# Engineering Hydroponic Systems for Sustainable Wastewater Treatment and Plant Growth
## 4 Discussion  
After the first week of the tomato and at the end of the ivy experiments, a dense mat of roots created a strong anchor and a large surface area for biofilm growth [26]. The complex root system posed challenges during plant removal, often necessitating cutting, while measured data offer a comprehensive representation of substantial system-wide growth rather than specific plant growth. Initial sewage samples exhibited high TSS and turbidity, but both decreased significantly within 48 h. After 72 h, levels were negligible. ANOVA confirmed a significant reduction (\(p\leq 0.001\)) in TSS and turbidity across all systems, including the control, which agrees with the literature [11; 13; 15]. Systems with plants showed significant (\(p\leq 0.05\)) reductions in TOC, as well as a decrease in ammonia levels (Tables S7 and S8). Ammonia oxidation to nitrite was faster in systems with plants. Adequate DO levels supported nitrification, resulting in decreased ammonium and increased nitrite and nitrate concentrations. It is expected that biofilms on roots aided in the nitrification process. Water plays a critical role in plant survival, but it can hinder gas exchange and result in oxygen deficiency in the rhizosphere [27]. Optimal water and oxygen access is vital for plant growth, giving an advantage to MT and AP systems that allow access to gaseous oxygen form, which is preferred, as saturated soil has limited access [28]. Despite an overall increase in theoretical saturated oxygen, the rice experiment showed lower levels, suggesting potential influences from wastewater flow, root structure, biofilm, and temperature [29]. No significant correlation was found between plant species and DO. Further research is necessary to elucidate these factors and address the existing knowledge gap. All systems started with a low DO; however, this increased rapidly over time, confirming the results for NFT found by [PERSON] [12]. Conversely, the C system consistently maintained lower DO levels, occasionally approaching inhibitory levels. Average DO across all systems was highest in the \"no plants\" experiment. No strong correlation was found between plant mass and DO levels. (Spearman correlation coefficient of 0.52, \(p\leq 0.05\)). Although the NFT system was anticipated to have the lowest DO levels, results indicated near saturation in the absence of plants. Contrary to expectations, the NFT system exhibited higher-than-anticipated DO levels, possibly due to turbulent water flow during \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & & **C** & **GP** & **NFT** & **MT** & **EF** & **FF** & **AP** \\ \hline \multirow{2}{*}{First 24 h} & Increase DO (\%) & 4.59 & 53.65 & 47.34 & 46.02 & 36.26 & 41.11 & 49.68 \\  & \(t\)-test & 0.13 & \(p\leq 0.05\) & \(p\leq 0.05\) & \(p\leq 0.05\) & \(p\leq 0.05\) & \(p\leq 0.05\) & \(p\leq 0.05\) \\ \hline \multirow{2}{*}{168 h} & Increase DO (\%) & 4.59 & 53.65 & 47.34 & 46.02 & 36.26 & 41.11 & 49.68 \\  & \(t\)-test & 0.16 & \(p\leq 0.05\) & \(p\leq 0.05\) & \(p\leq 0.05\) & \(p\leq 0.05\) & \(p\leq 0.05\) & \(p\leq 0.05\) \\ \hline \end{tabular} Note: geyser pump (GP), nutrient film technique (NFT), misting (MT), ebb and flow (EF), full flow (FF), aeroponics (AP), control (C).  
\end{table} 
Table 2: Average change of the DO for the first 24 h and the last 168 h of operation, and the \(p\)-values of the \(t\)-tests where \(p\leq 0.05\) was considered significant. return to the reservoir. Notably, the rice experiment displayed distinct patterns, with an initial decline in DO levels followed by a gradual increase throughout the entire duration compared to other runs. Roots require oxygen for respiration unless they are flood-tolerant [30]. Initially, wastewater with low DO levels enters due to high BOD. However, hydroponic systems create turbulence and agitation, increasing the water's surface area exposed to air and facilitating oxygenation, leading to a significant increase in DO levels.