Nature-based solutions as a pretreatment to enhance the removal of contaminant of emerging concern in wastewater

Abstract

The Sustainable Development Goal (SDG) number 6, established by the United Nations in 2015 as part of Agenda 2030, aims to ensure availability and sustainable management of water and sanitation for all. However, according to the United Nations' 2021 World Water Development Report, the current state of water is significantly far from achieving this goal. More than 2 billion people live in countries facing constant water stress, and approximately 4 billion suffer severe physical water scarcity for at least one month per year. Factors such as population growth, socio-economic development, and changes in consumption patterns are expected to increase water demand by 50% to 80% in the coming decades. Moreover, accelerated climate change could exacerbate this situation by rapidly reducing water availability globally. In response to this scenario, the reuse of wastewater emerges as a crucial sustainable development strategy to address the scarcity crisis. However, it is essential that wastewater undergoes proper treatment to remove all harmful elements resulting from various human activities. Emerging microcontaminants such as pharmaceuticals, pesticides, personal care products, and steroid hormones are examples of substances that must be eliminated from water bodies, despite being detected at very low concentrations (from ng/L to μg/L). Conventional wastewater treatment systems were not initially designed to completely remove these persistent compounds, underscoring the need to implement additional technologies such as advanced oxidation processes, activated carbon adsorption, or membrane filtration to significantly enhance their removal. Constructed Wetlands (CWs) are promising nature-based technologies for removing various types of microcontaminants due to their simplicity, low investment and operational costs. These wetlands are periodically flooded flat land areas with aquatic plants acting as natural filters. Through mechanisms like biodegradation, phytodegradation, photodegradation, rhizofiltration, and other processes, CWs can effectively eliminate heavy metals, nutrients, and organic matter.However, they face challenges such as long retention times, large space requirements, and may not be suitable for certain compounds unaffected by biological or adsorption processes. To improve the efficiency of the CWs and overcome these challenges, it is proposed to combine them with advanced oxidation processes. This can enhance treatment efficiency by reducing the load of organic matter and suspended solids before the oxidation stage. Additionally, the use of a natural waste product from the food industry in the CWs will be investigated due to its high adsorption capacity, and the effect of recirculation in these systems will be studied to optimize contaminant removal and nitrification-denitrification. The goal is to develop a more efficient and environmentally friendly treatment, also exploring the possibility of reusing the treated effluents for agricultural irrigation. By adding a layer of almond shell as a natural adsorbent, layers of 2 and 4 cm, greater removal was observed compared to the HC without this layer, with removal increasing by 70% for the 2 cm layer and 80% for the 4 cm layer. Therefore, the thickness of this layer influences the removal of microcontaminants. On the other hand, values such as DOC and TSS were also reduced by an average of 45% and 80% respectively. Nitrite levels decreased by almost 100%, and nitrate levels increased in all CWs due to the presence of oxygen. Finally, it was observed that the removal performance of the hybrid system (CW + AOP) increased compared to the processes separately, achieving an average removal of between 85-96%. In this case, the organic adsorbent layer is not as significant as in the separate CWs.