4.3 Nutrient Removal/Recovery from Wastewater
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4.3.1.2 Physico-Chemical Removal Methods
Ammonia could be removed from the wastewater using a physico-chemical process,
e.g., ammonia stripping, where lime or caustic soda is added to the wastewater until
the pH reaches 10.8 or 11.5 at which ammonia ions are converted into ammonia gas.
The wastewater then flows down through a tower to allow the gas to escape into the
air. This process is thus easier and less expensive than the nitrification–denitrification
process, and it is unaffected by toxic compounds that could disrupt the performance
of a biological system. It is also a controlled process for selected ammonia removal.
However, this method is not very efficient in cold weather and cannot remove nitrite
and organic nitrogen [4, 28].
Ammonium ions can also be physically adsorbed by acid solutions such as sulfuric
acid. In such process, ammonium ions are first converted into volatile ammonia using
high temperature and/or pH, and by addition of an acid solution such as sulfuric acid,
the volatile ammonia is transformed into ammonium salts, e.g., ammonium sulfate
[29].
Other physico-chemical processes include removal of ammonium using natural
clay and zeolite [30], aeration and air stripping [31], microwave radiation [32], as
well as the removal of dissolved organic nitrogen by coagulation/flocculation using
aluminum sulfate [33].
4.3.2 Phosphorous Removal/Recovery from Wastewater
Phosphorous exists in wastewater in the forms of soluble and particulate phosphate.
Particulate phosphate can be removed from wastewater using solid removal techniques such as filtration and clarification. The soluble phosphorous removal/recovery
can be achieved through chemical precipitation, physical–chemical methods, biological treatment or a combination of methods.
4.3.2.1 Chemical Precipitation
Chemical precipitation involves the addition of chemicals (coagulants/flocculants
and precipitants) to the wastewater and removes the inorganic forms of phosphate.
The most common chemical option is to add metal ions such as alum (aluminum
sulfate), ferric chloride, ferric sulfate, ferrous sulfate, ferrous chloride and lime
(calcium hydroxide) [34]. The required chemical dosage depends on soluble phosphorous concentration. According to water quality standards, the final phosphorous
concentration in the effluent should equal to or less than 1 mg/L for publicly/privately
owned wastewater treatment plants [35]. To achieve a final P concentration above
2 mg/L, 1 mol of alum or iron per mole of P is required, whereas a dosage of 1.2–4 mol
of alum or iron per mole of P is required for final P concentrations of 0.3–1 mg/L
[34]. Since the solubility of the precipitates depends on the pH values, it plays an
109
4.3.1.2 Physico-Chemical Removal Methods
Ammonia could be removed from the wastewater using a physico-chemical process,
e.g., ammonia stripping, where lime or caustic soda is added to the wastewater until
the pH reaches 10.8 or 11.5 at which ammonia ions are converted into ammonia gas.
The wastewater then flows down through a tower to allow the gas to escape into the
air. This process is thus easier and less expensive than the nitrification–denitrification
process, and it is unaffected by toxic compounds that could disrupt the performance
of a biological system. It is also a controlled process for selected ammonia removal.
However, this method is not very efficient in cold weather and cannot remove nitrite
and organic nitrogen [4, 28].
Ammonium ions can also be physically adsorbed by acid solutions such as sulfuric
acid. In such process, ammonium ions are first converted into volatile ammonia using
high temperature and/or pH, and by addition of an acid solution such as sulfuric acid,
the volatile ammonia is transformed into ammonium salts, e.g., ammonium sulfate
[29].
Other physico-chemical processes include removal of ammonium using natural
clay and zeolite [30], aeration and air stripping [31], microwave radiation [32], as
well as the removal of dissolved organic nitrogen by coagulation/flocculation using
aluminum sulfate [33].
4.3.2 Phosphorous Removal/Recovery from Wastewater
Phosphorous exists in wastewater in the forms of soluble and particulate phosphate.
Particulate phosphate can be removed from wastewater using solid removal techniques such as filtration and clarification. The soluble phosphorous removal/recovery
can be achieved through chemical precipitation, physical–chemical methods, biological treatment or a combination of methods.
4.3.2.1 Chemical Precipitation
Chemical precipitation involves the addition of chemicals (coagulants/flocculants
and precipitants) to the wastewater and removes the inorganic forms of phosphate.
The most common chemical option is to add metal ions such as alum (aluminum
sulfate), ferric chloride, ferric sulfate, ferrous sulfate, ferrous chloride and lime
(calcium hydroxide) [34]. The required chemical dosage depends on soluble phosphorous concentration. According to water quality standards, the final phosphorous
concentration in the effluent should equal to or less than 1 mg/L for publicly/privately
owned wastewater treatment plants [35]. To achieve a final P concentration above
2 mg/L, 1 mol of alum or iron per mole of P is required, whereas a dosage of 1.2–4 mol
of alum or iron per mole of P is required for final P concentrations of 0.3–1 mg/L
[34]. Since the solubility of the precipitates depends on the pH values, it plays an