## 3 Results and Discussion
### Reuse of Struvite Decomposition Residues  
The first tests were conducted to identify the optimal quantity of wet solid, obtained through the defined decomposition procedure, for ammonia removal from raw landfill leachate. In this regard, amounts of 7.5%, 8.5%, 10%, 20%, and 30% (\(w/v\)) of wet residues were used to treat 200 mL of leachate at pH = 9. The experimental results showed that NH4+ abatements of about 82% can be obtained by using amounts of struvite residues up to 8.5% (Figure 10). The efficiencies increased up to around 94% with the dosage of 10%, beyond which only further restricted performance increases were observed. The above results prove that ammonia removal can be efficiently performed by using struvite decomposition products as source of magnesium and phosphorus. In fact, in basic environment, newbeyite reacts with the NH4+ ion to again form MgNH4 PO4-6H2O, according to the following reaction [14]:  
\[\mathrm{MgHPO_{4}\cdot 3H_{2}O}\ +\mathrm{NH_{4}}^{+}+3H_{2}O\rightarrow\mathrm{MgNH _{4}PO_{4}\cdot 6H_{2}O}+\mathrm{H}^{+} \tag{2}\]  
Figure 10: N-NH4+ removal detected by changing the dosage of struvite residues for the treatment of raw leachate. Figure 9: Diffractogram of struvite residues obtained by using HCl at 50 \({}^{\circ}\)C and pH = 5.5. This was confirmed by the XRD analysis of solids recovered after the leachate treatment, which showed mainly the peaks characteristic of struvite, while those representative of MgHPO\({}_{4}\cdot\)3H\({}_{2}\)O were much less pronounced (Figure 11). Furthermore, the results of conducted tests showed that for used amounts of decomposition residues lower than 20%, residual phosphorus and magnesium concentrations were in line with those of raw leachate. Beyond this dosage, moderate increases, up to 35 mg(P)/L and 72 mg(Mg)/L, were observed. These increases are attributable to the dissolved phosphorus and magnesium contained in the liquid phase of struvite residues. On the basis of the above results, an amount of 10% was assumed as optimal for the treatment of raw leachate. Therefore, by considering the humidity (80%) of decomposition products, the actual ratio between the dry struvite residues and leachate is about 2% (\(w/v\)). The above dose was exploited to begin the multiple recycling tests. In particular, in the first reuse cycle amounts of wet struvite products, corresponding to the identified ratio (10%), were used to treat raw leachate. In the subsequent treatments, the quantity of decomposition residues obtained from the previous cycle was exploited. Specifically, as described in the Experimental section, the tests were conducted according to the sequential batch operating mode. Therefore, each phase of the process was sequentially executed in the reactor. In this way, the settled solids produced in each step were used in wet form without being subject to any further separation or drying treatments. This procedure simplifies the management of the process and could be easily applied in industrial plants. Three methods for the repeated treatment of raw leachate were tested. In two cases, the cycles were conducted without the supply of any Mg and P reactants and the pH was set, respectively, by means of NaOH and air insufflation. In the third method, MgCl\({}_{2}\cdot\)6H\({}_{2}\)O and H\({}_{3}\)PO\({}_{4}\) were fed at a molar ratio of _n_(N):_n_(Mg):_n_(P) 1:0.05:0.05, and NaOH addition was used for pH adjustment. The experimental results showed that, during the first reusing cycle, the ammonium removal was around 92% for each operating method tested (Figure 12). Without supplementation of reagents and by setting the pH with NaOH, the process performance resulted in around 86% for the second and third cycle, and then gradually decreased to about 54%. The performance reduction is a consequence of a progressive decrease of the precipitate that was produced from each treatment (Table 2). This reduction caused a corresponding decrease of solid residues obtained during the decomposition steps (Table 2).