# 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
#### 3.3.1 Methane Production
In the solid fraction of duckweed from T4, a higher increment in the methane production was observed from the 24 th day of incubation onwards. The lowest methane production was noticed in the solid fraction of duckweed from T3, suggesting that a higher Zn-enriched duckweed solid fraction inhibited methanogenesis. These data reveal that Zn contamination reduced the anaerobic digestibility of duckweed, particularly in the solid fractions, but NH\({}_{4+}\)-N could partially offset Zn's negative effects. The liquid fractions provided superior methane yields, emphasizing the importance of effective pretreatment to solubilize organics. These findings suggest that Zn-enriched duckweed can still be utilized for methane production though the optimization of pretreatment, and nutrient balance is key for unlocking its full bioenergy potential. Figure 5: Methane production from pre-treatment of different Zn-enriched duckweed samples and Zn concentration variation on the methane production using the solid (**a**,**c**) and liquid (**b**,**d**) fractions. Error bars show the standard deviation. Table 2 shows the kinetic parameters of methane production from the duckweed, derived by the modified Gompertz model. The modified Gompertz model fitted the methane production data of all the conditions tested very well (R\({}^{2}\) \(>\) 0.94). The maximum methane yield and methane production rate for T5 and T6 were higher than those for duckweed from T2 and T3. The liquid fractions of duckweed from T1 had a maximum methane yield (M\({}_{\rm max}\)) of 194.689 mL/g COD, a maximum methane production rate (R\({}_{\rm m}\)) of 18.936 mL/g COD d, a lag phase (\(\lambda\)) of 0.174 days, and a smaller peak time of 3.956 days. A maximum methane yield (M\({}_{\rm max}\)) of 112.207 mL/g COD and maximum methane production rate (R\({}_{\rm m}\)) of 5.448 mL/g COD d were predicted for T2, and the lowest maximum methane yield (95.569 mL/g COD) and a maximum methane production rate of 6.097 mL/g COD d were calculated for the liquid fraction from T3. For duckweed from the treatments with NH\({}_{4}^{+}\)-N (T4, T5, and T6), the liquid fraction from T4 had a maximum methane yield (M\({}_{\rm max}\)) of 141.182 mL/g COD and maximum methane production rate (R\({}_{\rm m}\)) of 8.309 mL/g COD d, whereas maximum methane yields (M\({}_{\rm max}\)) of 117.230 mL/g COD and 126.647 mL/g COD and maximum methane production rates (R\({}_{\rm m}\)) of 7.749 mL/g COD d and 6.584 mL/g COD d were estimated for the liquid fractions from T5 and T6, respectively. Compared to the liquid fractions, the kinetics of the duckweed solid fractions from all treatments were slower. Among the solid fractions of all treatments, the lowest maximum methane yield (M\({}_{\rm max}\)) of 28.130 mL/g COD, a maximum methane production rate (R\({}_{\rm m}\)) of 3.039 mL/g COD d, and a longer lag phase (\(\lambda\)) of 11.126 days were observed for duckweed from T3. The solid fraction of pretreated duckweed from T6 had the fastest kinetics, with a maximum methane yield (M\({}_{\rm max}\)) of 56.395 mL/g COD, a maximum methane production rate (R\({}_{\rm m}\)) of 2.316 mL/g COD d, and a lag phase (\(\lambda\)) of 2.922 days. The highest maximum methane yield (M\({}_{\rm max}\)) of 153.409 mL/g COD, a maximum methane production rate (R\({}_{\rm m}\)) of 3.364 mL/g COD d, and the longest lag phase (\(\lambda\)) of 11.763 days were calculated for the solid fraction of duckweed from T4. The kinetic analysis reaffirms earlier trends: (a) Zn negatively affected methane production, with the liquid fraction of uncontaminated duckweed (T1) being the most efficient substrate for anaerobic digestion; (b) NH\({}_{4}\)+-N showed the partial mitigation of Zn's effects, but not enough to match the control; (c) solid fractions showed significantly slower