# Engineering Hydroponic Systems for Sustainable Wastewater Treatment and Plant Growth
## 1 Introduction  
The urbanization trend has caused a spatial shift in human habitation, impacting the capacity of cities to match the historical ecological support once provided by rural areas through ecological services (ESs) [1] (which are products of nature, directly enjoyed, consumed, or used to yield human well-being [2]. Urban areas face ongoing challenges such as water scarcity, nutrient-depleted soils, food miles, and pollution, which are expected to intensify with population growth and climate change [3].  
To address these challenges and meet the growing needs of urban populations, innovative approaches are essential, as cities heavily depend on ESs for vital benefits such as water and air purification [4]. Sewage mining offers an ecologically and cost-effective decentralized approach for effluent treatment, mitigation of eutrophication, and local cultivation of valuable crops. Wetlands, known as \"nature's kidneys,\" serve as ESs for solar-driven, decentralized photoremediation and nutrient extraction from waste streams [5]. However,constructed wetlands face limitations in urban settings, such as the requirement for large horizontal space, mainly relying on anaerobic processes [6], and being limited in plant choices to those with roots adapted to low-oxygen environments [7].  
To overcome these limitations, a vertical wetland engineered on the facades of buildings, which increases oxygen to the root zone, is proposed. To do this, we need to ecologically engineer the wetland ES in a systematic approach. Hydroponics, a soilless method of agriculture, have traditionally been used to maximize crop yields by optimizing plant root access to nutrients and oxygen [8]. As a significant part of wastewater treatment in constructed wetlands occurs in the root zone [5], a hydroponic system offers a potential answer to wastewater that can be effectively treated to meet legislative standards using different hydroponic systems (Figure S1 in Supplementary Materials) [9; 10; 11]. Additionally, this approach enables the cultivation of food crops and oxygen-demanding plants, establishing an urban vertical farm. It promotes at-source treatment, reduces food transportation distances, facilitates nutrient recycling, and fosters circularity.  
[PERSON] [12] reported a 48% nitrogen removal efficiency with a gravel and ebb-flow system from brewery wastewater. [PERSON] et al. [13] demonstrated the effectiveness of the nutrient film technique (NFT) system for domestic sewage treatment through biofilm establishment [14]. However, [PERSON] et al. [15] found that the hydroponic system failed to meet the required phosphorus (P) and nitrogen (N) removal levels due to insufficient dissolved oxygen (DO). High biochemical oxygen demand (BOD) hindered the nitrification process by creating oxygen competition between nitriving bacteria and other microorganisms. Significant nitrification occurred only when BOD was below 45 mg L\({}^{-1}\)[15]. Limited research exists on the application of aeropoonics for wastewater treatment and the effects of different hydroponic methods on DO levels, plant growth, and microorganisms [9; 10; 15]. Oxygen and nutrient access in the rhizosphere are the key property for maximizing plant growth and wastewater treatment efficiency in these systems [16; 17; 18; 19]. Therefore, this research aimed to investigate the limiting criteria for such a hydroponic system, specifically access to oxygen. The experiment discussed in this paper aims to investigate how different methods of hydroponics influenced the accessibility of oxygen to the rhizosphere. To achieve this, the specific objectives of the experiment were to (1) confirm that oxygen is the key property for the growth of the plants and treatment of secondary wastewater, and (2) analyze how different hydroponic systems treat and affect growth in plants.