112
4 Nitrogen and Phosphorous Recovery from Municipal Wastewater …
– Electrocoagulation and floatation: In this method, low-intensity alternative electric current is applied between electrodes of aluminum or iron/steel. The phosphate
ion is removed by reacting with the iron or aluminum ions [36, 38].
– Crystallization: This method converts the phosphate ion to a solid phase of a
reactant, e.g., magnesium ammonium phosphate; Crystals of the reactants are
charged to the reactor and the phosphate deposits on the seed crystal when the pH
is in a region that does not allow the formation of new fine crystals or phosphorus
in the solution [39].
– Ion-exchange: In this method, the phosphate ions in the wastewater are interchanged with the ion exchanger, thus they are removed and recovered. Immobilized phosphorous-selective nanoparticles of metal cations are loaded on a
polymer exchange base. In this method, phosphorous anions are selectively
adsorbed on the polymer exchange base over other competing ions such as sulfates
or chlorides. However, due to the low concentration of phosphate ions in wastewater, this method is more difficult compared to other physico-chemical methods
[38].
4.3.2.3 Biological Process
Biological phosphorous removal is based on the phosphorous uptake by aerobic
heterotroph microorganisms in activated sludge in excess of phosphorus needed for
their biological growth requirements. The microorganisms are known as Phosphorous
Accumulating Organisms (PAOs) and they are present in the mixed liquor of the
conventional activated sludge process. PAOs contain 1.5–2% phosphorous based
on their dry weight and they are expected to remove approximately 20 to 40% of
the phosphorous from wastewater [17, 34, 40]. Under anaerobic conditions, PAOs
uptake readily available organic matter such as volatile fatty acids (VFAs) in the
absence of dissolved oxygen and nitrates, and store them as polyhydroxyalkanoates
(PHAs). The required energy for this process is obtained from the hydrolysis of
the internally stored polyphosphate. The breakdown of polyphosphate results in the
release of phosphorous in the form of orthophosphate. Under subsequent aerobic
conditions, the accumulated PHAs are oxidized by an available electron acceptor
and the PAOs use the released energy to uptake the phosphorous that was released
in the anaerobic zone, as well as any additional phosphate present in the wastewater
[17, 38]. Enhanced biological phosphorous removal (EBPR) is a process in which a
group of microorganisms is cultured in the mixed liquor of the conventional activated
sludge system. The microorganisms are able to store intercellular phosphorous up to
30% on a dry weight basis. As a result, the efficiency of the wastewater phosphorous
removal could increase to 80–90% by subsequent removal of waste activated sludge
[38].
EBPR is considered as a cost-effective and environmentally sustainable alternative to chemical treatment. The level of biological phosphorous removal is directly
proportional to the number of available PAOs. Currently, the available technologies
4 Nitrogen and Phosphorous Recovery from Municipal Wastewater …
– Electrocoagulation and floatation: In this method, low-intensity alternative electric current is applied between electrodes of aluminum or iron/steel. The phosphate
ion is removed by reacting with the iron or aluminum ions [36, 38].
– Crystallization: This method converts the phosphate ion to a solid phase of a
reactant, e.g., magnesium ammonium phosphate; Crystals of the reactants are
charged to the reactor and the phosphate deposits on the seed crystal when the pH
is in a region that does not allow the formation of new fine crystals or phosphorus
in the solution [39].
– Ion-exchange: In this method, the phosphate ions in the wastewater are interchanged with the ion exchanger, thus they are removed and recovered. Immobilized phosphorous-selective nanoparticles of metal cations are loaded on a
polymer exchange base. In this method, phosphorous anions are selectively
adsorbed on the polymer exchange base over other competing ions such as sulfates
or chlorides. However, due to the low concentration of phosphate ions in wastewater, this method is more difficult compared to other physico-chemical methods
[38].
4.3.2.3 Biological Process
Biological phosphorous removal is based on the phosphorous uptake by aerobic
heterotroph microorganisms in activated sludge in excess of phosphorus needed for
their biological growth requirements. The microorganisms are known as Phosphorous
Accumulating Organisms (PAOs) and they are present in the mixed liquor of the
conventional activated sludge process. PAOs contain 1.5–2% phosphorous based
on their dry weight and they are expected to remove approximately 20 to 40% of
the phosphorous from wastewater [17, 34, 40]. Under anaerobic conditions, PAOs
uptake readily available organic matter such as volatile fatty acids (VFAs) in the
absence of dissolved oxygen and nitrates, and store them as polyhydroxyalkanoates
(PHAs). The required energy for this process is obtained from the hydrolysis of
the internally stored polyphosphate. The breakdown of polyphosphate results in the
release of phosphorous in the form of orthophosphate. Under subsequent aerobic
conditions, the accumulated PHAs are oxidized by an available electron acceptor
and the PAOs use the released energy to uptake the phosphorous that was released
in the anaerobic zone, as well as any additional phosphate present in the wastewater
[17, 38]. Enhanced biological phosphorous removal (EBPR) is a process in which a
group of microorganisms is cultured in the mixed liquor of the conventional activated
sludge system. The microorganisms are able to store intercellular phosphorous up to
30% on a dry weight basis. As a result, the efficiency of the wastewater phosphorous
removal could increase to 80–90% by subsequent removal of waste activated sludge
[38].
EBPR is considered as a cost-effective and environmentally sustainable alternative to chemical treatment. The level of biological phosphorous removal is directly
proportional to the number of available PAOs. Currently, the available technologies