# 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
This section underscores the importance of monitoring COD dynamics in both phases during the anaerobic digestion of pretreated duckweed. While the system demonstrated a good performance in liquid-phase COD removal, especially under T1 and NH4+-N co-treatments, the persistent high COD in solids reflects limitations in hydrolysis and suggests that Zn, while partially mitigated by NH4+-N, still imposes constraints on biomass degradation. These findings pose the need for enhanced pretreatment or digester design when using metal-rich biomass. pH Variation During Anaerobic Digestion  
In all liquid fractions (Figure 6d), the pH increased with fluctuation during the entire incubation period, and the pH values were higher than 7 from the 4 th day onwards. The pH in the liquid fraction of duckweed from T1 was higher than that of the other liquid fractions during the incubation. The pH in the liquid fraction of duckweed from T5 and T6  
Figure 6: Variations in COD and pH in the solid (**a,c**) and liquid (**b,d**) fractions of Zn-enriched duckweed used for methane production after acid hydrothermal pre-treatments. Error bars show the standard deviation. was similar, and was higher than that of duckweed from T2 and T3, whereas the pH in the liquid fractions of duckweed from T2 and T3 was also similar. The pH fluctuated during the AD digestion tests with the solid fractions of duckweed from T1, T2, T3, and T4, whereas the pH values decreased initially and then increased in the solid fractions of duckweed from T5 and T6 during the AD tests (Figure 6c). The pH of the solid fractions of duckweed from T1, T2, and T4 was higher than that of duckweed from T3, T5, and T6 from the 4 th day onwards, while a difference in pH between the incubations with duckweed from T3, T4, and T6 was noticed. The highest pH was found in the solid fraction of duckweed from T4 from the 18 th day onwards, followed by duckweed from T1 and T2. Except the initial pH, the pH values were below 7 in all solid fractions. On the 4 th day, the minimum pH values were observed in duckweed from T1 (5.63 \(\pm\) 0.19), T2 (5.45 \(\pm\) 0.33), T3 (5.16 \(\pm\) 0.02), and T4 (5.81 \(\pm\) 0.22), and on the 7 th day in duckweed from T5 (5.15 \(\pm\) 0.02) and T6 (5.13 \(\pm\) 0.04). By the end of the incubation, the pH was 6.33 (\(\pm\)0.01) (duckweed from T1), 6.15 (\(\pm\)0.19) (duckweed from T2), 5.80 (\(\pm\)0.08) (duckweed from T3), 6.70 (\(\pm\)0.09) (duckweed from T4), 5.72 (\(\pm\)0.23) (duckweed from T5), and 5.76 (\(\pm\)0.11) (duckweed from T6). This section effectively highlights the pH challenges posed by Zn contamination in duckweed and the potential of NH\({}_{4+}\)-N supplementation to stabilize digestion conditions. While liquid fractions generally maintained a favorable pH for methane production, solid fractions were acidified, pointing to incomplete or unbalanced digestion processes. These findings underscore the importance of pretreatment strategies and pH control in maximizing the efficiency of anaerobic digestion using metal-enriched biomass. The variations in VFAs produced by the liquid (Figure 7) and solid fractions (Figure 8), which included formic acid (FA), acetic acid (AA), propionic acid (PA), isobutyric acid (IBA), butyric acid (BA), isovaleric acid (IVA), valeric acid (VA), isocaproic acid (ICA), and caproic acid (CA), were investigated during the entire AD incubation period. All VFAs in the liquid fractions of the AD tests were significantly lower than the AD tests with the solid fractions: (a) lower FA, AA, ICA, and CA concentrations were detected in the liquid fractions from the T1, T2, and T3 incubations; (b) lower AA and CA concentrations were detected in the liquid fractions from T4; and (c) lower FA, AA, and CA concentrations were detected in the liquid fractions from the T5 and T6 incubations, in which 9.28 (\(\pm\)5.48) mg/L and 17.18 (\(\pm\)4.94) mg/L FA was found on the 14 th day (Figure 7). CA was present at the highest concentration among the VFAs in each reactor.