Sewer process model

Wastewater in sewers includes different and varying species of heterotrophic microorganisms. A simple relationship between biomass growth and substrate utilization is needed. Several studies performed with different types of wastewater and sewer solids have shown that a simple description is possible and acceptable (Bjerre et al., 1995 Vollertsen and Hvitved-Jacobsen, 1999). 

Note rmaint is only relevant if Ss is not sufficient for maintaining the biomass. 

AEROBIC AND ANOXIC PROCESSES—PROCESS CONCEPT AND MODEL 

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This example supplements and extends what was illustrated in Example 5.3 related to the variability of DO in sewer networks. Example 5.3 was based on the simple DO mass balance expressed in Equation (5.10). This example will make use of the sewer process model that integrates the reaeration and the DO consuming processes (Table 5.3). 

FIGURE 5.12. Steady state equilibrium concentrations of DO at 11°C in the wastewater of the sewer dealt with in Figures 5.10 and 5.11. Simulation results are based on the sewer process model (Table 5.3). 

The concept expressed in Figure 6.8 is described in relatively simple terms. The most important parts are shown with full-drawn lines, whereas the dotted lines are generally less important for the formulation of a sewer process model. The processes can be described in further details, however, the major concern has been to establish a concept for which components and parameters can be experimentally determined without unrealistic resources for laboratory and field studies. Methods for this determination will be dealt with in Chapter 7. 

TABLE 6.7. Example of Model Parameters Used in the Sewer Process Model Outlined in Table 6.6. 

The use of the integrated aerobic-anaerobic sewer process model depicted in Table 6.6 will be exemplified in Chapter 8. Compared with the different empirical models that have been developed for prediction of transformations in sewers, it integrates and expands the conditions that can be taken into account ... 

The sewer model is designed from a conceptual point of view and has potential for further applications. In Section 4.3.3, it was concluded that the occurrence of sulfide can be used as a pragmatic measure of malodors. Therefore, the sewer process model also has potential for the prediction of odor problems. Furthermore, as dealt with in Section 8.5.2, the model also predicts the aerobic transformations of suspended sediment particles in sewers (Vollertsen and Hvitved-Jacobsen, 1998, 1999 Vollertsen et al 1998, 1999). The model is also a potential tool for simulation of the impacts from combined sewer overflows. 

This chapter focuses on two main subjects. It will first deal with knowledge and methodologies of good practice in the study of chemical and microbial processes in wastewater collection systems. The information on such processes is provided by investigations, measurements and analyses performed at bench, pilot and field scale. Second, it is the objective to establish the theoretical basis for determination of parameters to be used for calibration and validation of sewer process models. These main objectives of the chapter are integrated sampling, pilot-scale and field measurements and laboratory studies and analyses are needed to determine wastewater characteristics, including those kinetic and stoichiometric parameters that are used in models for simulation of the site-specific sewer processes. 

The main procedures for sewer process studies will be dealt with, however, primarily those that are directly related to the determination of process-relevant characteristics. Procedures and measurements of, e.g., sewer hydraulic and solids transport characteristics will not be included in the text. Although information from such measurements is relevant for sewer process model simulation and evaluation, literature is generally available for that purpose. The following are publications dealing with the hydraulic measurements in sewers ASCE (1983) and Bertrand-Krajewski et al. (2000). An overview of the physical processes in sewers is found in Ashley and Verbanck (1998). 

METHODS FOR DETERMINATION OF COMPONENTS AND PARAMETERS FOR SEWER PROCESS MODELING... 

A combination of laboratory and field experiments is required for determination of components and parameters for a sewer process model for simulation of the microbial transformations of organic matter (cf. specifically Sections 5.2-5.4,6.3 and 6.4). Furthermore, additional information is needed to include the sulfide formation. Explicit determination of model components and parameters are preferred to indirect and implicit methods. However, to some extent, model calibration is typically needed to establish an acceptable balance between process details of a model and possibilities for direct experimental determination of model parameters. 

