Dissolved pollutants

Sedimentation and dissolved air flotation are the most common clarification processes for removal of precipitates. Either sedimentation or flotation is often preceded by chemical coagulation or precipitation, which converts dissolved pollutants to a suspended form, and by flocculation, which enhances clarification by flocculating suspended solids into larger, more easily separating particles. Simple sedimentation normally requires a long retention time to adequately reduce the solids content. The detention time of dissolved air flotation, however, is much shorter. When chemicals are used, retention times are reduced and clarification removal efficiency of either sedimentation or flotation is increased. A properly operated clarification system is capable of efficient removal of suspended solids, metal hydroxides, and other wastewater impurities.10-12... 

FIGURE 44 Weathering. A weathered sandstone column. Calcite (composed of calcium carbonate) is dissolved by rain and groundwater (see Textbox 73). When stone in which calcite is a main component as, for example, sandstone, limestone, and marble, is in contact with water for long periods of time, it is weathered and partly or entirely dissolved. Pollutants such as sulfur dioxide are fundamental in accelerating the weathering and dissolution process. When sulfur dioxide, for example, dissolves in rainwater, it forms sulfuric acid, a strong acid that, at ambient temperatures, rapidly dissolves calcium carbonate. 

If this binding does occur, then one would expect very strongly bound compounds to show an unusual affinity for the aqueous phase. This could increase the mobility of these compounds in the environment. It is likely that the bound fraction will undergo phase transfers and degradation at different rates than the free truly dissolved fraction of a dissolved pollutant. If this is the case, then an observed equilibrium between a pollutant in the free and bound states could significantly affect its environmental behavior. 

Miller, G. C., and R. G. Zepp, Effects of suspended sediments on photolysis rates of dissolved pollutants , Water Res., 13, 453-459 (1979b). 

In the context of gas-phase photocatalysis, it is possible to identify four major t) es of applications indoor, outdoor, process gases, and dissolved pollutants. These four t) es have characteristic properties, which may (in certain cases may not) affect the effectiveness of photocatalysis relative to other means of treatment and the t) e of optimal photoreactor and material to be used. 

Photoreactors for dissolved pollutants operate by bubbling a carrier gas (usually air) through a reservoir of liquids (usually water). The pollutant-loaded gas is then passed though the photocatalytic reactor and released. Several t)q)es of reactors have been connected with this application, including PBRs and coated wall reactors. 

Dissolved pollutants can be transferred from a condensed (liquid or solid) phase into the vapor phase under the action of steam or air streams. As a mass transfer phenomenon, it is driven by a concentration gradient between the condensed and gas phases. It applies fundamentally to pollutants that show sufficiently high vapor pressures under the operating conditions. If the source phase is a liquid, Henry s law regulates the corresponding equilibrium (see Chapter 6). Steam can remove pollutants that may be difficult to remove with air. Obviously, the resulting (contaminated) gas stream or condensate must be treated before its release into the atmosphere. 

Adsorption flotation involves the removal of dissolved pollutants by activated carbon in a bubble reactor, and subsequent removal of activated carbon as well as other suspended particles by flotation technique (71). This process was found efficient for removing both dissolved organics and suspended solids from an industrial effluent (72), and for removing the emulsifled oil from water (73). The mechanism of removal has been proposed by Wang (72,73). 

Adsorption of aniline on minerals, such as kaolinite, montmorillonite and vermiculite as well as on a-alumina and iron powders, has been studied in order to better understand the interaction of the adsorbate with natural adsorbents154-157. However, the results, similarly to those obtained on a smooth polycrystalline platinum electrode158, are not useful for the removal of aniline from waste water. The commonly used activated charcoal is also deficient owing to slow kinetics of the removal of dissolved pollutants. In this situation, activated carbon fibers, woven as a C-cloth, seem to be the ideal adsorbents for removal of aniline from waste streams. The reports of Niu and Conway159,160 explained the adsorptive and electrosorptive behavior of aniline on C-cloth electrodes and suggested a methodology for clean-up of industrial waste waters. 

Surfactants and microemulsion systems can be used for ex situ treatment of contaminated soil or in situ soil decontamination. In situ remediation is usually preferred if excavation of the contaminated soil is not possible or expensive, e.g. beneath buildings or for contaminations at great depth. Often bioremediation or natural attenuation is used for decontamination. In most cases, these techniques only permit the effective degradation of contaminants in the plume formed by dissolved pollutants which may be very large. However, for the remediation of a contaminated site, it is also necessary to remove the source where the pollutants maybe adsorbed in large quantities or may be present as solid or liquid phases. The latter are called NAPL (non-aqueous phase liquids) and a differentiation is made between LNAPL (light non-aqueous phase liquids) with a lower density than water and DNAPL (dense non-aqueous phase liquids) with a higher density than water (see Fig. 10.1). 

Molecular Diffusion A micro-scale phenomenon occurring at slow groundwater flow velocities, which tends to spread dissolved pollutants into the finer pore space of the aquifer matrix and thereby greatly reduce their rate of migration, but greatly increase their persistence. 

By means of evaporation, dissolved pollution is concentrated with the aim of obtaining distilled purified water from wastewater. In mechanical vapor recompression (MVR), the influent is inserted in the system, where it is distributed across heat elements and as a consequence is partly evaporated. This vapor is compressed by a compressor and is then transported to the inner surface of the heat element where it condenses and is collected. The concentrated wastewater is deposited onto the bottom of the device and is subsequently transported by the concentrate pump, after which the cycle starts all over. The technique is effective (ca. 99%), which is dependent on the influent and the type of pollution. 

Kleine J, Peinemann K-V, Schuster C and Warnecke H-J (2002), Multifunctional system for treatment of wastewaters from adhesive-producing industries separation of solids and oxidation of dissolved pollutants using doted microfiltration membranes , Chem Eng Sci, 51,1661-1664. 

Injection of silica-rich solutions acts in the same direction as the before-mentioned procedures of chemical stabilization, namely by fixation of metals within a favourable pH-range. Grouting gels, in addition, by filling the pore space, will significantly reduce permeability for dissolved pollutants. 

These condensates are hot and since they have only accidentally been in contact with hydrocarbon-bearing products, they do not contain much dissolved pollution. The low paraffinic HC content must be eliminated before they can be recycled to LP boilers (see Chapter 4, Section 4.5). This type of recycling is increasingly advantageous. 

Certain petrochemical plant effluents, especially in polymer production, are low in dissolved pollution and loaded with SS and HC. In contrast, when solvent or alcohol processes are involved, or when sophisticated products and additives for the pharmaceutical industry are manufactured, the dissolved BOD5 becomes the main pollution fraction and can reach 500 to 1500 mg l . 

Process water, or sour water that includes dissolved pollution primarily (phenols, sulfides, etc.). 

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