Sewer Networks and Processes within Urban Water Systems
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Sewer Networks and Processes within Urban Water Systems
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Product Details
| ISBN-13: | 9781843395065 |
|---|---|
| Publisher: | IWA Publishing |
| Publication date: | 11/01/2004 |
| Series: | Water and Environmental Management Series (WEMS) |
| Pages: | 174 |
| Product dimensions: | 6.12(w) x 9.25(h) x 0.75(d) |
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CHAPTER 1
Modelling the Integrated Urban Drainage Systems
G. Mannina, G. Viviani and G. Freni
Dipartimento di Ingegneria Idraulica ed Applicazioni Ambientali, Università di Palermo, Viale delle Scienze, Palermo 90128, ITALIA
E-mail: mannina@idra.unipa.it, gviv@idra.unipa.it, freni@idra.unipa.it
Abstract In the last years, several researches were developed to analyse the problems linked with the polluting impact on water bodies because of the pollution produced by rainfall in the urban environment. In order to obtain a good description of the problem, it is important to analyse both quantity and quality aspects connected with all the transformation phases that characterise the water cycle in the urban drainage systems. In the paper, an integrated model has been developed which is able to estimate both the interactions between sewers system, WWTP, CSO and receiving water body and the modifications, in terms of quality, that urban storm water causes inside the receiving water body. Such a model is made up of three submodels: the rainfall-runoff and flow propagation sub-model in sewer system, which is able to evaluate the qualitative-quantitative characteristics of storm water; the WWTP sub-model, which is representative of the treatment processes; the receiving water body sub-model, that simulate the pollution transformations inside the water body. The proposed integrated model has been applied in order to evaluate the impact of urban developments on the Oreto river near Palermo city (Italy).
Keywords Integrated model, urban drainage, sewer system, WWTP, receiving water body.
INTRODUCTION
Integrated modelling can be defined as modelling able to simulate the interaction between two or more physical component of an urban drainage system, i.e. sewer system (SS), wastewater treatment plant (WWTP) and receiving water body (RWB). Integrated modelling development is basically connected with the attempt of simulating polluting impacts on receiving water body and, at the same time, the possible causes that generates them. This approach is usually aimed to a better management of all the components of the sewer system and to a better knowledge of the transformations of sewage in the different parts of the system (Rauch et al., 2002).
The need for analysing all the aspects of urban drainage systems is also represented in the EU Water Framework Directive 60/2000, that basically proposes a water-quality orientated view on the whole system and requires new ways of assessing their performance. The key to the required consideration is the change from an emission based approach (Emission Standard) to an ambient water-quality based approach (Stream Standard) (Chave, 2001).
Initially, integrated models have been limited to sewer system and WWTP simulation, neglecting receiving water body (Durchschlag, 1990); only in a second time, the integrated approach has been extended to the whole system; a comprehensive description of this kind of models has been provided by EU COST-682 working group (Vanrolleghem et al, 1999).
The most part of developed models couples separated approaches for simulating different system components using the output of each of them as input for the downstream element of the integrated system. As an example, the Integrated Catchment Simulator (ICS), basically links existing commercial models such as MOUSE for drainage systems (Danish Hydraulic Institute, 2003), STOAT for WWTP simulation and MIKE11 for rivers (Tomicic et al., 2000). Other examples of integrated models are WEST simulator for biochemical processes analysis and SIMBA that was developed in MATLAB/SIMULINK environment (Coen et al., 1997; Alex et al., 1999). The basic limitation of commercial models is connected to the unavailability of model source code, not allowing the final user for modifying and further developing model options in order to fit specific simulation needs.
Another problem frequently linked with commercial models regards the high parameterisation that requires long and expensive data acquisition activities (Vanrolleghem et al., 1999). For by-passing this kind of problem, Schutze et al. (1999) proposed the use of "semi-hypothetic" case studies for analysing integrated modelling potentialities. In details, the Authors simulated the different parts of the system (SS, WWTP, RWB) using data coming from real and well documented case studies not really linked together.
In the paper, an integrated model has been developed which is able to estimate both the interactions between the different systems and the modifications, in terms of quality, that urban storm water causes inside the receiving water body. The model simulate all urban drainage system taking into account the most important phenomena that regulate its behaviour with the objective to reduce the number of the parameters making its use, in real cases, easier. The model is able to simulate both synthetic and historical events.
