Sewer process concept

When considering the details of the sewer process concept, the processes of the subsystems are theoretically described at different levels. The most detailed description is done for the water phase by including the biomass/substrate relationship. Fewer details in terms of a description at an empirical level are included for the reaeration and the processes in the biofilm. 

Briefly expressed, the sustainable approach of the sewer process concept can be interpreted by changing wastewater management from an end-of-pipe treatment to a pipe and plant treatment. This is not the only way of approaching a sustainable solution for an urban wastewater system, however, it is a contribution. But, it is certainly true that the sewer process concept tends to put much more focus on the dry-weather performance of the sewer than is typically done. 

The objective of this chapter is to highlight fundamental chemical and physicochemical aspects of general importance for sewer systems and in-sewer processes. The contents are selected with this in mind, and the focus is on chemical and physicochemical concepts adapted to this purpose. The chapter is written to serve as a solid background for understanding process-related aspects considering the sewer as a reactor. 

From the very beginning, it is important to notice that activated sludge and wastewater, as they occur in sewer networks, are different matrices for microbial activity. The concepts developed when dealing with sewer processes may be different from what is known for activated sludge in a wastewater treatment system. 

The components referred to in Equation (3.5) are determined according to the activated sludge concept relevant at the influent to wastewater treatment plants. Basically, they are also present in the wastewater of sewer systems. However, when considering sewer processes, a slightly different approach compared with the activated sludge concept is needed. Details, in this respect, will be given in Chapters 5 and 6. Briefly, the explanation is as follows ... 

This chapter deals with the microbial transformations of wastewater under aerobic conditions in a sewer network. It emphasizes the transformations of the organic matter and includes processes in both the water phase and the biofilm. Furthermore, transformations of particles in suspension originating from sewer sediments are included. A concept and a corresponding model for the integration of the major microbial processes, i.e., growth of the heterotrophic biomass, the respiration and the hydrolysis, are also dealt with. The basic chemical and biological aspects of sewer processes are focused on in Chapters 2 and 3. The reaeration process is dealt with in Chapter 4. 

It is considered important to identify a rather simple, however, generally well-accepted concept for the microbial transformations that can be extended when further knowledge on sewer processes exists. Although details may be missing in a simple description, advantages in terms of possibilities for a sound... 

Microbial processes in terms of sludge (biomass) production, BOD and nutrient removal have been focused on when dealing with biological wastewater treatment. Therefore, and because the sewer is the system for input to the wastewater treatment plant, there is a basis as well as a perspective to establish a process concept for the sewer that can be integrated with the activated sludge processes. 

In addition to the kinetics of the sewer processes described in Section 5.3, the stoichiometry of the transformations of the components is crucial for the mass balance. The stoichiometry of the biomass/substrate relationships is, according to the activated sludge model concept, determined by the heterotrophic biomass yield constant, YH, in units of gCOD gCOD-1. As depicted in Figure 5.5, the yield constant is an important factor related to the consumption of both Ss and S0 for the production of XBw. 

Abdul-Talib, S., T. H vitved-Jacobsen, J. Vollertsen, and Z. Ujang (2001), Anoxic transformations of wastewater organic matter in sewers — process kinetics, model concept and wastewater treatment potential, Proceedings from the 2nd International Conference on Interactions between Sewers, Treatment Plants and Receiving Waters in Urban Areas (INTERURBAII), Lisbon, Portugal, February 19-22, 2001, pp. 53-60. 

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. 

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. 

Wet-weather processes have, in general, been excluded in the text, because they are based on a different concept and perform differently. Microbial and physicochemical processes are contrary to the physical processes dominating in sewers during dry-weather transport of the wastewater. When dealing with combined sewer networks in terms of pollutant loads during overflow events, dry-weather solids deposition and erosion and solids transport during high-flow events are, in addition to the rainfall/runoff hydraulic and sewer solids characteristics, the central physical in-sewer processes. Quite different process approaches are, therefore, required to describe dry-weather and wet-weather sewer performance. 

By adding a trickling water supply and drain, Q3, to the drag-out tank, the application of Kushner s concept can be extended to other metal finishing processes that may not be amenable to full reuse but can allow partial reuse. Figure 9.3 depicts the partial reuse scheme. The trickle concentrate can also be batch treated in a small volume on-site, recycled at a central facility, or mixed with Qx, for discharge, if the combined metal content is below sewer discharge standards. 