Calibration and validation of the sewer process model (cf. Section 7.2.4). OUR measurements of corresponding upstream and downstream waste-water samples followed by a simulation (calibration) with the sewer process model. 

These four procedures are all recommended to be performed in the order shown to achieve optimal parameter estimation followed by a final validation of the gravity sewer process model (Figure 7.7). In the case of design of a new sewer system, procedure number 4 is, of course, not relevant and kinetic parameters for the sewer biofilm must be evaluated and selected based on information from comparative systems. 

The procedures described in Sections 7.2.1 to 7.2.4 refer to the aerobic formulated sewer process model (cf. Table 5.3) whereas Section 7.2.5 deals with methods applied for determination of model parameters to include transformations of wastewater components under anaerobic conditions (cf. Table 6.6). 

FIGURE 7.7. Overview of procedures 1 to 4 for determination of wastewater components and parameters for the sewer process model. 

Procedures number 1 and 2 described in Sections 7.2.1 and 7.2.2, respectively, show that the COD components and central parameters for the sewer process model can be explicitly determined by means of rather simple OUR experiments. However, some of the process parameters for the hydrolysis still need to be determined. These are as follows ... 

Simulation procedure 4 is basically a calibration of the sewer process model for aerobic microbial transformations as described in the matrix formulation (Table 5.3). Both the biofilm processes and the reaeration are included. Initial values for the components and process parameters for this simulation originate from the sample taken at the upstream sewer station. When simulated values of the downstream COD components are acceptable, i.e., approaching the corresponding measured values, the calibration procedure is successfully completed. The major model parameters to be included in the calibration process are those relevant for the biofilm, especially km and K. After calibration, the model is ready for a successive validation process and later use in practice. 

For aerobic gravity sewers, procedure 4 is the ultimate calibration of the sewer process model. This is based on procedures 1 to 3 using information from upstream and downstream wastewater samples and by including local sewer systems and flow characteristics, temperature and DO concentration values of the wastewater in the sewer. Example 7.2 outlines the results of calibration and validation performed on a 5 km intercepting sewer line. 

Example 7.2 Calibration and validation of the sewer process model... 

Procedures 1 to4describedinSections7.2.1 through 7.2.4 are applied in this example for determination of wastewater COD fractions, model parameters and a corresponding calibration/validation of the sewer process model under aerobic and dry-weather conditions. The number of repeated tests — a total of 29 during different seasons — demonstrates not just the validity of the sewer process model depicted in Table 5.3 but also the validity of the concept behind the model formulated in Section 5.2. 

The field site used for calibration and validation of the sewer process model was an intercepting gravity sewer located between the city of Dronninglund and the wastewater treatment plant in Asaa in the northern part of Jutland, Denmark (Figure 7.11). 

FIGURE 7.12. Results for validation of the conceptual sewer process model for prediction of wastewater quality changes. Measured and simulated absolute values and changes of COD fractions for 29 dry-weather events are compared for wastewater transport in a 5.2 km gravity sewer line from Dronninglund to Asaa. 

Anaerobic processes in wastewater of sewer systems in terms of both the organic matter transformations and the sulfur cycle have been dealt with in Chapter 6. Particularly, Section 6.4 has focused on the integrated aerobic-anaerobic sewer process model. From a conceptual point of view, the anaerobic... 

As far as organic matter transformations are concerned, the process rates are significantly slower compared with aerobic transformations. Basically, readily biodegradable organic matter is preserved and even, to some extent, produced opposite to the situation when aerobic processes proceed. The sulfur cycle, until now included in the sewer process model, is relatively simply described following empirical expressions for sulfide formation. Other important processes in this respect, e.g., hydrogen sulfide emission and sulfide oxidation, still need to be included, however, and, most of all, investigated from a conceptual point of view. 

Focusing on the carbon flow in wastewater under anaerobic conditions, the corresponding process parameters shown in Figure 6.9 are based on utilization of results from the three mentioned procedures and a calibration of the aerobic-anaerobic sewer process model shown in Table 6.6. 

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