The Basics:
Characteristics of Oil in Water Mixtures
Before we immerse ourselves into the exciting world of oil water separation for oily waste water streams, a few basic definitions should be established. Although there are some variations in how industry views the types of phases in which oil exists, generally the following definitions are accepted.
Free Oil
Free oil is defined by the American Petroleum Institute as oil droplets larger than 150 microns in diameter. In almost all cases, these droplets are large enough that the difference in density between the water and oil allow for efficient separation in large open style separation chambers. These oil droplets follow Stokes Law which forms the basis of the API design.
Dispersed Oil
Historically dispersed oil are droplets that do not readily separate via gravity and lie between free oil (150 microns) and emulsified oil (.5 microns). With the development of the CPI separator, practical removal of droplets that are 20 micron or larger became feasible via gravity and coalescence. With coalescing separators in wide spread use it is common for droplets 20 microns or larger to be categorized as free oil, leaving droplets between roughly .5 micron and 20 micron as truly dispersed oil.
Emulsions
Droplets smaller than .5 microns will exhibit Brownian movement and may possibly never separate under the influence of gravity or coalescence. These droplets are considered emulsified and may be caused mechanically or chemically.
Before we immerse ourselves into the exciting world of oil water separation for oily waste water streams, a few basic definitions should be established. Although there are some variations in how industry views the types of phases in which oil exists, generally the following definitions are accepted.
Free Oil
Free oil is defined by the American Petroleum Institute as oil droplets larger than 150 microns in diameter. In almost all cases, these droplets are large enough that the difference in density between the water and oil allow for efficient separation in large open style separation chambers. These oil droplets follow Stokes Law which forms the basis of the API design.
Dispersed Oil
Historically dispersed oil are droplets that do not readily separate via gravity and lie between free oil (150 microns) and emulsified oil (.5 microns). With the development of the CPI separator, practical removal of droplets that are 20 micron or larger became feasible via gravity and coalescence. With coalescing separators in wide spread use it is common for droplets 20 microns or larger to be categorized as free oil, leaving droplets between roughly .5 micron and 20 micron as truly dispersed oil.
Emulsions
Droplets smaller than .5 microns will exhibit Brownian movement and may possibly never separate under the influence of gravity or coalescence. These droplets are considered emulsified and may be caused mechanically or chemically.
Applications
Characteristics of Oily Waste Water Streams in Various Industries
Next to sanitary wastes, oil is the most common pollutant in wastewater. Almost every industry, whether it is manufacturing or service orientated, produces oily wastewater somewhere in their operation. Some of the major sources of oily wastewater are:
Transportation
Railroads use diesel fuel and spillage is common. This spillage generally manifests itself in the run-off of rain from the property. Other forms of transportation also use diesel or gasoline, and while direct fuel spillage is not as common, the leakage of hydraulic fluids and lubricating oils can be significant at parking areas, and maintenance facilities. These types of oil contaminants are easily treated and do not normally require technology beyond an efficient oil/water separator. However, conditions at service centers for railroad equipment produces waste streams which generally have rather high concentrations of oil, both free and emulsified. In these cases chemical pre-treatment and physical separation is required after the basic gravity or coalescent oil/water separation.
Food Industry
The food industry produces oily wastewater with the oils and fats in an emulsified state along with a significant concentration of solids. The typical treatment scheme is usually screening followed by chemical treatment and dissolved air flotation separation.
Petrochemical
The type of oily wastewater generated in the petroleum industry and associated chemical plants varies greatly. At refineries the typical flow sheet is API designed separation followed by CPI type units, chemical treatment and DAF or induced gas flotation treatment. At some of the chemical plants where heavy crude oils and high solids loading do not occur, the flow sheet can eliminate the API unit from the treatment scheme. The wastewater from all of the plants in these industries generally contains a mixture of free and emulsified oils.
Heavy Manufacturing
Wastewater sources in the heavy manufacturing industry include rolling oils, hydraulic fluids, coolants, and lubricating oil leaks. All of the different types of wastewater contain a mixture of free and emulsified oils. At steel producers, the oily wastewater contains large quantities of solids which are removed in scale pits along with the largest oil droplets. Since much of the water is recycled in portions of the mills, the degree of removal required may be low.
In other heavy industries, such as automotive manufacturing, oil sources are from metal cleaning operations and spent coolants. In both operations the oil is highly emulsified. Besides the emulsified oil, lubricating oil and hydraulic oils are normally present in a non-emulsified state and are commonly called ‘tramp oils’. These industries typically use CPI type or other coalescing separators followed by various techniques for the treatment of the emulsified or soluble oils. Chemical coagulation and DAF flotation are commonly used except for the coolants which typically resist emulsion breaking or chemical coagulation techniques. In many cases only membrane separation or biological oxidation is effective.
