However, dissolved air floatation and dissolved gas floatation devices are more
widely used in petroleum and petrochemical industries, since they are more robust,
can produce better-quality effluent and can handle greater variances in flow and load.
Clarifiers and filters (sand or dual media) may be used as alternatives to floatation.
Sand and anthracite are commonly used as filtering media. Anthracite entraps large
particles, whereas sand filter catches the fine solids. Dual media filter means a
combination of anthracite and sand filters. These types of filters can separate the
oil particles with diameters of 2–5 μm and effluentoil concentration less than equal to
20 mg/L can be achieved. The filter is regularly cleaned up using a backwash cycle to
remove the trapped particles and oil by an air scour.
Equalization System
The equalization system aims to minimize fluctuations in flow and composition of
wastewater. Reducing flow variations decreases loads on the downstream units,
thereby reducing their sizes and costs. On the other hand, composition equalization
reduces shock generation in subsequent biological units. The performance of the
biological system largely depends on the adaptability of the microorganisms to the
changing environment of the wastewater both in terms of quality and quantity. The
equalization tank should be cleaned regularly (once or twice in a year) to prevent
accumulation of wastes (International Petroleum Industry Environmental Conservation Association 2010; Shahryar 2017; Alireza 2014).
Secondary Treatment
The secondary treatment further handles the effluents from primary treatment in the
petroleum refinery. After primary treatment, the residual organics, colloidal organic
matters, suspended solids and oily sludge are treated by using cultured microorganisms like bacteria (Arcobacter), fungi (Aspergillusniger), algae (Chlorella) in presence of oxygen, that is aerobic biological treatment or absence of oxygen, that is
anaerobic biological treatment. But, most refineries use the aerobic process, because
it is faster, produces no smell and forms stable products (Indus Eco Water, India
2019). In aerobic biological treatment, the aerobic microorganisms such as
Longilinea, Georgenia metabolize the organic matter, oily sludge, thereby increasing the growth of microorganisms and produce some inorganic compounds like
carbon dioxide, ammonia and hydrogen sulphide. The biological treatment process
is dependent on the volume of the reactor, growth rate of microorganisms and the
oxygen availability. If the reactor volume is small and high concentrations of
microorganisms are present, then the rate of the secondary treatment is high and
efficiency is also better. In aerobic process, the chemical oxygen demand and
biochemical oxygen demand of wastewater is significantly reduced (International
Petroleum Industry Environmental Conservation Association 2010; Central Pollution Control Board, India 2018; Shahryar 2017; Alireza 2014).
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widely used in petroleum and petrochemical industries, since they are more robust,
can produce better-quality effluent and can handle greater variances in flow and load.
Clarifiers and filters (sand or dual media) may be used as alternatives to floatation.
Sand and anthracite are commonly used as filtering media. Anthracite entraps large
particles, whereas sand filter catches the fine solids. Dual media filter means a
combination of anthracite and sand filters. These types of filters can separate the
oil particles with diameters of 2–5 μm and effluentoil concentration less than equal to
20 mg/L can be achieved. The filter is regularly cleaned up using a backwash cycle to
remove the trapped particles and oil by an air scour.
Equalization System
The equalization system aims to minimize fluctuations in flow and composition of
wastewater. Reducing flow variations decreases loads on the downstream units,
thereby reducing their sizes and costs. On the other hand, composition equalization
reduces shock generation in subsequent biological units. The performance of the
biological system largely depends on the adaptability of the microorganisms to the
changing environment of the wastewater both in terms of quality and quantity. The
equalization tank should be cleaned regularly (once or twice in a year) to prevent
accumulation of wastes (International Petroleum Industry Environmental Conservation Association 2010; Shahryar 2017; Alireza 2014).
Secondary Treatment
The secondary treatment further handles the effluents from primary treatment in the
petroleum refinery. After primary treatment, the residual organics, colloidal organic
matters, suspended solids and oily sludge are treated by using cultured microorganisms like bacteria (Arcobacter), fungi (Aspergillusniger), algae (Chlorella) in presence of oxygen, that is aerobic biological treatment or absence of oxygen, that is
anaerobic biological treatment. But, most refineries use the aerobic process, because
it is faster, produces no smell and forms stable products (Indus Eco Water, India
2019). In aerobic biological treatment, the aerobic microorganisms such as
Longilinea, Georgenia metabolize the organic matter, oily sludge, thereby increasing the growth of microorganisms and produce some inorganic compounds like
carbon dioxide, ammonia and hydrogen sulphide. The biological treatment process
is dependent on the volume of the reactor, growth rate of microorganisms and the
oxygen availability. If the reactor volume is small and high concentrations of
microorganisms are present, then the rate of the secondary treatment is high and
efficiency is also better. In aerobic process, the chemical oxygen demand and
biochemical oxygen demand of wastewater is significantly reduced (International
Petroleum Industry Environmental Conservation Association 2010; Central Pollution Control Board, India 2018; Shahryar 2017; Alireza 2014).
254
M. Samanta and D. Mitra