Considering energy efficiency in filtration of engineering nanoparticles

Engineering nanoparticles (ENPs) are an integral part of consumer products. Released to the atmosphere or disposed to sewage, ENPs quickly penetrate to surface and ground water sources. An absence of a dedicated ENP-retaining water treatment strategy imposes a potential health threat on drinking water consumers. The threat is met by gearing up an upgrade of treatment systems towards membrane filtration that provides a better barrier to ENP penetration at higher energy costs. The current study compares the energy demand of granular and membrane filtrations, with the retention efficiency of inorganic and organic ENPs. Dedicated experiments with gold and silver nanoparticles, dyed viruses, proteins, polysaccharides, and plasmid DNA showed that the energy demand increases from slow to river bank to rapid sand filtration, and to membrane ultrafiltration (UF). The UF alone consumes on average two times more energy than the entire coagulation-flocculation- sedimentation-sand filtration tray. The differences in retention efficiency however are much less pronounced. The traditional retention tray requires 0.4-0.45 kWh per m³ of effluent (kWh/m³) to provide between 90% and 99% (1 and 2 logs) ENP retention; 1 kWh/m³ on average is needed to secure the retention of 99.9% (3 log) ENPs by UF.