In conventional design and management practice, treatment of wastewater is assumed to take place entirely within the treatment plant, while a sewer network serves the sole purpose of collecting and transporting wastewater from source to treatment. The concept of considering the sewer as a process reactor... 

The fundamental understanding of the microbial processes in wastewater is based on the fact that substrate utilization for growth of biomass takes place parallel to its removal for energy purposes by an electron acceptor. Figure 2.2 shows the general concept and examples where an external electron acceptor is involved. These fundamental microbial transformations take place in the water phase, in the biofilms and in the sediments of the sewer. 

Substrate-limited growth in terms of reduced availability of both the electron donor and the electron acceptor is common in wastewater of sewer systems. Based on the concept of Michaelis-Menten s kinetics for enzymatic processes, Monod (1949) formulated, in operational terms, the relationship between the actual and the maximal specific growth rate constants and the concentration of a limiting substrate [cf. Equation (2.14)] ... 

Biomass and substrate must be separately described to establish a concept for classification of wastewater directed toward a description of the microbial processes. For several reasons, e.g., to allow widespread application and to observe a basic mass balance, the organic matter expressed in terms of COD is a central parameter for wastewater quality. According to the concepts used in the active sludge models, the classification of wastewater in a sewer network can also be subdivided as outlined in Figure 3.1 (Henze et al., 1987, 1995a, 2000). A direct interaction between sewer and treatment plant processes is therefore within reach. 

Basically, a concept for microbial transformations in sewer networks should cover soluble and particulate components and relevant processes in the water phase, in the biofilm and in the sewer sediments. In addition, mass transfer between these phases and an air-water transfer of oxygen should be taken into account (Figures 1.3 and 5.2). Although only the aerobic microbial activity will be focused on in the concept presented in this chapter, anoxic and anaerobic processes should be considered possible extensions (cf. Chapter 6). 

FIGURE 5.4. The flow of substrate and biomass according to the activated sludge concept for aerobic, heterotrophic transformations. The three major pathways of the flow, biomass growth, hydrolysis and biomass decay, were included in the first attempt to describe the corresponding processes in the sewer. The components are defined in Section 3.2.6. 

Based on this modified activated sludge concept, it was possible to produce acceptable model simulation results for the water-phase processes of the heterotrophic carbon transformations in sewers. However, problems were identified for the description of the heterotrophic biomass decay. A major problem was the magnitude of the 1-order decay rate constant with respect to the biomass concentration. Henze et al. (1987) and Kappeler and Gujer (1992)... 

The microbial transformations of the wastewater described in the concept shown in Figure 5.5 deal with the COD components defined in Section 3.2.6. The figure also depicts the major processes that include the transformations of the organic matter (the electron donors) in the two subsystems of the sewer the suspended wastewater phase and the sewer biofilm. The air-water oxygen transfer (the reaeration) provides the aerobic microbial processes with the electron acceptor (cf. Section 4.4). Sediment processes are omitted in the concept but are indirectly taken into account in terms of a biofilm at the sediment surface. Water phase/biofilm exchange of electron donors and dissolved oxygen is included in the description. 

TABLE 6.6. Integrated Aerobic and Anaerobic Process Model Concept for Transformations of Organic Matter and Sulfur Components of Wastewater in Sewers. Symbols of Components and Parameters are Defined in Tables 6.5, 6.7 and 6.8. Symbols Used are Given in Appendix A. 

The results from this study support what has been dealt with throughout the text and especially focused on in Chapter 5 the concept s ability to predict wastewater quality changes in a sewer. The example shows that the corresponding process model can be used to simulate the average dry-weather performance of the sewer and serve as a tool for process design and management. The sewer, with its relation to the subsequent treatment plant, is generally... 

A new concept for improved CSO impact assessment must include physical and microbial characteristics and processes. As far as the microbial heterotrophic transformations are concerned, intensive investigations have shown that suspended particles originating from sewer sediments follow the concept for wastewater depicted in Figure 5.5 (Vollertsen and Hvitved-Jacobsen, 1998 Vollertsen and Hvitved-Jacobsen, 1999 Vollertsen et al., 1999). This finding is important, because it shows that the concept and corresponding model developed for transformations of wastewater in sewers... 

---