9 Struvite Crystallization: An Effective Technology …
151
observed by others (Li et al. 2012; A. Siciliano et al. 2013). This disagreement is
due to the different characteristics of wastewaters and different experimental setup.
Although increasing PO 4
3− dosage could be more valuable in some cases, achieving
further removal of NH 4 −N by overdosing Mg
2+ is more preferable. Increasing the
dosage of PO 4
3− salts could produce unwanted compounds in the final effluents,
which means that additional treatment is required (Wang et al. 2006; Zhou and Wu
2012).
9.4.3 Reaction Time
Basically, struvite crystallization reaction can be accomplished rapidly. In fact, the
increase in reaction time could only enlarge the size of MAP crystals; however, the
development of actual struvite production would be negligible (Le Corre et al. 2009).
A study conducted by Wang et al. (2012) demonstrated that the practical reaction time
for effective MAP crystallization is around 20 min only. Reaction completion time
could be affected by other factors. For instance, Hao et al. (2013) needed three months
to obtain high-purity struvite (99.7%) at pH 7.0 (neutral) and ambient temperature
(25–30 °C). However, by raising the solution’s pH, reaction time could be reduced
to hours or minutes, which enhances the applicability of the process. Other factors
may also affect reaction time such as mixing intensity (G value) and reactivity of Mg
and P reagents.
9.4.3.1 Temperature
Controlling temperature in MAP crystallizers ensures the purity of the recovered
struvite. Practically, ambient temperatures are suitable for a thermally stable process
of struvite crystallization. On the other side, high temperatures significantly affect
the crystal phase and structure. Few studies illustrated the effect of temperature
on struvite recovery process. Bhuiyan et al. (2008) tested the effect of temperature
on the structure of struvite crystals. The authors found that thermal decomposition
of MAP starts at 55 °C, and MAP phase was transformed into other phases such
as bobierrite, newberyite and amorphous MgHPO 4 . This is because solubility of
struvite increases with increasing temperature (Le Corre et al. 2009). Accordingly,
struvite should be dried at a temperature between 30 and 45 °C to avoid mass loss
and phase transformation. These aspects should be carefully controlled during the
design of struvite reactors in order to achieve higher efficiency of struvite recovery.
9.4.3.2 Magnesium and Phosphorus Sources
In general, the applied sources of Mg and P considerably influence the time required
to complete the precipitation reaction, the quality of effluent, struvite purity, and the
151
observed by others (Li et al. 2012; A. Siciliano et al. 2013). This disagreement is
due to the different characteristics of wastewaters and different experimental setup.
Although increasing PO 4
3− dosage could be more valuable in some cases, achieving
further removal of NH 4 −N by overdosing Mg
2+ is more preferable. Increasing the
dosage of PO 4
3− salts could produce unwanted compounds in the final effluents,
which means that additional treatment is required (Wang et al. 2006; Zhou and Wu
2012).
9.4.3 Reaction Time
Basically, struvite crystallization reaction can be accomplished rapidly. In fact, the
increase in reaction time could only enlarge the size of MAP crystals; however, the
development of actual struvite production would be negligible (Le Corre et al. 2009).
A study conducted by Wang et al. (2012) demonstrated that the practical reaction time
for effective MAP crystallization is around 20 min only. Reaction completion time
could be affected by other factors. For instance, Hao et al. (2013) needed three months
to obtain high-purity struvite (99.7%) at pH 7.0 (neutral) and ambient temperature
(25–30 °C). However, by raising the solution’s pH, reaction time could be reduced
to hours or minutes, which enhances the applicability of the process. Other factors
may also affect reaction time such as mixing intensity (G value) and reactivity of Mg
and P reagents.
9.4.3.1 Temperature
Controlling temperature in MAP crystallizers ensures the purity of the recovered
struvite. Practically, ambient temperatures are suitable for a thermally stable process
of struvite crystallization. On the other side, high temperatures significantly affect
the crystal phase and structure. Few studies illustrated the effect of temperature
on struvite recovery process. Bhuiyan et al. (2008) tested the effect of temperature
on the structure of struvite crystals. The authors found that thermal decomposition
of MAP starts at 55 °C, and MAP phase was transformed into other phases such
as bobierrite, newberyite and amorphous MgHPO 4 . This is because solubility of
struvite increases with increasing temperature (Le Corre et al. 2009). Accordingly,
struvite should be dried at a temperature between 30 and 45 °C to avoid mass loss
and phase transformation. These aspects should be carefully controlled during the
design of struvite reactors in order to achieve higher efficiency of struvite recovery.
9.4.3.2 Magnesium and Phosphorus Sources
In general, the applied sources of Mg and P considerably influence the time required
to complete the precipitation reaction, the quality of effluent, struvite purity, and the