MODEL DESCRIPTION
The model is made up mainly of three sub-models (Figure 1):
a) the rainfall-runoff and flow propagation sub-model, which is able to evaluate the qualitative-quantitative characteristics of storm water in the sewer system; a CSO device was introduced at the outlet of the sewer system in order to contain the flow excursion in the WWTP during the storm event. To contain the CSOs and to achieve a RWB protection, a storm water tank (SWT) has been introduced.
b) the WWTP sub-model, which is representative of the treatment processes; the WWTP units considered in the sub-model consist of the activated sludge tank and the secondary sedimentation tank, that are the main units influenced by qualiquantitative characteristic variations of the storm sewage flow.
c) the receiving water body sub-model, that simulates the pollutants transformations inside the water body; the adopted algorithms are able to analysis of ephemeral natural stream that is typical of southern Europe.
Each sub-model has been set up according with the relative theory inherent the different part of the model to achieve a good simulation of the different processes take place in the whole system. All part of the system are computed simultaneously. The system compartments (SWT and WWTP) were previously designed in steady dry conditions. The main used algorithms are reported in Table 1. Table 2 summarised the algorithms parameters and variables, their physical meaning and the proposed values used for dynamic simulation.
Sewer system
This sub-model is able to simulate the main phenomena that take place both in the catchement and in the sewer network during a storm event. It is divided into two connected modules: a hydrological and hydraulic module, which calculates the hydrographs at the inlet and at the outlet of the sewer system, and a solid transport module, which calculates the pollutographs at the outlet for different pollutants (TSS, BOD and COD). The hydrological and hydraulic module starts to evaluate the net rainfall, from the hyetograph measured, by a loss function (initial and continuous). From the net rainfall, the model simulates the rainfall-runoff process and the flow propagation with a cascade of two reservoirs in series (eq. 1 in Tab. 1).
The solid transfer module reproduces the accumulation and propagation solids in the catchment and in the sewer network. The main phenomena simulated are: build-up and wash-off of pollutants from catchment surfaces, sedimentation and re-suspension of pollutants in sewers (Bertrand-Krajewski et al., 1993). To simulate the build-up on the catchment surfaces an exponential function (eq. 2) was adopted (Alley and Smith, 1981). The solid wash-off caused by overland flow during a storm event was simulated with the formulation in eq. 3 proposed by Jewell and Adrian (1978).
The solids deposits in the sewers during dry weather has been evaluated by adopting an exponential law (eq. 4); this phenomenon has been emphasized in the model because of its contribution to the storm water pollution. Two classes of particles have been considered: fines particles (d50=50 µm, specific gravity s=1,6) and the coarse particles (d50=500 µm, specific gravity s=2,0). The first classes are mainly transported in suspended load instead the second one are transported in bed load or suspended load in relation of the characteristics of the flow. In order to have a realistic and correct approach a particular care has been taken about sediments transformation in sewers, considering their cohesive-like behaviour linked to organic substances and to the physical-chemical changes during the sewer transport (Crabtree, 1989; Ristenpart, 1995). In particular, the transport equation proposed by Parchure and Metha (1985) couplet to the bed sediment structures hypothesised by Skipworth et al. (1999) to simulate the sediment erosion rate was considered (eq. 5). The pollutographs at the outlet of the sewer system have been evaluated by hypothesising the complex catchement sewer network as a reservoir and by considering an adapted version of Wiuff's model (Berthrand-Krajewski, 1993).
Finally, the WWTP inflow has been computed taking into account the presence of CSO device: its behaviour has been simulated by the eq. 6, where CSO efficiency has been taken into account by the kcso coefficient. The SWT inflow and outflows quali-quantitative characteristics has been considered by the eq. 7-8.
Wastewater treatment plant
Substrate and micro-organisms concentration in the activated sludge tank have been calculated with mass balances based on Monod's theory (eq 9-10). The sedimentation tank performance has been simulated using the solid flux theory according to the methodology proposed by Takacs et al. (1991). In particular, the solids concentration profile has been obtained by dividing the settler into 50 horizontal layers of constant thickness. Within each layer the concentration is assumed to be constant and the dynamic update is performed by imposing a mass balance for each layer. Three sets of equations have been adopted for the layers depending on their depth: the first set (eq. 11) has been used for the upper region of the tank (the clarification region), the second one (eq. 12) for the lower layer (the thickening zone) and the third for the feed layer (eq. 13). The settling velocity function (eq. 14) proposed by Takacs et al. (1991) has been adopted.
