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M. Darwish et al.
overall cost of the treatment process. Numerous types of Mg and P sources have been
investigated including analytical-grade reagents and other alternatives.
9.4.3.3 Magnesium Sources
Magnesium salts were found to possess different reaction completion-time periods.
By applying magnesium chloride (MgCl 2 · 6H 2 O), struvite reaction could be completed in 50 s only (Zhang et al. 2009a, b), while it takes more than 30 min to complete
the reaction if magnesium oxide (MgO) is used (Li et al. 2012). Fundamentally, this
distinction is referred to different solubility of the reagents; Mg or P salts with high
solubility rates result in higher ionic concentrations of Mg
2+ or HPO 4
2− in solution
and, therefore, improve struvite formation.
Considering the environmental aspect, applying MgCl 2 and magnesium sulphate
(MgSO 4 ) may not be preferred, as they raise the salts concentration in the final
effluents (Li et al. 2012). High salinity is a strong inhibitor for microbial activity in
the following biological treatment process. In addition, MgCl 2 and MgSO 4 are costly
materials, which make them economically unfeasible for large-scale applications.
MgO is one of the cheapest sources of Mg, which has been utilized in MAP
technology (Li et al. 2012; Wilsenach et al. 2007). Practically, the application of
MgO minimizes the salinity and electrical conductivity of the treated effluent, as
MgO does not add any extra anions (Li et al. 2012). However, the sole drawback of
MgO is its low solubility in water, which leads to longer reaction time periods.
Several studies have proposed different approaches to achieve an efficient application of MgO, including utilizing MgO as a dual function reagent (i.e. Mg
2+ source
and pH adjustment agent) (Huang et al. 2014) and the use of “magnesia suspension” (prepared by mixing pure MgO with de-ionized water, with a concentration of
100 g/L) (Capdevielle et al. 2013). These strategies contributed to significant cost
reductions. In addition, the efficiency of MgO could be improved by dissolving MgO
in an acidic media (Table 9.4). If phosphoric acid (H 3 PO 4 ) was used for this mission,
this approach could be competent. In the same context, instead of using pure Mg
salts, several alternatives can be effectively employed as low-cost sources of Mg,
which achieved outstanding levels of NH 4 −N recovery (Huang et al. 2014; Liu et al.
2013a, b).
The feasibility of using such alternative depends on several issues including abundance, quality and pre-treatment of the source, if required. Seawater bittern, RO and
NF brines, and low-grade MgO are waste materials, which contain different amounts
of Mg. Among Mg alternatives, seawater bittern contains the highest concentration
of Mg
2+ , which significantly promotes NH 4 −N recovery by MAP crystallization.
Natural Mg rocks such as magnesite (MgCO 3 ) (Gunay et al. 2008b) and brucite
(Mg(OH) 2 ) (Huang et al. 2011a, b) have low reactivity in wastewater. However, they
are good options in places where they are abundant. To overcome the low reactivity obstacle, these rocks should be dissolved by acids to increase their reactivity,
otherwise their dose needs to be increased.
M. Darwish et al.
overall cost of the treatment process. Numerous types of Mg and P sources have been
investigated including analytical-grade reagents and other alternatives.
9.4.3.3 Magnesium Sources
Magnesium salts were found to possess different reaction completion-time periods.
By applying magnesium chloride (MgCl 2 · 6H 2 O), struvite reaction could be completed in 50 s only (Zhang et al. 2009a, b), while it takes more than 30 min to complete
the reaction if magnesium oxide (MgO) is used (Li et al. 2012). Fundamentally, this
distinction is referred to different solubility of the reagents; Mg or P salts with high
solubility rates result in higher ionic concentrations of Mg
2+ or HPO 4
2− in solution
and, therefore, improve struvite formation.
Considering the environmental aspect, applying MgCl 2 and magnesium sulphate
(MgSO 4 ) may not be preferred, as they raise the salts concentration in the final
effluents (Li et al. 2012). High salinity is a strong inhibitor for microbial activity in
the following biological treatment process. In addition, MgCl 2 and MgSO 4 are costly
materials, which make them economically unfeasible for large-scale applications.
MgO is one of the cheapest sources of Mg, which has been utilized in MAP
technology (Li et al. 2012; Wilsenach et al. 2007). Practically, the application of
MgO minimizes the salinity and electrical conductivity of the treated effluent, as
MgO does not add any extra anions (Li et al. 2012). However, the sole drawback of
MgO is its low solubility in water, which leads to longer reaction time periods.
Several studies have proposed different approaches to achieve an efficient application of MgO, including utilizing MgO as a dual function reagent (i.e. Mg
2+ source
and pH adjustment agent) (Huang et al. 2014) and the use of “magnesia suspension” (prepared by mixing pure MgO with de-ionized water, with a concentration of
100 g/L) (Capdevielle et al. 2013). These strategies contributed to significant cost
reductions. In addition, the efficiency of MgO could be improved by dissolving MgO
in an acidic media (Table 9.4). If phosphoric acid (H 3 PO 4 ) was used for this mission,
this approach could be competent. In the same context, instead of using pure Mg
salts, several alternatives can be effectively employed as low-cost sources of Mg,
which achieved outstanding levels of NH 4 −N recovery (Huang et al. 2014; Liu et al.
2013a, b).
The feasibility of using such alternative depends on several issues including abundance, quality and pre-treatment of the source, if required. Seawater bittern, RO and
NF brines, and low-grade MgO are waste materials, which contain different amounts
of Mg. Among Mg alternatives, seawater bittern contains the highest concentration
of Mg
2+ , which significantly promotes NH 4 −N recovery by MAP crystallization.
Natural Mg rocks such as magnesite (MgCO 3 ) (Gunay et al. 2008b) and brucite
(Mg(OH) 2 ) (Huang et al. 2011a, b) have low reactivity in wastewater. However, they
are good options in places where they are abundant. To overcome the low reactivity obstacle, these rocks should be dissolved by acids to increase their reactivity,
otherwise their dose needs to be increased.