separating oil, water and solids, it is an efficient equipment. Some refineries also use
corrugated plate interceptors or parallel plate interceptors separators instead of
American petroleum institute separator, because these separators are more compact
than American petroleum institute separator.
Dissolved solids may cause corrosion, plugging problems in pipelines, heat
exchanger, etc. For this reason, secondary oil-water separators are involved for
separation of the three-phase mixture.
Second Stage
Oil-water-solid separation is carried out using dissolved air flotation separators or
induced air flotation separators or dissolved gas flotation or induced gas flotation or
sand filtration. The effluent from the first stage of oil-water separator is sent to the
second stage separator for the elimination of dissolved solids. The use of dissolved
air flotation and induced air flotation depends on amounts, condition and quality of
influent and also specification of effluents.
Dissolved Air Floatation
In this device, the first step is coagulation-flocculation. The scattered components
(oil, solids) carry same charge (negative) on their surface and hence repel each other.
These similarly charged components do not agglomerate to form flocs, hence do not
settle. So, for removal of these dispersed particles, chemical coagulation and flocculation are needed. This is done sequentially by a combination of chemical and
physical methods. At first, several coagulants like lime, certain iron-containing
compounds (such as ferric chloride or ferric sulphate) and alum (aluminium sulphate) are added to wastewater to assist the suspended solids to form stable bulky
particles which can settle down easily and can be removed. A portion of its effluent is
pressurized, saturated with dissolved air and added to the inflowing wastewater.
When the pressure of the recycle stream is released, the air bubbles liberated cause
free oil and solids to float up, which is skimmed off. In this device, the solids that
settle down are occasionally scraped off.
Induced Air Floatation
In the induced air floatation, a rotor-disperser system drags the air and combines with
the influent wastewater. The wastewater passes through a set of chambers before
being discharged from the unit while the float (~ 1–10% of the forward flow) moves
over the weir, thus enabling effective removal of free oil. These units operate
satisfactorily when the oil and dissolved solids level of the wastewater is less and
there is minute variance in flow as well as load. Low capital cost and compact size
are the main advantages of this system.
8 Treatment of Petroleum Hydrocarbon Pollutants in Water
253
corrugated plate interceptors or parallel plate interceptors separators instead of
American petroleum institute separator, because these separators are more compact
than American petroleum institute separator.
Dissolved solids may cause corrosion, plugging problems in pipelines, heat
exchanger, etc. For this reason, secondary oil-water separators are involved for
separation of the three-phase mixture.
Second Stage
Oil-water-solid separation is carried out using dissolved air flotation separators or
induced air flotation separators or dissolved gas flotation or induced gas flotation or
sand filtration. The effluent from the first stage of oil-water separator is sent to the
second stage separator for the elimination of dissolved solids. The use of dissolved
air flotation and induced air flotation depends on amounts, condition and quality of
influent and also specification of effluents.
Dissolved Air Floatation
In this device, the first step is coagulation-flocculation. The scattered components
(oil, solids) carry same charge (negative) on their surface and hence repel each other.
These similarly charged components do not agglomerate to form flocs, hence do not
settle. So, for removal of these dispersed particles, chemical coagulation and flocculation are needed. This is done sequentially by a combination of chemical and
physical methods. At first, several coagulants like lime, certain iron-containing
compounds (such as ferric chloride or ferric sulphate) and alum (aluminium sulphate) are added to wastewater to assist the suspended solids to form stable bulky
particles which can settle down easily and can be removed. A portion of its effluent is
pressurized, saturated with dissolved air and added to the inflowing wastewater.
When the pressure of the recycle stream is released, the air bubbles liberated cause
free oil and solids to float up, which is skimmed off. In this device, the solids that
settle down are occasionally scraped off.
Induced Air Floatation
In the induced air floatation, a rotor-disperser system drags the air and combines with
the influent wastewater. The wastewater passes through a set of chambers before
being discharged from the unit while the float (~ 1–10% of the forward flow) moves
over the weir, thus enabling effective removal of free oil. These units operate
satisfactorily when the oil and dissolved solids level of the wastewater is less and
there is minute variance in flow as well as load. Low capital cost and compact size
are the main advantages of this system.
8 Treatment of Petroleum Hydrocarbon Pollutants in Water
253