Ca2+ forming calcium phosphate with PO4
3- or interfering the process of struvite crystals to affect precipitation
reaction, which change the nature of struvite[14]. Table 3 shows the PRE change when the ratio of Mg2+ and Ca2+ is
different. It shows that PRE does not change significantly and is more than 90% when the ratio of Mg2+ and Ca2+ is
different. It means adding Ca2+ has no obvious effect on PRE.