Gaseous wastes treatment

Ozone is applied in three-phase systems where a selective ozone reaction, oxidation of residual compounds and/or enhancement of biodegradability is required. It can be used to treat drinking water and waste water, as well as gaseous or solid wastes. Especially in drinking water treatment full-scale applications are common, e. g. for particle removal and disinfection, while in waste water treatment sludge ozonation and the use of catalyst in AOP have been applied occasionally. Current research areas for three-phase ozonation include soil treatment and oxidative regeneration of adsorbers. Ozonation in water-solvent systems is seldom studied on the lab-scale and seems favorable only in special cases. In general, potential still exists for new developments and improvements in ozone applications for gas/watcr/solvent and gas/waler/solid systems. 

In many cases, this type of reactor provides the best way of carrying out a reaction between gaseous and liquid reactants in contact with a solid catalyst or an inert packing. That is the reason why these reactors are widely used in chemical and petrochemical industries as well as in biotechnology and waste water treatment. Reviews of all the applications have been published recently (1,2). 

Gas/liquid contacting is frequently encountered in chemical reaction and bioprocess engineering. For reactions in gas/liquid systems (oxidation, hydrogenation, chlorination, and so on) and aerobic fermentation processes (including biological waste water treatment), the gaseous reaction partner must first be dissolved in the liquid. In order to increase its absorption rate, the gas must be dispersed into fine bubbles in the liquid. A fast rotating stirrer (e.g. a turbine stirrer), to which the gas is supplied from below, is normally used for this purpose (see the sketch in Fig. 34). 

HDN-Technik GmbH (Rednitzhembach, Germany), K. W. Hofmann information concerning odor abatement (gaseous systems) and waste air treatment and a photograph of a photoreactor. 

One important chemical treatment of such waste is treatment with gaseous ozone. This method has not received adequate attention. Ozone has become important in recent years as an industrial raw material for the chemical industry. It is now available in tonnage quantities at a competitive cost for use as an oxidizing agent and as a chemical raw material. An excellent compilation of history, generation, and properties of ozone has been prepared by Hann and Manley (5). 

Electrolysis is the reaction of either oxidation or reduction taking place at the surface of conductive electrodes immersed in an electrolyte, under the influence of an applied potential. This process is used for reclaiming heavy metals from concentrated aqueous solutions. Application to waste water treatment may be limited because of cost factors. A frequent application is the recovery for recycle or reuse of metals, like copper, from waste streams. Pilot applications include oxidation of cyanide waste and separation of oil-water mixtures. Gaseous emissions may occur and, if they are hazardous and cannot be vented to the atmosphere, further treatment, such as scrubbing, is required. Waste water from the process may also require further treatment. 

Nitrogen( total) Wastewaters from urban waste water treatment plant Microwave-assisted oxidation of nitrogen-containing compounds to nitrate with an alkaline peroxydisulfate solution UV-Vis 0.21 mg L 1 Flow injection system chamber-like de- [434] bubbler for removal of the gaseous species formed during in-line oxidation nitrate reduction to nitrite Griess-Ilosvay reaction... 

Treatment of Caseous Waste. Gaseous wastes arise from the ventilation of process vessels and the concrete cells that house the plant and equipment used for reprocessing spent nuclear fuel. This gaseous waste is largely air contaminated with small entrained liquid or solid particles containing radioactive components. Some ventilation streams also are contaminated with oxides of nitrogen. 

It can be seen that a dashed line has been drawn around the block containing the environmental control operations. This identifies the unique role of environmental control operations in a chemical plant conplex. A single environmental control unit may treat the waste from several processes. For exanple, the waste water treatment facility for an oil refinery might treat the waste water from as many as 20 separate processes. In addition, the refinery may contain a single stack and incinerator to deal with gaseous wastes from these processes. Often, this common environmental control equipment is not shown in the PFD for an individual process, but is shown on a separate PFD as part of the off site section of the plant. Just because the environmental units do not appear on the PFD does not indicate that they do not exist or that they are uninportant. 

