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DESASS: A software tool for designing, simulating and optimising WWTPs
Environmental Modelling & Software 23 (2008) 19e26 www.elsevier.com/locate/envsoft
DESASS: A software tool for designing, simulating and optimising WWTPs J. Ferrer a, A. Seco b, J. Serralta a, J. Ribes b,*, J. Manga c, E. Asensi a, J.J. Morenilla d, F. Llavador d a
Dpto. Ingenierı´a Hidra´ulica y Medio Ambiente, Universidad Polite´cnica de Valencia, Camino de Vera, s/n, 46022 Valencia, Spain b Dpto. Ingenierı´a Quı´mica, Universitat de Vale`ncia, Doctor Moliner, 50, 46100 Burjassot (Valencia), Spain c Dpto. Ingenierı´a Civil, Universidad del Norte, Km 5, Antigua Vı´a Pto. Colombia, Barranquilla, Colombia d Entidad Pu´blica de Saneamiento de Aguas Residuales de la Comunidad Valenciana, C/ Alvaro de Baza´n, no. 10 Entl., 46010 Vale`ncia, Spain Received 13 January 2006; received in revised form 27 March 2007; accepted 3 April 2007 Available online 23 May 2007
Abstract This paper presents a very useful software tool to design, simulate and optimise wastewater treatment plants. The program is called DESASS (DEsign and Simulation of Activated Sludge Systems) and has been developed by CALAGUA research group. The mathematical model implemented is the Biological Nutrient Removal Model No.1 (BNRM1) which allows simulating the most important physical, chemical and biological processes taking place in treatment plants. DESASS calculates the performance under steady or transient state of whole treatment schemes including primary settlers, volatile fatty acid generation systems by primary sludge fermentation, activated sludge systems for biological organic matter and nutrient removal, chemical phosphorus precipitation, secondary settlers, gravity thickeners and sludge digesters (aerobic and anaerobic). Biological conversions occurring in settlers and thickeners (primary sludge fermentation, denitrification) are also taken into account, i.e. they are considered as reactive elements. DESASS also includes pH calculation coupled to biological processes in all the elements, so pH effect on biological processes can be directly simulated. Furthermore, the effect of sidestreams on nutrient removal efficiency can be estimated because the performance of the whole plant can be simulated. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: BNRM1; Wastewater; WWTPs design; Modelling; Simulation; Optimisation; Software tool; pH calculation
Software availability Name of software: DESASS (DEsign and Simulation of Activated Sludge Systems) Developer: CALAGUA Research Group Contact address: Jose´ Ferrer Polo, Dep. Ingenierı´a Hidra´ulica y Medio Ambiente, Universidad Polite´cnica de Valencia, Camino de Vera, s/n, 46022 Valencia, (Spain) Telephone: þ34 96 387 7617 Fax: þ34 96 387 7617 E-mail: [email protected] Web page: http://www.upv.es/calagua * Corresponding author. Tel.: þ34 96 354 3169; fax: þ34 96 354 4898. E-mail address: [email protected] (J. Ribes). 1364-8152/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.envsoft.2007.04.005
Year first available: 2004 (Spanish version), 2005 (English version) Hardware required: MS Windows 98 or more recent versions Software required: MS Excel (recommended) Program language: MS Visual Basic 6.0 Program size: 15 MB Availability and cost: Contact via e-mail Maintenance: The software will be periodically updated with the new research advances
1. Introduction Biological wastewater treatments are complex systems in which a range of physical, chemical and biological processes
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occur. Mathematical models are needed for a quantitative evaluation of these processes. Since Activated Sludge Model no. 1 (ASM1, Henze et al., 1987) was published, a great number of models for simulating biological processes have appeared. The most widely used models are those proposed by the International Water Association (IWA) (ASM series, Henze et al., 1987, 1995, 1999 and Anaerobic Digestion Model no. 1, ADM1, Batstone et al., 2002). These models are particularly interesting in nutrient removal WasteWater Treatment Plants (WWTP) for evaluating operational problems and implementing new operation strategies. Mathematical models for these processes are very complex and they are usually packaged in special software platforms called simulators. Simulators have been extensively used in many disciplines over the years and are powerful tools for design, planning, process analyses, operator guidance and education and training. There is a large number of applications reported in the literature were simulators have been used for WWTP design or optimisation. Some of the most used programs are GPS-X, AQUASIM, EFOR and SIMBA (COST, 2001). All of them include a biological model based on IWA models for simulating activated sludge systems and some of them include other models for simulating settling units, fixed film operations, anaerobic reactors, and algorithms for dissolved oxygen control. This paper presents DESASS (DEsign and Simulation of Activated Sludge Systems), a software tool for WWTP design, simulation and optimisation. This software includes the Biological Nutrient Removal Model no. 1 (BNRM1, Seco et al., 2004b) developed by CALAGUA research group. This model is based on a new concept of WWTP modelling: the model includes the most important biological and physico-chemical processes taking place in all treatment units, so the same model is used to design, simulate and optimise the whole plant including wastewater and sludge treatments. This paper focuses on illustrating the potential uses of the software rather than describing the implemented mathematical model. Software features and capabilities are described and demonstrated through the design of a full scale WWTP. 2. Software description DESASS is a powerful WWTP simulator developed for PC computers by using Microsoft Visual Basic 6.0 programming language and able to assist the design, upgrading, simulation and optimisation of municipal and industrial WWTPs. The software code and therefore the mathematical model cannot be changed by the user. DESASS has been financially supported by Entidad Pu´blica de Saneamiento de Aguas Residuales de la Comunidad Valenciana, the entity responsible for the management of all the urban WWTPs in the region of Comunidad Valenciana (Spain). Therefore, the program development was carried out with two main objectives: (a) to develop a useful tool for research work (modelling, calibration, devising and testing new control algorithms, studying treatment schemes for nutrient removal and recovery, sludge minimisation, etc.) and for engineering and consulting applications
(design, upgrading and optimisation of WWTPs), and (b) to develop a user-friendly program to be easily handled by plant operators in order to study the effects of modifying the plant operation criteria by simulating the WWTP performance. As a matter of fact, plant operators are using this software mainly as a decision support system. Also, DESASS is being used in several Spanish Universities for researching on nutrient removal processes as well as for educational purposes. Changing plant configuration and comparing the results over different influent conditions and different scenarios is really straightforward even for non skilled users. DESASS allows calculating the performance under steady or transient state of the following treatment units: primary settlers (considering fermentation processes inside), prefermenter tanks to generate volatile fatty acids, activated sludge reactors, secondary settlers (considering denitrification processes inside), gravity thickeners, aerobic and anaerobic sludge digesters and sludge dewatering systems. The effect of recycling the supernatants from the sludge dewatering system can be taken into account. The nitrogen and phosphorus load from this sidestream must be considered in an accurate nutrient removal WWTP design or simulation. For dynamic simulations, although the user can establish the initial concentrations of all the model components, it is advisable to start with a steady state calculation in order to obtain initial conditions for the dynamic simulation study. Computing time for a steady state depends on the number of reactor units and also the number of layers considered in the settlers and thickeners. For instance, a UCT configuration with 10 layer secondary settler can last 5 min, while the steady state solution of the whole WWTP presented in the example below (see Fig. 3), with 10 layers in settlers and thickener, was obtained after 30 min. These simulations were carried out in a conventional Pentium IV (CPU 2.80 GHz with 512 MB RAM). 2.1. Mathematical model The mathematical model implemented in DESASS is the BNRM1. This model considers the most important physical, chemical and biological processes taking place in a WWTP to maximise potential applicability without increasing neither model calibration nor wastewater characterisation efforts. The physical processes included are: settling and clarification processes (flocculated settling, hindered settling and thickening), Volatile Fatty Acids (VFA) elutriation and gas-liquid transfer. The chemical interactions included comprise acid-base processes, where equilibrium conditions are assumed, and phosphorus precipitation processes in the same way as in ASM2. The biological processes included are: organic matter, nitrogen and phosphorus removal; acidogenesis, acetogenesis and methanogenesis. The effect of temperature on the processes rates is considered in this model by using the general Arrhenius equation. The settling processes model consists in a one-dimensional model based on the solids flux concept and the conservation of mass law. This model uses the settling velocity function
