Predicting the Photolytic Removal of Emerging Contaminants in Wastewater Treatment Lagoons
Creators: name: Um, Michelle
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 400410
subject: Wastewater treatment processes

description: Contaminants of emerging concern (CECs) such as pharmaceuticals, pesticides and herbicides are increasingly prevalent in household and industrial waters and add strain on wastewater treatment facilities to treat or remove them prior to discharge to the natural environment. This can reduce the water quality of drinking water, recycled water used for irrigation of crops, and environmental water, resulting in unprecedented negative impacts on communities and ecosystems. Photodegradation or photolysis is a sunlight-driven degradative process of CECs that can occur naturally in surface waters, reducing their persistence and harm to the environment. While CEC photolysis is widely studied for natural environments such as marine, lake and river waters, there is limited knowledge about photolysis occurring in wastewater treatment systems, such as waste stabilisation ponds (WSPs). This research investigated the direct photolysis of four CECs by considering the application of the findings for Melbourne Water’s Western Treatment Plant (WTP) which utilises 10,500 hectares of surface area of WSPs. The direct photolysis rate constant and half-life of four representative CECs, diclofenac (DCF), naproxen (NPX), sulfamethoxazole (SMX), and triclosan (TCS) in buffered water (pH 7.5) were experimentally obtained under simulated sunlight and natural sunlight conditions to be used in the validation of modelling predictions by GCSolar, ABIWAS and APEX software.

Under simulated sunlight conditions, TCS and DCF photodegradation were rapid obtaining half-lives of 0.59 h and 0.50 h, respectively. TCS photodegradation at pH 7.5 was rapid because the compound was in equilibrium with its neutral form and its more photolabile anionic form, indicating its non-persistence at the WTP. For DCF, literature reported enhanced photodegradation of DCF with increasing pH from 2 to 8 which may explain the rapid half-life measured in the current study. Additional experiments for TCS and DCF were conducted in filtered wastewater (0.2 μm) containing quenchers of photochemically-produced reactive intermediates (PPRI, e.g., •OH, •CO3-, 1O2, 3EfOM*) such as isopropanol (IPA), N,N-dimethylaniline (DMA), isoprene and 1,4-diazabicyclo[2.2.2]octane (DABCO). Both TCS and DCF half-lives in the wastewater solution were approximately doubled, obtaining half-lives of 1.14 h and 1.03 h, respectively. For SMX, the half-life was 6.18 h which was within the range of literature values of half-lives up to 17.3 h. Literature indicates that SMX does not readily undergo direct or indirect photolysis in natural waters. However, studies have reported faster photodegradation in wastewater solutions, suggesting that indirect photolysis may play a role in its removal within wastewater systems. For NPX, the half-life of 9.43 h found in the current study was longer than the literature reported values of approximately less than 1 h.

The direct photolysis rate experiments under natural sunlight for the four CECs were conducted in February 2024 on a building rooftop (latitude 37.8°S, longitude 144°E). Photodegradation rates were faster under natural sunlight of late-summer conditions (February 2024) compared to the simulated sunlight with photon intensity measured to be 0.311 Einstein m-2 h-1 for simulated sunlight and 0.531 to 0.598 Einstein m-2 h-1 for natural sunlight. The half-lives of TCS and DCF in buffered water (pH 7.5) under natural sunlight were 0.42 and 0.31 h, respectively, and were shorter than the simulated sunlight half-lives by 29-40%. The half-life for NPX under natural sunlight showed the greatest difference of about 7.2-fold. This was linked to differences in the photon intensity and spectra of the simulated and natural sunlight which showed a 71-92% difference in photon density. For SMX, the rate constant under natural sunlight was less discernible, obtaining a half-life range of 3.85 h to 27.7 h for the February experiment which was due to uncertainties related to longer sunlight exposure of SMX. The SMX rate experiments were repeated in May and July 2024, yet their degradation curves C vs. t and ln(Ct/C0) vs. t showed poor correlations (R2 &lt;0.5) and insignificant degradation.

The initial evaluation of the existing modelling software programs such as GCSolar, ABIWAS, APEX suggested that they were not fully suitable for predicting direct photolysis rates in wastewater systems under Australian climate conditions because they were developed for modelling in natural water systems using solar intensities in the northern hemisphere. Modifications to the software were undertaken for location-specific wastewater and climate conditions of the WTP and model validation was conducted by comparing experimentally obtained direct photolysis rates and half-lives of the four CECs. All software required rigorous calibrations to improve their prediction accuracy to match those of the experimental half-lives. APEX was the most flexible software for modifications of the solar intensity and water attenuation database. ABIWAS was limited to modelling in pure water systems and its solar intensity data could not be modified. An advantage of GCSolar was its ability to model photolysis as a function of time and water depth, providing useful insights of the attenuation of photodegradation. The equivalent monochromatic wavelength (EMW) approximation was conducted for the four CECs and were found to be less accurate when modelling at a shallower water depth of 0.1 m. This has implications for modelling in the WTP because photodegradation mainly occurs within the first 10 cm of wastewater surfaces. Sensitivity analyses of GCSolar, ABIWAS and APEX showed that these models were not sensitive (&lt;4.2% change in model output) to the input parameters of water depth and quantum yields.

Recommendations for future work in this area include more experimentally obtained rate constants of CECs in wastewater solutions under natural sunlight during different seasons. This will assist in the further validation of the modelling work. Strategies can be developed using the models so that WWTPs can maximise the photolytic removal of CECs, thereby improving wastewater treatment and preventing CECs from entering the environment. The optimised model will assist wastewater operators, planners and managers to monitor and track priority CECs in WWTPs. Overall, the research output improves scientific understanding of photolytic removal of CECs in wastewater systems as well as provides useful insight into the challenges of modelling direct photolysis in wastewater systems.