In this type of reactors, the gas and the liquid phase flow over a fixed bed of catalysts. The fixed bed reactors can be mainly classified into three types, (i) co-current down-flow of both gas and liquid phases (ii) downward flow of liquid with gas in the countercurrent upward direction and (iii) co-current up-flow of both gas and liquid. Reactors with co-current down-flow of gas and liquid is called as trickle bed reactors (TBR) and the co-current up-flow reactors are also referred to as packed bubble column reactors. Trickle bed reactors, wherein, the liquid reactant trickles down concurrently along with the gaseous reactant, over a fixed bed of catalyst pellets finds its application in wide variety of chemical, petrochemical and biochemical processes along with its application in waste water treatment. The examples of application of trickle bed reactors are given in detail in several monographs. (Satterfield (1975), Shah (1979), Al-Dahhan (1997) and Saroha (1996)). These include oxidation, hydrogenation, isomerisation, hydrodesulfurisation, hydroprocessing. These types of reactors are also applicable for esterification reactions (Hanika (2003)). 

Membrane processes are very important in our everyday life, but also in industry, for example, for water and waste water treatment, in medical applications, or separation of petrochemicals. Membrane processes are an energy saving method for the separation of mixtures, which occur in nearly all production processes in the chemical industry. Membrane-based devices are much smaller and work at lower temperatures compared to conventional separation facilities with distillation, extraction, or adsorption processes. Classical separation methods used for purification of chemical products, notably distillation, extraction, and crystallization are energy and cost intensive. Over 50% of the energy costs in the chemical industry are used for the separation of gaseous or liquid mixtures. With membrane technology, the costs for difficult separations, for example, of azeotropic mixtures. 

The performance of SCWO for waste treatment has been demonstrated (15,16). In these studies, a broad number of refractory materials such as chlorinated solvents, polychlorinated biphenyls (PCBs), and pesticides were studied as a function of process parameters (17). The success of these early studies led to pilot studies which showed that chlorinated hydrocarbons, including 1,1,1-trichloroethane /7/-T5-6y,(9-chlorotoluene [95-49-8] and hexachlorocyclohexane, could be destroyed to greater than 99.99997, 99.998, and 99.9993%, respectively. In addition, no traces of organic material could be detected in the gaseous phase, which consisted of carbon dioxide and unreacted oxygen. The pilot unit had a capacity of 3 L/min of Hquid effluent and was operated for a maximum of 24 h. 

Waste treatment prior to disposal may introduee phase ehanges whieh result in quite different pollution eontrol eonsiderations. For example, the gases generated by ineineration of a solid waste ean be serubbed with liquid in order to meet an aeeeptable diseharge eriterion henee, in addition to ash for disposal, a liquid effluent stream is produeed and requires treatment. Other waste treatment proeesses may result in the liberation of flammable or toxie gaseous emissions as exemplified in Table 16.5. 

Treatment of ammoniabearing waste from chemical industry Liberation of gaseous ammonia Ammonium chloride Nitric acid... 

A 3 Gaseous (gases, vapors, airborne particulates) W = Wastewater (aqueous waste) B11 Biological Treatment - Aerobic... 

An enclosure is usually an air-supported structure which permits the collection and treatment of gaseous wastes produced by surface impoundments. Enclosures are susceptible to wind damage and can be harmed by the wastes they cover. Subject to these limitations, control effectiveness approaches 100 percent (University of Arkansas and Louisiana State University, 1985). 

Release of trichloroethylene also occurs at treatment and disposal sites. Water treatment facilities may release trichloroethylene from contaminated water through volatilization and air-stripping procedures (EPA 1985e). Trichloroethylene is also released to the atmosphere through gaseous emissions from landfills. The compound may occur as either an original contaminant or as a result of the decomposition of tetrachloroethylene. Trichloroethylene has also been detected in stack emissions from the incineration of municipal and hazardous waste (James et al. 1985 Oppelt 1987). 

Direct acidihcation of cyanide waste streams was once a relatively common treatment. Cyanide is acidified in a sealed reactor that is vented to the atmosphere through an air emission control system. Cyanide is converted to gaseous hydrogen cyanide, treated, vented, and dispersed. 

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