Pollutants dilution

Ventilation Ventilation systems have served as the primary and supplementary sources of control for reducing miners imderground exposures to toxic air pollutants. Dilution ventilation supplemented with other types of controls should be used to reduce exposure to diesel emissions. As a minimum, ventilation rates must meet statutory and regulatory requirements for imderground mines. 

In this section we discuss the origins of dispersion in turbulent flow. This discussion is especially relevant to problems common in environmental engineering, problems like pollutant dilution in rivers or the spreading of plumes. Not surprisingly, the origin of the effect turns out to be a consequence of turbulent fluctuations in velocity and concentration. The coupling between these fluctuations is the cause of dispersion. In more informal terms, gusts and eddies cause dispersion. 

Selection of pollution control methods is generally based on the need to control ambient air quaUty in order to achieve compliance with standards for critetia pollutants, or, in the case of nonregulated contaminants, to protect human health and vegetation. There are three elements to a pollution problem a source, a receptor affected by the pollutants, and the transport of pollutants from source to receptor. Modification or elimination of any one of these elements can change the nature of a pollution problem. For instance, tall stacks which disperse effluent modify the transport of pollutants and can thus reduce nearby SO2 deposition from sulfur-containing fossil fuel combustion. Although better dispersion aloft can solve a local problem, if done from numerous sources it can unfortunately cause a regional one, such as the acid rain now evident in the northeastern United States and Canada (see Atmospheric models). References 3—15 discuss atmospheric dilution as a control measure. The better approach, however, is to control emissions at the source. 

The bottoms from the stripper (40—60 wt % acid) are sent to an acid reconcentration unit for upgrading to the proper acid strength and recycling to the reactor. Because of the associated high energy requirements, reconcentration of the diluted sulfuric acid is a cosdy operation. However, a propylene gas stripping process, which utilizes only a small amount of added water for hydrolysis, has been described (63). In this modification, the equiUbrium quantity of isopropyl alcohol is stripped so that acid is recycled without reconcentration. Kquilibrium is attained rapidly at 50°C and isopropyl alcohol is removed from the hydrolysis mixture. Similarly, the weak sulfuric acid process minimizes the reconcentration of the acid and its associated corrosion and pollution problems. 

Pollution Control in the Bleachery. The quantity of water necessary for bleaching, and consequently the volume of effluents, has been decreased significantly by various schemes for recycle of Hquors, eg, pulp washing using dilute spent Hquors and countercurrent flow. Effort is underway to close bleach plants and further reduce water consumption. 

Effluent Guidelines and Standards. The Clean Water Act requires specific levels of control for dischargers. These are outlined in the Effluent Guidelines and Standards for various industrial categories. These standards limit the discharge of pollutants, usually in terms of a unit weight of pollutant per unit of either product or raw material, rather than a concentration in the discharge stream, in order to eliminate the use of dilution to meet limits. 

Water Quality Standards. Water quaUty standards are usually based on one of two primary criteria, stream standards or effluent standards. Stream standards are based on dilution requirements for the receiving water quaUty based on a threshold value of specific pollutants or a beneficial use of the water. Effluent standards are based on the concentration of pollutants that can be discharged or on the degree of treatment required. 

Air or biological oxidation of pyrite leads to sulfate formation and dilute sulfuric acid in the mine drainage. This pollutes streams and the water supphes into which the mine water is drained. Means of controlling this problem are under study. 

From the viewpoint of air pollution, both stable surface layers and low-level inversions are undesirable because they minimize the rate of dilution of contaminants in the atmosphere. Even though the surface layer may be unstable, a low-level inversion will act as abarrier to vertical mixing, and contaminants will accumulate in the surface layer below the inversion. Stable atmospheric conditions tend to be more frequent and longest in persistence in the autumn, but inversions and stable lapse rates are prevalent at all seasons of the year. 

For example in paint shops, TCE evaporates and causes air pollution. The contaminated air has 250 ppm TCE in it and this can be fed to a moving bed reactor at 300°C that is charged with OXITOX (Chovan et al, 1997) The kinetics must be studied experimentally. The experimental setup is shown in Figure 4.5.1 and the following description explains the recommended procedure. In the experimental unit shown, the feed is contained under pressure in a gas cylinder. Two percent of the feed is saturated by TCE and diluted with the rest of the feed. The rate is calculated as ... 

