Designing a PAC/NF advanced process for controlling pharmaceutical compounds in reclaimed water

Abstract

"Water scarcity and asymmetric space-time distribution is driving many countries to seek nonconventional water sources. Treated wastewater is an emerging alternative water source but, depending on its use(s), it may require an increased level of water treatment. At the same time, the list of emerging contaminants (ECs), including pharmaceutical compounds (PhCs), of great relevance to the environment and to public health continues to grow. Further, many of these contaminants are partly or strongly resistant to conventional treatment in water and wastewater treatment plants (WTPs and WWTPs). Therefore, advanced treatment processes are being developed or optimised for improved PhC control. Amongst them, hybrid processes of adsorption – low pressure membranes are promising for their low risk of oxidation by-product formation and low operating pressures (< 2 bar). The fine powdered activated carbon (PAC), if properly selected for the target contaminants and background water matrix, can effectively adsorb the small dissolved molecules (such as the PhCs) passing through low-pressure membranes, whereas these membranes (usually microfiltration or ultrafiltration membranes and more recently “loose” nanofiltration membranes) safely retain the PAC particles, turbidity and bacteria. In LIFE aWARE project “Innovative hybrid MBR-(PAC-NF) systems to promote Water Reuse"", the combination of a membrane bio-reactor (MBR) with PAC / loose Nanofiltration (PAC/NF) is demonstrated at El Prat WWTP (Barcelona, Spain), being LNEC responsible for the PAC/NF design, namely for (i) selecting the PAC suitable for PhC control and (ii) assessing, at laboratory scale, different PAC/NF configurations to select the one to be implemented for demonstration at pilot scale, using a recently developed NF membrane (Pentair HFW 1000). This hollow fibre, hydrophilic polyethersulfone membrane with around 1000 Da cut-off constitutes a safe barrier also against viruses and provides higher retention of high molar mass organics. This paper presents LNEC’s PAC selection methodology and the bench-scale results of PhC removal by adsorption and by PAC/NF. The methodology developed allowed (i) identifying four target compounds representative of the set of ECs previously detected in El Prat WWTP and in Llobregat River and (ii) selecting a PAC suitable for controlling the four target ECs. The adsorption studies and modelling further developed allowed calibrating predictive models of the PAC adsorption capacity and rate. The PAC/NF bench-scale results showed high removal efficiencies of the target PhCs and the PAC continuous dosing to be more efficient than the pulse dosing, achieving an overall PhC removal of 68% (ranging from 58% to 89%). The results were successfully used in the design and operation of the PAC/NF pilot for the technology demonstration in El Prat WWTP."