biomass called sloughs are generated, they are immediately cleaned up. There are
two types of attached growth equipment like trickling filter and rotary biological
contactor (International Petroleum Industry Environmental Conservation Association 2010).
Trickling Filter
Trickling filter or bio-filter is a basin which is supported with permeable media such
as shredded polyvinyl chloride, rocks, gravels, tubes, corrugated blocks, etc. The
specific surface and diameter of the permeable materials are about 30–900 m
2 /m
3
and 7–9 cm, respectively. The microorganisms are attached to these substances for
their suspended growth. The microorganisms are added to the wastewatereffluents
from primary treatment or equalization system. These microorganisms consume the
wastes (oil, grease), producing carbon dioxide, methane, ammonia, etc. Generally,
oxygen is introduced either up or down to the bed by natural air flow. Occasionally,
air is introduced forcefully by blower, pump and diffuser. The underwater air moves
through the void space of the filter bed by inlet draught tubes. After treatment of
wastewater in trickling bed, the treated water is drained out through the under drain
channel. The trickle filter is cleaned up periodically (once or twice in year) by
scraping the dead microorganisms. It may be located upstream of the activated
sludge process to reduce organic loads. Sometimes wastes microbes are recycled
to the filter for enhancing hydraulic distribution of the wastewater into the filter. The
removal efficiency of trickling bed is 60–85% for biochemical oxygen demand,
30–70% for chemical oxygen demand, 60–85% for suspended solids and 50–40%
for oil (International Petroleum Industry Environmental Conservation Association
2010; Shahryar 2017; Alireza 2014).
Rotating Biological Contactor
Rotating biological contactors contain rotating discs that are closely placed on a
horizontal shaft. The distance between two rotating discs is 2–3 cm, approximate
diameter of discs is 3 cm and the speed of the disc is about 1–2 rpm. The rotating
discs are made of plastic materials, polyurethane, polyvinyl chloride, polystyrene,
etc. These plastic discs are partially immersed (approximately 40–45%) into the
wastewater and are continuously rotated along a horizontal shaft using an air-driven
machine. Amount of dissolved oxygen in wastewater is increased by surface turbulence (bubbles formation) due to disc rotation. The microorganisms, like
oil-degrading bacteria (Burkholderia cepacia), protozoa, attach to the discs and
digest the organic pollutants as feed and also grow vigorously on this media. Excess
biomass generated is cleared off regularly from the disc surface. Some advantages of
this contactor are low energy consumption, simple operation and maintenance.
Moreover, they do not need addition of aeration devices, because oxygen transfer
facility is present in the system by the rotation of discs. According to the Environmental Protection Agency, the removal of efficiency of this equipment is 85% for
biochemical oxygen demand and suspended solids. Some industry rotating
8 Treatment of Petroleum Hydrocarbon Pollutants in Water
257
two types of attached growth equipment like trickling filter and rotary biological
contactor (International Petroleum Industry Environmental Conservation Association 2010).
Trickling Filter
Trickling filter or bio-filter is a basin which is supported with permeable media such
as shredded polyvinyl chloride, rocks, gravels, tubes, corrugated blocks, etc. The
specific surface and diameter of the permeable materials are about 30–900 m
2 /m
3
and 7–9 cm, respectively. The microorganisms are attached to these substances for
their suspended growth. The microorganisms are added to the wastewatereffluents
from primary treatment or equalization system. These microorganisms consume the
wastes (oil, grease), producing carbon dioxide, methane, ammonia, etc. Generally,
oxygen is introduced either up or down to the bed by natural air flow. Occasionally,
air is introduced forcefully by blower, pump and diffuser. The underwater air moves
through the void space of the filter bed by inlet draught tubes. After treatment of
wastewater in trickling bed, the treated water is drained out through the under drain
channel. The trickle filter is cleaned up periodically (once or twice in year) by
scraping the dead microorganisms. It may be located upstream of the activated
sludge process to reduce organic loads. Sometimes wastes microbes are recycled
to the filter for enhancing hydraulic distribution of the wastewater into the filter. The
removal efficiency of trickling bed is 60–85% for biochemical oxygen demand,
30–70% for chemical oxygen demand, 60–85% for suspended solids and 50–40%
for oil (International Petroleum Industry Environmental Conservation Association
2010; Shahryar 2017; Alireza 2014).
Rotating Biological Contactor
Rotating biological contactors contain rotating discs that are closely placed on a
horizontal shaft. The distance between two rotating discs is 2–3 cm, approximate
diameter of discs is 3 cm and the speed of the disc is about 1–2 rpm. The rotating
discs are made of plastic materials, polyurethane, polyvinyl chloride, polystyrene,
etc. These plastic discs are partially immersed (approximately 40–45%) into the
wastewater and are continuously rotated along a horizontal shaft using an air-driven
machine. Amount of dissolved oxygen in wastewater is increased by surface turbulence (bubbles formation) due to disc rotation. The microorganisms, like
oil-degrading bacteria (Burkholderia cepacia), protozoa, attach to the discs and
digest the organic pollutants as feed and also grow vigorously on this media. Excess
biomass generated is cleared off regularly from the disc surface. Some advantages of
this contactor are low energy consumption, simple operation and maintenance.
Moreover, they do not need addition of aeration devices, because oxygen transfer
facility is present in the system by the rotation of discs. According to the Environmental Protection Agency, the removal of efficiency of this equipment is 85% for
biochemical oxygen demand and suspended solids. Some industry rotating
8 Treatment of Petroleum Hydrocarbon Pollutants in Water
257