# Anaerobic Digestion of Duckweed Used to Remediate Water Contaminated with Zinc and Ammonium
## 3 Results
### Methane Production Using the Solid and Liquid Fractions of the Duckweed
#### 3.3.2 Variation in Process Parameters  
Zn Removal in the Methane Production Process  
Figure 5c,d show the Zn concentrations in the solid and liquid fractions from different duckweed digestions. The highest Zn concentrations were found in the solid and liquid fractions of duckweed digestion from T3 (0.70 \(\pm\) 0.01 mg/L and 18.89 \(\pm\) 0.60 mg/L, respectively), followed by duckweed from T2 (0.42 \(\pm\) 0.12 mg/L and 11.32 \(\pm\) 0.95 mg/L, respectively) and duckweed from T6 (0.20 \(\pm\) 0.0 mg/L and 4.46 \(\pm\) 0.35 mg/L, respectively). This means that the adsorbed Zn content of duckweed was directly related to the zinc content of the solid and liquid fractions after pretreatment. During anaerobic digestion, the Zn concentrations sharply dropped in the solid and the liquid fractions of duckweed grown in the presence of Zn. The Zn concentrations in the solid and liquid fractions of duckweed cultured with Zn and NH\({}_{4}\)+-N were lower than those of duckweed grown in the presence of Zn (Figure 5c,d). The Zn concentrations in the solid and liquid fractions from all treatments were lower than 0.02 mg/L and 0.22 mg/L until the end of the incubation, respectively. The removal efficiencies of Zn in the solid fractions of duckweed from T2, T3, T5, and T6 were 92.63% (\(\pm\)2.33%), 90.50% (\(\pm\)1.26%), 98.22% (\(\pm\)1.23%), and 96.59% (\(\pm\)3.13%) on the 4 th day, respectively, and their removal efficiencies in the liquid fractions were 99.65% (\(\pm\)0.10%), 99.72% (\(\pm\)0.10%), 96.22% (\(\pm\)0.21%), and 91.15% (\(\pm\)1.77%) on the 4 th day, respectively, indicating that Zn was likely removed from the incubation medium by sulfide precipitation [51]. A removal efficiency above 91% was achieved. However, the Zn concentrations fluctuated in the solid and liquid fractions throughout 30 days, indicating that Zn was released due to the microbial degradation of duckweed biomass in the solid fractions and the solubilization of Zn precipitates in the liquid fractions, and was later used by the anaerobic sludge towards the end of the incubation. This section highlights the strong Zn removal capability of anaerobic digestion systems using duckweed biomass, especially in the presence of sulfide-generating conditions. Zn is mostly removed in the early stages via chemical precipitation, but the biological degradation of Zn-loaded biomass can lead to temporary re-mobilization, underscoring the need for monitoring metal dynamics throughout the digestion process. These insights are crucial for scaling up AD systems where metal-laden biomass is used and for ensuring biosafety in digestate management. COD Variation in the Methane Production Processes  
Figure 6a,b present the COD concentration in the medium in both the solid and liquid fractions. For the liquid fractions, the COD concentrations of all treatments initially decreased rapidly and fluctuated until the end of the incubation (Figure 6b). The COD removal efficiencies in all liquid fractions amounted to 61.37-81.34%. The lowest COD concentrations of duckweed were found in T1 throughout the entire incubation. The COD concentrations in the solid fractions of duckweed cultured with solely Zn were less than those in the solid fraction of control duckweed from the 4 th day onwards, indicating that Zn affected the release of organic matter from the solid fraction of the pretreated duckweed biomass. The COD concentrations in the solid fractions of duckweed from T1, T2, and T3 were higher than those of duckweed from T4, T5, and T6 throughout the entire incubation period (Figure 6a). From the 7 th day onwards, lower COD concentrations were observed in the solid fraction of duckweed from T4, followed by the solid fractions in the duckweed from T5 and T6. However, the COD concentrations in the solid fractions of duckweed from T5 and T6 were similar during the incubation period. The COD concentrations were more than 3270 mg/L in AD digestion tests with the solid fractions at the end of incubation (Figure 6a).