Abstract on the Master`s work
ANALYTICAL REVIEW OF METHODS OF SEWAGE TREATMENT FROM ORGANIC IMPURITIES
Removal of tars and oils from the sewage waters of cokechemical plants can be carried out in various ways. The choice of a method of division the oil-emulsioned drains is connected with the size of oil particles, structure of a tar phase, requirements to quality of the cleared water and other accompanying parameters. There are next basic ways of sewage treatment: mechanical, physico-chemical, chemical and biological.
Mechanical ways of cleaning assume the use of filtering, upholding and filtering of industrial sewage. Upholding process is applied to waters with high content of tars and oils of various degree of dispersion, and the most simple devices are applied for that purpose: various type of sediment bowls and oiltraps. The given way of clearing allows to take particles 80–100 microns in diameter. For a considerable intensification of oil globules upholding it is possible to use centrifugation in hydrocyclones (pressure head, free-flow, stacked, combined), centrifuges and separators. But application of centrifugal devices for cleaning from emulsified oils is connected with considerable energy and operational expenses. Therefore centrifugation it is possible to recommend only for local cleaning of small on volume oil-emulsioned drains.
Widespread way of sewage treatment and condensates is the filtration. As a filtering material inert granular loading is used: sand, gravel, rubble, and also various production wastes. During the work loading of granular filters is quickly hammered and for its regeneration from oils a considerable quantity of washing waters is required. Filters are very sensitive to a hydraulic mode of washing. Application of filters in sewage treatment practice restrains by their high cost, necessity of rather frequent replacement of loading and pressure maintenance on an input to the filter. [1]
Methods with use the basic chemical ways of cleaning from organic impurities are connected with application of oxidizers. With their help sewage is released from toxic and other components. For example, the method of oxidation by ozone doesn't find wide application in the industry. High cost of ozone reception, its toxicity, low quality of such equipment doesn't allow to apply this method for deep sewage treatment.
The biological (biochemical) method is used for cleaning of economic-household and industrial sewage pulp-and-paper, oil refining, cokechemical and food enterprises. This method is based on microorganisms ability to use for the development the organic and some inorganic substances containing in sewage as pollutants. Biochemical cleaning is made by means of natural and artificial methods. Natural methods of biocleaning are carried out with application of fields of an irrigation and a filtration, and also biological ponds, and artificial methods — with the help of biofilters. Cleaning in airtanks the sewage clarified in primary sediment bowls is carried out by the slow admission through them of the sewage enriched with oxygen, and their mixing with active silt. Active silt represents itself the community of microorganisms and small invertebrate animals (a mold, yeast, water mushrooms, etc.) and also a firm substratum. Thus it is very important to pick up temperature correctly, the reaction of environment, stimulating additives, careful hashing and an effective oxidizer that in the maximum degree to promote an intensification of the hydrobiocenosis which is making an active silt, absorbing pollution. The given method is rather effective and is applied only to additional cleaning of sewage as high concentration of components-pollutants perniciously influence on microorganisms which live in active silt.
Physicochemical sewage treatment is carried out basically with application of such methods, as coagulation, flotation and sorption.
Coagulation is made by introduction in sewage of coagulants (salts of ammonium, iron, copper, wastes, etc.) for formation of deposits which then easily leave.
Flotation is carried out by the admission through air sewage which vials grasp at movement upwards surface-active substances, oil, oils and other pollutants and form easily deleted spumy layer on a water surface. The given way of cleaning is seldom applied in industrial conditions because of its small efficiency, high turbulence of streams in flotation chamber leading to destruction of flocculent particles and necessity to apply surface-active substances.
Sorption is based on ability of some substances named sorbents (the clay, the activated coal, zeolites, peat, etc.) to adsorb pollution therefore there is an extraction from sewage not only harmful, but also valuable substances [2].
Among sorptional methods the most useful are adsorptional which apply to deep sewage treatment from the dissolved organic and resinous substances before biochemical cleaning.
Advantages of a method are high efficiency, possibility of the sewage treatment, substances containing some, and also recuperation of these substances, possibility to clear sewage without entering of any secondary pollution, removals of biologically indestructible substances, reliability in the conditions of fluctuation of volumes and sewage structure. [3]
Cleaning by means of adsorption can be regenerative, i.e. with extraction of substance from an adsorbent both its recycling, and destructive at which the substances are taken from sewage and destroyed together with an adsorbent. Efficiency of such way of cleaning depends on the chemical nature of an adsorbent, size of adsorptional surface and its availability and from a chemical structure of substance.
As adsorbents natural and artificial porous materials are applied. Use of asbestos-like material – a withdrawal of manufacture of asbestine papers and a cardboard, a porous polymeric sorbent-copolymer of styrene, a sorbent on the basis of a basalt fiber, wood shavings, sawdust, the fibers placed in porous woven covers, various modifications of active coals and other adsorbents. As sorbents slags, coke, ashes also can be used. [4]
Adsorption by active coals are one of methods of cleaning of waters containing organic impurities, and allows to achieve deep water treating to norms of maximum concentration limit of harmful substances in water of industrial, turnaround use with simultaneous recycling or destructive destruction of the taken substances. Adsorbtion ability of active coals is a consequence of strongly developed surface and porosity. The specific surface of active coals makes 400-900 m2/g; adsorptional properties appreciably depend on structure, size of pores. [5]
Initial raw materials for reception of active coal are any materials which contain carbon: coal, peat, wood, etc. Process of manufacturing of high-quality active coals is combined and long, therefore their cost is sufficiently high. It leads to necessity of repeated use of active coals or their use as secondary raw materials, for example, as an additive to coalmixture for coking [6] . Therefore the wide application of active coals at sewage treatment is restrained by economic frameworks.
Advantages of the given method – possibility to spend adsorption of substances from multicomponent impurities, and also high efficiency, especially at cleaning low concentrated sewages.
To our opinion, the most perspective direction of sorptional sewage treatment are methods with use of sorbents on the basis of mineral fibers. It is necessary to notice that the given sorbents (fiber glass, a mineral fiber of diabase group) are received from cheap raw materials, therefore finished article cost is insignificant; they provide high adsorbing ability at filtering of the ammonia sewage containing tar-like organic substances. Therefore the expediency of carrying out of the further researches in this direction is doubtless.
Apparatus of sorptional cleaning - standard in chemical technology - pressure head filters with a dense layer of granulated active coals before which mechanical filters are located. Two-level filtering is applied for deep sewage treatment, containing emulsified and dissolved oil products. For example, coalescing filter, then a sediment bowl and then a filter with a fibrous adsorbent.
Thus, the given methods can provide deep sewage treatment from high-dispersed and dissolved oils and tars, and requirements to quality of cleared water become more and more rigid, the perspective of application of these methods remains demanding.
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