# Engineering Hydroponic Systems for Sustainable Wastewater Treatment and Plant Growth
## 5 Conclusions  
In pursuit of these objectives, the experiments successfully demonstrated that oxygen availability is indeed a crucial factor for both plant growth and wastewater treatment in hydroponic systems. Results revealed that different hydroponic systems exhibit varying abilities to facilitate oxygen accessibility to the rhizosphere. The experiments conducted using different hydroponic systems, including GP, NFT, MT, EF, FF, and AP, demonstrated significant growth in plant mass and length. Results indicated minor variations in pH, EC, and TDS over time, while turbidity and TOC levels decreased significantly in the presence of plants. BOD and COD levels were reduced similarly with and without plants, while ammonium levels decreased, and nitrate levels increased significantly in plant sys  
Figure 7: Utilizing a schematic inspired by the concept of plant growth on a waterfall, the AP hydronic system is transformed into a vertical AP system for vertical farming. tems. Moreover, notable increases in plant height and biomass were observed across all hydroponic systems, accompanied by higher levels of DO compared to the control. The hydroponic systems that provided a moist atmosphere through misting or aeroponics exhibited the greatest growth, confirming the importance of oxygen availability in the rhizosphere for plant growth and wastewater treatment. The MT and AP systems showed the greatest root growth due to their saturated environments. However, all systems exhibited overall growth and demonstrated the potential for plant cultivation in hydroponics. These findings underscore the importance of considering oxygenation strategies when designing hydroponic systems for optimal plant growth and wastewater treatment. DO levels in the wastewater were a critical parameter for the success of the hydroponic systems. While initially, all systems showed low DO levels, they quickly increased over time. The NFT system, contrary to expectations, showed higher DO levels compared to other systems during experiments without plants. The control system consistently showed lower DO levels, sometimes dropping close to the inhibitory level. The rice experiment showed different patterns with a temporary dip in DO levels before gradually increasing. Our results contribute to the development of sustainable urban vertical farming strategies by emphasizing the integration of ecological services and hydroponic systems for efficient wastewater treatment, resource recycling, and enhanced food production. Findings highlight the importance of optimizing access to both oxygen and nutrients in the rhizosphere to achieve maximum plant growth and wastewater treatment efficiency. Further research is crucial to comprehend the impact of hydroponic methods on DO and plant-microorganism interactions. This will enhance engineering criteria, enabling more efficient and sustainable wastewater treatment and urban farming solutions, ultimately contributing to circular cities and environmental sustainability. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app13148032/s1](https://www.mdpi.com/article/10.3390/app13148032/s1)]([https://www.mdpi.com/article/10.3390/app13148032/s1](https://www.mdpi.com/article/10.3390/app13148032/s1)). Figure S1. Diagrams of different hydroponic systems, NFT, DFT, aeroponic and fogponics. Figure S2. An illustration of the complete setup and the location of the different systems. Figure S3. An illustration of the proposed setup. The system that is gray is removed for illustrative purposes so that the system in color can be seen. The pipe is supported by its reservoir (colored) and that of the opposite system (gray). The pump pushed the water to the far end of the pipe. It flowed through the system via its specific method (illustrated is the MT system) and returned the other end to the reservoir by the return pipe. Figure S4. The six systems with tomato plants. Clearly, the fogponic system (MT) and the aeroponic system (AP) show the greatest growth. Figure S5.