Treatment of Waste Streams

The industrial research efforts on coffee decaffeination, spice extraction, and flavors concentration are, to a great extent, shrouded by the cloak of proprietary security, but the investigations of the use of supercritical fluids to treat various waste streams is reasonably well publicized. Most familiar, perhaps, is the supercritical waste water detoxification process developed by Modar Inc. This is potentially attractive for detoxifying refractory chemicals such as polychlorinated biphenyls, dioxin, and other toxic materials (Anon., 1982 Modell, 1982). In the Modar process, the toxic chemicals are homogeneously reacted with oxygen in supercritical water, the solvent for the organics and the oxygen. The main feature of the process is a chemical reaction discussed in more detail in chapter 11. 

A waste process directed to the cleanup of drilling fluids is being developed by Critical Fluid Systems Inc. (Anon., 1982). Oil-based drilling fluids (commonly termed muds) are used to lubricate and cool drill bits and to 

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The radicals are then involved in oxidations such as formation of ketones (qv) from alcohols. Similar reactions are finding value in treatment of waste streams to reduce total oxidizable carbon and thus its chemical oxygen demand. These reactions normally are conducted in aqueous acid medium at pH 1—4 to minimize the catalytic decomposition of the hydrogen peroxide. More information on metal and metal oxide-catalyzed oxidation reactions (Milas oxidations) is available (4-7) (see also Photochemical technology, photocatalysis). 

Flocculation and sedimentation arc two processes used to separate waste streams that contain both a liquid and a solid phase. Both are well-developed, highly competitive processes, which arc oflcii used in the complete treatment of waste streams. They may also be used instead of, or in addition to, filtration. Some applications include the removal of suspended solid particles and soluble heavy metals from aqueous streams. Many industries use both processes in the rcmowal of pollutants from their wastewaters. These processes work best when the waste stream contains a low concentration of the contaminating solids. Although they are applicable to a wide variety of aqueous waste streams, these processes arc not generally used to treat nonaqueous or semisolid waste streams such as sludges and slurries. 

Generally, a distinction can be made between membrane bioreactors based on cells performing a desired conversion and processes based on enzymes. In ceU-based processes, bacteria, plant and mammalian cells are used for the production of (fine) chemicals, pharmaceuticals and food additives or for the treatment of waste streams. Enzyme-based membrane bioreactors are typically used for the degradation of natural polymeric materials Hke starch, cellulose or proteins or for the resolution of optically active components in the pharmaceutical, agrochemical, food and chemical industry [50, 51]. In general, only ultrafiltration (UF) or microfiltration (MF)-based processes have been reported and little is known on the application of reverse osmosis (RO) or nanofiltration (NF) in membrane bioreactors. Additionally, membrane contactor systems have been developed, based on micro-porous polyolefin or teflon membranes [52-55]. 

Oxidation of organic and inorganic species in aqueous solutions can find applications in fine chemical processes and wastewater treatment. Here, the oxidant, often either air or pure oxygen, must undergo all the mass transfer steps mentioned above in order for the reaction to proceed. During the last decade, increased environmental constraints have resulted in the application of novel processes to the treatment of waste streams. An example of such a process is wet air oxidation. Here, the simplest reactor design is the cocurrent bubble column. However, the presence of suspended organic and inert solids makes the use of monolith reactors favorable. 

REDUCTION AND OXIDATIVE TREATMENT OF FOSSIL FUELS AND OTHER CHEMICAL TREATMENTS OF WASTE STREAMS... 

Traditional chemical manufacturing is resource demanding and wasteful, and often involves the use of hazardous substances. Resources are used throughout the production and including the treatment of waste streams and emissions (Figure 1.1). 

A number of technologies are now available and under development for the treatment of waste streams containing solvents. These cover waste in the vapour phase and as liquid or immobile residues. 

Earlier we pointed out that when cyanide ions encounter silver ions, a precipitate forms. There are relatively few cations that precipitate cyanide, but other anions such as phosphate or carbonate precipitate readily with a wide range of cations. Eor the engineers and chemists who design industrial processes, solubility often is vital in both the isolation of desired products and the treatment of waste streams. Because industrial processes are often carried out at high temperatures, the characteristics of solubility must be viewed within the context of the equilibrium (and the equilibrium constant) of the reactions at those temperatures. 

To summarize, BES for wastewater treatment may become attractive as a method for the recovery of energy from the treatment of waste streams, especially when value-added by-products can be generated simultaneously. However, due to the intrinsic limitations detailed above and the limited economy of scale of their modular design, it will remain highly unlikely that BES will completely replace existing wastewater treatment systems any time soon. 

Aelterman, P. Microbial fuel cells for the treatment of waste streams with energy recovery. PhD Thesis, Gent University, Belgium, 2009. 

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