# Anaerobic Digestion of Duckweed Used to Remediate Water Contaminated with Zinc and Ammonium
## 4 Discussion  
### Phytoremediation of Zn/NH\({}_{4}\)\({}^{+}\)-Polluted Water by Duckweed  
#### 4.1.1 Influence of Zn on Duckweed Growth and Zn Removal Efficiency  
This study showed that duckweed growth was seriously affected by the presence of the pollutants Zn and NH\({}_{4}\)\({}^{+}\) in the growth medium. Figure 2a shows that duckweed was able to grow at a Zn concentration of 5.0 mg/L, indicating that duckweed can be used for Zn accumulation; however, both Zn concentrations of 2.5 mg/L and 5.0 mg/L seriously decreased the growth rate compared to the control. Zn directly destroys the structure and function of the chloroplast and inhibits overall chlorophyll biosynthesis. Zn also enters the fond chloroplast and can damage the chloroplast membranes due to oxidative stress via peroxidation [52; 53]. In addition, Zn may inhibit the uptake and transportation of manganese and iron by antagonistic effects [53], which inhibits duckweed growth.  
Previous work showed that Zn concentrations of 4 mg/L inhibited _Lemma gibba_ (L.) growth by 50% and 10 mg/L Zn reduced the specific biomass growth of _Lemma gibba_ by 90%, while less than 1.82 mg/L Zn promoted the growth of _Lemma minor_ (L.) [54]. The addition of Zn reduced the growth rate of _Lemma minor_ by 15.5% and 56.6% when the Zn concentration increased from 0.205 mg/L to 0.819 mg/L [55]. [PERSON] et al. also observed a reduction in the growth rate of _Lemma minor_ by 70% when Zn concentrations increased from 0 to 10 mg/L [56]. [PERSON] et al. found a reduction in the dry weight of _Lemma minor_ from 0.12 g to 0.05 g for 5.0 mg/L Zn [57]. Hence, duckweed has potential for the remediation of Zn-polluted water, while the growth rate parameter indicates that duckweed is sensitive to elevated Zn levels.  
Zn can be removed from water by duckweed bioaccumulation or chemical precipitation (Table 3). At a Zn concentration of 5.0 mg/L, on the 7 th day, precipitation could have  
Figure 8: Variations in the concentrations of volatile fatty acids (VFAs) in different Zn-enriched duckweed solid fractions from (**a**) T1, (**b**) T2, (**c**) T3, (**d**) T4, (**e**) T5, and (**f**) T6. Error bars show the standard deviation.  
played an important role in Zn removal by producing Zn\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)[58], resulting in a low Zn recovery efficiency and greater removal efficiency by duckweed (Figure 2b). Meanwhile, a Zn concentration of 5.0 mg/L inhibited duckweed growth and reduced the fresh biomass production (Figure 2), resulting in a lower Zn recovery efficiency. [PERSON] and [PERSON] observed that _Lemma minor_ showed the maximum accumulation of 10 mg/L of Zn after 4 days, and the Zn removal efficiency was approximately 70-80% [53]. [PERSON] and [PERSON] found the highest removal efficiency for _Lemma trisulca_ (97%) as compared to _Lemma minuta_ (89%) and _Lemma minor_ (83%) for 10 days of incubation with 1, 5, 10, 15, and 20 mg/L Zn [59]. [PERSON] and [PERSON] found that the duckweed _Lemma gibba_ removed 61-71% of Zn after 7 days of exposure [58]. Figure 2 shows that a removal efficiency of more than 82% was achieved when the hybrid of _Lemma minor_ and _Lemma turionifera_ in this study was grown in the presence of solely Zn. This indicates that duckweed is a significant option for the phytoremediation of Zn-contaminated waters.