## 3 Nutrients Balance and Distribution into Liquid and Solid Digestate during Anaerobic Digestion  
AD is an appropriate technology to convert agricultural residues into biogas [37] and to transform the nutrients contained in the substrates into plant-available forms so that the final digestate can be used as fertilizers [43, 44]. The effective use of nutrients available in the digestate is a way to improve the overall efficiency of AD technology [37]. The mass balance of nutrients in the AD is crucial for the quantification of their recovery potential [45]. It is also essential to evaluate the fate of nutrients like N, P, K, Ca, Mg, and Sulfur (S) and their distribution into LD and SD to assess the agronomic, environmental, and economic benefits of AD technology [46]. The mass balance of nutrients has been investigated in the AD of different substrates [37, 47, 48, 49, 46]. The recovery rate (RR) of nutrients is defined as the amount of nutrients available in the digestate after the AD process relative to their initial value (before the AD) and expressed as a percentage (%) [37]. The RR is usually calculated by the authors and provided in the studies. Various substrates have been used in the studies for the investigation of the mass balance of nutrients. The categories are defined here on the basis of the nature of the substrates. The category \"straws\" consists of rice, corn, and wheat straw while, \"manures\" contains swine, cattle, and chicken manure. The RR of N from various studies using different substrates is summarized in Table 3. N showed up to 82% RR for different substrates used in these various experiments at different scales, specifically, 93-100% for manures [37, 46], 98.4% for pig slurry (PS) [47], 85-91% for straws [37], 90.8% for organic fraction of municipal waste (OFMSW) [48], and 82-94% for mixed substrates [48, 49]. Total N is often reported to be conserved during AD [43]. However, the primary cause of less than \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**References** & **Unit** & **P** & **Ca** & **Mg** & **K** & **Na** & **Ni** & **Cu** & **Zn** & **Fe** \\ \hline
[36] & (mg/kgTS) & 1082 + 118 & 6669 + 17 & 1572 + 12 & 9676 + 23 & 712 + 11 & 4.4 \(\pm\) 0.6 & 18.3 \(\pm\) 1.2 & 54.3 \(\pm\) 1.6 & 320 + 24 \\
[37] & (mg/kgTS) & - & 1940 + 23 & 990 + 10 & 25,050 + 13 & 1230 + 80 & - & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Nutrients and TEs in RS reported in the literature. (“\(\pm\)” stands for the standard deviation of the mean as reported by the authors). Figure 2: Statistical evaluation of RS characteristics reported in the literature. “n” is the number of values to calculate the median, maximum, minimum, and quartiles [10, 26, 28, 36, 37, 38, 39, 40, 41, 42]. 100% RR of N may be gaseous NH\({}_{3}\) migration with the biogas flux [48; 49]. The RRs of N in these studies are comparable; however, the results of one study are contradictory, as the amount of total N in the digestate after AD was 8-23% higher than before AD [45]. The authors explained that the amount of hydrolyzed microalgae and the quantity of N released to the aqueous phase might have been overestimated since all the parameters were calculated as the difference between the microalgae tests and the control test [45]. There is also a wide range (64-100%) of RRs of P when the different substrates are used in AD (Table 3). A RR of P of up to 91% during AD is not considered far from 100% [37], while up to 75% [46] and 64% [47] of P RR have been observed in two studies. The retention of the sludge in psychrophilic anaerobic sequencing batch reactors (PASBR) could be the possible reason for such a low RR of P [46]. In another study, crystal lining was observed on the opening of the digesters, and scanning electron microscopy (SEM-EDS) clearly showed them to be composed of P, Ca, Mg, and Mn (Manganese) [47]. K is conserved in the digestate in all these studies during the AD as shown in Table 3.  
There are a few studies found addressing the mass balance of Ca, Mg, and S in AD.