4.1 Wastewater Treatment
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of smaller particles and colloids. Materials that will settle out by gravity are removed
and form the primary sludge, while grease and oil rise to the surface and are skimmed
off by mechanical surface skimming devices. Dissolved and colloidal organics as well
as nutrients such as nitrogen and phosphorous will remain in the water during the
primary treatment and will be removed in the secondary or biological treatment [4].
4.1.3 Secondary Treatment
Secondary treatment removes the dissolved organic matter and most of the remaining
suspended solids through biological treatment and secondary sedimentation. Biological treatment aims to degrade the organic contents of sewage and is mostly performed
using activated sludge and the trickling filter processes. During the biological process,
microorganisms consume the organic matter (reducing COD) to sustain their own
growth and reproduction, while producing carbon dioxide and water [4]. The microorganisms in secondary treatment can be applied in fixed film or suspended growth
systems [3].
– Fixed films or attached growth systems include Rotating Biological Contactors
(RBC) and Trickling Filters in which the biomass grows on the media and the
effluent passes over its surface.
– Suspended growth systems include conventional activated sludge processes,
membrane bioreactors, sequencing batch reactors and lagoons, where the microorganisms are suspended in water.
Trickling Filter: Trickling filter is a tank consisting of a fixed bed of stones,
rocks, gravel, slag or plastic media over which, the swage is sprayed continuously
and trickles to the bottom. The downward flow of the wastewater causes a layer of
microbial slime (biofilm) to grow on the stones and allows the microorganisms to
absorb organics. Aerobic conditions by forced-air or natural air convection provide
sufficient oxygen for the metabolic processes. Trickling filters are followed by settling
tanks or secondary clarifiers. The clarifiers remove the solids and microorganisms
(mostly dead microorganisms) that are washed off the filter by the flow of wastewater.
The discharge from the bottom of the tank is known as the secondary sludge [4].
Waste Activated Sludge: This process consists of an aeration tank followed by a
secondary clarifier. A mixture of the settled sludge and the fresh sludge is recirculated
from the secondary clarifier into the aeration tank. Air or oxygen is injected into the
mixed liquor through porous diffusers located at the bottom of the tank. The air
provides oxygen and a rapid mixing action to reduce the organic matter content of
the sewage by microbiological aerobic digestion. The mixture then flows into the
secondary clarifier where the activated sludge is settled by gravity. The clear treated
water is skimmed from the surface of the tank, disinfected, and discharged as the
secondary effluent [4].
Lagoons: Also known as oxidation ponds or stabilization ponds are large, shallow
ponds and provide settlement and further biological treatment through storage in the
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of smaller particles and colloids. Materials that will settle out by gravity are removed
and form the primary sludge, while grease and oil rise to the surface and are skimmed
off by mechanical surface skimming devices. Dissolved and colloidal organics as well
as nutrients such as nitrogen and phosphorous will remain in the water during the
primary treatment and will be removed in the secondary or biological treatment [4].
4.1.3 Secondary Treatment
Secondary treatment removes the dissolved organic matter and most of the remaining
suspended solids through biological treatment and secondary sedimentation. Biological treatment aims to degrade the organic contents of sewage and is mostly performed
using activated sludge and the trickling filter processes. During the biological process,
microorganisms consume the organic matter (reducing COD) to sustain their own
growth and reproduction, while producing carbon dioxide and water [4]. The microorganisms in secondary treatment can be applied in fixed film or suspended growth
systems [3].
– Fixed films or attached growth systems include Rotating Biological Contactors
(RBC) and Trickling Filters in which the biomass grows on the media and the
effluent passes over its surface.
– Suspended growth systems include conventional activated sludge processes,
membrane bioreactors, sequencing batch reactors and lagoons, where the microorganisms are suspended in water.
Trickling Filter: Trickling filter is a tank consisting of a fixed bed of stones,
rocks, gravel, slag or plastic media over which, the swage is sprayed continuously
and trickles to the bottom. The downward flow of the wastewater causes a layer of
microbial slime (biofilm) to grow on the stones and allows the microorganisms to
absorb organics. Aerobic conditions by forced-air or natural air convection provide
sufficient oxygen for the metabolic processes. Trickling filters are followed by settling
tanks or secondary clarifiers. The clarifiers remove the solids and microorganisms
(mostly dead microorganisms) that are washed off the filter by the flow of wastewater.
The discharge from the bottom of the tank is known as the secondary sludge [4].
Waste Activated Sludge: This process consists of an aeration tank followed by a
secondary clarifier. A mixture of the settled sludge and the fresh sludge is recirculated
from the secondary clarifier into the aeration tank. Air or oxygen is injected into the
mixed liquor through porous diffusers located at the bottom of the tank. The air
provides oxygen and a rapid mixing action to reduce the organic matter content of
the sewage by microbiological aerobic digestion. The mixture then flows into the
secondary clarifier where the activated sludge is settled by gravity. The clear treated
water is skimmed from the surface of the tank, disinfected, and discharged as the
secondary effluent [4].
Lagoons: Also known as oxidation ponds or stabilization ponds are large, shallow
ponds and provide settlement and further biological treatment through storage in the