Stormwater herbicides removal via advanced oxidation process

Abstract

Urban stormwater runoff possesses the properties of intermittent occurrence, unexpectable volume and variable pollution which lead to different environmental issues, including flooding and waterlogging, pollution transportation, damage to downstream and contamination of the receiving waters. On the other hand, the low-level contamination (relative to sewerage) and large volume supply of stormwater makes it suitable as an alternative water resource to relieve the water shortage in the urban areas. Stormwater harvesting is under the concept of Water Sensitive Urban Design (WSUD) trying to treat stormwater properly for the different end-uses (like irrigation, toilet flushing and even for the uses close to human contact). Several treatment technologies (e.g., biofilters, constructed wetlands) have already been implemented to purify the stormwater with effective performance prior to reuse. However, the refractory organic micropollutants (especially herbicides) presented resistance to these nature-based solutions by showing variable treatment outcomes. In order to provide harvested stormwater for end-uses with high quality requirement (e.g., close to human contact recreational waters), a reliable treatment technology for organic micropollutants is desired as a post-treatment method in the stormwater harvesting system. This thesis aims to develop advanced oxidation processes (AOPs), in particular, photoelectrochemical oxidation (PECO), as the post-WSUD treatment approach for stormwater using its oxidation capacity towards the refractory organic micropollutants. Following the technology development procedure, three steps have been conducted: (1) testing the feasibility of AOPs for stormwater herbicides treatment; (2) investigating the intrinsic mechanism in the stormwater herbicides degradation process; and (3) assessing the operation conditions impact towards PECO stormwater treatment system. Boron-doped diamond (BDD) anode was used in the preliminary lab-scale tests for the feasibility study of AOPs towards stormwater organic micropollutants (two representative herbicides, diuron and atrazine - selected as the target pollutants in the study). The results showed that the effective herbicides degradation could be achieved by PECO process under 5 V operation (which was regarded as the optimal voltage in the system). The positive impact coming from voltage increase has been found in the study. BDD showed a remarkably durable property with stable removal performance under challenging voltage application (9 V) without observed deterioration. The catalysts loading showed negligible effect in removal performance. While the thermal effect was observed as a supporting factor for the process (higher temperature supported the oxidation process). Since BDD is not a perfect choice for scaled-up implementation due to its high manufacture cost, carbon fiber anode was chosen in the following studies and operated under low voltage (2 V) to avoid the possible anode deterioration under high voltage application. In the mechanism investigation, the superoxide radicals were found to be the major reactive species in PECO process. Meanwhile, hydroxyl radicals and free chlorine also demonstrated supporting impact for the oxidation process. With the identified intermediate products, degradation pathways of diuron and atrazine were proposed for the first time for three AOPs (PECO, electrochemical oxidation (ECO) and photocatalytic oxidation (PCO)) in stormwater herbicides degradation process. PECO was certified to be the preferrable stormwater treatment technology with the ability for further oxidation reactions towards herbicides degradation compared with ECO and PCO. In the third study, a flow reactor was designed and used to test the impacts of operational conditions (flow rate, light intensity, and initial pollutant concentration) for PECO process. An obvious improvement was observed for flow rate towards removal performance, while the light intensity was found to influence atrazine removal only. The initial pollutant concentration study demonstrated the robust performance of PECO flow reactor towards herbicides removal under challenging (240 μg L-1) pollutant concentration condition. The real stormwater experiments suggested the possible impacts coming from the stormwater chemistry towards PECO process. Further based on the energy consumption analysis, high flow rate (610 mL min-1) and normal light intensity (100 mW cm-2) were regarded as the optimal operational conditions for flow reactor system. Also, the effective PECO degradation performance of herbicides under the real stormwater environment has been verified by using the stormwater collected from field as supporting electrolyte in the experiments. Overall, this thesis confirms PECO as a promising stormwater herbicides treatment technology (potentially for all organic micropollutants) to provide further purification for stormwater high-quality targets. It also discusses the implications for the practical implementation and points out the future research directions for the system optimization.