Sewer atmosphere

The sewer processes take place in a complex system. They proceed in one or more of the five phases the suspended water phase, the biofilm, the sewer sediments, the sewer atmosphere and the sewer walls, and by exchange of relevant substances across the interphases. Processes that proceed in the sewer system affect other parts of the urban system, i.e., the urban atmosphere with malodorous substances. Furthermore, wastewater treatment plants and local receiving waters receive not just those substances discharged into the sewer but also products that are the result of the sewer processes (Figures 1.1 and 1.3). 

Under such conditions, exchange of substances between the sewer atmosphere and the wastewater phase may depend on and affect sewer processes. The following two phenomena are, to a great extent, affected by the equilibrium conditions and the mass transfer that takes place across the air-water interface in a sewer. 

The concept of mole fraction of a component used in Equation (4.1) is a convenient measure of concentration when dealing with trace quantities and dilute solutions, often experienced in environmental systems. This is especially the case with transport phenomena and equilibrium between phases, where it results in simple quantitative expressions. The phenomena of interest when dealing with the exchange of odorous compounds and oxygen between wastewater and a sewer atmosphere are, in this respect, relevant examples. 

The release to the atmosphere is strongly dependent on the pH because only the molecular form and not the dissociated forms can be emitted, e.g., at a pH about 7, an equal amount of H2S and HS- exists in the water phase. Increase of the pH will, therefore, at equilibrium conditions and at a constant total sulfide concentration, reduce the hydrogen sulfide concentration in the overlying sewer atmosphere (Figure 4.1). Therefore, when applying Henry s law [Equation (4.8)], only the nondissociated molecular form, H2S, should be taken into account. 

Group 1 (C02) indicates that microbial degradation of wastewater organic matter takes place in the sewer. In terms of odor, the other groups (2-4) are relevant. In spite of the fact that the investigation did not include the sources of the components found in the sewer atmosphere, group 2 probably is a result of... 

The fundamentals of the first aspect are dealt with in Sections 4.1 and 4.2, concerning the equilibrium relations and the transport processes, respectively. Furthermore, equilibrium aspects of the emission of hydrogen sulfide from the water phase into the sewer atmosphere are included in Section 4.1 as relevant and illustrative. 

The occurrence of hydrogen sulfide in the sewer atmosphere is an important example for illustrating odor problems and other negative effects associated with sulfide that will be further dealt with in Section 6.2.6. According to Table 4.1, HB2S = 563 atm (mole fraction)-1, and H2S, therefore, observes the... 

A number of approaches have been suggested for the determination of the molecular diffusion coefficient, D, of a component in water (Othmer and Thakar, 1953 Scheibel, 1954 Wilke and Chang, 1955 Hayduk andLaudie, 1974 Thibodeaux, 1996). Based on these five references, the diffusion coefficient ratio />/Jl2s / Dlq2 was found to vary within the interval 0.78-0.86 with an arithmetic mean value equal to 0.84. This value can be inserted in Equation (4.22) as a first estimate to determine Km. Equation (4.22) and the empirical expressions for KLC>2 outlined in Table 4.7 are the basis for the determination of the mass transfer coefficient for H2S, KL i S, and thereby, the emission of H2S from the wastewater into the sewer atmosphere. Further details relevant in this respect are dealt with in Section 4.4. 

Matos and de Sousa (1992) and Matos and Aires (1995) have, based on empirical expressions for the emission rate and adsorption rate on the sewer walls of H2S, used Equation (4.23) as a basis for a model for forecasting the buildup of H2S in the sewer atmosphere. The expressions included in this model are detailed in Matos and de Sousa (1992). 

Although the hydrogen sulfide concentration in the sewer atmosphere or the... 

The partial pressure of H2S on a volumetric basis in the atmosphere in equilibrium with a water phase of sulfide (H2S + HS ) is at a pH of 7, approximately equal to 100 ppm (gS m-3)-1 (Figure 4.2). It is clear that under equilibrium conditions, much lower concentrations than those corresponding to the values shown in Table 4.6 may result in odor and human health problems. This is also seen from the fact that Henry s constant for H2S is rather high, //H2S =563 atm (mole fraction)-1 at 25°C (Table 4.1). However, under real conditions in sewer networks, conditions close to equilibrium rarely exist because of, for example, ventilation and adsorption followed by oxidation on the sewer walls. Typically, the gas concentration found in the sewer atmosphere ranges from 2-20% and is normally found to be less than 10% of the theoretical equilibrium value (Melbourne and Metropolitan Board of Works, 1989). 

The temperature dependency of Sos is generally more important than the dependency of the pressure. As seen from Equation (4.24), Sos is close to 14.65 g m-3 when the temperature is approaching 0°C, and at 15°C, it is 10.04 g m-3. However, a reduced partial pressure of 02 in a sewer atmosphere should be considered (cf. Example 4.1). 