J. Ferrer et al. / Environmental Modelling & Software 23 (2008) 19e26
proposed by Taka´cs et al. (1991), which is corrected by a compression function in the lower layers. It has been linked to the biological model in order to consider biological processes taking place in primary and secondary settlers and gravity thickeners, i.e. they are considered as reactive elements. This model is described in detail elsewhere (Ribes et al., 2002). BNRM1 considers 27 components including those considered in the Activated Sludge Model No. 2d and those required to simulate primary sludge fermentation and anaerobic digestion. The bacterial groups considered are: heterotrophic, autotrophic, polyphosphate accumulating, acidogenic, acetogenic and two groups of methanogenic bacteria: acetoclastic and hydrogenotrophic. The environmental conditions (mainly concentrations of electron acceptors) determine which bacterial groups can proliferate in the different treatment units. The processes considered are divided into two groups: kinetically governed and equilibrium governed processes. The kinetically governed processes are those related to the different bacterial groups, the stripping of the gaseous species (oxygen, carbon dioxide, hydrogen and methane) and chemical phosphorus precipitation. The equilibrium governed processes are the acid-base interactions in which any of the components considered are involved. Coupling these chemical interactions to ASM2d has proven to be a suitable way of calculating pH variations in biological nutrient removal systems (Serralta et al., 2004). This way, pH inhibition can be considered in the kinetic expressions of biological processes. The formulation of biological and physical processes around the mass-balance for total concentration of each component gives rise to a set of independent partial differential equations. These equations need to be integrated over time to calculate the gradual change of concentrations. In each timestep the concentration of the species are calculated by equilibrium chemistry-based algorithms with no effect on the total concentration of the components. Thus, the calculation procedure involves a sequential iteration among the differential and the algebraic equations (see Fig. 1). The equilibrium composition in each time-step is calculated using MINTEQA2 (Allison et al., 1991). MINTEQA2 is a program capable of computing equilibria among the dissolved, adsorbed, solid and gas phases. This program predicts all the species present in the equilibrium and calculates their concentrations. The data required to predict the equilibrium composition consists of total dissolved concentrations for the components of interest and in this case they are provided by the solution of mass-balance equations. MINTEQA2 includes an extensive database of equilibrium constants and their variations with temperature, so calibration tasks are not required for these chemical interactions. MINTEQA2 original code has been slightly modified and compiled through the dynamic
t=0
SOFTWARE
MINTEQA2
Kinetic processes→Tj
Equilibrium processes → C i
t=t+ t
t > tMax? no
Fig. 1. Schematic diagram of the solution procedure.
yes
End
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link library (DLL) technique in order to be used by DESASS in an iterative calculation. A schematic diagram of the solution procedure is shown in Fig. 1. 2.2. Features and capabilities of DESASS The main features of the program DESASS can be summarised as follows: The program allows designing, simulating and optimising the whole WWTP performance as the most important physical, chemical and biological processes are considered in the model implemented. DESASS calculates a great variety of treatment schemes and allows the establishment of tank volumes, flow rates, operation criteria, etc. with high flexibility. The program calculates the required volumes and the performance under steady-state conditions of the different process units. Default design criteria are taken from WEF and ASCE (1998a,b), but the user can change this criteria according to its own experience. It also allows simulating the plant performance under daily variations of the influent flow rate and loads, and the return and waste sludge flow rates. These variations can be introduced through an Excel file. The software uses the available data by spline interpolation, so data should be first filtered to avoid outliers. For steady state calculation the program automatically detects the independent loops in the flow scheme that can be solved separately. The computing time can be significantly reduced by solving the loops separately. To simulate whole plants, it is recommended to initially create the flow scheme without recycling the sidestreams in order to solve the existing loops separately. Once the iteration process is converged the sidestreams should be recycled and the flow scheme should be calculated again by using the previous solution as initial condition. Concentrations of all the components involved in the different treatment units can be plotted during the calculations. Furthermore, in primary and secondary settlers and gravity thickeners, the concentrations profile along the settler can be plotted as well. Different influent flow rates, loads, temperature and operation criteria can be established during winter and summer seasons, and the program calculates the plant performance under both conditions, choosing the highest required volume for each treatment unit. DESASS calculates the aeration system required to supply the oxygen requirements in activated sludge tanks and aerobic digesters and selects the suitable equipment from a database included in the program. This database can be enlarged with new data. The program contains a control module to simulate the performance of a supervisory fuzzy logic control system to control dissolved oxygen (DO) concentration in activated sludge tanks and aerobic digesters aerated by diffusers and blowers. The control system implemented in DESASS
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modifies the air control valves opening according to DO concentration in the tanks and the rotational speed of the blowers according to the discharge pressure in the air pipelines. In order to evaluate different operation strategies under transient periods the user can modify every operational variable during the simulations. These variables are: DO concentration in the aerobic tanks, percentage of aerobic, anoxic and anaerobic zones in an activated sludge reactor, volumes of the treatment units and flow rates (return and waste activated sludge flow rates, sludge withdrawal flow rate in primary settlers and gravity thickeners, internal recycling flow rates). These flow rates can be modified manually during the simulation or set through an Excel file (.xls) before the simulations. The results obtained can be exported to an Excel file (.xls). In summary, DESASS allows engineers and plant operators to test the consequences of modifying the operation conditions. The sensitivity of the system with respect to these parameters can be studied for a wide range of treatment schemes. 2.3. Using the software The utilisation of DESASS is illustrated with an example of a WWTP including biological nutrient removal (BNR) and anaerobic digestion as sludge treatment. The main window of DESASS, in which the user creates the WWTP flow sheet, appears when the program is launched. The two tool bars shown in Fig. 2 must be used in order to set up the WWTP layout, which is the first step that should be carried out before starting the calculations. Fig. 3 shows the main window of DESASS with the layout of the whole WWTP calculated in this example. As can be seen in this figure, the WWTP consists in a primary sludge prefermentation process, an activated sludge process for BNR, and an anaerobic digestion of the waste sludge. The supernatants from the gravity thickener and the sludge dewatering system are recycled to the inlet of the primary settler. Once the treatment scheme is created, the next step consists in defining the influent wastewater characteristics on the window
Fig. 2. Elements and Streams toolbars used to set up the WWTP layout.
Fig. 3. Main window of DESASS with the WWTP layout used in this example.
shown in Fig. 4. The user introduces the influent flow rate, temperature, pH, the concentrations of all the components considered in the model and the settling ability characteristics of the suspended solids. The influent peak flow rate must be also introduced to calculate the diameter of primary and secondary settlers and to design the aeration system. Table 1 shows all the analytic parameters required to fully characterise the influent wastewater according to BNRM1. The concentrations of all the model components are obtained from these analyses (see Penya-Roja et al., 2002 for more details on this characterisation method). It is not usual to have at one’s disposal all these analyses before designing or upgrading a WWTP. In these cases, some assumptions must be carried out to estimate the missing parameters. The last step before starting the calculations consists in introducing the design criteria of each element in the flow sheet. Each element has a specific window to establish its design criteria. Fig. 5 shows this window for the element Secondary settler. In this case, the main design criteria are the overflow rate, solids loading rate, retention time and the weir loading rate.
