After wastewater treatment, the used microorganisms are separated from the
treated water. These used microorganisms are called biological solids which are
combined with the sludge after wastewater treatment (International Petroleum
Industry Environmental Conservation Association 2010).
The most important processes involved in the secondary aerobic wastewater
treatment are suspended growth process (activated sludge, stabilization pond),
attached growth process (trickling filter, rotating biological contactor), nitrification
and denitrification (International Petroleum Industry Environmental Conservation
Association 2010; Shahryar 2017).
Suspended Growth
In this process, the microorganisms are systematically combined with organic wastes
(non-settleable, colloidal particles) to maintain suspended growth of microbes for
removal of oil, grease, suspended hydrocarbons in presence of oxygen. There are
two types of suspended growth: (1) activated sludge and (2) stabilization pond.
Activated Sludge
The most widely used suspended growth process is the activated sludge process. It is
an incessant suspension growth process of aerobic microorganisms in wastewater
that may consist of suspended colloidal, dissolved organic-inorganic matter, which
is used as food for growth of microbes and evolving carbon dioxide, ammonia and
methane gas by oxidation. The whole process is done in a dispersed growth reactor:
an aeration tank or basin. Air is constantly injected using a submerged diffuser
(aeration device) into the system to control the aerobic sludge in suspension medium.
The requirement of oxygen is about 0.6–1.5 kg O 2 /kg of biochemical oxygen
demand removal in this system. The mixtures that occur in this aeration basin such
as wastewater-sludge are called mixed liquor suspended solids. The organic fraction
of the mixed liquor is referred to as mixed liquor volatile suspended solids. The
mixed liquor suspended solids of a typical refinery wastewater system consists of
70–90% active mixed liquor volatile suspended solids and 10–30% inert solids
(International Petroleum Industry Environmental Conservation Association 2010).
From the aeration tank, the mixed liquor suspended solids or mixed liquor volatile
suspended solids is passed through a clarifier. Here, the sludge containing the
biomass is separated. A portion of the concentrated sludge is recycled to the aeration
tank to maintain an adequate microbial mass and is called the return activated sludge.
The rest is regularly discharged as waste activated sludge (International Petroleum
Industry Environmental Conservation Association 2010).
According to the Environmental Protection Agency, the removal efficiency of
activated sludge is 80–99% for biochemical oxygen demand, 50–95% for chemical
oxygen demand, 60–85% for suspended solids, 80–99% for oil, 95–99% for phenol,
33–99% for ammonia and 97–100% for sulphides (Shahryar 2017).
8 Treatment of Petroleum Hydrocarbon Pollutants in Water
255
treated water. These used microorganisms are called biological solids which are
combined with the sludge after wastewater treatment (International Petroleum
Industry Environmental Conservation Association 2010).
The most important processes involved in the secondary aerobic wastewater
treatment are suspended growth process (activated sludge, stabilization pond),
attached growth process (trickling filter, rotating biological contactor), nitrification
and denitrification (International Petroleum Industry Environmental Conservation
Association 2010; Shahryar 2017).
Suspended Growth
In this process, the microorganisms are systematically combined with organic wastes
(non-settleable, colloidal particles) to maintain suspended growth of microbes for
removal of oil, grease, suspended hydrocarbons in presence of oxygen. There are
two types of suspended growth: (1) activated sludge and (2) stabilization pond.
Activated Sludge
The most widely used suspended growth process is the activated sludge process. It is
an incessant suspension growth process of aerobic microorganisms in wastewater
that may consist of suspended colloidal, dissolved organic-inorganic matter, which
is used as food for growth of microbes and evolving carbon dioxide, ammonia and
methane gas by oxidation. The whole process is done in a dispersed growth reactor:
an aeration tank or basin. Air is constantly injected using a submerged diffuser
(aeration device) into the system to control the aerobic sludge in suspension medium.
The requirement of oxygen is about 0.6–1.5 kg O 2 /kg of biochemical oxygen
demand removal in this system. The mixtures that occur in this aeration basin such
as wastewater-sludge are called mixed liquor suspended solids. The organic fraction
of the mixed liquor is referred to as mixed liquor volatile suspended solids. The
mixed liquor suspended solids of a typical refinery wastewater system consists of
70–90% active mixed liquor volatile suspended solids and 10–30% inert solids
(International Petroleum Industry Environmental Conservation Association 2010).
From the aeration tank, the mixed liquor suspended solids or mixed liquor volatile
suspended solids is passed through a clarifier. Here, the sludge containing the
biomass is separated. A portion of the concentrated sludge is recycled to the aeration
tank to maintain an adequate microbial mass and is called the return activated sludge.
The rest is regularly discharged as waste activated sludge (International Petroleum
Industry Environmental Conservation Association 2010).
According to the Environmental Protection Agency, the removal efficiency of
activated sludge is 80–99% for biochemical oxygen demand, 50–95% for chemical
oxygen demand, 60–85% for suspended solids, 80–99% for oil, 95–99% for phenol,
33–99% for ammonia and 97–100% for sulphides (Shahryar 2017).
8 Treatment of Petroleum Hydrocarbon Pollutants in Water
255