Treatment technologies for emerging contaminants in wastewater treatment plants: A review

The "emerging contaminants" (ECs) are predominantly unregulated anthropogenic chemicals that occur in air, soil, water, food, and human/animal tissues in trace concentrations. The ECs are persistent in the environment, capable of perturbing the physiology of target receptors and, therefore, are regarded as contaminants of emerging environmental concerns in recent years. The prominent classes of ECs include pharmaceuticals and personal care products (PCPs), surfactants, plasticizers, pesticides, fire retardants, and nanomaterials. Some of the ECs with harmful effects on endocrine systems have been recognized as endocrine disrupting chemicals (EDCs). Since the 1990s intensive research has been done covering environmental occurrence, fate, ecological effects, and treatment technologies of ECs. However, a comprehensive summary of the EC removal techniques, particularly in wastewater treatment plants (WWTPs) are limited. Though the WWTPs are inefficient when it comes to ECs removal, they act as primary barriers against the spread of ECs. Therefore, this paper reviews the treatment technologies currently engaged for ECs removal in WWTPs for further possible upgrades of the existing designs. Results of this review indicate that the fate and distribution of ECs can be approximately estimated based on physicochemical properties like octanol-water partitioning coefficient (e.g., log K_OW > 4, maximum sorption potential) and solid-water distribution coefficient [e.g., K_d < 300-500 L/kg MLSS (mixed liquor suspended solids), insignificant sorption into sludge]. Biodegradation potential of ECs can be predicted from biodegradation constant values (e.g., K_bio < 0.01 = low biodegradation and >10 = high biodegradation). In WWTPs, the EC removal efficiency varies in the range of 20-50%, 30-70%, and >90% during the primary, secondary, and tertiary treatment steps, respectively. Tertiary treatment technologies are considered as the most suitable alternatives for ECs treatment, but complete ECs removal is yet to be achieved. Further advancements in the treatment technologies will unquestionably be necessary in the future.