Fig. 4. Window of influent wastewater characteristics specified by the user. Definition of the components shown in the window can be found in Seco et al. (2004b).
J. Ferrer et al. / Environmental Modelling & Software 23 (2008) 19e26 Table 1 Influent wastewater measurements required for using DESASS Parameter Qinf COD % sol BODlim % sol NHþ 4 NO 3 N sol N tot
Value 10000 720 39.6 615 41.5 16 0 32.6 49.3
Units
Parameter
Value
Units
3
PO4 P sol P tot SS % VSS % BVSS VFA pH Alkalinity
3.6 5.7 10.1 344.5 88.4 82.8 45 7 208
mg P/l mg P/l mg P/l mg/l e e mg COD/l e mg CaCO3/l
m /day mg/l e mg/l e mg N/l mg N/l mg N/l mg N/l
The values shown are those used in the example.
The user decides whether the settling and biological processes are calculated or not by selecting the options of this window. If settling processes are going to be calculated, the user must introduce the settling parameters of the activated sludge entering the settler and the number of layers in which the settler is split. DESASS provides two calculation modes: design and simulation modes. In design mode, the program establishes the required volumes of each treatment unit as well as the return and waste sludge flow rates and calculates the performance of the plant under steady state. The oxygen requirements are also calculated in order to design the aeration system. In simulation mode DESASS calculates the evolution of the plant from the starting condition detailed by the user and according to the daily variations of the influent flow rate and loads. This mode is mainly used to simulate existing or previously designed WWTPs, so the user must specify all the volumes and flow rates of the plant, as the design criteria are not used. As commented before, the volumes and flow rates can be modified during the simulation. The results are shown either numerically or graphically. Numerically, the user can see the results by clicking on each stream. This way, a window that includes all the information associated to that stream (flow rate, temperature, pH and concentrations of all components) is launched. Graphically, the user
Fig. 5. Window of design properties of the element Secondary settler.
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can see the evolution along the time (in simulation mode) or along the number of iterations (in design mode) of all the model components in every element. Also, in settlers and thickeners, the user can see the concentrations profile along their height, either for one given iteration or its evolution during the calculations. So the sludge blanket height, which is one of the most important parameters of these elements, can be determined from this plot. As an example, Fig. 6 shows the evolution during the iteration procedure of pH and autotrophic bacteria concentration in the aerobic reactor (Fig. 6a) and the suspended solids profile along the primary settler at steady state (Fig. 6b). Table 2 shows the organic matter, suspended solids, ammonium, nitrate and phosphate effluent concentrations obtained in this example. As can be seen comparing Tables 1 and 2, the biological organic matter, nitrogen and phosphorus removal efficiencies are very high. These results have been obtained taking into account the effect of recycling the sidestreams and considering that 60% of phosphate released in the sludge treatment is chemically precipitated. Several authors have shown that a significant part of the released phosphorus is fixed in different forms as struvite, iron and calcium phosphate inside the anaerobic digesters (Jardin and Po¨pel, 1994; Wild et al., 1997; Seco et al., 2004a). DESASS allows the user to establish the percentage of phosphorus released that is fixed in the anaerobic digestion.
Fig. 6. Examples of graphs shown in DESASS. (a) Evolution of pH and autotrophic bacteria concentration in the aerobic reactor. (b) Suspended solids profile along the height of primary settler at the steady state.
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Table 2 Effluent concentrations obtained from the simulation of the example plant design Effluent COD (mg/l) BOD (mg/l) TSS (mg/l) NHþ 4 (mg N/l) NO 3 (mg N/l) TN (mg N/l) PO4 (mg P/l) TP (mg P/l) pH Alkalinity (mg CaCO3/l)
63.2 5.3 13.5 1.9 3.5 6.6 1.2 2.1 7.4 162
3. Main applications of DESASS DESASS can be used for many different purposes related with wastewater treatments. The main applications are described next. Some of them can also be found in literature. 3.1. Design of new WWTPs and upgrading of existing ones DESASS has been used for designing and upgrading several WWTPs located in Spain. These plants incorporate biological phosphorus and nitrogen removal, extended aeration processes, aerobic and anaerobic digestions, plug flow reactors and phosphorus recovery. As an example, the upgrading of Denia WWTP is described in Seco et al. (2005). The treatment scheme of Denia WWTP was modified according to BNRM1 simulations. It consisted on two primary settlers, a conventional activated sludge process, two secondary settlers and an aerobic digester. Now, the plant is operated under extended aeration conditions, the old primary settlers are used as anoxic reactors and the aerobic reactor is formed by the old biological reactor and aerobic digester. Model simulations predicted adequately the WWTP performance. 3.2. Diagnosis and optimisation of existing WWTPs The first step for optimising the performance of an existing WWTP is the calibration of model parameters. Different methods have been proposed in literature for this purpose (see e.g. Koch et al., 2000; Hulsbeek et al., 2002; Penya-Roja et al., 2002; Melcer et al., 2003; Insel et al., 2006). Particularly, our research group uses the calibration methodology proposed by Penya-Roja et al. (2002), which is based on off-line laboratory batch experiments using WWTP biomass and a following adjustment of model predictions to historical data. Once the model is calibrated, DESASS is a very useful tool to detect non optimal performance and to study the consequences of different modifications of treatment scheme or operation criteria. The results obtained in the calibration of two full scale WWTPs, the simulations carried out and the modifications proposed can be found in Ferrer et al. (2004a).
