# Anaerobic Digestion of Duckweed Used to Remediate Water Contaminated with Zinc and Ammonium
## 1 Introduction  
The sustainable management of wastewater containing both nutrients and heavy metals remains a critical environmental challenge worldwide [1; 2]. Livestock operations, particularly swine farming, generate large volumes of effluents rich in ammonium and phosphorus, as well as heavy metals such as zinc and copper, which are commonly added to animal feed but poorly assimilated by livestock [3; 4; 5]. Therefore, zinc appears at elevated concentrations in waste streams or the environment, such as in anaerobically digested swine wastewater (ADSW) (ca. 1.5-30 mg Zn/L) [6], in agricultural runoff, or in aquaculture wastewater (ca. 2-3 mg Zn/L) after the application of Zn minerals [7]. Zn-enriched wastewater not only affects the activities of aquatic organisms, but also results in excessive human Zn intake through the food chain. This can result in nonspecific gastrointestinal symptoms such as abdominal pain, diarrhea, nausea, and vomiting [8]. However, zinc is an essential trace element for all forms of life, playing a crucial role in a large number of enzyme functions, acting as a stabilizer of the molecular structures of subcellular constituents and membranes, and protecting biological structures from the damage caused by free radicals [9]. As an essential micronutrient for plant growth, Zn is an important cofactor for the enzyme carbonic anhydrase, which is essential to carbon uptake during photosynthesis and a component of many enzymes of cellular processes, including protein metabolism, gene expression, and chromosome construction, while also constructive to the synthesis of vitamin E [10; 11; 12]. However, excessive Zn concentrations not only have toxic effects in humans, but also in plants due to osmoregulatory disturbances and reduction in the metabolic activity of microorganisms in the plant rhizosphere [13; 14]. On the other hand, Zn deficiency results in plant deficiency and affects plant yields and the activities of rhizosphere microorganisms [15]. Traditional physicochemical and biological treatment methods often struggle to simultaneously remove nutrients and heavy metals, and may incur high operational costs or generate secondary pollution. In this context, phytoremediation, which is a plant-based and environmentally sustainable approach, is used to eliminate, recover, extract, sequester, or detoxify pollutants [16]. It is extensively used for pollutant removal from wastewater due to low installation and operation costs [17; 18]. As one of the smallest aquatic floating plants, duckweed--part of the _Lemnaceae_ family--is commonly used for the phytoremediation of industrial and municipal wastewater due to its rapid growth, high nutrient uptake capacity, tolerance to polluted environments, removal of micropollutants, and ease of cultivation and harvesting [19; 20]. Duckweed also has great potential as a feedstock to produce biofuel by fermentation and anaerobic digestion [21]. [PERSON] et al. reported high CH\({}_{4}\) yields (60-468 mL CH\({}_{4}\)/g) from the anaerobic digestion (AD) of pretreated duckweed [22]. Some studies have reported that acid pretreatment is preferable for obtaining a larger biofuel yield and produces higher-grade fuels [23; 24]. Some _Lemma_ species can accumulate Zn concentrations 2700 times higher than in the medium [25]. Trace amounts of Zn in anaerobic digestion systems are necessary to maintain the cellular metabolism of methanogens, while high Zn concentrations inhibit the growth of methanogens [26]. [PERSON] et al. showed that the excess release of Zn ions inhibited biogas production in the anaerobic digestion of cattle manure [27]. Duckweed has also a high tolerance for NH\({}_{4}\)\({}^{+}\)-N and is an ideal plant for nutrient capture from NH\({}_{4}\)\({}^{+}\)-N-rich wastewater [28]. However, exposure to high NH\({}_{4}\)\({}^{+}\)-N concentrations is harmful to duckweed [29].