## 3 Results and Discussion
### Phosphate Recovery as Hydroxyapatite  
The compositions of hydroxyapatite at the two molar ratios of phosphate to calcium used in the experiment are presented in Table 3. The phosphate contents of the precipitates formed from the desorbed solution collected during the first 30 min were 25.1% and 38.5% for the phosphate to calcium ratios 1:0.5 and 1:2, respectively, while the corresponding calcium contents were 35.1% and 32.8% (Table 3). The corresponding P contents were 8.2% and 12.6%, respectively, which are within the range of values reported in the literature for natural phosphate rocks (8.9%-17.2%) [30]. The P content of 12.6% obtained for the higher phosphate to calcium ratio (1:2) matches well with the P content of 13.1% reported by [PERSON] _et al._[21] for a hydroxyapatite recovered from municipal wastewater using a metal oxide/polymer adsorbent and NaOH desorbing solution. The increase of P content with an increase in calcium in the phosphate to calcium ratio indicates that calcium limited the formation of the hydroxyapatite at the lower calcium in the ratio. Limitation of calcium may be because that part of the calcium was used up in the formation of calcium sulphate complex or precipitate. This is a likely possibility because of the high concentration of sulphate in the MBR water and the resin having preference for adsorption of sulphate over phosphate. As in the case of struvite precipitation, the phosphate contents of the hydroxyapatite produced from the desorbed solution collected during the last 30 min were smaller than those produced from the first 30 min desorbed solutions. The amount of phosphate adsorbed by the column used in the study was 746 mg (breakthrough capacity of 13.3 mg/g \(\times\) weight of resin in column of 56 g). The treated volume of water was 78.8 L. Given that the Gordon plant produces 110 ML/year, it is therefore possible that the amount of phosphate recoverable from this plant as phosphate fertiliser is 1041 kg/year. The Gordon plant \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Desorption Time** & **Phosphate Concentration** & \multicolumn{2}{c}{**Chemical Composition, Weight (\%)**} \\
**(min)** & **of Desorbed Solution (M)** & **Phosphate** & **Ammonium** & **Magnesium** \\ \hline
30 & 0.007 & 41.7 \(\pm\) 0.29 & 10.9 \(\pm\) 0.68 & 12.6 \(\pm\) 0.55 \\
60 & 0.004 & 21.7 \(\pm\) 0.49 & 7.9 \(\pm\) 0.68 & 8.6 \(\pm\) 0.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of the recovered struvite precipitate (mean \(\pm\) standard error). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Desorption Time (min)** & **Phosphate Concentration** & **Molar Ratio** & **Chemical Composition, Weight (\%)** \\ \cline{3-5}
**30** & 0.007 & 1.0:0.5 & 25.1 \(\pm\) 0.28 & 35.1 \(\pm\) 0.27 \\  & & 1.0:2.0 & 38.5 \(\pm\) 0.28 & 32.8 \(\pm\) 0.45 \\  & & 1.0:0.5 & 26.4 \(\pm\) 0.19 & 28.4 \(\pm\) 0.49 \\
60 & 0.004 & 1.0:2.0 & 29.9 \(\pm\) 0.58 & 33.7 \(\pm\) 0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical composition of the recovered hydroxyapatite precipitate (mean \(\pm\) standard error). is a relatively small one compared to other plants in Sydney and therefore the amount of phosphate that can be recovered is also small. At the average rate of phosphate applied to pasture in Australia, _i.e._, 20 kg/ha/year [31] the amount of phosphate fertiliser able to be produced even with this small plant can meet the phosphate requirement of 50 ha of pasture land. Regeneration and reusability of the adsorbent was not studied. However, a recent study on the use of this same adsorbent for nitrate removal has shown that the adsorbent can be reused for two more adsorption/desorption cycles without significantly losing its adsorption capacity [32]. The desorption in that study was conducted using 1 M KCl which is stronger than the 0.1 M NaCl concentration used in the current study. Considering this, it is expected that the adsorbent can be satisfactorily reused for few more cycles for phosphate removal as well.