3.3. Research and development of new process schemes The BNRM1 potential (biological processes under any environmental condition) joined to the software capabilities allows combining, in a flexible way, aerobic, anoxic and anaerobic stirred tanks with anaerobic digesters and sedimentation units. This feature is especially useful for testing and developing treatment schemes for biological nutrient removal from either urban or industrial wastewaters. 3.4. Teaching and staff training An increasing number of plant operators are using this software to simulate the WWTP performance and to evaluate different operation alternatives and control actions when changes in influent wastewater or system conditions occur. Also, DESASS is being used by students of civil and chemical engineering in different Spanish universities and makes it easier to understand the whole plant performance, taking into account the interactions between main stream and side streams. 3.5. Test of new control systems by simulation studies DESASS contains the required data to design and simulate the performance of aeration systems (membrane diffusers, control valves and blowers) and recycling pumps. Also, the time evolution of dissolved oxygen concentration in each aerobic reactor as well as waste sludge, recycling sludge and internal recycling flow rates can be established. This feature allows the user to analyse the dynamic plant performance under variable operation strategies and to obtain valuable information to develop new control systems. For instance, it allowed us the development and test of a fuzzy logic control system for optimising nitrogen removal (Serralta et al., 2002). This control system modifies the internal recycling flow rate according to nitrate concentration in the anoxic reactor and effluent total nitrogen concentration. 4. Discussion The complexity of the models which are commonly used to describe wastewater treatment processes requires the use of simulation platforms. The use of these software tools has improved design operation efficiency and many other aspects related to wastewater treatment processes. However, the application of mathematical models to full scale plants requires exhaustive wastewater characterisation, model parameters calibration, operational data and hydraulics knowledge. The use of simulation tools without understanding the processes involved can lead engineers or plant operators to obtain wrong solutions. The benefits and drawbacks of simulating full scale plants are described in detail in Ferrer et al. (2004b). The trends of standard benchmark community for plantwide modelling consist on using ASM models linked to ADM1. This connection involves the application of artificial conversions between the different components considered by both models. The use of transforming/interfacing methods
J. Ferrer et al. / Environmental Modelling & Software 23 (2008) 19e26
has lead to strong discrepancies among the scientific community. These methods make model application more difficult as they add more parameters to calibrate (Zaher et al., 2007). One of the main advantages of using DESASS is that the same model is applied to simulate all the treatment units included in a WWTP. This way, DESASS can simulate processes from both models (AS and AD models) that take place in the same treatment unit, as for instance, the phosphorus release from polyphosphate stored in PAO bacteria and VFA production observed in the bottom of sludge thickeners. 5. Conclusions The program DESASS has been presented in this paper. DESASS is a very useful tool for designing, simulating and optimising WWTPs. The mathematical model implemented in this program is the BNRM1 and it considers the main physical, chemical and biological processes taking place in WWTPs. DESASS can be applied to calculate the performance under steady or transient state of complete treatment schemes including primary settlers, perfermenter tanks, biological reactors, secondary settlers, gravity thickeners and aerobic and anaerobic digesters. Recycling the supernatants from the sludge treatment can be considered in order to take into account the nitrogen and phosphorus loads from these sidestreams. The utilisation of DESASS has been illustrated with an example of a WWTP design including biological nutrient removal and anaerobic digestion as sludge treatment. The results obtained can be seen either numerically or graphically and can also be exported to an Excel file. So far, DESASS has been applied for: designing new WWTPs, diagnosis and upgrading of existing WWTPs, development of new treatment schemes and control systems, teaching and staff training. The main features and capabilities presented in this paper make DESASS a very useful tool for engineers to calculate the WWTPs performance but also for plant operators to test the consequences of modifying the operation criteria. Changing plant configuration and comparing the results over different influent conditions and different scenarios has been shown to be straightforward even for non skilled users. Acknowledgements Financial support from Entidad Pu´blica de Saneamiento de Aguas Residuales de la Comunidad Valenciana, SEARSA y AQUAGEST is gratefully acknowledged. References Allison, J.D., Brown, D.S., Novo-Gradac, K.J., 1991. MINTEQA2/ PRODEFA2, A Geochemical Assessment Model for Environmental Systems, Version 3.0. USEPA, Washington, DC. EPA/600/3e91/021. Batstone, D.J., Keller, J., Angelidaki, I., Kaliuzhnyi, S.V., Pavlostathis, S.G., Rozzi, A. Sanders, W.T.M., Siegrist, H., Vavilin, V.A., 2002. Anaerobic digestion model No. 1 (ADM1). Scientific and Technical Report No. 13. IWA Publishing, London.
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COST, 2001. The COST Simulation BenchmarkdDescription and Simulator Manual. COST (European Cooperation in the field of Scientific and Technical Research), Brussels, Belgium. Ferrer, J., Morenilla, J.J., Bouzas, A., Garcia-Usach, F., 2004a. Calibration and simulation of two large wastewater treatment plants operated for nutrient removal. Water Science and Technology 50 (6), 87e94. Ferrer, J., Seco, A., Garcia-Usach, F., Bouzas, A., Barat, R., 2004b. Simulation of full scale plants: benefits and drawbacks. Water and environmental management series. 2nd IWA Leading-Edge Conference on Water and Wastewater Treatment Technologies. IWA Publishing, London. Henze, M., Grady, C.P.L. Jr., Gujer, W., Marais, G.v.R., Matsuo, T., 1987. Activated Sludge Model No.1 IAWPRC Scientific and Technical Report No.1 IAWPRC, London. Henze, M., Gujer, W., Mino, T., Matsuo, T., Wentzel M., Marais, G.v.R., 1995. Activated Sludge Model No.2. IAWQ Scientific and Technical Report, IAWQ, London. Henze, M., Gujer, W., Mino, T., Matsuo, T., Wentzel, M., Marais, G., van Loosdrecht, M.C.M., 1999. Activated Sludge Model No.2d, ASM2d. Water Science and Technology 39 (1), 165e182. Hulsbeek, J.J.W., Kruit, J., Roeleveld, P.J., van Loosdrecht, M.C.M., 2002. A practical protocol for dynamic modelling of activated sludge systems. Water Science and Technology 45 (6), 127e136. Insel, G., Sin, G., Lee, D.S., Nopens, I., Vanrolleghem, P.A., 2006. A calibration methodology and model-based systems analysis for SBRs removing nutrients under limited aeration conditions. Journal of Chemical Technology and Biotechnology 81 (4), 679e687. Jardin, N., Po¨pel, H.J., 1994. Phosphate releases of sludges from enhanced biological P-removal during digestion. Water Science and Technology 20 (6), 281e292. Koch, G., Ku¨hni, M., Gujer, W., Siegrsit, H., 2000. Calibration and validation of activated sludge model No.3 for Swiss municipal wastewater. Water Research 34 (14), 3580e3590. Melcer, H., Dold, P.L., Jones, R.M., Bye, C.M., Takacs, I., Stensel, H.D., Wilson, A.W., Sun, P., Bury, S., 2003. Methods for wastewater characterization in activated sludge modelling. Water Environment Research Foundation (WERF). Alexandria, VA, USA. Penya-Roja, J.M., Seco, A., Ferrer, J., Serralta, J., 2002. Calibration and validation of Activated Sludge Model No.2d for Spanish municipal wastewater. Environmental Technology 23, 849e862. Ribes, J., Ferrer, J., Bouzas, A., Seco, A., 2002. Modelling of an activated primary settling tank including the fermentation process and VFA elutriation. Environmental Technology 23, 1147e1156. Seco, A., Pastor, L., Barat, R., Ferrer, J., Mangin, D., 26e29 January 2004a. Nutrient recovery by struvite crystallization: an improvement for enhanced biological phosphorus removal treatment plants. Proceedings of the International Conference on Wastewater Treatment for Nutrient Removal and Reuse, Asian Institute of Technology, Thailand 1, 117e124. Seco, A., Ribes, J., Serralta, J., Ferrer, J., 2004b. Biological Nutrient Removal Model No.1 (BNRM1). Water Science and Technology 50 (6), 69e78. Seco, A., Ribes, J., Serralta, J., Ferrer, J., 2005. Upgrading the Denia WWTP according to BNRM1 simulations. Proceedings of the IWA Specialised Conference on Nutrient Management in Wastewater Treatment Processes and Recycle Streams, Krakow, pp. 1113e1118. Serralta, J., Ferrer, J., Borra´s, L., Seco, A., 2004. An extension of ASM2d including pH calculation. Water Research 38 (19), 4029e4038. Serralta, J., Ribes, J., Seco, A., Ferrer, J., 2002. A supervisory control system for optimising nitrogen removal and aeration energy consumption in wastewater treatment plants. Water Science and Technology 45 (4-5), 309e316. Taka´cs, I., Patry, G.G., Nolasco, D., 1991. A dynamic model of the clarification-thickening process. Water Research 25 (10), 1263e1271. WEF and ASCE, 1998a. Design of municipal wastewater treatment plants. Liquid treatment processes. In: ASCE manuals and reports on engineering practice no. 76, WEF manual of practice no. 8, Volume 2. American Society of Civil Engineers and Water Environment Federation, Alexandria. WEF and ASCE, 1998b. Design of municipal wastewater treatment plants. Solids processing and disposal. In: ASCE manuals and reports on engineering practice no. 76, WEF manual of practice no. 8, Volume 3.
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American Society of Civil Engineers and Water Environment Federation, Alexandria. Wild, D., Kisliakova, A., Siegrist, H., 1997. Prediction of recycle phosphorus loads from anaerobic digestion. Water Research 31 (9), 2300e2308.
Zaher, U., Grau, P., Benedetti, L., Ayesa, E., Vanrolleghem, P.A., 2007. Transformers for interfacing anaerobic digestion models to pre- and posttreatment processes in a plant-wide modelling context. Environmental Modelling and Software 22, 40e58.
