Removal of micropollutants contained in wastewaters by a hybrid system: Constructed wetland and Advanced oxidation process

Abstract

Water is a vital resource for life on Earth, with incalculable economic, social and environmental value. However, due to the ever-increasing demand for water and ongoing climate change, water scarcity has become the main challenge we face in the 21st century. Against this critical backdrop, it is expected that wastewater reuse will continue to be necessary to ensure that water demand is met for the foreseeable future. Water for agriculture accounts for approximately 70 % of total freshwater demand, and this percentage is as high as 90 % in some developing countries. Therefore, the reuse of wastewater in agriculture appears to be essential to reduce the share of freshwater used in this sector. However, the quality of this reclaimed wastewater must meet some minimum requirements to ensure its safe use as an alternative resource for crop irrigation, such as biochemical oxygen demand (BOD), turbidity and pathogens are defined as the main parameters to be monitored. However, wastewater may also contain micropollutants (MPs), as wastewater treatment plants are not designed to remove them. Moreover, they are not fully regulated. However, since the presence of these substances in water can be harmful to ecosystems and human health, it is expected that new quality criteria will soon be included in water reuse regulations related to this type of pollution. Due to this scenario, specific treatments are needed to remove these MPs and preserve the environment. Advanced Oxidation Processes (AOPs) are chemical processes that involve the generation of transient species, mainly the hydroxyl radical (•OH), and have demonstrated their efficiency in removing MPs. However, they are costly options. Constructed wetlands, on the other hand, are engineered systems that recreate conditions and processes occurring in nature and have been shown to be effective in improving water quality. These involve lower implementation and operating costs, but require more space, treatment time but are less efficient. In some cases, no studies have been carried out on the possible efficiency of a hybrid process combining AOP and constructed wetlands (CWs). For this purpose, this study will investigate a hybrid process combining a constructed wetland with different advanced oxidation processes (solar photo-Fenton at neutral pH and UVC/H2O2) to treat different types of effluents (different physicochemical characteristics) containing MCP from different families. To carry out the research, 3 laboratory-scale wetlands were constructed, one for each waste effluent to be studied (MBR, MIX, IFAS). The effluents resulting from this first stage (CWs) will be used to carry out the two advanced oxidation processes. In addition, these processes will also be carried out without pretreatment of the CWs in order to investigate which would be the most efficient combination depending on the wastewater to be treated. Therefore, the goal of this combination is to make the treatment still more efficient and environmentally friendly and to explore the possibility of reusing the treated effluent for agricultural purposes. After all the research, we can conclude that for the less organic matter content effluent (MBR) the hybrid process will not be optimal as it becomes more turbid after passing through the CW and to achieve a higher removal rate only an AOP would be needed. (70 % CW+AOP-80 % AOP). On the other hand, for the mixed water and IFAS, being much more turbid, with more organic matter, nitrites, more alkalinity, the hybrid process was optimal as it considerably reduces the concentrations of the MPs, eliminating some completely, suspended solids(80 % IFAS), BOD and nitrites in the pre-treatment, so the effluent arrives much less turbid and the photolysis of the AOPs is much more efficient than without the pre-treatment. Therefore, it was concluded that the mixed water is the optimal water for the hybrid process as it has the highest removal percentages of MPs with 97.2 %. Finally, a survey will be conducted to find out if people are willing to live with this recycled water and implement it in their lives.