Oily wastewater treatment may be applied to both combined and segregated wastewaters. However, effective removal of characteristic parameters is most efficiently carried out in segregated waste streams. This point is of importance for priority pollutant and total toxic organics removal. Two main parameters of the oily wastewaters are oil and grease, and toxic organics. The treatment scheme for oily waste is determined by the form of the oil and organic content. Oil can be found as free, emulsified, and soluble forms. The main treatment methods for these forms are summarized below.
Free oils separate easily by decantation. Gravity oil separators of various types can be used to separate and remove free oils. Detailed information about gravity separators can be found elsewhere .
Coalescing and centrifugation can be used for oily waste treatment. For diluted aqueous suspensions, use of aluminum silicate-based powders may also be considered for the adsorption of oil .
Emulsion breaking and skimming is a basic technology for the removal of emulsified oil. Chemical emulsion breaking aims to break the ion bond between the oil and water, and to coagulate the freed oils to form separable particles by gravity. A variety of chemicals can be used for this purpose. Acids, being effective emulsion breakers, are commonly used. Adjustment of pH to 3 or lower provides high efficiency without causing high sludge production. Cost of the process and need of pH correction after the process are limiting factors. Inorganic flocculants such as ferric chloride and alum function by both lowering the pH and providing effective coagulation. Although they are reasonably priced, they produce excessive sludge. Organic polymers do not cause sludge production, but when used alone their effectiveness needs to be evaluated in terms of type and dosage. Good mixing agitation is required for homogeneous dissipation of chemicals and for the process of emulsion breaking. Retention times of 15-20 minutes are commonly employed. At least two hours separation time should be allowed after emulsion breaking for effective oil removal. Skimming is the operation to skim off the floating oil layer, and can be carried out using several types of skimming devices, such as rotating drum and belt skimmers. Heat application reduces the separation time. Ultrafiltration may replace the emulsion breaking and skimming, particularly for the cases where recovery of oils is considered. Thermal emulsion breaking is similar to chemical emulsion breaking except with the use of heat instead of chemicals. This very effective but costly method is used for recovery rather than for treatment .
Flotation, particularly dissolved air flotation (DAF), is a very effective way of separating oil following emulsion breaking. Dissolved air flotation, as explained in the metal hydroxide separation applications section, is a very well established technology. Design and operating information about DAF application to oily wastewater can be found in the literature [110,111]. Electroflotation is particularly effective for oily waste treatment.
Electrocoagulation, an electrochemical method using iron or aluminum, has been suggested for oil-water emulsion treatment .
Ultrafiltration (UF) uses a semipermeable membrane that physically separates oil from oil-water emulsions. The separation is realized by pressurizing the liquid so that it permeates the membrane. The membrane works as a molecular screen. Pore sizes vary between 0.0025 and 0.01 microns. Water, solvent, and low molecular weight solutes pass through the membrane, while dispersed material and substances with high molecular weight (generally in the range of 1000-100,000 g) are retained. The fluid is pumped to the membrane unit under pressures of 0.7-3.5 kg/cm2. Retained oil droplets are removed continuously. Because the retained particle sizes are much greater than the pore size of the membrane, clogging generally does not pose a problem. A wide range of membranes are available. Membranes may be tubular, hollow fiber, or spiral wound. Tubular membranes are preferred for small flows. A very broad range of capacities, beginning with a few m3/m2-day to about 1000 m3/m2-day are also available. The limitations of ultrafiltration are temperature and the substances that are hazardous to the membrane. Membranes can withstand up to 70°C. Oxidizing agents, some solvents, and organic compounds can cause dissolution of the membrane. Ultrafiltration is a proven technology with relatively lower capital and operating costs. It is highly efficient for the recovery of solids and liquids. In metal finishing it is used for the separation of oils, toxic organics, and solids. Oily wastewater of all kinds and electropainting wastes are major areas of use. It also has a very common application for the treatment and recovery of metalworking fluids [2,57].
Microfiltration employs larger pore sizes (0.1 micron or greater) and retains only fine solids and micro emulsions. It can be used in the recovery of aqueous cleaners and after hydroxide precipitation for solid separation. Cross-flow type units are most commonly used in metal finishing because of their self-cleaning ability .
Adsorption, chemical oxidation, and biological treatment are used as polishing treatments for oily waste, mostly aiming at soluble oil and toxic organics removal. Adsorption is a very effective technology with a wide area of application. Activated carbon obtained from resins of phenol formaldehydes or acrylic esters, and from a variety of other materials, can be used for adsorption. Activated carbon can be applied in powdered or granulated form. Powdered activated carbon (PAC) application is limited and it is generally used in combination with other methods such as biological treatment, due to operational difficulties. Granulated activated carbon is used in columns similar to filters. Suspended solids and emulsified oil must be treated efficiently before activated carbon applications to prevent frequent backwashing and poisoning of the carbon. Increased temperatures reduce the adsorption rate. Regeneration of activated carbon can be made thermally or by oxidation. If regeneration is not economically justifiable, the carbon is disposed of as solid or hazardous waste. Activated carbon efficiently removes organic matter except very low molecular weight and highly solute ones, and inorganic pollutants such as heavy metals, chlorine, and cyanide.
Air stripping is applied to aqueous waste to separate low concentration of volatile substances having Henry's law constants of 10 atm by passing air through the waste solution. volatile organic carbon (VOC) removal from wastewaters up to 10 mg/L concentration by air stripping is a commonly used technology. Steam stripping is similar to air stripping, but steam is used instead of air. It is a more versatile technology and used to separate and concentrate volatile substances existing in the solution in high concentrations. VOC concentration is a common application .
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