DESASS: A software tool for designing, simulating and optimising WWTPs J. Ferrer a, A. Seco b, J. Serralta a, J. Ribes b,*, J. Manga c, E. Asensi a, J.J. Morenilla d, F. Llavador d a
Dpto. Ingenierı´a Hidra´ulica y Medio Ambiente, Universidad Polite´cnica de Valencia, Camino de Vera, s/n, 46022 Valencia, Spain b Dpto. Ingenierı´a Quı´mica, Universitat de Vale`ncia, Doctor Moliner, 50, 46100 Burjassot (Valencia), Spain c Dpto. Ingenierı´a Civil, Universidad del Norte, Km 5, Antigua Vı´a Pto. Colombia, Barranquilla, Colombia d Entidad Pu´blica de Saneamiento de Aguas Residuales de la Comunidad Valenciana, C/ Alvaro de Baza´n, no. 10 Entl., 46010 Vale`ncia, Spain Received 13 January 2006; received in revised form 27 March 2007; accepted 3 April 2007 Available online 23 May 2007
Abstract This paper presents a very useful software tool to design, simulate and optimise wastewater treatment plants. The program is called DESASS (DEsign and Simulation of Activated Sludge Systems) and has been developed by CALAGUA research group. The mathematical model implemented is the Biological Nutrient Removal Model No.1 (BNRM1) which allows simulating the most important physical, chemical and biological processes taking place in treatment plants. DESASS calculates the performance under steady or transient state of whole treatment schemes including primary settlers, volatile fatty acid generation systems by primary sludge fermentation, activated sludge systems for biological organic matter and nutrient removal, chemical phosphorus precipitation, secondary settlers, gravity thickeners and sludge digesters (aerobic and anaerobic). Biological conversions occurring in settlers and thickeners (primary sludge fermentation, denitrification) are also taken into account, i.e. they are considered as reactive elements. DESASS also includes pH calculation coupled to biological processes in all the elements, so pH effect on biological processes can be directly simulated. Furthermore, the effect of sidestreams on nutrient removal efficiency can be estimated because the performance of the whole plant can be simulated. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: BNRM1; Wastewater; WWTPs design; Modelling; Simulation; Optimisation; Software tool; pH calculation
Software availability Name of software: DESASS (DEsign and Simulation of Activated Sludge Systems) Developer: CALAGUA Research Group Contact address: Jose´ Ferrer Polo, Dep. Ingenierı´a Hidra´ulica y Medio Ambiente, Universidad Polite´cnica de Valencia, Camino de Vera, s/n, 46022 Valencia, (Spain) Telephone: þ34 96 387 7617 Fax: þ34 96 387 7617 E-mail: [email protected] Web page: http://www.upv.es/calagua * Corresponding author. Tel.: þ34 96 354 3169; fax: þ34 96 354 4898. E-mail address: [email protected] (J. Ribes). 1364-8152/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.envsoft.2007.04.005
Year first available: 2004 (Spanish version), 2005 (English version) Hardware required: MS Windows 98 or more recent versions Software required: MS Excel (recommended) Program language: MS Visual Basic 6.0 Program size: 15 MB Availability and cost: Contact via e-mail Maintenance: The software will be periodically updated with the new research advances
1. Introduction Biological wastewater treatments are complex systems in which a range of physical, chemical and biological processes
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occur. Mathematical models are needed for a quantitative evaluation of these processes. Since Activated Sludge Model no. 1 (ASM1, Henze et al., 1987) was published, a great number of models for simulating biological processes have appeared. The most widely used models are those proposed by the International Water Association (IWA) (ASM series, Henze et al., 1987, 1995, 1999 and Anaerobic Digestion Model no. 1, ADM1, Batstone et al., 2002). These models are particularly interesting in nutrient removal WasteWater Treatment Plants (WWTP) for evaluating operational problems and implementing new operation strategies. Mathematical models for these processes are very complex and they are usually packaged in special software platforms called simulators. Simulators have been extensively used in many disciplines over the years and are powerful tools for design, planning, process analyses, operator guidance and education and training. There is a large number of applications reported in the literature were simulators have been used for WWTP design or optimisation. Some of the most used programs are GPS-X, AQUASIM, EFOR and SIMBA (COST, 2001). All of them include a biological model based on IWA models for simulating activated sludge systems and some of them include other models for simulating settling units, fixed film operations, anaerobic reactors, and algorithms for dissolved oxygen control. This paper presents DESASS (DEsign and Simulation of Activated Sludge Systems), a software tool for WWTP design, simulation and optimisation. This software includes the Biological Nutrient Removal Model no. 1 (BNRM1, Seco et al., 2004b) developed by CALAGUA research group. This model is based on a new concept of WWTP modelling: the model includes the most important biological and physico-chemical processes taking place in all treatment units, so the same model is used to design, simulate and optimise the whole plant including wastewater and sludge treatments. This paper focuses on illustrating the potential uses of the software rather than describing the implemented mathematical model. Software features and capabilities are described and demonstrated through the design of a full scale WWTP. 2. Software description DESASS is a powerful WWTP simulator developed for PC computers by using Microsoft Visual Basic 6.0 programming language and able to assist the design, upgrading, simulation and optimisation of municipal and industrial WWTPs. The software code and therefore the mathematical model cannot be changed by the user. DESASS has been financially supported by Entidad Pu´blica de Saneamiento de Aguas Residuales de la Comunidad Valenciana, the entity responsible for the management of all the urban WWTPs in the region of Comunidad Valenciana (Spain). Therefore, the program development was carried out with two main objectives: (a) to develop a useful tool for research work (modelling, calibration, devising and testing new control algorithms, studying treatment schemes for nutrient removal and recovery, sludge minimisation, etc.) and for engineering and consulting applications
(design, upgrading and optimisation of WWTPs), and (b) to develop a user-friendly program to be easily handled by plant operators in order to study the effects of modifying the plant operation criteria by simulating the WWTP performance. As a matter of fact, plant operators are using this software mainly as a decision support system. Also, DESASS is being used in several Spanish Universities for researching on nutrient removal processes as well as for educational purposes. Changing plant configuration and comparing the results over different influent conditions and different scenarios is really straightforward even for non skilled users. DESASS allows calculating the performance under steady or transient state of the following treatment units: primary settlers (considering fermentation processes inside), prefermenter tanks to generate volatile fatty acids, activated sludge reactors, secondary settlers (considering denitrification processes inside), gravity thickeners, aerobic and anaerobic sludge digesters and sludge dewatering systems. The effect of recycling the supernatants from the sludge dewatering system can be taken into account. The nitrogen and phosphorus load from this sidestream must be considered in an accurate nutrient removal WWTP design or simulation. For dynamic simulations, although the user can establish the initial concentrations of all the model components, it is advisable to start with a steady state calculation in order to obtain initial conditions for the dynamic simulation study. Computing time for a steady state depends on the number of reactor units and also the number of layers considered in the settlers and thickeners. For instance, a UCT configuration with 10 layer secondary settler can last 5 min, while the steady state solution of the whole WWTP presented in the example below (see Fig. 3), with 10 layers in settlers and thickener, was obtained after 30 min. These simulations were carried out in a conventional Pentium IV (CPU 2.80 GHz with 512 MB RAM). 2.1. Mathematical model The mathematical model implemented in DESASS is the BNRM1. This model considers the most important physical, chemical and biological processes taking place in a WWTP to maximise potential applicability without increasing neither model calibration nor wastewater characterisation efforts. The physical processes included are: settling and clarification processes (flocculated settling, hindered settling and thickening), Volatile Fatty Acids (VFA) elutriation and gas-liquid transfer. The chemical interactions included comprise acid-base processes, where equilibrium conditions are assumed, and phosphorus precipitation processes in the same way as in ASM2. The biological processes included are: organic matter, nitrogen and phosphorus removal; acidogenesis, acetogenesis and methanogenesis. The effect of temperature on the processes rates is considered in this model by using the general Arrhenius equation. The settling processes model consists in a one-dimensional model based on the solids flux concept and the conservation of mass law. This model uses the settling velocity function
