Atmospheric reaeration

Atmospheric Reaeration. Interfacial properties and phenomena that govern oxygen concentrations in river systems include 1) oxygen solubility (temperature, partial pressure and surface dependency), 2) rate of dissolution of oxygen (saturation level, temperature and surface thin film dependency, i.e., ice, wind), and 3) transport of oxygen via mixing and molecular diffusion. A number of field and empirically derived mathematical relationships have been developed to describe these processes and phenomena, the most common of which is (32) ... 

Prior to simulating stream conditions and determining responses to imposed organic loads, it is desirable to investigate the individual factors that influence oxygen balance, namely atmospheric reaeration, photosynthetic production, respiration, and benthic uptake of oxygen. The aerobic or anaerobic conditions of a stream may drastically influence transport of ions, but the effect of reaeration may be difficult to monitor in a stream. 

While these equations are descriptive of the flume, they are reasonably close to the prediction model derived by Churchill, Elmore, and Buckingham from their analysis of atmospheric reaeration of streams in the Tennessee Valley (11). The Churchill et al. formula for prediction of k2 at 20°C is... 

Figure 6. Atmospheric reaeration as related to velocity and depth (in flume)...

Table II. Predicted Atmospheric Reaeration Coefficients (k2) as a Function of Stream Flow in the Jackson River"...

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],... 

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. 

Exchange of volatile compounds across the air-water interface, e.g., oxygen (reaeration that affects aerobic or anaerobic conditions) and release of odorous substances Release of odorous substances to the urban atmosphere and change of reaeration due to a lower atmospheric oxygen concentration Extent of the processes... 

We turn in the next illustration to an examination of the reverse process, that of uptake of a substance of a body of water. The system considered differs from the basin examined previously in two important ways First, the substance in question is a benign one, namely, oxygen, which is taken up by the water from the atmosphere. This process is termed reaeration, and is a highly desirable one as it helps maintain aquatic life and aids in the biodegradation of objectionable substances. Second, the body of water is assumed to be in steady flow, such as a river. The system is no longer considered to be well mixed except in the vertical direction and we can therefore expect a steady increase in oxygen concentration in the direction of flow. This calls... 

In the case of oxygen-depleted streams, such as typically occur downstream of wastewater outfalls, the flux of O2 is from the atmosphere into the streams. In a stream with steady, uniform flow and no sources or sinks of oxygen other than the atmosphere, an oxygen deficit decays exponentially with downstream travel time. The classic Streeter-Phelps model, discussed in Section 2.5, considers not only dissolution of O2 into a stream but also simultaneous O2 consumption due to microbial degradation of organic waste within the stream. By tradition, the reaeration coefficient in Streeter-Phelps modeling is designated fCoj. 

The rate of O2 reaeration (the third term in Eq. 2.62) is proportional both to the O2 deficit, which is the difference between the saturated O2 concentration and the actual O2 concentration, and to the reaeration coefficient. The reaeration coefficient for oxygen equals the gas exchange velocity (the piston velocity) for oxygen divided by average stream depth (Section 2.3.2) thus, the gas exchange velocity equals the product of the reaeration coefficient and depth. Total flux [M/T] of atmospheric O2 into the control volume by reaeration is thus... 

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