From a general point of view, but still related to sewer conditions, the anaerobic processes in wastewater are dealt with in Chapter 3, especially in Section 3.2.2. In the following, the sulfur cycle is focused on. A part of this cycle proceeds under anaerobic conditions, and another part is aerobic. In a sewer system with changing aerobic and anaerobic conditions, this combined cycle is of particular interest but, at the same time, also complex to deal with. The nature of the sulfur cycle in a sewer is further complicated because the processes proceed in and between the biofilm, the sewer sediments, the water phase, and the sewer atmosphere. 

The oxidation of sulfide to elementary sulfur (S) or sulfate (SO4-) may take place when aerobic conditions exist. If sulfide is produced in the deep part of a biofilm in a gravity sewer, it may be oxidized in an aerobic upper layer of the biofilm or in the water phase (Figure 6.2). The details of the oxidation are not well known and may be due to chemical and biological processes. The final step of this process is sulfate, although sulfur in an oxidation step of 0 may be temporarily generated. Oxidation of sulfide that is released into the sewer atmosphere will be dealt with in Section 6.2.6. 

The second term in Equation (6.8) corresponds to the sinks for sulfide in the water phase that, according to Figure 4.4, are primarily caused by oxidation in the water phase and emission into the sewer atmosphere. Pomeroy and Parkhurst (1977) propose values for Nat two levels,/V=0.96 and A=0.64. The first value corresponds to a median buildup of sulfide, whereas the last value is a conservative estimate for prediction of sulfide buildup in a sewer. The second term of Equation (6.8) shows that the removal of sulfide from the water phase is considered a 1-order reaction in the sulfide concentration. The term also includes elements related to the reaeration and, thereby, the emission of hydrogen sulfide [cf. Equations (3) and (6) in Table 4.7 and Section 4.3.2],... 

A reliable concrete corrosion rate is difficult to predict. As already mentioned and also shown in Figure 4.4, it requires that several process and exchange rates in terms of primarily sulfide formation, emission to the sewer atmosphere, sulfide absorption and sulfide oxidation on the sewer walls can be determined. 

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. 

Ventilation of sewers may not only reduce the hydrogen sulfide concentration in the sewer atmosphere but also the moisture that is a fundamental requirement for establishment of microbial activity on the sewer walls. It is important that the ventilation be well controlled otherwise, odorous problems in the vicinity of the sewer network may occur. In some cases, operational procedures like treatment of the vented air, e.g., by wet oxidation, by chemical scrubbing or by passing a biofilter, may need to be considered. 

It is important to note that addition of iron salts keeps the sulfide as a precipitate in the wastewater and hinders the emission to the sewer atmosphere and the following negative effects. Precipitation of sulfide normally has no effect on the formation of sulfide. 

The model is, in several aspects, a simplification of the processes that occur. As an example, it is important—and also possible when further data are available from experiments — to expand the model by including the oxidation of sulfide in a gravity sewer at low DO concentrations and the emission of hydrogen sulfide into the sewer atmosphere. From a general point of view, however, it is considered important only to deal with the most important aspects to keep the number of processes low and not include too many process parameters that are site specific. 

Extension of the WATS model to integrate further dry-weather processes is considered important. Examples of such extensions are the description of the wastewater quality and nitrite/nitrate transformations under anoxic conditions and the emission of hydrogen sulfide into the sewer atmosphere followed by its transformation (oxidation) at the sewer walls. 

The integrated aerobic-anaerobic WATS model has changed this situation. As an example, it is possible to use the model in a gravity sewer with changing aerobic and anaerobic conditions. As previously stressed, a number of in-sewer processes still need to be dealt with. Examples are the anoxic transformations and the processes related to the extended sulfur cycle, particularly, the oxidation of sulfide and the emission of hydrogen sulfide into the sewer atmosphere, including its further oxidation at the sewer walls. Combined use of empirical and conceptual models is still needed. 

The simulations depicted in Figure 8.8 also show that arather low hydrogen sulfide concentration is predicted in the gravity sewer. Only minor problems related to hydrogen sulfide production may therefore arise. Until now, the WATS model did not include sulfide release to the sewer atmosphere, sulfide oxidation or sulfide precipitation that may further reduce the concentrations shown. The predicted sulfide concentrations are, therefore, maximum levels. In case a natural capacity of iron salts in the wastewater to precipitate sulfide is inadequate, the sulfide concentrations are considered at a level that can be relatively easily controlled. 

Elemental sulfur, thiosulfates, metal sulfides, H2S, and tetrathionates can be oxidized to sulfuric acid by microorganisms generically referred to as thiobacilli. Corrosion in sewers and other concrete structures is often due to oxidation of sulfides generated by the activities of SRB and may occur in many steps. Concrete is a moderately porous mixture of alkaline inorganic precipitates and mineral aggregates. Anaerobic conditions in sewage support SRB that convert sulfate to H2S, which volatilizes to the sewer atmosphere and redissolves in condensate on the sewer crown [43] (Fig. 6). A second community of microorganisms at the... 

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