## 3 Nutrients Balance and Distribution into Liquid and Solid Digestate during Anaerobic Digestion
For example, on average, 8.7% of Ca, 18.7% of Mg, and 67.7% of S were retained in the bioreactors. However, the authors did not find statistically significant Ca and Mg retention in the digesters except for S [46]. Significant retention of Ca and Mg with 44% and 32.5% has been found in another study as they crystallize as carbonates and phosphates [47]. [PERSON] et al. [37] investigated the mass balance of nutrients (N and P) during the AD of RS in batch experiments. The RRs of N and P were 85-91% and 91-97%, respectively. The mass balance of other nutrients (K, Ca, Mg, and S) were not investigated in this study. This is the only study found in which RS was used and mass balance of nutrients (N and P) was performed, thus indicating the scarcity of nutrient mass balance on AD of RS. The ranges of all nutrients' RRs for specific substrates are shown in Table 3. The results mentioned in Table 3 are reported as a single value or as a range in the studies. The arithmetic mean is first determined based on the minimum and maximum values if the data points in the study are available as a range. Then these means are further used to calculate the overall average RR of N and P in the AD irrespective of a specific substrate. The results of 12 and 14 experiments are used to calculate the average RR of N and P, respectively, in AD, which is shown in Figure 3. It is assumed that the RR of N is not 100% due to its release over the gas phase, whereas P is suspected to crystalize with other elements and remains on the reactor walls [47]. The overall average RRs of Ca, Mg, and S was not calculated because only two studies were available. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Substrates** & **RR of N** & **RR of P** & **RR of K** & **RR of Ca** & **RR of Mg** & **RR of S** & **References** \\ \hline Straws & 85–91\% & 91–97\% & - & - & - & - & [37] \\ Manures & 93–100\% & 74.5–97\% & 101.9\% & 91.3\% & 81.3\% & 32.3\% & [37; 46] \\ PS & 98.4\% & 64\% & 106.2\% & 56\% & 67.5\% & 83.4\% & [47] \\ OFMSW & 90.8\% & 94\% & 94\% & - & - & - & [48] \\ Mixed substrates & 82–94\% & 70–94\% & 95–98\% & - & - & - & [48; 49] \\ Microalgae & 108–123\% & 99–100\% & - & - & - & - & [45] \\ \hline \hline
**RR, Recovery rate; AD**, Anaerobic digestion; **Straws** (Rice straw, com straw, wheat straw); **Manure** (Swine manure, cattle manure, chicken manure); **PS**, Pig slurry; **OFMSW**, Organic fraction of municipal solid waste; **Mixed substrates**, (swine manure, maize silage, triticale, sorghum, barley thresh from beer industry, molasses, bakery-industry waste, and olive mill sludge; swine manure, cow manure, milk whey, and rice culture by-products; cow manure, blood, fish, paper pulp, out of date beverages, and grease trap waste). & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: RR of nutrients during AD of various substrates. Concerning the effect of AD on the RR of nutrients, Figure 3 shows that 92.5% of N and 89.3% of P, on average, are recoverd. It can be concluded that N, P, and K are conserved in the digestate, as their RRs are close to 100%. The separation of digestate into LD and SD with the use of separation techniques like press screws have been considered simple and low-cost practices for the management of the digestate from biogas plants and have been widely used in the European Union [43]. It has been reported that LD has a higher concentration of N in the form of ammonia, and it can be used as a fertilizer to substitute mineral fertilizer to some extent [43]. Moreover, the N/P ratio tends to increase in the LD because of separation, therefore, reducing the accumulation of P in the soil, as it offers a more balanced nutrient content for the crops [50]. The resulting SD generally used in the soil as an organic amendment [51], and it is also exported to other nearby farms to provide the solution of the excess of nutrients problem in that farm [52]. [PERSON] et al.