## 3 Results and Discussion  
### Struvite Production  
The leachate used in this study showed a characteristic composition of wastewaters produced in landfills in a methanogenic phase (Table 1). Indeed, a basic pH of around 8.15 and a high alkalinity, close to 13 gCaCO\({}_{3}\)/L, were detected. Moreover, the value of conductivity, almost 23 mS/cm, denotes a great quantity of dissolved salts. The low BOD\({}_{5}\)/COD ratio indicates a low content of degradable compounds. The N-NH\({}_{4}\)\({}^{+}\) amount was higher than 2.6 g(N)/L and, with respect to this value, the concentrations of magnesium and phosphorus were negligible, as highlighted by the \(n\)(M):\(n\)(Mg):\(n\)(P) molar ratios. This clearly makes the use of great amounts of reactants necessary to allow struvite production. With the aim of identifying the most favorable dosages of magnesium and phosphorous reactants, a set of tests was carried out by testing \(n\)(Mg):\(n\)(N) and \(n\)(P):\(n\)(N) ratios between 1 and 1.5. In these tests, all the reagents were instantly dosed. In particular, the pH of mixture was corrected after the initial \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline pH & 8.16 & - \\ Conductivity & 28.70 & mS/cm \\ Alkalinity & 12.98 & gCaCO\({}_{3}\)/L \\ COD (chemical oxygen demand) & 9.7 & g/L \\ BOD\({}_{5}\) (biochemical oxygen demand) & 0.56 & g/L \\ TS & 14.8 & g/L \\ VS & 5.12 & g/L \\ N-NH\({}_{4}\)\({}^{+}\) & 2.62 & g/L \\ P-PO\({}_{4}\)\({}^{3-}\) & 25.5 & mg/L \\ Mg\({}^{2+}\) & 63.1 & mg/L \\ Ca\({}^{2+}\) & 27.8 & mg/L \\ K\({}^{+}\) & 11.2 & mg/L \\ \(n\)(N):\(n\)(Mg):\(n\)(P) & 1:0.014:0.004 & mol(N):mol(Mg):mol(P) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of landfill leachate. addition of Mg and P solutions. This operating modality was selected on the basis of our previous works, which demonstrated that the initial setting of pH, followed by the dosages of magnesium and phosphorus, is not suitable for struvite precipitation [34]. Indeed, the acid characteristics of P and Mg reagents, if they were fed after the base addition, produce an instantaneous reduction of pH, which hinders the struvite crystallization. The results of the tests conducted in this study, instead, showed satisfactory process performances with the adopted procedure. In particular, ammonium removals of about 72% were obtained using \(n\)(P):\(n\)(N) = 1 and adjusting the \(n\)(Mg):\(n\)(N) ratio between 1 and 1.2 (Figure 1). By holding the phosphorus dosage constant and increasing the magnesium molar amount, the efficiency slightly increased to 78%. The increase of the phosphorus dose to 1.2, for a given \(n\)(Mg):\(n\)(N) ratio, produced an almost proportional increase in ammonium abatement. This growth was contextually accentuated by the increase of magnesium dosage up to a value of 1.2, reaching maximum abatements close to 95% (Figure 1). The necessity to dose higher amounts of Mg and P reagents, with respect to the stoichiometric ratio (\(n\)(N):\(n\)(Mg):\(n\)(P) = 1:1:1), is imputable to the precipitation, besides to struvite particles, of further salts composed of magnesium and phosphorus, such as magnesium and calcium phosphates [22]. The formation of these products removes some of the Mg\({}^{2+}\) and PO\({}_{4}\)\({}^{3-}\) available for struvite crystallization, therefore, an overdose of reagents is necessary. In further researches, the benefit of overdosing magnesium and phosphorus was verified [18; 19; 32]. Clearly, the optimal dosages reported in the literature are related to many factors, such as the characteristics of treated wastewaters, the exploited reactants, and the applied process conditions. In the present study, as abovementioned, the highest ammonium abatements were reached with \(n\)(N):\(n\)(Mg):\(n\)(P) = 1:1.2:1.2. In addition to NH\({}_{4}\)\({}^{+}\) abatement, the phosphorus recovery was monitored since this element is undesirable in treated wastewaters.