Generalities
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1.2.2.1.3. Secondary treatments (Biological treatments)
The main objective of these treatments is to eliminate the soluble and organic fractions of
pollution, by exploiting the activity of microorganism’s populations capable of consuming them
(Moulin et al., 2013).
Biological treatment processes are based on the ability of a purifying biomass of
microorganisms (essentially bacteria) to oxidize mineral matter (Ní µí°»3) and the constituent
materials of COD and BOD on the one hand, and to reduce oxygen-bearing molecules on the
other: Ní µí±3, Sí µí±4 and Cí µí±2. Depending on the type of bacteria present in the water, biological
treatments are classified into two groups :Anaerobic treatments and aerobic treatments
(Grosclaude, 1999).
Anaerobic treatment
This biological treatment is based on anaerobic fermentation or methanization
processes. It consists on putting the wastewater to be treated into contact with a biomass
of microorganisms mainly methanogenic bacteria that are capable of degrading organic
matter under anaerobic conditions (absence of oxygen) to produce methane-rich biogas
(50-70%) (Bernet, 2015).
Aerobic treatment
Aerobic biological treatment involves the degradation of organic matter by microorganisms in
the presence of oxygen, while controlling temperature and pH. A distinction can be made
between:
Free culture (activated sludge)
Activated sludge is the most common aerobic biological treatment.It involves the development
of a bacterial culture dispersed in flocculent form in the form of flakes (activated sludge), in a
brewed and aerated basin (aeration tank) supplied with water to be treated . In this tank, the
purpose of mixing is to prevent deposits and homogenize the mixture of bacterial flakes and
wastewater (mixed liquor); aeration can be based on oxygen from water, oxygen-enriched gas,
or even pure oxygen, the aim being to dissolve this gas in the mixed liquor, in order to meet the
needs of aerobic purification bacteria. After a sufficiently long contact time, the mixed liquor
is sent to a clarifier also known as a secondary clarifier, designed to separate the purified water
24
1.2.2.1.3. Secondary treatments (Biological treatments)
The main objective of these treatments is to eliminate the soluble and organic fractions of
pollution, by exploiting the activity of microorganism’s populations capable of consuming them
(Moulin et al., 2013).
Biological treatment processes are based on the ability of a purifying biomass of
microorganisms (essentially bacteria) to oxidize mineral matter (Ní µí°»3) and the constituent
materials of COD and BOD on the one hand, and to reduce oxygen-bearing molecules on the
other: Ní µí±3, Sí µí±4 and Cí µí±2. Depending on the type of bacteria present in the water, biological
treatments are classified into two groups :Anaerobic treatments and aerobic treatments
(Grosclaude, 1999).
Anaerobic treatment
This biological treatment is based on anaerobic fermentation or methanization
processes. It consists on putting the wastewater to be treated into contact with a biomass
of microorganisms mainly methanogenic bacteria that are capable of degrading organic
matter under anaerobic conditions (absence of oxygen) to produce methane-rich biogas
(50-70%) (Bernet, 2015).
Aerobic treatment
Aerobic biological treatment involves the degradation of organic matter by microorganisms in
the presence of oxygen, while controlling temperature and pH. A distinction can be made
between:
Free culture (activated sludge)
Activated sludge is the most common aerobic biological treatment.It involves the development
of a bacterial culture dispersed in flocculent form in the form of flakes (activated sludge), in a
brewed and aerated basin (aeration tank) supplied with water to be treated . In this tank, the
purpose of mixing is to prevent deposits and homogenize the mixture of bacterial flakes and
wastewater (mixed liquor); aeration can be based on oxygen from water, oxygen-enriched gas,
or even pure oxygen, the aim being to dissolve this gas in the mixed liquor, in order to meet the
needs of aerobic purification bacteria. After a sufficiently long contact time, the mixed liquor
is sent to a clarifier also known as a secondary clarifier, designed to separate the purified water