J. Ferrer et al. / Environmental Modelling & Software 23 (2008) 19e26
proposed by Taka´cs et al. (1991), which is corrected by a compression function in the lower layers. It has been linked to the biological model in order to consider biological processes taking place in primary and secondary settlers and gravity thickeners, i.e. they are considered as reactive elements. This model is described in detail elsewhere (Ribes et al., 2002). BNRM1 considers 27 components including those considered in the Activated Sludge Model No. 2d and those required to simulate primary sludge fermentation and anaerobic digestion. The bacterial groups considered are: heterotrophic, autotrophic, polyphosphate accumulating, acidogenic, acetogenic and two groups of methanogenic bacteria: acetoclastic and hydrogenotrophic. The environmental conditions (mainly concentrations of electron acceptors) determine which bacterial groups can proliferate in the different treatment units. The processes considered are divided into two groups: kinetically governed and equilibrium governed processes. The kinetically governed processes are those related to the different bacterial groups, the stripping of the gaseous species (oxygen, carbon dioxide, hydrogen and methane) and chemical phosphorus precipitation. The equilibrium governed processes are the acid-base interactions in which any of the components considered are involved. Coupling these chemical interactions to ASM2d has proven to be a suitable way of calculating pH variations in biological nutrient removal systems (Serralta et al., 2004). This way, pH inhibition can be considered in the kinetic expressions of biological processes. The formulation of biological and physical processes around the mass-balance for total concentration of each component gives rise to a set of independent partial differential equations. These equations need to be integrated over time to calculate the gradual change of concentrations. In each timestep the concentration of the species are calculated by equilibrium chemistry-based algorithms with no effect on the total concentration of the components. Thus, the calculation procedure involves a sequential iteration among the differential and the algebraic equations (see Fig. 1). The equilibrium composition in each time-step is calculated using MINTEQA2 (Allison et al., 1991). MINTEQA2 is a program capable of computing equilibria among the dissolved, adsorbed, solid and gas phases. This program predicts all the species present in the equilibrium and calculates their concentrations. The data required to predict the equilibrium composition consists of total dissolved concentrations for the components of interest and in this case they are provided by the solution of mass-balance equations. MINTEQA2 includes an extensive database of equilibrium constants and their variations with temperature, so calibration tasks are not required for these chemical interactions. MINTEQA2 original code has been slightly modified and compiled through the dynamic
t=0
SOFTWARE
MINTEQA2
Kinetic processes→Tj
Equilibrium processes → C i
t=t+ t
t > tMax? no
Fig. 1. Schematic diagram of the solution procedure.
yes
End
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link library (DLL) technique in order to be used by DESASS in an iterative calculation. A schematic diagram of the solution procedure is shown in Fig. 1. 2.2. Features and capabilities of DESASS The main features of the program DESASS can be summarised as follows: The program allows designing, simulating and optimising the whole WWTP performance as the most important physical, chemical and biological processes are considered in the model implemented. DESASS calculates a great variety of treatment schemes and allows the establishment of tank volumes, flow rates, operation criteria, etc. with high flexibility. The program calculates the required volumes and the performance under steady-state conditions of the different process units. Default design criteria are taken from WEF and ASCE (1998a,b), but the user can change this criteria according to its own experience. It also allows simulating the plant performance under daily variations of the influent flow rate and loads, and the return and waste sludge flow rates. These variations can be introduced through an Excel file. The software uses the available data by spline interpolation, so data should be first filtered to avoid outliers. For steady state calculation the program automatically detects the independent loops in the flow scheme that can be solved separately. The computing time can be significantly reduced by solving the loops separately. To simulate whole plants, it is recommended to initially create the flow scheme without recycling the sidestreams in order to solve the existing loops separately. Once the iteration process is converged the sidestreams should be recycled and the flow scheme should be calculated again by using the previous solution as initial condition. Concentrations of all the components involved in the different treatment units can be plotted during the calculations. Furthermore, in primary and secondary settlers and gravity thickeners, the concentrations profile along the settler can be plotted as well. Different influent flow rates, loads, temperature and operation criteria can be established during winter and summer seasons, and the program calculates the plant performance under both conditions, choosing the highest required volume for each treatment unit. DESASS calculates the aeration system required to supply the oxygen requirements in activated sludge tanks and aerobic digesters and selects the suitable equipment from a database included in the program. This database can be enlarged with new data. The program contains a control module to simulate the performance of a supervisory fuzzy logic control system to control dissolved oxygen (DO) concentration in activated sludge tanks and aerobic digesters aerated by diffusers and blowers. The control system implemented in DESASS
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modifies the air control valves opening according to DO concentration in the tanks and the rotational speed of the blowers according to the discharge pressure in the air pipelines. In order to evaluate different operation strategies under transient periods the user can modify every operational variable during the simulations. These variables are: DO concentration in the aerobic tanks, percentage of aerobic, anoxic and anaerobic zones in an activated sludge reactor, volumes of the treatment units and flow rates (return and waste activated sludge flow rates, sludge withdrawal flow rate in primary settlers and gravity thickeners, internal recycling flow rates). These flow rates can be modified manually during the simulation or set through an Excel file (.xls) before the simulations. The results obtained can be exported to an Excel file (.xls). In summary, DESASS allows engineers and plant operators to test the consequences of modifying the operation conditions. The sensitivity of the system with respect to these parameters can be studied for a wide range of treatment schemes. 2.3. Using the software The utilisation of DESASS is illustrated with an example of a WWTP including biological nutrient removal (BNR) and anaerobic digestion as sludge treatment. The main window of DESASS, in which the user creates the WWTP flow sheet, appears when the program is launched. The two tool bars shown in Fig. 2 must be used in order to set up the WWTP layout, which is the first step that should be carried out before starting the calculations. Fig. 3 shows the main window of DESASS with the layout of the whole WWTP calculated in this example. As can be seen in this figure, the WWTP consists in a primary sludge prefermentation process, an activated sludge process for BNR, and an anaerobic digestion of the waste sludge. The supernatants from the gravity thickener and the sludge dewatering system are recycled to the inlet of the primary settler. Once the treatment scheme is created, the next step consists in defining the influent wastewater characteristics on the window
Fig. 2. Elements and Streams toolbars used to set up the WWTP layout.
Fig. 3. Main window of DESASS with the WWTP layout used in this example.
shown in Fig. 4. The user introduces the influent flow rate, temperature, pH, the concentrations of all the components considered in the model and the settling ability characteristics of the suspended solids. The influent peak flow rate must be also introduced to calculate the diameter of primary and secondary settlers and to design the aeration system. Table 1 shows all the analytic parameters required to fully characterise the influent wastewater according to BNRM1. The concentrations of all the model components are obtained from these analyses (see Penya-Roja et al., 2002 for more details on this characterisation method). It is not usual to have at one’s disposal all these analyses before designing or upgrading a WWTP. In these cases, some assumptions must be carried out to estimate the missing parameters. The last step before starting the calculations consists in introducing the design criteria of each element in the flow sheet. Each element has a specific window to establish its design criteria. Fig. 5 shows this window for the element Secondary settler. In this case, the main design criteria are the overflow rate, solids loading rate, retention time and the weir loading rate.
