4.3 Nutrient Removal/Recovery from Wastewater
111
1. Before the primary clarifier to treat raw/primary wastewater: In this process,
phosphorous is removed with around 90% efficiency and the final P concentration
can be about 0.5 mg/L. The required chemical dosage is the same as the dosage
needed for BOD and suspended solids removal.
2. Upstream secondary clarifier or co-precipitation: This method is usually used for
the activated sludge process and the chemicals are fed directly into the aeration
tank or on the effluent line of the aeration tank. The circulation of the activated
sludge along with the coagulation-flocculation and adsorption processes results
in a reduction in chemical consumption and thus a lower cost. However, the P
removal efficiency with this method is lower than 85% and the final P concentration is about 1 mg/L. Another disadvantage of this method is that the biological
sludge is mixed with the chemical contaminated sludge and they cannot be used
separately for the next treatments.
3. After the secondary clarifier and upstream of filters to treat the final effluent of
the biological plant (activated sludge plant): This is also called post-precipitation
and usually only metal ions are used at this location. The efficiency of the method
is around 95% and the final P concentration can be lower than 0.5 mg/L. This
method guarantees higher purification efficiency as it provides good removal of
suspended solids that escape the final sedimentation in case the biological process
is not efficient. However, it is a costly method and some iron might remain in the
effluent if ferric salts are used.
Chemical addition would result in increasing sludge production in wastewater
treatment plants (by around 40% in primary treatment and 26% in activated sludge
process) [34]. The capital costs of chemical precipitation systems are lower than
biological removal processes (explained in the next sections) but the operational
costs are higher due to the costs of chemical addition. The chemical addition method
can be used in combination with the biological removal process when the biological
method is not efficient enough to meet a required effluent P concentration.
4.3.2.2 Physico-Chemical Methods
The physico-chemical methods include the following:
– Adsorption: Adsorption is a promising method for nutrient recovery due to its
simple design and operation, low cost and high stability. Phosphorous can be
adsorbed and removed in filter systems using active media. Adsorbents can be
natural products such as apatite, bauxite or limestone, industrial waste products
such as fly ash, ochre or steel slag or man-made products. In this method, the
inorganic P from the wastewater adsorbs on the reactive component in the media
and accumulates by the process of sorption or direct precipitation. The phosphorous removal capacity depends on the mineral content of the active media.
Operating parameters such as the pH value, temperature, co-existent ions such as
CO
2−
3 , NO
−
3 , F
− and SO
2−
4 , and desorption methods such as solvent washing and
calcinations affect the phosphate recovery through adsorption [29, 36, 38].
111
1. Before the primary clarifier to treat raw/primary wastewater: In this process,
phosphorous is removed with around 90% efficiency and the final P concentration
can be about 0.5 mg/L. The required chemical dosage is the same as the dosage
needed for BOD and suspended solids removal.
2. Upstream secondary clarifier or co-precipitation: This method is usually used for
the activated sludge process and the chemicals are fed directly into the aeration
tank or on the effluent line of the aeration tank. The circulation of the activated
sludge along with the coagulation-flocculation and adsorption processes results
in a reduction in chemical consumption and thus a lower cost. However, the P
removal efficiency with this method is lower than 85% and the final P concentration is about 1 mg/L. Another disadvantage of this method is that the biological
sludge is mixed with the chemical contaminated sludge and they cannot be used
separately for the next treatments.
3. After the secondary clarifier and upstream of filters to treat the final effluent of
the biological plant (activated sludge plant): This is also called post-precipitation
and usually only metal ions are used at this location. The efficiency of the method
is around 95% and the final P concentration can be lower than 0.5 mg/L. This
method guarantees higher purification efficiency as it provides good removal of
suspended solids that escape the final sedimentation in case the biological process
is not efficient. However, it is a costly method and some iron might remain in the
effluent if ferric salts are used.
Chemical addition would result in increasing sludge production in wastewater
treatment plants (by around 40% in primary treatment and 26% in activated sludge
process) [34]. The capital costs of chemical precipitation systems are lower than
biological removal processes (explained in the next sections) but the operational
costs are higher due to the costs of chemical addition. The chemical addition method
can be used in combination with the biological removal process when the biological
method is not efficient enough to meet a required effluent P concentration.
4.3.2.2 Physico-Chemical Methods
The physico-chemical methods include the following:
– Adsorption: Adsorption is a promising method for nutrient recovery due to its
simple design and operation, low cost and high stability. Phosphorous can be
adsorbed and removed in filter systems using active media. Adsorbents can be
natural products such as apatite, bauxite or limestone, industrial waste products
such as fly ash, ochre or steel slag or man-made products. In this method, the
inorganic P from the wastewater adsorbs on the reactive component in the media
and accumulates by the process of sorption or direct precipitation. The phosphorous removal capacity depends on the mineral content of the active media.
Operating parameters such as the pH value, temperature, co-existent ions such as
CO
2−
3 , NO
−
3 , F
− and SO
2−
4 , and desorption methods such as solvent washing and
calcinations affect the phosphate recovery through adsorption [29, 36, 38].