Next to sanitary wastes, oil is the most common pollutant in wastewater. Almost every industry, whether it is manufacturing or service orientated, produces oily wastewater somewhere in their operation. Some of the major sources of oily wastewater are:
Transportation
Railroads use diesel fuel and spillage is common. This spillage generally manifests itself in the run-off of rain from the property. Other forms of transportation also use diesel or gasoline, and while direct fuel spillage is not as common, the leakage of hydraulic fluids and lubricating oils can be significant at parking areas, and maintenance facilities. These types of oil contaminants are easily treated and do not normally require technology beyond an efficient oil/water separator. However, conditions at service centers for railroad equipment produces waste streams which generally have rather high concentrations of oil, both free and emulsified. In these cases chemical pre-treatment and physical separation is required after the basic gravity or coalescent oil/water separation.
Food Industry
The food industry produces oily wastewater with the oils and fats in an emulsified state along with a significant concentration of solids. The typical treatment scheme is usually screening followed by chemical treatment and dissolved air flotation separation.
Petrochemical
The type of oily wastewater generated in the petroleum industry and associated chemical plants varies greatly. At refineries the typical flow sheet is API designed separation followed by CPI type units, chemical treatment and DAF or induced gas flotation treatment. At some of the chemical plants where heavy crude oils and high solids loading do not occur, the flow sheet can eliminate the API unit from the treatment scheme. The wastewater from all of the plants in these industries generally contains a mixture of free and emulsified oils.
Heavy Manufacturing
Wastewater sources in the heavy manufacturing industry include rolling oils, hydraulic fluids, coolants, and lubricating oil leaks. All of the different types of wastewater contain a mixture of free and emulsified oils. At steel producers, the oily wastewater contains large quantities of solids which are removed in scale pits along with the largest oil droplets. Since much of the water is recycled in portions of the mills, the degree of removal required may be low.
In other heavy industries, such as automotive manufacturing, oil sources are from metal cleaning operations and spent coolants. In both operations the oil is highly emulsified. Besides the emulsified oil, lubricating oil and hydraulic oils are normally present in a non-emulsified state and are commonly called ‘tramp oils’. These industries typically use CPI type or other coalescing separators followed by various techniques for the treatment of the emulsified or soluble oils. Chemical coagulation and DAF flotation are commonly used except for the coolants which typically resist emulsion breaking or chemical coagulation techniques. In many cases only membrane separation or biological oxidation is effective.
Techonolgies
API Designed Separators
The API designed oil water separator continues to be the primary treatment step when wastewater contains heavy solids, heavy oils or a combination of these items. Despite the almost pre-historic appearance of the equipment, for the conditions mentioned, there is generally no effective substitute. However in situations with light to moderate solids loading and oil of a lower viscosity, other equipment may be more effective from both cost and process evaluations. The API designed separator is one designed in accordance with parameters contained in the API manual. Common usage has led to any rectangular, open channel, mechanically skimmed oil/water separator to be called an API separator. To obtain an API design you want to specify a design in accordance with the API manual. The API design is for 150 micron droplet removal.
CPI Separators
A very well known mechanism for oil removal involves the installation of oleophilic (oil loving) coalescing media in wastewater streams. As the oil and water flows through the media, oil droplets impinge on the oleophilic surface and coalesce on the surface. The coalesced oil will rise to the surface of the separation chamber and accumulate. The separated free oil can be manually or automatically decanted from the oil/water separator. The coalescing oil/water separator or CPI separator is best suited for petroleum hydrocarbons that are fluid and where the suspended solids concentration is less than 300 mg/l. Most CPI separators are designed for 60 micron removal, and newer coalescing separators are designed for 20 micron removal.
The API designed oil water separator continues to be the primary treatment step when wastewater contains heavy solids, heavy oils or a combination of these items. Despite the almost pre-historic appearance of the equipment, for the conditions mentioned, there is generally no effective substitute. However in situations with light to moderate solids loading and oil of a lower viscosity, other equipment may be more effective from both cost and process evaluations. The API designed separator is one designed in accordance with parameters contained in the API manual. Common usage has led to any rectangular, open channel, mechanically skimmed oil/water separator to be called an API separator. To obtain an API design you want to specify a design in accordance with the API manual. The API design is for 150 micron droplet removal.
CPI Separators
A very well known mechanism for oil removal involves the installation of oleophilic (oil loving) coalescing media in wastewater streams. As the oil and water flows through the media, oil droplets impinge on the oleophilic surface and coalesce on the surface. The coalesced oil will rise to the surface of the separation chamber and accumulate. The separated free oil can be manually or automatically decanted from the oil/water separator. The coalescing oil/water separator or CPI separator is best suited for petroleum hydrocarbons that are fluid and where the suspended solids concentration is less than 300 mg/l. Most CPI separators are designed for 60 micron removal, and newer coalescing separators are designed for 20 micron removal.