Are Membrane Bioreactors Better than Conventional Activated Sludge Systems to Remove Compounds of Emerging Concern?

Endocrine disrupting compounds (EDCs), pharmaceutically active compounds (PhACs) and personal care products (PCPs), are a substantial subset of emerging contaminants recently named as contaminant of emerging concern (CEC) that have been detected at trace concentrations in municipal wastewater effluents, groundwater and drinking water. CECs have already been detected at concentrations capable of inducing fish feminization and sexual development defects (known as “intersex”). Even though these compounds are not regulated, they have captured the public's attention, and as a result there is intense interest in preventing their release to surface waters, groundwaters and public water supplies. Wastewater treatment facilities are not designed to remove specific compounds (i.e., EDCs, PhACs) but are designed to remove gross contaminants (such as TSS, BOD, and COD). Removal efficiencies of CECs cannot be determined using gross contamination surrogates and NPDES permits do not require WWTPs to monitor for CECs. As a result, limited information is available for treatment efficiencies of CECs during wastewater treatment processes. Most work on CEC research in wastewater has involved in activated sludge systems including conventional activated sludge systems, MBR and extended aeration activated sludge systems. Findings and conclusions of these studies are not consistent. In this study, using literature findings, simple linear (one variable), multi variable linear and non-linear models are developed to establish CECs removal as function of operating parameters and physical-chemical properties of the compounds in CAS and MBR systems. The study results showed that both biodegradability and adsorption into the solids particles (reflected by Kow) strongly influence removal efficiencies of the CECs via CAS and MBR. However, the p-values of multiple regression lines suggest that adsorption takes more important role in MBRthan CAS. The findings in this study support that sorption is the dominant mechanism for the removal of slowly biodegradable CECs rather than biodegradation as suggested by Takigami et al. (2000) and Holbrook et al. (2002). These findings also explain why MBR exhibits better removal efficiencies than CAS for certain CECs when CAS an MBR operated at identical SRTs.