# Anaerobic Digestion of Duckweed Used to Remediate Water Contaminated with Zinc and Ammonium
## 5 Conclusions  
This research showed that duckweed can accumulate a high amount of Zn, indicating that phytoremediation using duckweed can be an effective method. However, the presence of NH\({}_{4}\)+-N inhibited Zn uptake. The study subsequently showed the feasibility of using acid-pretreated Zn-enriched duckweed biomass for biomethane production by anaerobic digestion. The acid pretreatment of the duckweed resulted in a liquid hydrolysate with high Zn concentrations that inhibited methane production. The residual solid fraction had low methane production because of the fast acidification of the incubation medium to about pH 5. Hence, liquid fractions of Zn-enriched pretreated duckweed could be used as substrates for methane production by AD, while the AD of solid fractions requires appropriate neutralization of the pH during anaerobic conversion. This research offers a practical blueprint for advancing both wastewater treatment and renewable energy generation through an integrated duckweed-based system. By cultivating duckweed in zinc- and ammonium-contaminated wastewater, this process enables the simultaneous removal of heavy metals and nutrients-two major classes of pollutants that frequently co-occur in agricultural, industrial, and livestock effluents. This phytoremediation step not only purifies the water, but also results in a nutrient- and metal-rich biomass. Rather than discarding this biomass, our approach valorizes it through acid hydrolysis followed by anaerobic digestion (AD), producing methane-rich biogas that can be used as a renewable energy source. This integration transforms a treatment challenge into a circular system that recovers both purified water and energy. A key practical insight from this work is the need for the pH neutralization of the acid-hydrolyzed solids before AD, as residual acidity can inhibit methane production. The study also highlights that high zinc concentrations in the hydrolysate can suppress biogas yields, underlining the importance of monitoring and managing metal content throughout the process. Importantly, the research reveals that most zinc remains in the digestate after AD, raising environmental concerns if the digestate is to be used as fertilizer. This finding points to the necessity of additional steps for zinc recovery or stabilization to prevent secondary pollution. For practitioners, this research provides actionable recommendations for optimizing integrated phytoremediation-biogas systems, including process control for pH and metals and careful management of the digestate. For policy makers and engineers, it demonstrates a scalable, sustainable approach to circular wastewater management that aligns with environmental protection and resource recovery goals, while also identifying future research needs in digestate treatment and metal recovery technologies. Conceptualization, Y.Z., X.Z. and A.H.; methodology, Y.Z. ; investigation Y.Z., formal analysis, Y.Z. ; resources, Y.Z. ; data curation, Y.Z. and A.H.; writing--original draft preparation, Y.Z. ; writing--review and editing, Y.Z., X.Z., A.H. and P.N.L.L. ; supervision, A.H. and P.N.L.L. ; project administration, P.N.L.L. ; funding acquisition, Y.Z. and P.N.L.L. All authors have read and agreed to the published version of the manuscript.