Fig. 4. Window of influent wastewater characteristics specified by the user. Definition of the components shown in the window can be found in Seco et al. (2004b).
J. Ferrer et al. / Environmental Modelling & Software 23 (2008) 19e26 Table 1 Influent wastewater measurements required for using DESASS Parameter Qinf COD % sol BODlim % sol NHþ 4 NO 3 N sol N tot
Value 10000 720 39.6 615 41.5 16 0 32.6 49.3
Units
Parameter
Value
Units
3
PO4 P sol P tot SS % VSS % BVSS VFA pH Alkalinity
3.6 5.7 10.1 344.5 88.4 82.8 45 7 208
mg P/l mg P/l mg P/l mg/l e e mg COD/l e mg CaCO3/l
m /day mg/l e mg/l e mg N/l mg N/l mg N/l mg N/l
The values shown are those used in the example.
The user decides whether the settling and biological processes are calculated or not by selecting the options of this window. If settling processes are going to be calculated, the user must introduce the settling parameters of the activated sludge entering the settler and the number of layers in which the settler is split. DESASS provides two calculation modes: design and simulation modes. In design mode, the program establishes the required volumes of each treatment unit as well as the return and waste sludge flow rates and calculates the performance of the plant under steady state. The oxygen requirements are also calculated in order to design the aeration system. In simulation mode DESASS calculates the evolution of the plant from the starting condition detailed by the user and according to the daily variations of the influent flow rate and loads. This mode is mainly used to simulate existing or previously designed WWTPs, so the user must specify all the volumes and flow rates of the plant, as the design criteria are not used. As commented before, the volumes and flow rates can be modified during the simulation. The results are shown either numerically or graphically. Numerically, the user can see the results by clicking on each stream. This way, a window that includes all the information associated to that stream (flow rate, temperature, pH and concentrations of all components) is launched. Graphically, the user
Fig. 5. Window of design properties of the element Secondary settler.
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can see the evolution along the time (in simulation mode) or along the number of iterations (in design mode) of all the model components in every element. Also, in settlers and thickeners, the user can see the concentrations profile along their height, either for one given iteration or its evolution during the calculations. So the sludge blanket height, which is one of the most important parameters of these elements, can be determined from this plot. As an example, Fig. 6 shows the evolution during the iteration procedure of pH and autotrophic bacteria concentration in the aerobic reactor (Fig. 6a) and the suspended solids profile along the primary settler at steady state (Fig. 6b). Table 2 shows the organic matter, suspended solids, ammonium, nitrate and phosphate effluent concentrations obtained in this example. As can be seen comparing Tables 1 and 2, the biological organic matter, nitrogen and phosphorus removal efficiencies are very high. These results have been obtained taking into account the effect of recycling the sidestreams and considering that 60% of phosphate released in the sludge treatment is chemically precipitated. Several authors have shown that a significant part of the released phosphorus is fixed in different forms as struvite, iron and calcium phosphate inside the anaerobic digesters (Jardin and Po¨pel, 1994; Wild et al., 1997; Seco et al., 2004a). DESASS allows the user to establish the percentage of phosphorus released that is fixed in the anaerobic digestion.
Fig. 6. Examples of graphs shown in DESASS. (a) Evolution of pH and autotrophic bacteria concentration in the aerobic reactor. (b) Suspended solids profile along the height of primary settler at the steady state.
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Table 2 Effluent concentrations obtained from the simulation of the example plant design Effluent COD (mg/l) BOD (mg/l) TSS (mg/l) NHþ 4 (mg N/l) NO 3 (mg N/l) TN (mg N/l) PO4 (mg P/l) TP (mg P/l) pH Alkalinity (mg CaCO3/l)
63.2 5.3 13.5 1.9 3.5 6.6 1.2 2.1 7.4 162
3. Main applications of DESASS DESASS can be used for many different purposes related with wastewater treatments. The main applications are described next. Some of them can also be found in literature. 3.1. Design of new WWTPs and upgrading of existing ones DESASS has been used for designing and upgrading several WWTPs located in Spain. These plants incorporate biological phosphorus and nitrogen removal, extended aeration processes, aerobic and anaerobic digestions, plug flow reactors and phosphorus recovery. As an example, the upgrading of Denia WWTP is described in Seco et al. (2005). The treatment scheme of Denia WWTP was modified according to BNRM1 simulations. It consisted on two primary settlers, a conventional activated sludge process, two secondary settlers and an aerobic digester. Now, the plant is operated under extended aeration conditions, the old primary settlers are used as anoxic reactors and the aerobic reactor is formed by the old biological reactor and aerobic digester. Model simulations predicted adequately the WWTP performance. 3.2. Diagnosis and optimisation of existing WWTPs The first step for optimising the performance of an existing WWTP is the calibration of model parameters. Different methods have been proposed in literature for this purpose (see e.g. Koch et al., 2000; Hulsbeek et al., 2002; Penya-Roja et al., 2002; Melcer et al., 2003; Insel et al., 2006). Particularly, our research group uses the calibration methodology proposed by Penya-Roja et al. (2002), which is based on off-line laboratory batch experiments using WWTP biomass and a following adjustment of model predictions to historical data. Once the model is calibrated, DESASS is a very useful tool to detect non optimal performance and to study the consequences of different modifications of treatment scheme or operation criteria. The results obtained in the calibration of two full scale WWTPs, the simulations carried out and the modifications proposed can be found in Ferrer et al. (2004a).
