Assessment and Prediction of Organic Micropollutant Removal Efficiency during Enhanced Wastewater Treatment with Activated Carbon Adsorption Process

Abstract

Due to large-scale production and use of synthetic chemicals in industrialized countries, various chemicals are found in the aquatic environment, which are often termed as organic micropollutants (OMPs). OMPs such as pharmaceuticals, personal care products, industrial chemicals, and pesticides are introduced to wastewater treatment plants (WWTPs), but they are not fully removed by conventional WWTP treatment processes. Enhanced treatment of WWTP effluents has been considered to minimize the discharge of problematic OMPs. Ativated carbon adsorption has received increasing attention as an enhanced effluent treatment technology, however, the information is currently limited for its treatability for various OMPs and economic/technical feasibility. Main objective of this PhD investigation was to assess the potential of powdered activated carbon (PAC) adsorption process for the OMP removal in WWTP treatment and to develop the prediction model for adsorptive OMP removal in various operational and water matrix conditions. The investigation consisted of the following four sub-topics: i) assessment of the priority OMPs in domestic WWTPs by quantitative monitoring and data-driven prioritization, ii) assessment of combined coagulation and PAC processes as an alternative primary wastewater treatment option to remove organic matter and OMPs, iii) development of a linear free energy relationship (LFER)-based prediction model for estimating isotherm parameters of OMPs, and iv) investigation of the effect of dissolved organic matter (DOM) on PAC adsorption and development of a model for predicting removal efficiency of OMPs in various matrices. In the first part, the occurrence, removal efficiency, and potential risk of 43 OMPs in four WWTPs were investigated to determine the priority OMPs. The biological treatment processes contributed the most to the overall removal efficiencies of target OMPs, and the A2O-MBR process exhibited higher performance compared to other biological treatment processes. Pesticides (atrazine, DEET), and corrosion inhibitors (4-methyl-1H-benzotriazole (4-TTR), 1H-benzotriazole (BTR)) showed the highest contribution to the sum of toxic units based on ecotoxicological risk assessment. Eight OMPs (4-TTR, atrazine, BTR, butyl-paraben, cimetidine, DEET, estrone, and propyl-paraben) were determined as priority OMPs based on the data-driven prioritization method. Overall, this study identified OMPs with potential ecological hazards in the effluent of domestic WWTPs, which supported the need for enhanced WWTP treatment for controlling OMPs. In the second part, different coagulants (Al2(SO4)3, PACl, and FeCl3) and PACs (SPC, F400, BL, and SPO) were compared and assessed to determine the major operational variables, such as dosage, and dosing sequence, influencing the removal of organic matter and 43 OMPs during primary wastewater treatment. In addition, the sequential and combined coagulation-PAC process was assessed to determine the impact of coagulation on the adsorption of DOM and OMPs. The coagulation process using 20 mg (Al or Fe)/L of inorganic coagulants was sufficient to achieve significant removal of dissolved organic carbon (DOC) and total phosphorous (TP) but insufficient for OMP removal. The adsorption process at a specific dose of 4.3 mgPAC/mgDOC was effective for removing most OMPs, with removal efficiencies of 63 – 99%. The studied PAC materials were effective in removing DOM and OMPs. Mesopores (5 – 10 nm), as well as the specific surface area and micropores, were important properties for enhancing the adsorption of OMPs. Adsorption was mainly responsible for removing OMPs in a combined process of coagulation and adsorption, and coagulation did not interfere with the adsorption of OMPs by PAC in the wastewater influent matrix. The simultaneous application of coagulation and PAC is proposed as a promising option for primary wastewater treatment to allow sufficient contact for adsorption and compact reactor size. In the third part, LFER-based models for predicting partition coefficients and Freundlich isotherm parameters were developed for diverse OMPs with different charge. The prediction model was constructed by integrating the conventional LFER model with intermolecular and coulombic interaction terms to predict the partition coefficients with improved prediction accuracy. The developed prediction model successfully simulated the partition coefficients of diverse OMPs under the condition of different residual concentrations (R2 = 0.75 with a standard deviation of 79%). Freundlich constants (log Kf) and exponents (1/n) were also well simulated with standard deviations of 21% and 83%, respectively. The molecular volume-related hydrophobic interaction (V) was found to be the most important solvation parameter affecting the adsorption strength. Overall, the models from this study improved the prediction accuracy for the adsorption factors (Kd, Kf, and 1/n) for diverse OMPs in a wide OMP concentration range (ng/L – mg/L). In the last part, the effect of DOM on PAC adsorption was investigated to develop a model for predicting the removal efficiency of OMPs. Quantitative and qualitative analysis of DOM using size exclusion and fluorescence excitation-emission methods were conducted to identify the competitive adsorption-inducing characteristics of DOM on the adsorption of OMPs during PAC treatment. Based on the partitioning coefficients predicted by the model of this study, the 43 OMPs could be categorized into the groups, which showed similar removal behaviors. The OMP removal prediction model was constructed by simplifying the existing equivalent background component model (EBCM). The WWTP effluent DOM showed similar molecular size distributions but different fluorescence properties. The microbially derived DOM (fluorescence index, FI) was the main DOM characteristics causing competitive adsorption in WWTP effluent. The simplified EBCM well predicted the OMP removal efficiency except for weakly adsorbable OMPs in different water matrices with standard deviations within 44%. Overall, the developed model can be useful for predicting OMP removal efficiency in PAC treatment based on the basic information on the OMP characteristis (Freundlich parameters) and water quality parameters (DOC). This dissertation identified several OMPs (e.g., 4-TTR, atrazine, BTR) with potential ecological hazards in the effluent of domestic WWTPs. To minimize their discharge, the feasibility of PAC adsorption process as an enhanced wastewater treatment was evaluated and demonstrated. In addition, the models developed in this study successfully predicted the removal efficiency of diverse OMPs under a wide range of process operating (PAC dose, dosing sequence) and different water matrix conditions. A large number of OMPs could be removed by 60 – 80% under a specific PAC dose of 2 mgPAC/mgDOC. In conclusion, PAC adsorption is an effective option for controlling OMPs from WWTP, and further investigation is proposed to confirm the applicability of PAC adsorption process through pilot- and full-scale studies.