Receiving water body
This sub-model simulates the processes that take place in the river. It is divided into quantity and quality module. The first module allows to determinate the flow distribution in the river instead the second module simulates the distribution of a generic pollution along the river. The exemplified form of the De Saint Venant equation (cinematic wave) for the quantity module and the dispersion advection equation for the quality module were adopted (eq.15-16). The dispersion coefficient E has been evaluated by using the expression suggested by Elder (eq. 17), where only the vertical velocity gradient is considered important in stream flow (Brown and Barnwell, 1987).
APPLICATION OF THE MODEL
The model has been applied to a semi-hypothetical case study; this approach was developed to cope with the lack of date and to verify, in this first phase, the performances of the model (Schutze et al., 1999). The model has been tested by using the databases of Fossolo (Bologna) catchment, for the sewer system, and Oreto river (Palermo) for the receiving water body. Fossolo experimental catchment collects water coming from a residential area on the outskirts of Bologna (Artina et al., 1997). The drained area measures 40.71 ha, with an impermeable percentage of 75%. The catchment has an average slope of 0.3%. The number of inhabitants is about 10.000. The drainage network length is about 5 km and it ends in a polycentric section pipe 144 cm high and 180 cm wide. Discharge has been estimated by an ultrasonic probe. A refrigerated automatic sampler with 24 bottles has been used for sewage quality analysis. 12 events have been measured.
Oreto catchment area is about 112 km. The river has an ephemeral natural stream flow with alternation of intense short rainfall and long dry periods. The upstream reach is still natural instead the downstream part has been urbanized during the last 50th years. The upstream reach has got a strong auto-depuration capacity due to its turbulent hydraulic regime. Twelve cross section have been selected in order to estimate the whole stream behaviour in terms of pollutants and dissolved oxygen concentration.
RESULTS
The model has been applied using the 12 events recorded in Fossolo catchment. Fig 2 shows the hydrograph and pollutograph in sewer for the event recorded on 25/4/94. The pollutograph is characterized by a first flush effect. Fig. 3 shows the CSO pollutograph during the storm event. At the first part of the event the pollution load rises rapidly; this causes the fill of the retention tank and the discharge in the RWB. In Fig. 4 the settler behaviour is shown. Fig.5 shows flows and dissolved oxygen concentrations along the water body, in different sections of the river.
The model allows for evaluating the variations of the WWTP performances during a storm event and the dissolved oxygen reduction in the RWB due to the discharge. In particular, the increase of flow rates causes a reduction in treatment efficiency both regarding the activated sludge tank and the sedimentation tank. This fact can cause the overflow of TSS and then the consequent discharging in the receiving water body. Moreover, the loss of the biomass from the system causes a general reduction in the treatment process efficiency worsening the quality conditions of the receiving water body also in the dry period subsequent the event. Indeed normal working conditions can't take place immediately after the end of the storm event but it involves usually a time period, for the re-establishment of the biomass in the activated tank reactor.
The consequent dissolved oxygen reduction in the RWB is much more emphasized in the upstream sections, near the WWTP and SWT outlets. This aspect could cause the water fauna dead and the consequent decay of the river environmental quality characteristics.
CONCLUSIONS
An integrated model has been developed to analyse the impact on RWB quality coming from urban sewage discharge. In this first phase of the research the model has been applied to an hypothetical site to test the accuracy during different storm events. The model is able to estimate the effect of the introduction of mitigation and control measures to reduce the CSOs. In order to preserve the water body, the temporal accumulation of the first part of the event in a SWT and its treatment at the end of the storm event has been considered.
The model adopts simple but accurate algorithms to reproduce main phenomena that characterize the integrated system component behaviour. This approach allows to overcame problem of lack of data due to the several processes to simulate. Further development of the research will be given to the application of a real pilot case.