3.3. Research and development of new process schemes The BNRM1 potential (biological processes under any environmental condition) joined to the software capabilities allows combining, in a flexible way, aerobic, anoxic and anaerobic stirred tanks with anaerobic digesters and sedimentation units. This feature is especially useful for testing and developing treatment schemes for biological nutrient removal from either urban or industrial wastewaters. 3.4. Teaching and staff training An increasing number of plant operators are using this software to simulate the WWTP performance and to evaluate different operation alternatives and control actions when changes in influent wastewater or system conditions occur. Also, DESASS is being used by students of civil and chemical engineering in different Spanish universities and makes it easier to understand the whole plant performance, taking into account the interactions between main stream and side streams. 3.5. Test of new control systems by simulation studies DESASS contains the required data to design and simulate the performance of aeration systems (membrane diffusers, control valves and blowers) and recycling pumps. Also, the time evolution of dissolved oxygen concentration in each aerobic reactor as well as waste sludge, recycling sludge and internal recycling flow rates can be established. This feature allows the user to analyse the dynamic plant performance under variable operation strategies and to obtain valuable information to develop new control systems. For instance, it allowed us the development and test of a fuzzy logic control system for optimising nitrogen removal (Serralta et al., 2002). This control system modifies the internal recycling flow rate according to nitrate concentration in the anoxic reactor and effluent total nitrogen concentration. 4. Discussion The complexity of the models which are commonly used to describe wastewater treatment processes requires the use of simulation platforms. The use of these software tools has improved design operation efficiency and many other aspects related to wastewater treatment processes. However, the application of mathematical models to full scale plants requires exhaustive wastewater characterisation, model parameters calibration, operational data and hydraulics knowledge. The use of simulation tools without understanding the processes involved can lead engineers or plant operators to obtain wrong solutions. The benefits and drawbacks of simulating full scale plants are described in detail in Ferrer et al. (2004b). The trends of standard benchmark community for plantwide modelling consist on using ASM models linked to ADM1. This connection involves the application of artificial conversions between the different components considered by both models. The use of transforming/interfacing methods
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has lead to strong discrepancies among the scientific community. These methods make model application more difficult as they add more parameters to calibrate (Zaher et al., 2007). One of the main advantages of using DESASS is that the same model is applied to simulate all the treatment units included in a WWTP. This way, DESASS can simulate processes from both models (AS and AD models) that take place in the same treatment unit, as for instance, the phosphorus release from polyphosphate stored in PAO bacteria and VFA production observed in the bottom of sludge thickeners. 5. Conclusions The program DESASS has been presented in this paper. DESASS is a very useful tool for designing, simulating and optimising WWTPs. The mathematical model implemented in this program is the BNRM1 and it considers the main physical, chemical and biological processes taking place in WWTPs. DESASS can be applied to calculate the performance under steady or transient state of complete treatment schemes including primary settlers, perfermenter tanks, biological reactors, secondary settlers, gravity thickeners and aerobic and anaerobic digesters. Recycling the supernatants from the sludge treatment can be considered in order to take into account the nitrogen and phosphorus loads from these sidestreams. The utilisation of DESASS has been illustrated with an example of a WWTP design including biological nutrient removal and anaerobic digestion as sludge treatment. The results obtained can be seen either numerically or graphically and can also be exported to an Excel file. So far, DESASS has been applied for: designing new WWTPs, diagnosis and upgrading of existing WWTPs, development of new treatment schemes and control systems, teaching and staff training. The main features and capabilities presented in this paper make DESASS a very useful tool for engineers to calculate the WWTPs performance but also for plant operators to test the consequences of modifying the operation criteria. Changing plant configuration and comparing the results over different influent conditions and different scenarios has been shown to be straightforward even for non skilled users. Acknowledgements Financial support from Entidad Pu´blica de Saneamiento de Aguas Residuales de la Comunidad Valenciana, SEARSA y AQUAGEST is gratefully acknowledged. References Allison, J.D., Brown, D.S., Novo-Gradac, K.J., 1991. MINTEQA2/ PRODEFA2, A Geochemical Assessment Model for Environmental Systems, Version 3.0. USEPA, Washington, DC. EPA/600/3e91/021. Batstone, D.J., Keller, J., Angelidaki, I., Kaliuzhnyi, S.V., Pavlostathis, S.G., Rozzi, A. Sanders, W.T.M., Siegrist, H., Vavilin, V.A., 2002. Anaerobic digestion model No. 1 (ADM1). Scientific and Technical Report No. 13. IWA Publishing, London.
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COST, 2001. The COST Simulation BenchmarkdDescription and Simulator Manual. COST (European Cooperation in the field of Scientific and Technical Research), Brussels, Belgium. Ferrer, J., Morenilla, J.J., Bouzas, A., Garcia-Usach, F., 2004a. Calibration and simulation of two large wastewater treatment plants operated for nutrient removal. Water Science and Technology 50 (6), 87e94. Ferrer, J., Seco, A., Garcia-Usach, F., Bouzas, A., Barat, R., 2004b. Simulation of full scale plants: benefits and drawbacks. Water and environmental management series. 2nd IWA Leading-Edge Conference on Water and Wastewater Treatment Technologies. IWA Publishing, London. Henze, M., Grady, C.P.L. Jr., Gujer, W., Marais, G.v.R., Matsuo, T., 1987. Activated Sludge Model No.1 IAWPRC Scientific and Technical Report No.1 IAWPRC, London. Henze, M., Gujer, W., Mino, T., Matsuo, T., Wentzel M., Marais, G.v.R., 1995. Activated Sludge Model No.2. IAWQ Scientific and Technical Report, IAWQ, London. Henze, M., Gujer, W., Mino, T., Matsuo, T., Wentzel, M., Marais, G., van Loosdrecht, M.C.M., 1999. Activated Sludge Model No.2d, ASM2d. Water Science and Technology 39 (1), 165e182. Hulsbeek, J.J.W., Kruit, J., Roeleveld, P.J., van Loosdrecht, M.C.M., 2002. A practical protocol for dynamic modelling of activated sludge systems. Water Science and Technology 45 (6), 127e136. Insel, G., Sin, G., Lee, D.S., Nopens, I., Vanrolleghem, P.A., 2006. A calibration methodology and model-based systems analysis for SBRs removing nutrients under limited aeration conditions. Journal of Chemical Technology and Biotechnology 81 (4), 679e687. Jardin, N., Po¨pel, H.J., 1994. Phosphate releases of sludges from enhanced biological P-removal during digestion. Water Science and Technology 20 (6), 281e292. Koch, G., Ku¨hni, M., Gujer, W., Siegrsit, H., 2000. Calibration and validation of activated sludge model No.3 for Swiss municipal wastewater. Water Research 34 (14), 3580e3590. Melcer, H., Dold, P.L., Jones, R.M., Bye, C.M., Takacs, I., Stensel, H.D., Wilson, A.W., Sun, P., Bury, S., 2003. Methods for wastewater characterization in activated sludge modelling. Water Environment Research Foundation (WERF). Alexandria, VA, USA. Penya-Roja, J.M., Seco, A., Ferrer, J., Serralta, J., 2002. Calibration and validation of Activated Sludge Model No.2d for Spanish municipal wastewater. Environmental Technology 23, 849e862. Ribes, J., Ferrer, J., Bouzas, A., Seco, A., 2002. Modelling of an activated primary settling tank including the fermentation process and VFA elutriation. Environmental Technology 23, 1147e1156. Seco, A., Pastor, L., Barat, R., Ferrer, J., Mangin, D., 26e29 January 2004a. Nutrient recovery by struvite crystallization: an improvement for enhanced biological phosphorus removal treatment plants. Proceedings of the International Conference on Wastewater Treatment for Nutrient Removal and Reuse, Asian Institute of Technology, Thailand 1, 117e124. Seco, A., Ribes, J., Serralta, J., Ferrer, J., 2004b. Biological Nutrient Removal Model No.1 (BNRM1). Water Science and Technology 50 (6), 69e78. Seco, A., Ribes, J., Serralta, J., Ferrer, J., 2005. Upgrading the Denia WWTP according to BNRM1 simulations. Proceedings of the IWA Specialised Conference on Nutrient Management in Wastewater Treatment Processes and Recycle Streams, Krakow, pp. 1113e1118. Serralta, J., Ferrer, J., Borra´s, L., Seco, A., 2004. An extension of ASM2d including pH calculation. Water Research 38 (19), 4029e4038. Serralta, J., Ribes, J., Seco, A., Ferrer, J., 2002. A supervisory control system for optimising nitrogen removal and aeration energy consumption in wastewater treatment plants. Water Science and Technology 45 (4-5), 309e316. Taka´cs, I., Patry, G.G., Nolasco, D., 1991. A dynamic model of the clarification-thickening process. Water Research 25 (10), 1263e1271. WEF and ASCE, 1998a. Design of municipal wastewater treatment plants. Liquid treatment processes. In: ASCE manuals and reports on engineering practice no. 76, WEF manual of practice no. 8, Volume 2. American Society of Civil Engineers and Water Environment Federation, Alexandria. WEF and ASCE, 1998b. Design of municipal wastewater treatment plants. Solids processing and disposal. In: ASCE manuals and reports on engineering practice no. 76, WEF manual of practice no. 8, Volume 3.
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American Society of Civil Engineers and Water Environment Federation, Alexandria. Wild, D., Kisliakova, A., Siegrist, H., 1997. Prediction of recycle phosphorus loads from anaerobic digestion. Water Research 31 (9), 2300e2308.
Zaher, U., Grau, P., Benedetti, L., Ayesa, E., Vanrolleghem, P.A., 2007. Transformers for interfacing anaerobic digestion models to pre- and posttreatment processes in a plant-wide modelling context. Environmental Modelling and Software 22, 40e58.