# 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
No VFAs were detected in the liquid fraction of duckweed from T1 on the 24 th day and T3 on the 30 th day, respectively, and lower CA concentrations (8.55 \(\pm\) 2.28 mg/L) were found in the liquid fraction of duckweed from T2 on the 30 th day, indicating that the presence of Zn in the liquid fraction prolonged the VFAs' degradation time. Compared with duckweed cultured without NH\({}_{4}\)\({}^{+}\)-N, larger variations in the types of VFAs produced were observed in the liquid fractions of duckweed from the T4, T5, and T6 incubations. Only traces of AA were found from the 4 th day onwards, and AA disappeared in the liquid fractions of these three duckweed types by the 30 th day. CA was observed from the 4 th day onwards, and the CA concentrations in the AD tests with hydrolyses from T4, T5, and T6 duckweed reached 42.07 (\(\pm\)7.91) mg/L, 47.54 (\(\pm\)2.46) mg/L, and 33.11 (\(\pm\)2.75) mg/L, respectively, at the end of the incubation. In all the solid fractions, the VFAs constituted only lower concentrations of AA at the start of the AD experiment, and the highest concentration among the VFAs was AA in each vial during the incubation period (Figure 8). The lowest AA concentrations were found in duckweed from T4 in the whole incubation. FA was only detected in duckweed from T2 and T3. The only trace of FA was observed in the solid fraction of duckweed from T2 on the 30 th day, while FA in duckweed from T3 was noticed on the 11 th day and increased until the end of the incubation (86.22 \(\pm\) 9.66 mg/L). ICA was not detected in AD runs with the solid fraction of duckweed from T5, and only traces of ICA from T5 and T6 were observed during the incubation. AA, PA, IBA, BA, IVA, VA, ICA, and CA were detected throughout the 30 days of the operation (duckweed from T1, T2, and T3). The total VFA concentrations in the solid fraction from T1 were higher than those in duckweed from T2 and T3 during the initial 24 days, indicating that duckweed biomass with Zn inhibited VFAs' release, however, the types of VFAs increased by the end of these three incubations. For duckweed cultured in the presence of NH\({}_{4}\)^-N, the total VFAs in the fractions of duckweed from T5 and T6 were higher than those from T4, meaning that the presence of Zn enhanced the total VFAs under the applied experimental conditions. However, no difference in the total VFAs between duckweed from T5 and T6 was noticed. This section effectively shows that Zn toxicity suppresses early fermentation, especially in solid fractions, but microbial communities may partially adapt over time. NH\({}_{4}\)+-N acts as a modulator, helping to buffer or recover microbial activity. VFA accumulation, particularly FA and CA, in Zn-rich treatments also implies the inhibition of methanogenesis, which could limit methane yield and cause system instability if not managed. To optimize AD with Zn-rich duckweed, the careful control of NH\({}_{4+}\)-N levels, monitoring of VFA profiles, and possibly targeted inoculation strategies are, thus, essential. Figure 7: Variations in volatile fatty acid (VFA) concentrations in different Zn-enriched duckweed liquid fractions from (**a**) T1, (**b**) T2, (**c**) T3, (**d**) T4, (**e**) T5, and (**f**) T6. Error bars show standard deviation.