Oily wastewater separation

Oily wastewater is often treated or stored in unlined wastewater treatment ponds in refineries. These wasfes appear to be similar to API separator waste. 

The technology actually employed in treating a given waste depends upon waste characteristics primarily, but it is also influenced by such other factors as available land area, volume of discharge, and disposition of treated waste. Oil separation, for example, is necessary only for oily wastewater and, among timber products industries, is employed only in the pretreatment of wood preserving wastes. Adjustment of pH is required only if the pH value falls outside the range of 6.0 to 9.0. 

Ultrafiltration — This process has been successful with mixtures difficult to separate, such as oily machining wastes and oily wastewater. A pressure-driven filtration membrane separates multicomponent solutes from solvents, according to molecular size, shape and chemical bonding. Substances below a preselected molecular size are driven through the membrane by hydraulic pressure, while larger molecules, such as oil droplets, are held back. Effluent oil concentration depends on influent concentration, but properly operated ultrafiltration units can produce oilfree water (less than 0.1 ppm for all practical purposes). 

Oily wastewater treatment can be classified into two categories primary and secondary treatment systems. The primary treatment is employed to separate floatable oils from water and emulsified oil. This system is suitable for oil or grease that is of lower specific gravity than water. Skimmer and gravity separation are the major treatment systems belonging to this group. A secondary treatment system s goal is to treat or... 

To select an appropriate oil-water separation technique, knowledge of droplet size distributions in oily wastewater is the most crucial parameter that has to be identified (17). Three techniques commonly used to determine size distributions are ... 

Membrane processes are widely used in oil water separation. In general, a membrane is classified into two groups pressure-driven membrane and electrical membrane, known as electrodialysis. The most applicable process for oily wastewater removal is the former type. The pressure-driven membrane applications include microfiltration (MF), ultrafil-tration (UF), nanofiltration (NF), and reverse osmosis (RO). All of them are categorized by the molecular weight or particle size cut-off of the membrane as shown in Table 5. 

A. Fleischer, Separation of Oily Wastewaters—the State-of-the-Art, paper presented at the Annual Technical conference Canadian Instimte of Marine Engineers. MARI-TECH 84, Ottawa, May 25, 1984. 

Characterization of Volatile Fraction. Volatile organic compounds found in oily wastewaters consist primarily of lower-molecular-weight aliphatic and aromatic hydrocarbons. Because of its relatively high vapor pressure, this fraction is quite often lost during analysis of oily wastes. For this reason a separate procedural step was incorporated into the overall scheme for analysis of the volatile fraction. An unfiltered sample of oily waste is used in this determination. The volatile fraction is separated from water by means of nitrogen sparging and collected in an activated carbon absorption column. The collected compounds are desorbed into carbon disulfide and analyzed by GC. 

Abcor (now a division of Koch Industries) installed the first industrial ultrafiltration plant to recover electrocoat paint from automobile paint shop rinse water in 1969. Shortly afterwards, systems were installed in the food industry for protein separation from milk whey and for apple juice clarification. The separation of oil emulsions from effluent wastewaters has also become a significant application. The current ultrafiltration market is approximately US 200 million/ year, but because the market is very fragmented no individual end-use segment is more than US 10-30 million/year. In the chemical and refining industries, the principal application of ultrafiltration is the treatment of oily wastewater. 

L. Yan, S. Hong, M.L. Li, and YS. Li, Application of the AljOj-PVDF nanocomposite tubular ultrafiltration (UF) membrane for oily wastewater treatment and its antifouling research. Separation and Purification Technology 66 (2009) 347-352. 

Other plant-scale applications to pollution control include the flotation of suspended sewage particles by depressurizing so as to release dissolved air [Jenkins, Scherfig, and Eckhoff, Applications of Adsorptive Bubble Separation Techniques to Wastewater Treatment, in Lemlich (ed.). Adsorptive Bubble Separation Techniques, Academic, New York, 1972, chap. 14 and Richter, Internat. Chem. Eng, 16,614 (1976)]. Dissolved-air flotation is also employed in treating waste-water from pulp and paper mills [Coertze, Prog. Water TechnoL, 10, 449(1978) and Severeid, TAPPl 62(2), 61, 1979]. In addition, there is the flotation, with electrolytically released bubbles [Chambers and Cottrell, Chem. Eng, 83(16), 95 (1976)], of oily iron dust [Ellwood, Chem. Eng, 75(16), 82 (1968)] and of a variety of wastes from surface-treatment processes at the maintenance and overhaul base of an airline [Roth and Ferguson, Desalination, 23, 49 (1977)]. 

In addition, oily sludge from a wastewater treatment facility that results from treating sour wastewaters may be a hazardous waste (unless recycled in the refining process). These include API separator sludge, primary treatment sludge, sludge from various gravitational separation units, and float from dissolved air flotation units. 

Dissolved air flotation (DAF) is a process commonly used in refineries to enhance oil and suspended solids from gravity-separator effluent. In some refineries it is used as a secondary clarifier for activated sludge systems and as a sludge thickener. The process involves pressurizing the influent or recycled wastewater at 3-5 atm (40-70 psig) then releasing the pressure, which creates minute bubbles that float the suspended and oily particulates to the surface. The float solids are removed by a mechanical surface collector. 

Is the wastewater from high pressure cleaning of heat exchangers treated separately and not with all the other water streams These often carry fine particles that provide a large surface area for oil and water to stabilize creating an oily sludge in the wastewater which is difficult to separate. 

Bhave, R.R., and H.L. Fleming, 1988, Removal of oily contaminants in wastewater with microporous alumina membranes, in New membrane materials and processes for separation (K.K. Sirkar and DR. Lloyd, eds.), AIChE Symp. Ser. No. 261, Vol. 84, p.l9. 

Induced Gas Flotation. Mechanically induced gas dotation (IGF) is employed extensively to remove suspended solids, oil, and other organic matter from oil-field and refinery wastewaters. Consequently, these IGF units are particularly suited to the treatment of oil-in-water or reverse emulsions. Such units generally follow gravity oil-water separation units such as FWKOs, gun barrels, and skim tanks in oil-field-produced water-treatment schemes, and also handle the oily water streams generated from all treaters in a specific produced-fiuid treatment plant. 

Oily process wastes and oil-free wastes are collected separately in some refineries so that the oily wastes can be treated for oil removal before mixing with other waste streams. The spent caustics and spent acids are generally collected and sold or disposed by other means. Few refineries neutralize these wastes for discharge, to the wastewater collection system. 

Primary treatment uses a settling pond to allow most hydrocarbons and suspended solids to separate from the wastewater. The solids drift to the bottom of the pond, hydrocarbons are skimmed off the top, and oily sludge is removed. Difficult oil-in-water emulsions are heated to expedite separation. 

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