The different methods of treatment of waste water from petroleum refining and petrochemical industries may be classified as follows:
1. Physical Methods:
Physical methods include gravity separation, air floatation, filtration, centrifugation, vacuum filtration and carbon adsorption.
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Gravity separation is the first step in the treatment of refinery waste. This is accomplished on the basis of the difference in specific gravity of oil and water. The gravity chamber usually consists of a grit chamber and a main separator.
Such separators are used by all types of refineries to remove free oil and oil containing waste. The skimmed oil from the separator is reprocessed and, then, recovered. The settled sludge may be dewatered and either incinerated or disposed of as land fill.
In the air floatation process, a portion of waste water feed is saturated with air at a pressure of 40 to 50 psi. The waste water is held at this pressure for a period of 1 to 5 minutes in a retention tank and, then, released at atmospheric pressure into the floatation chamber.
The sudden reduction, in the pressure, results in the release of microscopic air bubbles which attach themselves to oil and suspended particles in the waste water in the floatation chamber. These results in the agglomerates, which due to entrained air have greatly increased their vertical-rise velocity of about 0.5 to 1.0 ft/minute
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The floated material, then, rises to the surface and a layer of froth is formed which is continuously removed by special devices. A retention time of usually 10 to 30 minutes is provided in the floatation chamber. The efficiency of the dissolved floatation technique depends upon the attachment of bubbles to the suspended oil and other particles which are to be removed.
Flocculating agents are added for high efficiency of the air floatation unit. The clarified effluent from an air floatation unit may be further treated in a biological unit or may be discharged as final effluent, depending upon the BOD content and effluent requirements
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2. Chemical Methods:
The chemical methods of waste-water treatment include coagulation, precipitation, chemical oxidation, ion exchange and chemical pretreatment or sludge conditioning. These methods remove oil and solid, particularly from oil emulsions.
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The delayed coking operation, in the petroleum industry, results in a substantial amount of oil-laden waste water which cannot be reused before being chemically treated. The chemical treatment of waste water from the delayed coking operation permits the reuse of oil-laden water in decoking, by reducing oil content to below 100 ppm.
The chemically treated waste water from the delayed coking operation is used as a coolant in the decoking operation. Any emulsion, that may be formed when using the oil-laden water for cooling, does not interfere with the cooling of the bed and is broken when brought into contact with the hot coke.
3. Biological Methods:
The biological waste treatment methods include
1. Activated sludge process, conventional and modified
2 Trickling filters
3. Aerated lagoons
4. Oxidation pond.
Prior to waste being treated by biological methods, it is pretreated to remove oils, suspended solids and toxic substances and to provide neutralization.
Activated Sludge Process This is considered to be the most effluent and effective biological method for the removal of organic materials. The activated sludge process operates in the efficiency range of 70 to 95 percent for BODs, 30 to 70 percent for CODs and 65 to 99 percent for phenols and cyanides.
The conventional activated sludge process is most widely applied in contact stabilization and is mostly applicable when a large fraction of the organic constituents is in a suspended and colloidal form. Extended aeration is particularly adaptable to industrial applications as longer detention-periods allow micro-organisms more time to degrade the complex organics and sludge production is minimized.
Activated sludge is an aerobic biological treatment process in which, high concentrations of newly grown and recycled micro-organisms, are suspended uniformly throughout a holding tank to which raw waste-waters are added.
Oxygen is introduced via mechanical aerators, diffused air systems or by other means. The organic materials in the waste are removed from the aqueous phase by microbial growth and are stabilized by biochemical synthesis and oxidation reaction.
The activated sludge process consists of aeration, followed by a sedimentation tank. The microbial growth of flocculants is removed in the sedimentation tank and recycled to the activated sludge tank to maintain a high concentration of active micro-organisms.
Trickling Filter:
The trickling filter uses an aerobic biological treatment process. It has a porous bed, made of solid particles, of stone or synthetic material usually PVC, on the surface of which a fixed growth of micro-organism is contained. The waste is allowed to pass through the bed.
A microbial film develops on the surface of the bed-media and removes the organic materials from the waste water by absorption, bioflocculation and sedimentation. As this is an aerobic biological system, oxygen is required for rapid metabolism of the removed organic matter.
The large surface area of the filter-media permits a rapid transfer of oxygen by simple diffusion from the void space into the liquid layer. The filter-media may be stone or synthetic material like polyvinyl chloride.
Trickling filter units have been used in the petroleum industry, both as complete secondary treatment, and as roughing devices to reduce the organic leaf, on subsequent activated sludge units. They have been put to extensive use to remove phenolic compounds.
Prior to the application of the trickling filter, the waste water from the petroleum industry is pretreated to remove oil and to limit concentrations of sulphides, mercaptan and phenols. Oil removal is accomplished by gravity separation and air floatation. The sulphides and mercaptans are removed by stripping.
Aerated Lagoon:
This also provides aerated biological treatment by mixing dilute concentrations of micro-organisms with waste water in a large, relatively deep basin. For aerobic stabilization of organic matter, oxygen is essential which is supplied by diffused aeration units. Turbulence is maintained in the basin to ensure proper distribution of oxygen and biological solids throughout the basin. The detention time in the aerated lagoon is greater than that in the activated sludge process for removal of an equivalent amount of BOD.
Oxidation Pond:
This method of treatment is applicable only at places where land is plentiful and cheap. Basically, the oxidation pond utilizes bacteria for aerobic stabilization of organic matter added to the pond. Bacteria consume oxygen, supplied from atmospheric aeration at the pond surface and from oxygen production by algae in the pond.
Algae produce oxygen by synthesis of cellular protoplasm, from carbon dioxide, in the presence of sunlight. As in the pond, organic loadings are low, and very little biological sludge settles down in the pond. Refinery wastes are generally treated in an oxidation pond, which is also a polishing step for the effluent from physical, chemical and other biological waste treatment processes,. Normally, pre-treatment is required to limit concentrations of sulphides, mercaptan and phenols.
Cooling Towers Cooling towers are used to biologically treat refinery effluent where cooling water supply is short. Under controlled loading conditions, cooling towers are capable of removing 99 percent phenol, 75 to 90 percent oxygen demand (COD) and 90 percent biochemical oxygen demand (BOD). The oil-bearing waste is passed through gravity separators to remove oil. The sulphides and mercaptans may be removed by treatment in a stripper.