & Technology, 39(4), Denot, T. et Lang, P. (2000). Apports de la modélisation numérique à l'étude de la dynamique hydro-sédimentaire de l'estuaire de la Rance. 6èmes Journées Nationales Génie côtier- Génie Civil, Caen, France. Fluent Inc. (1998). Users Guide, Fluent Incorporated, Centerra Park, Fluent 5. User's Guide Cavendish Court Labanon, NH03766, USA. Gogien, F. et Zug, M. (2005). Water quality modelling from sewer network to coastal water: a case study. The 1st International Conference on Environmental Science and Technology (January 23-26, 2005), New Orleans, USA. Gogien, F., Zug, M. et Le Luherne, J. (2002). Diagnosis of the real time controlled sewage system of Saint-Malo using modelling. 9th International Conference on Urban Drainage (ICUD), Portland, USA. Gogien, F., Zug, M., Le Luherne, J. et Jamet, G. (2004). Pollution modelling on the site of Saint- Malo: an interesting tool for sewer optimization. 5ème conférence Novatech, Lyon, France. Gruber, G., Winkler, S. et Pressl, A. (2004). Continuous Monitoring in Sewer Networks - an Approach for Quantification of Pollution Loads from CSO's into Surface Water Bodies. 4th IWA World Water Congress, Marrakech, Marocco. Lesouëf, A., Payraudeau, M., Rogalla, K. et Kleiber, B. (1992). Optimizing nitrogen removal reactor configurations by on-site calibration of the IAWPRC activated sludge model. Water Science & Technology, 25(6), Manic, G., Printemps, C., Zug, M. et Lemoine, C. (2006). Development and commissioning of decision support tools for sewerage management. Water Science & Technology, 53(4-5), Printemps, C. (2004). Apports de la modélisation pour la gestion des systèmes d'assainissement urbains. Thèse de doctorat, Université de Poitiers. Printemps, C. et Amerlinck, Y. (2004). Creating an operator interface for wastewater treatment works simulations. Water 21, Issue of April, Printemps, C., Zug, M. et Meinhold, J. (2006). Modélisation de la station d'épuration de Saint- Malo et développement d'un outil d'aide à la décision à l'échelle du couple réseau-step. 4ème colloque STIC et Environnement, Narbonne, France. Takács, I., Patry, G.G. et Nalasco, D. (1991). A dynamic model of clarification-thickening process. Water Research, 25(10), Vanhooren, H., Meirlaen, J., Amerlinck, Y., Claeys, F., Vangheluwe, H. et Vanrolleghem, P.A. (2003). WEST: Modelling biological wastewater treatment. Journal of Hydroinformatics, 5(1),