# 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  
The applicability of Zn-enriched duckweed biomass as a feedstock in anaerobic digestion (AD) was tested with duckweed from six different treatments (T1, T2, T3, T4, T5, and T6). The solid fractions of duckweed from all pretreatments had lower methane production compared to the liquid fractions (Figure 5a vs. Figure 5b). Methane production from the liquid hydrolysate of T1 was significantly higher than that from the liquid fractions from the other treatments (\(p<0.05\)). Only a negligible increment in the methane production of the liquid fraction from T1 from the 24 th day onwards was observed (Figure 5b), and  
Figure 3: (**a**) SEM-EDX analysis of Zn-enriched duckweed from T1 (sample 1), T2 (sample 2), T3 (sample 3), T4 (sample 4), and T5 (sample 5). (**b**) SEM-EDX analysis of Zn-enriched duckweed from T6 (sample 6), T7 (sample 7), T8 (sample 8), and T9 (sample 9). the greatest value of methane production was 201.67 (\(\pm\)5.72) mL/g COD by the 30 th day of incubation. A decrease in methane production from the pretreated duckweed liquid hydrolysate from treatments with Zn or NH4+ was observed. Among the different incubations, the duckweed liquid fraction from T4 had a higher methane production than that from Zn-enriched duckweed liquid fractions (T2, T3, T5, and T6), indicating that the presence of Zn suppressed methane production. The methane production from pretreated duckweed hydrolysate from T5 and T6 was higher than that from T2 and T3, indicating that the liquid fraction from duckweed grown with NH4+-N produced more methane under the experimental conditions tested. No statistical difference in methane production from the liquid fractions in T5 and T6 was found (\(p\) > 0.05), and there was also no significant difference between the duckweed liquid fractions from T2 and T3, indicating that the same inhibition level in methanogenesis was observed in both treatments, with zinc at 2.5 and 5.0 mg/L. The duckweed liquid fractions from T2, T3, T5, and T6 on the 4 th day depicted 38.41 (\(\pm\)0.06) mL/g COD, 41.43 (\(\pm\)1.62) mL/g COD, 39.32 (\(\pm\)0.45) mL/g COD, and 44.54 (\(\pm\)6.42) mL/g COD, respectively, which increased to 75.90 (\(\pm\)4.39) mL/g COD, 74.26 (\(\pm\)5.60) mL/g COD, 91.75 (\(\pm\)10.27) mL/g COD, and 90.29 (\(\pm\)4.91) mL/g COD by the 14 th day, respectively. The methane production of duckweed liquid fractions from treatments with Zn increased with incubation time, and a maximum of 111.32 (\(\pm\)8.48) mL/g COD, 99.88 (\(\pm\)10.49) mL/g COD, 120.90 (\(\pm\)12.03) mL/g COD, and 129.82 (\(\pm\)10.65) mL/g COD was achieved in duckweed liquid fractions from T2, T3, T5, and T6 by the 30 th day of incubation, respectively. This meant that prolonging the incubation time could increase the overall methane production. Figure 4: FTIR spectra of dried Zn-enriched duckweed grown in the different treatments. The highest methane production achieved in pretreated duckweed solid fractions from T1, T2, T3, T4, T5, and T6 by the 30 th day of incubation amounted to 46.52 (\(\pm\)4.74) mL/g COD, 55.91 (\(\pm\)6.94) mL/g COD, 27.85 (\(\pm\)2.93) mL/g COD, 61.45 (\(\pm\)1.69) mL/g COD, 40.051 (\(\pm\)2.52) mL/g COD, and 48.23 (\(\pm\)3.30) mL/g COD, respectively (Figure 5a). Methane production in the duckweed solid fractions from T1 was significantly higher than that from the other treatments before the 18 th day of incubation (\(p<0.05\)). Methane production in the duckweed solid fraction from T6 was significantly greater than that from T2, T3, T4, and T5 before the 14 th day, while there was no difference in methane production between T2, T3, T4, and T5 during the initial 14 days (\(p>0.05\)). Except for duckweed from T4, the methane production in duckweed solid fractions from all treatments showed only a negligible increment from the 24 th day of incubation onwards.