Reaeration in Sewer Networks

TABLE 4.6. Levels of Total Hydrogen Sulfide Concentration in Wastewater of a Sewer System and Associated Problems in Terms of Malodors, Health and Corrosion. 

Problems related to the occurrence of hydrogen sulfide have been intensively reported in the literature (cf. this chapter and Section 6.2.6). Although a great number of factors affect the relation between the occurrence and the problems identified, Table 4.6 can be considered a relevant approach. The table gives a simplified estimate, and indication of medium problems must not be considered equivalent to no need for control.  

Another reason why a total concentration of 0.5 gS m-3 typically does not give rise to problems is that wastewater contains small amounts of heavy metals that will combine with free sulfide to form insoluble metal sulfides. Iron is typically present in wastewater in relatively high concentrations. 

The DO mass balance in wastewater of sewer systems is fundamental for the microbial processes. The low solubility of oxygen in water, relatively high resistance to mass transfer across the air-water interface and potentially high removal rate of DO are maj or reasons for the fact that DO is often a limiting factor 

The first basic aspect is the solubility of oxygen in water and wastewater in equilibrium with an overlaying atmosphere [cf. Henry s law, Equation (4.8), and Example 4.1], 

---

Reaeration in sewer networks the presence of dissolved oxygen in wastewater of sewer systems determines if, and to what extent, aerobic and anaerobic processes proceed. The air-water oxygen transfer (the reaeration) determines the potential of aerobic transformation and corresponding removal of wastewater components in many sewer... 

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). 

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. 

When designing sewer networks, particularly gravity sewers, reaeration is the major process that should be focused on to reduce sulfide formation and the formation of organic odorous substances (cf. Section 4.4). A number of hydraulic and systems characteristics can be managed to increase the reaeration rate and avoid or reduce sulfide-related problems. The hydraulic mean depth, the hydraulic radius, the wastewater flow velocity and the slope of the sewer pipe are, in this respect, important factors that are dealt with in Section 4.4. It should be stressed that it is not necessarily the objective to avoid sulfide formation (in the sewer biofilm), but the sulfide that occurs in the bulk water phase should be at a low concentration level. Therefore, the DO concentration in the bulk water phase should not be lower than about 0.2-0.5 g02 m-3, sufficiently high to oxidize sulfide before a considerable amount is emitted to the sewer atmosphere. 

---