158
M. Darwish et al.
COD concentration during MAP precipitation was attributed to co-precipitation of
other compounds alongside with struvite (Yetilmezsoy and Sapci-Zengin 2009). In
fact, the excessive dosages of Mg reagents contribute to more removal of COD, SS,
colour and turbidity, as Mg ions have similar behaviour to flocculants, which remove
particulate organic matter (Akkaya et al. 2010; Ryu et al. 2008).
Moreover, adsorption of some organic substances onto the surface of struvite
improves COD reduction. A similar removal trend of organics, expressed as TC
and TOC, was reported by Zhang et al. (2012), but with less removal percentage as
compared to COD. The removal of other pollutants during struvite crystallization
was found to be influenced by pH, Mg and P dosages and mixing intensity (Akkaya
et al. 2010; Gunay et al. 2008a).
High concentrations of some pollutants could have negative impact on NH 4 −N
recovery. Barnes et al. (2007) found that NH 4 −N removal from a mature LL was
98% when initial TSS was 320 mg/L. However, the removals were reduced to 78%
and 71% when initial TSS concentrations were 3,600 and 5,160 mg/L, respectively.
On the contrary, Tarragó et al. (2018) confirmed that high concentrations of TSS (1–
3 g-TSS/L) in digested swine manure have favoured the agglomeration of struvite
fine crystals, thus obtaining larger crystalline structure.
The removal of some pollutants could be affected by the type of Mg reagent.
As reported by Yetilmezsoy and Sapci-Zengin (2009), using MgO as Mg source has
resulted in lower COD and colour reduction than that of MgCl 2 or MgSO 4 . This could
be due to the vast difference between the solubility of Mg reagents in wastewater.
The high reactivity of Mg reagent (e.g. MgCl 2 · 6H 2 O) contributes to higher rate of
MAP formation, in parallel with higher coagulation effect, which is the main reason
for COD and colour reduction. On the other hand, the opposite may occur when using
Mg reagent with low reactivity (e.g. MgO), leading to lesser reductions in COD and
colour.
9.5 Integration of MAP Precipitation Technology
with Other Treatment Techniques
The integration of struvite technology with other treatment processes has been shown
to be useful for highly polluted wastewaters. As recommended by Pal and Kumar
(2014), this kind of integration promotes the sustainability of wastewater treatment,
owing to the substantial economic advantages that could be obtained. As a pretreatment process, MAP precipitation was found to be an effective solution for high
NH 4 −N levels that inhibit the bacterial activity in biological treatment stages (Chen
et al. 2013; Jaafarzadeh et al. 2010). Additionally, struvite precipitation could be
applied as a post-treatment process to enhance the final effluent quality in matching
with the discharge limits. For example, Akkaya et al. (2010) achieved only 7.9%
NH 4 −N removal during anaerobic treatment, whereas the efficiency was increased
to 98% after applying struvite precipitation as a post-treatment process. Moreover,
M. Darwish et al.
COD concentration during MAP precipitation was attributed to co-precipitation of
other compounds alongside with struvite (Yetilmezsoy and Sapci-Zengin 2009). In
fact, the excessive dosages of Mg reagents contribute to more removal of COD, SS,
colour and turbidity, as Mg ions have similar behaviour to flocculants, which remove
particulate organic matter (Akkaya et al. 2010; Ryu et al. 2008).
Moreover, adsorption of some organic substances onto the surface of struvite
improves COD reduction. A similar removal trend of organics, expressed as TC
and TOC, was reported by Zhang et al. (2012), but with less removal percentage as
compared to COD. The removal of other pollutants during struvite crystallization
was found to be influenced by pH, Mg and P dosages and mixing intensity (Akkaya
et al. 2010; Gunay et al. 2008a).
High concentrations of some pollutants could have negative impact on NH 4 −N
recovery. Barnes et al. (2007) found that NH 4 −N removal from a mature LL was
98% when initial TSS was 320 mg/L. However, the removals were reduced to 78%
and 71% when initial TSS concentrations were 3,600 and 5,160 mg/L, respectively.
On the contrary, Tarragó et al. (2018) confirmed that high concentrations of TSS (1–
3 g-TSS/L) in digested swine manure have favoured the agglomeration of struvite
fine crystals, thus obtaining larger crystalline structure.
The removal of some pollutants could be affected by the type of Mg reagent.
As reported by Yetilmezsoy and Sapci-Zengin (2009), using MgO as Mg source has
resulted in lower COD and colour reduction than that of MgCl 2 or MgSO 4 . This could
be due to the vast difference between the solubility of Mg reagents in wastewater.
The high reactivity of Mg reagent (e.g. MgCl 2 · 6H 2 O) contributes to higher rate of
MAP formation, in parallel with higher coagulation effect, which is the main reason
for COD and colour reduction. On the other hand, the opposite may occur when using
Mg reagent with low reactivity (e.g. MgO), leading to lesser reductions in COD and
colour.
9.5 Integration of MAP Precipitation Technology
with Other Treatment Techniques
The integration of struvite technology with other treatment processes has been shown
to be useful for highly polluted wastewaters. As recommended by Pal and Kumar
(2014), this kind of integration promotes the sustainability of wastewater treatment,
owing to the substantial economic advantages that could be obtained. As a pretreatment process, MAP precipitation was found to be an effective solution for high
NH 4 −N levels that inhibit the bacterial activity in biological treatment stages (Chen
et al. 2013; Jaafarzadeh et al. 2010). Additionally, struvite precipitation could be
applied as a post-treatment process to enhance the final effluent quality in matching
with the discharge limits. For example, Akkaya et al. (2010) achieved only 7.9%
NH 4 −N removal during anaerobic treatment, whereas the efficiency was increased
to 98% after applying struvite precipitation as a post-treatment process. Moreover,