One of the effects of wind speed is to dilute continuously released pollutants at the point of emission. Whether a source is at the surface or elevated, this dilution takes place in the direction of plume transport. Figure 19-2 shows this effect of wind speed for an elevated source with an emission of 6 mass units per second. For a wind speed of 6 m s", there is 1 unit between the vertical parallel planes 1 m apart. When the wind is slowed to 2 m s there are 3 units between those same vertical parallel planes 1 m apart. Note that this dilution by the wind takes place at the point of emission. Because of this, wind speeds used in estimating plume dispersion are generally estimated at stack top. 

At times when the surface pressure gradient is weak, resulting in light winds in the atmosphere s lowest layers, and there is a closed high-preSsure system aloft, there is potential for the buildup of air pollutant concentrations. This is especially true if the system is slow-moving so that light winds remain in the same vicinity for several days. With light winds there will be little dilution of pollutants at the source and not much advection of the polluted air away from source areas. 

The concentration of indoor pollutants is a function of removal processes such as dilution, filtration, and destruction. Dilution is a function of the air exchange rate and the ambient air quality. Gases and particulate matter may also be removed from indoor air by deposition on surfaces. Filtration systems are part of many ventilahon systems. As air is circulated by the air-conditioning system it passes through a filter which can remove some of the particulate matter. The removal efficiency depends on particle size. In addition, some reactive gases like NOj and SOj are readily adsorbed on interior surfaces of a building or home. 

Provision and use of appropriate health surveillance, e.g. for signs of dermatitis, asthma, effects of specific solvent exposures. Full use of any spray booth, enclosure, exhaust ventilation or dilution systems, and automatic handling equipment. (The efficiency of all local exhaust ventilation and other control systems should be maintained, and checked by testing.) Where appropriate, atmospheric monitoring of airborne pollution levels. 

For continuous release of gas or vapour the steady-state dilution ventilation required to reduce the atmospheric pollutant to a level below its hygiene standard is given by... 

If there is substantial airflow through the pathway, the peppermint oil odor could be diluted so that it is imperceptible. Tracer gases such as sulfur hexafluoride (SF ) can provide qualitative and quantitative information on pollutant pathways and ventilation rates. Use of tracer gases to obtain quantitative results requires considerable technical expertise. If it appears that a sophisticated study of pathways (or ventilation rates) is required, you need to use trained investigators. 

In Gaussian plume computations the change in wind velocity with height is a function both of the terrain and of the time of day. We model the air flow as turbulent flow, with turbulence represented by eddy motion. The effect of eddy motion is important in diluting concentrations of pollutants. If a parcel of air is displaced from one level to another, it can carry momentum and thermal energy with it. It also carries whatever has been placed in it from pollution sources. Eddies exist in different sizes in the atmosphere, and these turbulent eddies are most effective in dispersing the plume. 

Temperature-. Typieally, gas temperatures up to about 260°C (500°F), with surges to about 290 C (550 F) ean be aceommodated routinely, with the appropriate fabrie material. Spray eoolers or dilution air ean be used to lower the temperature of the pollutant stream. This prevents the temperature limits of the fabrie from being exeeeded. Lowering the temperature, however, inereases the humidity of the pollutant stream. Therefore, the minimum temperature of the pollutant stream must remain above the dew point of any eondensable in the stream. The baghouse and assoeiated duetwork should be insulated and possibly heated if eondensation may oecur. 

Segregation simply refers to avoiding the mixing of streams. In many cases, segregating waste streams at the source renders several streams environmentally acceptable and hence reduces the pollution-prevention cost. Furthermore, segregating streams with different compositions avoids unnecessary dilution of streams. This reduces the cost of removing the pollutant from the more concentrated streams. It may also provide composition levels that allow the streams to be recycled directly to process units. 

Equation (3.5) can be used to establish a one-to-one correspondence among all composition scales for which mass exchange is feasible. Since most environmental applications involve dilute systems, one can assume that these systems behave ideally. Hence, the transfer of the pollutant is indifferent to the existence of other species in the waste stream. In other words, even if two waste streams contain species that are not identical, but share the same composition of a particular pollutant, the equilibrium composition of the pollutant in an MSA will be the same for both waste streams. Hence, a single composition scale, y, can be used to represent the concentration of the pollutant in any waste stream. Next, (3.5) can be employed to generate Ns scales for the MSAs. For a given set of corresponding composition scales y,x, X2,..., xj,..., it is thermodynamically and practically feasible to transfer the pollutant from any waste stream to any MSA. In addition, it is also feasible to transfer the pollutant from any waste stream of a composition y/ to any MSA which has a composition less than the xy obtained from (3.5b). 

Of particular importance is the case of wastewater with dilute pollutants at ambient temperature. In this case, one can use the following conversion from ppm to mass concentration ... 

---