Application of a QWASI model to produce validated insights into the fate and transport of six emerging contaminants in a wastewater lagoon system

Abstract

The occurrence and fate of emerging contaminants (ECs) in surface water bodies is of increasing interest to water quality managers and environmental regulators throughout the world. Wastewater treatment plants are a major source of ECs in many aquatic environments. A modified Quantitative Water Air Sediment Interaction (QWASI) fugacity model was developed for a municipal wastewater lagoon system to study the behaviour of six representative ECs. As the wastewater lagoons were exposed to extensive periods of sunlight, the original model was modified by the addition of photolytic degradation as a removal mechanism. Laboratory studies were conducted over different seasons of a year to obtain the rate constants for the key processes of sunlight photodegradation, water and sediment transformation, as well as sediment sorption coefficients for the target ECs in the system to serve as model inputs. The model predicted the pathways for the different ECs and that at least 65% of the concentration of the ECs remained in the outflow of the first lagoon of the lagoon system after treatment. The greatest removal was predicted for sulfamethoxazole (35%) and the least for carbamazepine (5%). Multi-segment theory was applied to the single lagoon model and the predictions for the sequential six lagoon system were validated through field sampling. Sensitivity analysis revealed that the mass transfer coefficient between the water and sediment phases was the most influential parameter, with the four key process rate constants having various impacts depending on the EC. These results suggest that the modified QWASI model could be used to more accurately represent the fate and transport of ECs in this unique wastewater lagoon/stabilisation pond treatment system. Furthermore, it can be adapted to model a wide range of ECs in other wastewater treatment lagoon systems and thus assist with process optimisation and risk assessment of the treated water.