On the other hand, the
average concentrations of calcium ions in chamber A and C were
50.3 mg/L and 37.0 mg/L. Le Corre et al. and Hwang and Choi
[34,35] reported that the ratio of Ca2+ to Mg2+ should be below 1
for an effective struvite formation. The average ratios of Ca2+ to
Mg2+ in chamber A and C were 3.77 and 4.10 in this study. This evidently
shows that calcium ions inhibit the struvite crystallization
in chamber A and/or chamber C. Pearson correlation between
PO4–P removal and each cation concentration in chamber C is
shown in Supplementary Table 7. As shown in Supplementary
Table 7, PO4–P removal in the IEBR was caused by magnesium ions
and/or calcium (P < 0.05). In addition, it is evidently shown that
magnesium ions were competed with calcium. Specifically, under
high pH conditions, the amount of calcium precipitation increases
with a low Mg:P ratio because of the competition between Mg–P
bindings and Ca–P [36]. The pH at chamber A or C was weak basic
conditions during the study period because of the characteristics of
swine wastewater/denitrification (Table 3). As shown in Fig. 4,
each amorphous crystal rather than pure struvite was caused by
the calcium ions consumed in each chamber. Thus, considering
the concentrations of ammonia and phosphate in swine wastewater,
removal of phosphorus in the IEBR is determined by competition
between magnesium ions and calcium.