In wastewater treatment, granular activated carbon (GAC) adsorption has been routinely used as a tertiary or advanced treatment option prior to the chlorine disinfection for controlling effluent organic matter (EfOM), the major precursor of halogenated disinfection byproducts (DBPs). These unintended halogenated DBPs may be chronically toxic to the aquatic species in receiving water bodies. As EfOM is usually hydrophilic and large in molecular weight, the DBP control (precursor removal) with the GAC adsorption prior to chlorination (GAC+Cl₂, i.e., the traditional approach) might not be effective. Recently, a new approach has been successfully developed to remove intermediate halogenated aromatic DBPs in chlorinated drinking water by GAC adsorption. This new approach (i.e., chlo...[ Read more ]

In wastewater treatment, granular activated carbon (GAC) adsorption has been routinely used as a tertiary or advanced treatment option prior to the chlorine disinfection for controlling effluent organic matter (EfOM), the major precursor of halogenated disinfection byproducts (DBPs). These unintended halogenated DBPs may be chronically toxic to the aquatic species in receiving water bodies. As EfOM is usually hydrophilic and large in molecular weight, the DBP control (precursor removal) with the GAC adsorption prior to chlorination (GAC+Cl₂, i.e., the traditional approach) might not be effective. Recently, a new approach has been successfully developed to remove intermediate halogenated aromatic DBPs in chlorinated drinking water by GAC adsorption. This new approach (i.e., chlorine disinfection prior to GAC adsorption, Cl₂+GAC) has been demonstrated to be substantially more effective in controlling overall halogenated DBPs and overall toxicity in drinking water than the traditional approach (i.e., GAC+Cl₂). However, it is unclear whether the effective DBP control with the new approach in drinking water treatment can be applied to wastewater treatment. Many intermediate halogenated aromatic DBPs, which generally have high affinity toward GAC have been detected and identified in chlorinated wastewater effluents. Besides, the chlorine residual in disinfected wastewater effluent can be harmful to aquatic species in the receiving water body, and dechlorination should be conducted before wastewater discharge. By reversing the sequence of GAC adsorption and chlorine disinfection (i.e., the new approach), the chlorine residual in the chlorinated effluent might be largely reduced by GAC, which may save the costs for purchasing, storing and applying a dechlorinating agent. Accordingly, the application of the new approach for controlling DBPs in chlorinated wastewater was evaluated in terms of removing DBPs, reducing toxicity, and saving the costs for dechlorination.

In this study, a lab-scale GAC adsorption module was coupled with a batch-scale chlorination test to simulate the new and traditional GAC treatment approaches. Saline and non-saline secondary sewage effluents were collected locally and subjected to the treatment with new and traditional approaches. For each wastewater effluent, the treated effluent samples with each approach were collected and a control sample was prepared by chlorination only without GAC adsorption. The new and traditional approaches were compared for DBP removal, toxicity reduction, and dechlorination agent consumption. Despite the wastewater matrix, significant reductions (60% to 99%) of aromatic halogenated DBPs in samples treated with the new approach were observed using ultra performance liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry. Besides, the removals of overall polar halogenated DBPs by the new approach were always 1.5 to 2.0 times higher than those with the traditional approach. Total organic halogen (TOX) measurements indicated that the new approach reduced the TOX by 5.6% to 24% more than the traditional approach. The developmental toxicity of the DBP mixtures in effluent samples treated with both approaches and in the control samples was evaluated using the embryos of the marine polychaete Platynereis dumerilii. Compared with the control samples, the new approach lowered the overall toxicity of the chlorinated wastewater effluents by 55% to 74%, which were significantly higher than those by the traditional approach (i.e., 41% to 66%). Furthermore, the chlorine residuals in wastewater samples treated with the new approach were substantially lower than those with the traditional approach, which substantially lowered the cost for purchasing a dechlorinating agent by around 90%. These results indicated that the new approach was cost-effective for wastewater treatment, as it not only significantly lowered DBP levels and the toxicity risks of chlorinated wastewater, but also substantially reduced the cost for dechlorination.