# Engineering Hydroponic Systems for Sustainable Wastewater Treatment and Plant Growth
## 4 Discussion
The growth of oxidizing bacteria and decreased ammonium oxidation rates contribute to higher DO levels, observed in most systems except for the rice experiment, where continuous DO increase is attributed to oxygen transfer from aerial parts to roots, causing a leaching effect in the rhizosphere [31; 32; 33]. The DO concentration increases through surface interaction with the atmosphere [34]. In hydroponic systems, the water movement increases DO compared to stagnant water, such as in wetlands [35]. This increased availability of DO improved efficiency of biofilm oxidation of organic matter and increased survival chances for non-flood-tolerant plants. Successful plant growth of all systems affirms the stated advantages, while the reduced frequency of sewage-air interaction in the EF system led to lower DO. The AP and MT systems, utilizing mist generation, exhibited enhanced root development. The observed rapid growth in the MT and AP systems resulted in the blockage and malfunction of the mixer (Figure S12) and spray nozzle, respectively. In the MT system, root growth obstructed the outlet (Figure S6), leading to flow impeding and initial tomato run flooding. Extensive root growth in these systems was facilitated by the mist zone and the presence of air, especially oxygen (Figure 6). Figure 6: Schematics of the AP (**a**) and MT (**b**) systems showing the zone of moisture that encourage the growth of the rhizosphere. NFT (**c**) system showing the stream of sewage above the rhizosphere zone. A schematic of an FF (**d**) system showing that the rhizosphere was smothered in sewage restricting its access to oxygen. The AP and MT systems create a mist zone that provides oxygen and nutrients to the roots and biofilm through microdroplets, ensuring proper moisture in the rhizosphere [36]. However, inadequate root length in our experiment hindered sewage access [28]. Conversely, the FF system showed excess moisture but limited oxygen, smothering the rhizosphere (Figure 6d). Rhizosphere narcosis (Figure S13) occurred in plant samples with minimal development, while the EF system differed from FF, allowing adjustment of oxygen-moisture ratio through drainage and flood cycles. Ensuring adequate root volume is essential in hydroponic systems to facilitate maximize operation and promote biofilm development. Factors such as root architecture, dimension, surface area, and presence of root hairs influence biofilm formation and nutrient absorption [28, 37]. Oxygen availability is critical for ideal root growth, with environments with abundance of both oxygen and moisture supporting the best results. AP and MT systems demonstrated uniform root development, while NFT systems concentrated roots at flow surfaces. In water-limited environments, oxygen availability plays a vital role by providing increased access to oxygen and nutrients for root uptake. The rhizosphere of the root system facilitates the breakdown of organic matter through the interaction of water droplets, leading to the production of ammonium and nitrate. These compounds, along with water and oxygen, are assimilated by the root system. In the design of a vertical urban farm, a vertical AP system is employed, incorporating an internal waterfall and fine droplets (Figure 7) to optimize this process. Our findings highlight the significance of oxygen availability in the degradation of organics and root health, thereby influencing plant growth and symbiotic biofilm formation. In constructed wetland systems, oxygen is scarce, but plants survive due to oxygen transport via aerenchyma, supporting some biofilm in the rhizosphere [5]. In contrast, hydroponic systems provide ample oxygen and DO, leading to accelerated aerobic oxidation of organics compared to anaerobic degradation [38].