150
M. Darwish et al.
9.4 Factors Influencing Struvite Recovery
The process of struvite recovery is basically affected by several factors. These include
pH, molar ratio of Mg, N and P, type of reagents, temperature, reaction time, presence
of foreign ions and mixing conditions.
9.4.1 pH of Solution
The percentage of struvite in precipitates is significantly influenced by the pH of
reaction, as the supersaturation increases with increasing pH. Generally, struvite
can be formed within pH range of 8–10. At pH greater than 10, MAP crystallization could be inhibited due to the formation of competing compounds such as
Mg 3 (PO 4 ) 2 , Mg(OH) 2 , and other amorphous compounds such as hydroxyapatite
(Ca 10 (PO 4 ) 6 (OH) 2 ) (Ryu et al. 2008).
The effect of pH on MAP recovery has been reported in many studies. Focusing
on NH 4 −N removal, Ryu et al. (2008) found that the optimum pH was 9.2 for
semiconductor wastewater, whereas for rare-earth wastewater, Huang et al. (2011a,
b) obtained the best NH 4
+ removal within pH 8.5–9.5. For LL, a wide range of
optimum pH has been found through several studies, as demonstrated in Table 9.2.
Apparently, different types of pollutants present in LL could affect the optimum pH
value (Zhang et al. 2012). On the opposite side, other researchers found that highpurity struvite could not be produced at pH higher than 8. This is possible because
of the formation of Mg 3 (PO 4 ) 2 and Ca 3 (PO 4 ) 2 instead of the targeted struvite (Hao
et al. 2008). In fact, the large differences between wastewaters’ characteristics, as well
as the possibility of ammonia to volatile at higher pH values, cause inconsistency
between studies’ results. Therefore, pH value should be optimized to ensure high
NH 4 −N removal with significant production of pure struvite.
9.4.2 Molar Ratios (Mg:N and P:N)
Theoretically, struvite is formed at the stoichiometric molar ration of Mg:N:P (1:1:1)
(Eqs. 9.1, 9.2, 9.3). However, the practical ratios are usually different from the theoretical one, because of the presence of other species that form other by-products
(Siciliano and De Rosa 2014; Zhang et al. 2009a, b). In that case, the optimum molar
ratio should be determined for each individual case to achieve the highest efficiency of
MAP recovery, as well as the least cost. Some previous studies have shown that when
increasing Mg:N and/or P:N molar ratios, removal of NH 4 −N could be improved
(Akkaya et al. 2010; Li et al. 2012). While a number of research works have stated
that increasing PO 4
3− dosage could enhance NH 4 −N recovery more than increasing
the dosage of Mg
2+ (Ryu et al. 2008; Zhou and Wu 2012), the opposite has been
M. Darwish et al.
9.4 Factors Influencing Struvite Recovery
The process of struvite recovery is basically affected by several factors. These include
pH, molar ratio of Mg, N and P, type of reagents, temperature, reaction time, presence
of foreign ions and mixing conditions.
9.4.1 pH of Solution
The percentage of struvite in precipitates is significantly influenced by the pH of
reaction, as the supersaturation increases with increasing pH. Generally, struvite
can be formed within pH range of 8–10. At pH greater than 10, MAP crystallization could be inhibited due to the formation of competing compounds such as
Mg 3 (PO 4 ) 2 , Mg(OH) 2 , and other amorphous compounds such as hydroxyapatite
(Ca 10 (PO 4 ) 6 (OH) 2 ) (Ryu et al. 2008).
The effect of pH on MAP recovery has been reported in many studies. Focusing
on NH 4 −N removal, Ryu et al. (2008) found that the optimum pH was 9.2 for
semiconductor wastewater, whereas for rare-earth wastewater, Huang et al. (2011a,
b) obtained the best NH 4
+ removal within pH 8.5–9.5. For LL, a wide range of
optimum pH has been found through several studies, as demonstrated in Table 9.2.
Apparently, different types of pollutants present in LL could affect the optimum pH
value (Zhang et al. 2012). On the opposite side, other researchers found that highpurity struvite could not be produced at pH higher than 8. This is possible because
of the formation of Mg 3 (PO 4 ) 2 and Ca 3 (PO 4 ) 2 instead of the targeted struvite (Hao
et al. 2008). In fact, the large differences between wastewaters’ characteristics, as well
as the possibility of ammonia to volatile at higher pH values, cause inconsistency
between studies’ results. Therefore, pH value should be optimized to ensure high
NH 4 −N removal with significant production of pure struvite.
9.4.2 Molar Ratios (Mg:N and P:N)
Theoretically, struvite is formed at the stoichiometric molar ration of Mg:N:P (1:1:1)
(Eqs. 9.1, 9.2, 9.3). However, the practical ratios are usually different from the theoretical one, because of the presence of other species that form other by-products
(Siciliano and De Rosa 2014; Zhang et al. 2009a, b). In that case, the optimum molar
ratio should be determined for each individual case to achieve the highest efficiency of
MAP recovery, as well as the least cost. Some previous studies have shown that when
increasing Mg:N and/or P:N molar ratios, removal of NH 4 −N could be improved
(Akkaya et al. 2010; Li et al. 2012). While a number of research works have stated
that increasing PO 4
3− dosage could enhance NH 4 −N recovery more than increasing
the dosage of Mg
2+ (Ryu et al. 2008; Zhou and Wu 2012), the opposite has been