CHAPTER 2
Model connectors for integrated simulations of urban wastewater systems
L. Benedetti, J. Meirlaen and P.A. Vanrolleghem
BIOMATH, Ghent University, Coupure Links 653, B-9000 Gent (Belgium) tel. +32 9 264 6196, fax. +32 9 264 6220, e-mail lorenzo.benedetti@biomath.ugent.be
Abstract In order to perform simulations of the integrated wastewater system, state variables of sub-system models (sewer, activated sludge, river) must be translated from one model to the other. A connector between ASM1 (Henze et al. 2000) and RWQM1 (Shanahan et al. 2001) is described in this paper to introduce the general approach developed to overcome this necessity. Inherent features of this connector are its closed elemental mass balances. The COD fractions of ASM1 have been split over the COD fractions of RWQM1, while balance terms were used to close elemental balances. Also the different environmental conditions in the systems (activated sludge, sewer and river) have been taken into account. An evaluation of the influence of several connector and model parameters on connector balance terms and outputs has been performed. Some results of a case study concerning the river Lambro (Italy) are described to show an application of such model connection approach.
Keywords ASM1, integrated modelling, mass balances, model connectors, RWQM1, urban wastewater system.
1 Introduction
In recent years the possibility of performing integrated simulations has largely improved due to the enhanced models described in literature and the continuous development of simulation software together with the increasing computational power at the desktop. Recently research on the integrated urban drainage system has received ever-increasing attention, especially after the approval by the European Union of the EU Water Framework Directive.
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Table of Contents
Foreword, viii,
Part One: Selected Papers, 1,
Modelling the integrated urban drainage systems G. Mannina, G. Viviani and G. Freni, 3,
Model connectors for integrated simulations of urban wastewater systems L. Benedetti, J. Meirlaen and P.A. Vanrolleghem, 13,
Combined sewer overflows into the Crati River (Cosenza, Italy) G. Palma, F. Calomino and P. Piro, 21,
Comparison between the performances of two combined sewer overflow devices in the reduction of water volume and pollutant discharges A.P. Campisano, E. Creaco and C. Modica, 31,
Spatial evolution of the characteristics of urban wastewater in combined sewers M. Kafi-Benyahia, M. G. Gromaire and G. Chebbo, 40,
Needs and influence of calibration and validation data sets in stormwater quality models of various levels of complexity M. Mourad, J.-L. Bertrand-Krajewski and G. Chebbo, 49,
Prediction of sediment deposition in Raja River concrete drainage system: a case study M. Kassim, A. Ab. Ghani, R. Abdullah and N.A. Zakaria, 59,
Experimental validation of the DORA model for bed load transport in storm sewers C. Arico, R. Gaudio and A. Miglio, 66,
Advection and dispersion of solutes in sewer systems: verification of water quality models by tracer experiments C.M.L. Flamink, J.G. Langeveld and F.H.L.R. Clemens 76,
Measuring oxygen transfer in sewer drops A. Soares, M. C. Almeida and J. Matos, 86,
Electron acceptor utilization rate in municipal wastewater under anoxic condition S. Maizurah Misuan and Suhaimi Abdul-Talib, 94,
Anoxic transformations of wastewater in sewer networks W. Yang, J. Vollertsen and T. Hvitved-Jacobsen, 102,
Nitrate utilization rate in the bulkwater and biofilms phases of sewer networks N. Samsina Johari and S. Abdul-Talib, 112,
Infiltration and exfiltration from sewer network with a focus on house connections in city of Prague I. Princ and D. Kohout, 119,
Measurement of infiltration rates in urban sewer systems: use of oxygen isotopes J. De Benedittis and J.-L. Bertrand-Krajewski, 128,
Exfiltration from sewers pipes - effects of flowrates, different leakage area and soil beddings M.A. Ab-Wahab, S. Abdul-Talib, B. Baharom and M.S. Marwi, 138,
Part Two: Workshop Guidelines, 147,
Guidelines for abstracts J.-L. Bertrand-Krajewski, 149,
Guidelines for oral presentations and preparation of slides J.-L. Bertrand-Krajewski, 152,
Guidelines for session Chairpersons J.-L. Bertrand-Krajewski, 155,
Guidelines for session Rapporteurs J.-L. Bertrand-Krajewski, 158,
Guidelines for "Advocatus Diaboli" M. Almeida and J.-L. Bertrand-Krajewski, 160,
Keyword Index, 163,